802.11Ax/Ay/Az Training

Apple’s Bluetooth Classic, BLE and 802.11ax/ay/az Training

1 802.11ax

2 802.11ax

Overview of 802.11ax 3 About the Instructor and Course Authors

Charles Alexi Senior Technology Consultant and Instructor [email protected] Mobile: +1-214-385-8366

4 Learning Objectives

• Upon completion of Overview of 802.11ax (Wi-Fi 6) Course, the participant will: • Learn about the fundamental concepts of 802.11ac evolution towards 802.11ax • Highlight concepts and ideas behind Wi-Fi® Alliance Wi-Fi® CERTIFIED 6 • Illustrate the core concepts on 802.11ax new features, PHY and MAC layers • Explain the rational behind 802.11ax PHY and MAC modifications • Explain about ideas behind OFDMA, MIMO spatial streams and Multi-user MIMO (MU-MIMO) features • Describe Modulation/Coding and Other elements/features behind 802.1ax • Learn about 802.11-MC Fine Timing Measurement (FTM) protocol

• Learn about 802.11s Mesh Networking and EasyMesh 5 Day 2’s Course Modules

Module 1: Overview of 802.11ax “High Efficiency WLAN (HEW)” Module 2: 802.11ac Evolution to 802.11ax-2019 Module 3: Key 802.11ax Technologies and Building Blocks: 802.11 ax PHY and MAC Enhancements: OFDMA/RU, MU- MIMO DL/UL Module 4: Overview of Spatial frequency reuse/BSS coloring, power savings, Target Wake Time/TWT, Trigger-based Random Access, MAC Enhancement Module 5: Overview of 802.11az and 802.11ay

6 6 h N H i g E f f i c i e n c y W L A

Overview of 802.11ax “High Efficiency WLAN (HEW)”

7 Overview of 802.11ax “High Efficiency WLAN (HEW)”

• IEEE 802.11ax: The Sixth Generation of Wi-Fi – IEEE 802.11ax will enhance existing 802.11a/g/11n/11ac deployments even if they are not fully upgraded to 802.11ax immediately. Its OFDMA-based channel access is fully backward-compatible with traditional EDCA/CSMA (enhanced distributed channel access, Carrier- sense multiple access)

• IEEE 802.11ax achieves many benefits by pushing on three different dimensions: – Denser modulation using 1024 Quadrature Amplitude Modulation (QAM), enabling a more-than-35-percent speed burst – Orthogonal Frequency Division Multiple Access (OFDMA)-based scheduling to reduce overhead and latency – Robust high-efficiency signaling for better operation at a significantly lower Received Signal Strength Indication (RSSI)

8 The Goals of 802.11ax/HE

• The Goals of 802.11ax are: Orthogonal Frequency Division – Enhancing operation in the 2.4 Multiple Access (OFDMA)-based GHz and 5 GHz band scheduling to reduce overhead and – latency; OFDMA-based channel Increasing average throughput per access is fully backward-compatible station by at least four times in with traditional EDCA/CSMA. dense deployment scenarios – Enhancing in both indoor and Denser modulation using 1024 outdoor environments Quadrature Amplitude Modulation – Maintaining or improving power (QAM), enabling a more-than-35- efficiency in STAs and IoT devices percent speed burst. Robust high-efficiency signaling for – Improving the efficiency of traffic better operation at a significantly management in a variety of lower Received Signal Strength environments Indication (RSSI). 9 The Goals of 802.11ax

• The goals of 802.11ax include:

– Enhancing operation in the 2.4 GHz and 5 GHz band

– Increasing average throughput per station by at least four times in dense deployment scenarios

– Enhancing in both indoor and outdoor environments

– Maintaining or improving power efficiency in stations » Improving the efficiency of traffic management in a variety of environments

10 802.11ax is Different than 802.11ac

•Focus is on increasing •Focus is on improving user total throughput to at Objectives performance by a factor of 4. least 1 Gbps •Dense deployment environments •application for a single with a mix of clients/AP and traffic client in indoor scenarios Scenarios types including outdoor situations Peak rate driven Metrics reflecting user experience •Link throughput KPIs/Metrics • average per STA throughput •Aggregate throughput • Area Throughput

•At least 4 times capability/better goodput at L2 for multiple STA, not a L1 throughput for a single STA •Modifications PHY and MAC improvement in the average throughput •Operations in frequency bands between 1 GHz and 6 GHz. •Backward compatibility and coexistence with legacy devices 11 Highlights of Key Features in 802.11ax

• Designed for high density connectivity with high overall capacity

• Uplink resource scheduling – Supports simultaneously serving lots of devices per AP – Increases capacity and efficiency – Improves device battery life

• MU-MIMO and OFDMA – Efficiently serves multiple traffic types with multiple APs on shared channels – MAC enhancements support newly introduced mechanisms

• Long OFDM symbols & higher order modulation – Provide increased efficiency

• Extended guard interval coverage – For improved coverage

12 Main Elements of 802.11ax High Efficiency WLAN (HEW)

– OFDMA – MU-MIMO • OFDM: use full bandwidth per user • Up to 8x8 MIMO in downlink and • OFDMA: scales resource for different uplink types of traffic -> increase overall efficiency, reduce latency • Serving up to 8 users • For high-band applications • For low-band applications 13 Main Elements of 802.11ax High Efficiency WLAN (HEW)

– Long OFDM Symbol – 1024-QAM •The OFDM symbol duration and cyclic •The 802.11ax standard now mandates prefix also increased 4X, keeping the raw support for 1024-QAM. Additionally, link data rate the same as 802.11ac, but the subcarriers are only 78.125 KHz improving efficiency and robustness in away from each other. 1024-QAM and indoor/outdoor and mixed environments. smaller cyclic prefix ratios for indoor

environment, which will increase the 14 maximum data rate. MU-MIMO and OFDMA: Scheduled UL multi- user access

Contention based resource Scheduling based resource allocation for legacy WLAN tech. VS. allocation 11ax – Un-coordinated resource – UL resource allocation by AP management – A must for dense scenarios – Devices compete to get resource until they succeed – QoS

Target Wake Time (TWT) • AP and device negotiate a specific time (awake) to access the medium, otherwise device sleeps • Reduce contention between users • Increase the device sleep time to reduce power consumption

15 Key 802.11ax (Wi-Fi CERTIFIED 6 ) Capabilities and Benefits

• 802.11ax or Wi-Fi CERTIFIED 6 devices operate in the 2.4 and 5 GHz bands and deliver greater capacity than the prior generation of Wi- Fi. Wi-Fi CERTIFIED 6 devices bring reliable performance indoors, outdoors, and in dense environments. – Client devices will also demonstrate longer battery life.

• Key features enabling the benefits of Wi-Fi CERTIFIED 6 are listed below. – Orthogonal frequency division multiple access (OFDMA) enables more users to simultaneously operate in the same channel and therefore improves efficiency, latency, and throughput – Multi-user multiple input, multiple output (MU-MIMO) allows more data to be transferred at once and enables an access point to handle a larger number of concurrent clients (MU-MIMO UL is new in 802.11x) – Transmit beamforming improves signal power resulting in significantly higher rates at a given range – 1024 quadrature amplitude modulation mode (1024-QAM) enables throughput increases by as much as 25 percent over Wi-Fi 5 – Target wake time (TWT) makes Wi-Fi CERTIFIED 6 devices more power efficient 16 The Need: Issues Facing Current Wi-Fi Networks

– Preponderance of short data frames that are not aggregated; large number of users – Significantly degrading system efficiency

1 2 3 1 4 1 – Overlapping BSS’s in dense 3 2 deployments unnecessarily 4 1 4 blocking each other from 2 3 transmitting 1 4 1 3 2 1

– Improving performance in outdoor hotspots to better compete with cellular

17 The Demand for High Efficiency Wi-Fi

Dense Wi-Fi Public access & Outdoor use & Large number of deployments offloading extended range devices

Targeting 4x throughput increase per station in dense environments over 11ac.

802.11ax builds upon 802.11ac, improving performance in dense deployment, using WLAN outdoors, and offers better support of real-time IoT real-time applications. 18 Objectives of 11ax/HE Efficiency: Scenarios Categories of objectives to improve WLAN efficiency: 1. Make more efficient use of spectrum resources in scenarios with a high density of STAs per BSS. 2. Significantly increase spectral frequency reuse and manage interference between neighboring overlapping BSS (OBSS) in scenarios with a high density of both STAs and BSSs. 3. Increase robustness in outdoor propagation environments and uplink transmissions.

• Capability to handle multiple simultaneous communications in both the spatial and frequency domains, in both the uplink (UL) and downlink (DL) direction. • Power efficiency is intended to measure consumption of devices which can reasonably be assumed to be powered by batteries and will take into account average power consumption for a given scenario 19 802.11ax Key Focus of Areas

Increasing per-user throughput 4X in high density scenarios: Need for the 802.11ax •Both indoor and outdoor • Very dense deployments •Pico-cell WiFi • Growing use of WLAN outdoors •Airport Wi-Fi • Better support of real-time applications with •Stadium Wi-Fi improved power efficiency •Many AP • Focusing on improving metrics that reflect •Heterogeneous Traffic, Many STAs user experience in typical conditions •Outdoor efficiency

20 New and Enhanced Applications

• Cellular Offloading • IoT and Smart Homes • Cloud Computing - including VDI • Display Sharing - 1-to-1, 1-to-many, Many-to-1 • Interactive Multimedia & Gaming • Progressive Streaming • Real-time Video Analytics & Augmented Reality • Support of wearable devices • Unified Communications - Including Video conf. • User Generated Content (UGC) Upload & Sharing • Video conferencing/tele-presence • Video distribution at home – (VHD, UHD) • Wireless docking 21 Main Scenarios

A Residential D Station

C e-Education

B Public transportation E Outdoor (Street)

802.11ax will handle client density more efficiently through a new channel-sharing capability, improve battery life using negotiated wake- time scheduling between APs and clients to preserve energy, and deliver efficiency improvements with at least four times more throughput than 22 802.11ac. 802.11ax Major Changes

• 1) Bidirectional multiplexing in space, frequency and time – In space, frequency and time domains – What is multiplexing? • Multiple users share a medium with minimum or no interference. • Example of interference: two STA talking at the same time. – Frequency: Bidirectional OFDMA up to 37 STAs independent MCS (modulation and coding scheme) – Space: Bidirectional MU-MIMO up to 8 spatial streams via trigger frames – Space: MU-MIMO flexibility (DU/UL cascading) and efficiency ( combining with OFDMA, control frame muxing) – Time: Spatial reuse via BSS coloring

23 Problem Statement

IEEE 802.11 HEW Focused primarily on 2.4 GHz and the 5 GHz frequency bands

MAC and PHY modifications in focused directions:

– (1) To improve efficiency in the use of spectrum resources in dense networks with large no. of STAs and large no. of Aps

– (2) To improve efficiency and robustness in outdoor deployments

– (3) To improve power efficiency

24 HEW Differentiating Features Previous 802.11 HEW Amendments being Amendments considered Increase the per-link Increase the average per STA Objectives peak throughput throughput in dense environments

Single application for a Dense deployment environments with Scenarios single client in indoor a mix of clients/APs and traffic types situations including outdoor situations

User Experience Driven Peak rate driven - Average per station throughput, KPIs/ - Link throughput, - 5th %ile per station throughput, Metrics - Aggregate throughput - Area throughput - Power efficiency

25 802.11ax Requirements

• Based on the aforementioned scenarios and expected use-cases, there are four key requirements for the ax-2019 amendment.

– 1. Coexistence: WLANs operate as unlicensed devices in the ISM (Industrial, Scientific and Medical) bands. Therefore, the ax-2019 amendment has to include the required mechanisms to coexist both with the other wireless networks that also operate there and with the licensed devices. – 2. Higher throughput: Improving both the system and user throughput requires the improved use of channel resources. ax-2019 aims for a 4-fold throughput increase compared with ac-2013. To achieve this goal, some new wireless technologies such as Dynamic CCA, OFDMA (Orthogonal Frequency Division Multiple Access), and advanced multiple-antenna techniques may be used. – 3. Energy efficiency: The target in ax-2019 is - at least - to not consume more than the previous amendments, considering the aforementioned 4-fold throughput increase, which requires both new low-power hardware architectures and new low-power PHY/MAC functionalities. – 4. Backward Compatibility: Because WLANs implementing ax-2019 must also support devices using any previous PHY/MAC amendments, mechanisms must also be implemented to make it backward compatible (i.e., common frame headers and transmission rates), although it is a clear source of inefficiency. 26 WiFi Alliance Requirements for WiFi Certified 6 (2019) • New PHY and MAC modifications of 802.11ax has a few pre-requisite:

– All WiFi Certified 6 (AX) might require to be WiFi Certified AC (5) and WiFi Certified N (4).

– All WiFi Certified 6 (AX) might require to be WiFi Certified Agile Multiband (broader visibility into network loading, and the ability to move (or be moved) to the optimum band and AP.

– Wi-Fi Protected Access Version 3 (WPA3) compliant.

27 Wi-Fi Protected Access Version 3 (WPA3)

• All 802.11ax equipment will have to meet new security for authentication, authorization and encryption.

• The Wi-Fi Protected Access Version 3 (WPA3) security protocol, introduces security enhancements, most importantly, Simultaneous Authentication of Equals (SAE) as a replacement for WPA2-Personal’s Pre-Shared Key (PSK). – WPA3 also requires Protected Management Frames (PMF) for more robust protection against brute force attacks. – Additionally, WPA3 provides for an optional 192-bit encryption suite.

28 Convergence with LTE and 5G

• Drivers for convergence: – Spectral efficiency – Higher data rates – Long range – Good battery life

• 5G Radio technologies and 802.11ax share many characteristics: – OFDMA – MU-MIMO – Spatial diversity – Beamforming – Channel Aggregation – And others 29 Relative Performance of 802.11ax

• Absolute bits/s target

• Use what we have in a more efficient way

• Overall goal is to increase multi-STA L2 goodput – Not a single STA L1 data rate

802.11ax addresses some of today’s biggest high density and performance challenges – increasing capacity by up to 4x and improving spectral efficiency to benefit both 2.4 GHz and 5 GHz bands in a variety of environments.

30 Scenarios Defined in HEW

Scenario Name Topology Management Channel Homogeneity Traffic Model Model A - Apartmentbuilding e.g. ~10m x 10m apartments in a 1 Residential multi-floor building Unmanaged Indoor Flat Home ~10s of STAs/AP, P2Ppairs B - Dense small BSSs with clu sters e.g. ~10-20m inter APdistance, 2 Enterprise Enterprise ~100s of STAs/AP, P2Ppairs

Managed Indoor Flat C - Dense small BSSs, uniform Indoor Small B 3 e.g. ~10-20m inter APdistance Mobile SS Hotspot ~100s of STAs/AP, P2Ppairs

Outdoor Large B D - Large BSSs, uniform 4 SS Hotspot e.g. 100-200m inter APdistance Managed Flat Mobile ~100s of STAs/AP, P2Ppairs Outdoor Outdoor Large B Managed + Un Mobile + 4a SS Hotspot D+A managed Hierarchical Home + Residential

31 Residential Scenario • In each apartment, AP is placed in random xy-locations at z = 1.5 m above the floor level of the apartment • In each apartment, 5 STAs are placed in random xy-locations at z = 1.5m above the floor level of the apartment • Channel model: TGac channel B • Primary channel: Random assignment of 3 non overlapping channels • AP Tx power: 23dBm • STA Tx power: 17dBm

32 Enterprise Scenario

• Office floor configuration 8 offices 64 cubicles per office Each cubicle has 4 STAs • 4 APs per office installed on the ceiling • STAs are placed in random position within a cubicle (x,y,z=2) • Channel model: TGac channel D • Primary channel: mod(BSS_index,4) • AP Tx power: 24dBm • STA Tx power: 21dBm

33 Indoor Small BSS Scenario

• BSSs are placed in a regular and symmetric grid with 7 meter BSS

BSS BSS BSS radius (frequency reuse 3 configuration) BSS BSS BSS BSS • AP is placed at the center of the BSS • 30 STAs are placed randomly in a BSS BSS BSS BSS BSS BSS

BSS BSS BSS BSS • Channel model: TGac channel D for AP-AP, AP-STA, TGac channel B for BSS BSS BSS STA-STA • Primary channel: same for all BSSs • AP Tx power: 17dBm • STA Tx power: 15dBm

34 Outdoor Large BSS Scenario

• Outdoor street deployment • overlap of 3 operators • define a 19 hexagonal grid with ICD = 130 meters • AP is placed at the center of theBSS • 50 STAs are placed randomly in a BSS • Channel model: UMi channel model • Primary channel: same for all BSSs • AP Tx power:30dBm • STA Tx power:15dBm

35 REVIEW: 802.11ax Requirements

• Based on the aforementioned scenarios and expected use-cases, there are four key requirements for the ax-2019 amendment.

• High Efficiency (HE) basic service set (BSS): A BSS in which a Beacon frame transmitted by an HE station (STA) includes the HE Operation element.

• High Efficiency (HE) beamformee: An HE station (STA) that receives an HE physical layer (PHY) protocol data unit (PPDU) that was transmitted using a beamforming steering matrix.

• High Efficiency (HE) beamformer: An HE station (STA) that transmits an HE physical layer (PHY) protocol data unit (PPDU) using a beamforming steering matrix.

• High Efficiency (HE) extended range (ER) single-user (SU) physical layer (PHY) protocol data unit(PPDU): An HE PPDU transmitted with HE ER SU PPDU format that carries one PHY service data units (PSDU) for one user.

• High Efficiency (HE) modulation and coding scheme (HE-MCS): A specification of the HE physical layer (PHY) parameters that consists of modulation order (e.g., BPSK, QPSK, 16-QAM, 64-QAM, 256-QAM, 1024-QAM) and forward error correction (FEC) coding rate (e.g., 1/2, 2/3, 3/4, 5/6) and that is used in an HE PHY protocol data unit (PPDU).

37 Basic 802.11ax (HE) Definitions

• Orthogonal frequency division multiple access (OFDMA): – An OFDM-based multiple access scheme in which groups of subcarriers are allocated to different users of the channel, allowing simultaneous data transmission to or from several users of the channel.

• Orthogonal frequency division multiple access (OFDMA) high efficiency (HE) physical layer (PHY) protocol data unit (PPDU): – A 20 MHz HE PPDU with RUs smaller than 242-tone, or a 40 MHz HE PPDU with RUs smaller than 484-tone, or an 80 MHz HE PPDU with RUs smaller than 996-tone, or a 160 MHz or 80+80 MHz HE PPDU with RUs smaller than 2x996-tone.

38 Basic 802.11ax (HE) Definitions

• Target Wake Time (TWT): – Target wake time (TWT) is a power-saving mechanism originally defined in the 802.11ah-2016 amendment. A TWT is a negotiated agreement, based on expected traffic activity between the access point (AP) and Wi-Fi clients, to specify a scheduled target wakeup time for clients in power-save (PS) mode. – In addition to the power-saving benefits, the negotiated TWTs allow an AP to manage client activity by scheduling client stations to operate at different times and therefore minimize contention between the clients.

39 Basic 802.11ax (HE) Definitions • Target wake time (TWT) scheduling access point (AP): An AP that schedules broadcast TWTs and provides these schedules in a broadcast TWT element.

• Target wake time (TWT) scheduled STA: A STA that follows the schedules provided in a broadcast TWT element.

• Triggered uplink access (TUA): A mechanism by which one or more non-AP stations (STAs) simultaneously participate in an uplink (UL) transmission to an access point (AP) using resource units (RUs) allocated in the preceding Trigger frame.

• Uplink (UL) orthogonal frequency division multiple access (OFDMA)- based random access (UORA): – A random access mechanism for non-AP HE STAs to participate in uplink OFDMA transmissions in one or more designated random access resource units (RUs).

40 802.11ax Abbreviations

MUEDCATimer Multi-user EDCA timer OBO Orthogonal frequency division multiple access (OFDMA) random access backoff OCW Orthogonal frequency division multiple access (OFDMA) contention window OFDMA Orthogonal frequency division multiple access OM Operating mode OMI Operating mode indication PPE Physical layer (PHY) packet extension PPET Physical layer (PHY) packet extension threshold QTP Quiet time period RA-RU Random access resource unit RDP Reverse direction protocol ROM Receive operating mode RPL Received power level RU Resource unit SF Scaling factor SRP Spatial reuse parameters SR Spatial reuse SRG Spatial reuse group TB Trigger-based TOM Transmit operating mode TRS Triggered response scheduling TUA Triggered uplink access UL Uplink UL MU Uplink multi-user UORA Uplink orthogonal frequency division multiple access (OFDMA) based random access UPH Uplink power headroom

41 802.11ax Abbreviations

A-Control Aggregated control BQR Bandwidth query report BQRP Bandwidth query report poll BFRP Beamforming report poll BSR Buffer status report BSRP Buffer status report poll CAS Command and status CCDF Complementary cumulative distribution function CQI Channel quality indication DCM Dual carrier modulation DL MU Downlink multi-user HE High efficiency LA Link adaptation MU-BAR Multi-user block ack request MU-RTS Multi-user request to send

42 h N H i g E f f i c i e n c y W L A

802.11ac Evolution to 802.11ax-2019

43 Wi-Fi Generation Comparison Overview

Wi-Fi generation names generation names Wi-Fi 4, Wi-Fi 5, and Wi-Fi 6 are intended to be used widely throughout the Wi-Fi ecosystem by Wi-Fi Alliance members, non-members, industry partners, media and analysts. Generation names shall be used in text format to refer to the corresponding Wi-Fi technology for the generation. Adoption of generation names Wi-Fi 4, Wi-Fi 5, and Wi-Fi 6 as industry terminology is encouraged for use in marketing materials and promotion with consumers, media and analysts. Generations of Wi-Fi prior to Wi-Fi 4 will not be assigned names. If a company adopts Wi-Fi generation names, the following guidelines shall be followed: References to 802.11ax technology shall use the generation name Wi-Fi 6 References to 802.11ac technology shall use the generation name Wi-Fi 5 References to 802.11n technology shall use the generation name Wi-Fi 4

Source: WiFi Alliance 44 802.11ac vs. 802.11ax: Main Differences

45 802.11ac Evolution to 802.11ax • “Enhanced Wi-Fi- 802.11ax” or WiFi 6 – 802.11ax is an evolution of 802.11ac. It delivers an improved experience (higher speeds, lower latency) for more users and devices at homes, in outdoor or dense environments, even when using high-bandwidth applications (AR/VR), while also providing flexible support for the differing requirements (low power, wide area) of the IoT.

– In aggregate, 802.11ax can deliver up to 40% higher peak data rates for a single device, and improve average throughput per user by at least four times in dense or congested environments. • It also can increase network efficiency and extend the battery life of devices.

– IEEE 802.11ax will enhance existing 802.11a/g/11n/11ac deployments even if they are not fully upgraded to 802.11ax immediately.

– With 802.11ax, the Evolution of Wi-Fi Brings Us Closer to a 5G Future

46 802.11n, 802.11ax, and 802.11ax Comparison

802.11n 802.11ac 802.11ax High Throughput (HT) WLAN Very High Throughput (VHT) WLAN High Efficiency (HE) WLAN

Frequency band (GHz) 2.4 and 5 5 2.4 and 5 Multiplexing scheme OFDM OFDM OFDMA Channel bandwidth (MHz) 20, 40 20, 40, 80, 160, 80+80 20, 40, 80, 160, 80+80 Subcarrier spacing 312.5 kHz 312.5 kHz 78.125 kHz (for non-legacy portion) Symbol duration, not including guard 3.2 3.2 3.2, 6.4 or 12.8 interval (µsec) Guard interval/cyclic prefix (µsec) 0.8 0.4 or 0.8 0.8, 1.6 or 3.2 Number of spatial streams 1~4 1~8 1~8 Multi-user (MU) technology Not available MU-MIMO: downlink only, up to 4 MU-MIMO: downlink and uplink, users up to 8 users OFDMA: downlink and uplink Resource unit (RU) size (# of Full channel bandwidth Full channel bandwidth 26, 52, 106, 242, 484, 996, 2*996 subcarriers, also known as tones) Data subcarrier modulation BPSK, QPSK, 16QAM, 64QAM BPSK, QPSK, 16QAM, 64QAM, BPSK, QPSK, 16QAM, 64QAM, 256QAM 256QAM, 1024QAM Channel coding BCC (mandatory) LDPC BCC (mandatory) LDPC (optional) BCC (mandatory) LDPC (optional) (mandatory) Uplink scheduling (managed by No No Yes access point) Maximum theoretical data rate 600 6933.3 9607.8 (Mbps)

47 Wi-Fi CERTIFIED 6

• Wi-Fi CERTIFIED 6™, the industry certification program based on the IEEE 802.11ax standard, enables next generation Wi-Fi connectivity providing the capacity, coverage, and performance required by users—even in environments with many connected devices such as stadiums and other public venues. • Wi-Fi CERTIFIED 6 networks enable lower battery consumption in devices, making it a solid choice for any environment, including smart home and Internet of Things (IoT) uses.

• Key benefits of Wi-Fi CERTIFIED 6 technology include: – Higher data rates – Increased capacity – Performance in environments with many connected devices – Improved power efficiency

Source: WiFi Alliance 48 802.11ac Evolution to 802.11ax Nine Main Components of Enhancement

• 802.11ax focuses on nine main components of enhancement: 1. 5 GHz and 2.4 GHz support 2. Orthogonal frequency-division multiple access (OFDMA) uplink and downlink (UL/DL) 3. Longer orthogonal frequency-domain multiplexing (OFDM) symbol 4. Multi-user multiple-input multiple-output (MU-MIMO) 8×8 and UL/DL 5. 1024 quadrature amplitude modulation (1024-QAM) 6. New PHY headers 7. Spatial reuse, also referred to as BSS Coloring 8. Target Wake Time (TWT): power saving 9. Enhanced outdoor robustness 49 802.11ac vs. 802.11ax

50 802.11ac vs. 802.11ax: OFDM vs. OFDMA

802.11ac OFDM 802.11ax OFDMA Bandwidth wasted - Higher latency Bandwidth fully utilized - Lower latency OFDM is sometimes referred to as discrete multi-tone OFDMA distributes subcarriers among users so all users can modulation because, instead of a single carrier being transmit and receive at the same time within a single modulated, a large number of evenly spaced subcarriers channel on what are called subchannels. What’s more, are modulated using some m-ary of QAM. This is a spread- subcarrier-group subchannels can be matched to each user spectrum technique that increases the efficiency of data to provide the best performance, meaning the least communications by increasing data throughput because problems with fading and interference based on the location there are more carriers to modulate and propagation characteristics of each user. 51 802.11ac vs. 802.11ax: Main Differences

52 h N H i g E f f i c i e n c y W L A

Key 802.11ax Technologies and Building Blocks: 802.11 ax PHY and MAC Enhancements- OFDMA/RU, MU-MIMO DL/UL 53 REVIEW: 802.11ax Enhancements vs. 802.11ac

54 REVIEW: 802.11ax Categories of Enhancements

Spectral Efficiency & Area Throughput High Density DL/UL MU-MIMO OFDMA 1024 QAM w/ 8 clients Long OFDM Spatial Reuse 25% increase Symbol in data rate ac

8x8 AP ax 2x increase Up to 20% in throughput increase in data rate Power Saving Outdoor / Longer Range Scheduled sleep and wake times Next TWT B TWT WakeInterval B e e a a c c o T DL/UL T DL/UL T DL/UL T DL/UL o n F MU F MU F MU F MU n

TWT element: Implicit TWT, Next TWT, TWT Wake Interval

80 MHz Capable 20 MHz-only clients 20 MHz-only

0.8us Enhanced delay 11ac spread protection- 1.6us 11ax long guard interval 3.2us 11ax

55 Elements in the 802.11ax Building Blocks

• Network Capacity – OFDMA • Power Efficiencies • Device Battery Life – MU-MIMO – BSS Coloring • Outdoor Reliability • Long OFDMA Symbol • Peak Throughput Increase • Enhanced Wi-Fi Coexistence – 1024-QAM • BSS Coloring – Long OFDMA Symbol

56 802.11ax Major Features: Mandatory and Optional

57 How Fast is 802.11ax?

• Let’s say we take the more conservative 4x estimate, and assume a massive 160MHz channel.

• In that case, the maximum speed of a single 802.11ax stream will be around 3.5Gbps (compared with 866Mbps for a single 802.11ac stream).

• Multiply that out to a 4×4 MIMO network and you get a total capacity of 14 Gb/s.

• Multiply that out to a 8×8 MIMO network and you get a total capacity of 28 Gb/s.

• In a more realistic setup with 80MHz channels, we’re probably looking at a single- stream speed of around 1.6Gbps, which is still a reasonable 200MB/sec.

• If your device supports MIMO, you could be seeing 400 or 600MB/sec.

• And in an even more realistic setup with 40MHz channels (such as what you’d probably get in a crowded apartment block), a single 802.11ax stream would net you 800Mbps (100MB/sec), or a total network capacity of 3.2Gbps.

58 HE PHY – Modulation

• The HE PHY data subcarriers are modulated using: – Binary phase shift keying (BPSK), quadrature phase shift keying (QPSK), 16-quadrature amplitude modulation (16-QAM), 64-QAM, 256- QAM and 1024-QAM. – Forward error correction (FEC) coding (convolutional or LDPC coding) is used with coding rates of 1/2, 2/3, 3/4 and 5/6.

59 Multiuser Technologies used in 802.11ax

60 Technical Highlights Multi-user support: MU-MIMO and OFDMA – Increase network efficiency by multiplexing users in both frequency

and space Freq

PHY OFDMA Header Sub- STA#10 Band • 4 STAs STA#35 STA#26 • 52 Subcarriers each

0MHz 20 STA#54 • Option 1: No MIMO

SS 1,2 STA#3 • Option 2: Each 2 SS

SS 3,4,5 STA#8 0MHz 20 Time • STA #3, #8 and #19 SS 6 STA #19 • Using MIMO features • No OFDMA

MU-MIMO Space

Frames are transmitted employing either OFDMA, MU-MIMO or a mixture of both

OFDMA allows for multiple-user access by subdividing a channel. MU-MIMO allows for multiple-user access by using different spatial streams. Access points will send unique steams of data to multiple clients simultaneously. 61 802.11ax Channelization

5170 5330 5490 5710 5735 5835 MHz MHz MHz MHz MHz MHz 36 40 44 48 52 56 60 64 100 104 108 112 116 120 124 128 132 136 140 149 153 157 161 165 IEEE channel 144 # 20 MHz 40 MHz 80 MHz 160 MHz

62 REVIEW: 802.11ax Nine Components • 802.11ax focuses on nine main components of enhancement:

1. Orthogonal frequency-division multiple access (OFDMA) uplink and downlink (UL/DL) 2. Multi-user multiple-input multiple-output (MU-MIMO) 8×8 and UL/DL 3. Longer orthogonal frequency-domain multiplexing (OFDM) symbol 4. 1024 quadrature amplitude modulation (1024-QAM) 5. Spatial reuse, also referred to as BSS Coloring 6. Target Wake Time (TWT) — power saving 7. New PHY headers 8. Enhanced outdoor robustness 9. 5 GHz and 2.4 GHz support

63 Review: New Features in 802.11ax

1.Downlink and uplink OFDMA 2.Downlink and uplink multi-user MIMO (MU-MIMO) 3.Transmit beamforming 4.Higher order modulation (1024-QAM) 5.Outdoor operation 6.Reduced power consumption 7.Spatial re-use

64 New Features in 802.11ax

•Downlink and Uplink OFDMA: •OFDMA is one of the more complex features in 802.11ax. It allows a single transmission (for downlink OFDMA, the access point transmits) to be split by frequency within a channel, such that different frames addressed to different client devices use groups of subcarriers. •Uplink OFDMA is equivalent to downlink OFDMA, but in this case multiple client devices transmit simultaneously, on different groups of subcarriers within the same channel. •Uplink OFDMA is more difficult to manage than the downlink variety, as many different clients must be coordinated: the access point transmits trigger frames to indicate which sub- channels each client can use.

65 New Features in 802.11ax

• Downlink and uplink multi-user MIMO – The downlink version extends an existing 802.11ac feature where an access point determines that multipath conditions allow it to send, in a single time-interval, frames to different client devices.

– 802.11ax increases the size of downlink MU-MIMO groups, allowing more efficient operation.

• For uplink multi-user MIMO like uplink OFDMA, the access point must coordinate the simultaneous transmissions of multiple clients.

66 New Features in 802.11ax

•Transmit Beamforming: This is another existing feature where an access point uses a number of transmit antennas to land a local maximum signal on a receiver’s antennas. It improves data-rates and extends range. •Higher-Order Modulation: 802.11a/g introduced 64-QAM, and 802.11ac 256 QAM: in 802.11ax, the highest-order modulation is extended to 1024-QAM. •This increases peak data-rates under good conditions (high SNR). •OFDM symbols, subcarrier spacing and FFT size are all changed to allow efficient operation of small OFDMA sub- channels: these changes allow an increase in the length of guard interval without loss of symbol efficiency. •Outdoor Operation: A number of features improve outdoor performance. The most important is a new packet format where the most sensitive field is now repeated for robustness. Other features that contribute to better outdoor operation include longer guard intervals and modes that introduce redundancy to allow for error recovery. •Reduced Power Consumption: Existing power-save modes are supplemented with new mechanisms allowing longer sleep intervals and scheduled wake times. Also, for IoT devices, a 20MHz-channel-only mode is introduced, allowing for simpler, less powerful chips that support only that mode.

67 New Features in 802.11ax

•Spatial re-use: When contending for a transmit opportunity, a device is allowed to transmit over the top of a distant transmission, which would previously have forced it to wait. •This increases network capacity by allowing more simultaneous transmissions in a given geographic area. •In historical context, it can be seen that the new features in 802.11ax are mostly extensions or improvements on previous work – with the standout exceptions of OFDMA and spatial re-use, which are new territory.

68 New Subcarrier Spacing and Symbol Duration

• The OFDM symbol is the basic building-block of a Wi-Fi transmission. It is a small segment in time of the modulated waveform of a subcarrier, carrying information: the more variants of a symbol are available, the more information (binary bits) it can carry.

– The fundamental characteristics: fast Fourier transform (FFT) size, subcarrier spacing and OFDM symbol duration are linked, given a fixed channel width.

– In 802.11ax, the subcarrier spacing is reduced by a factor of 4x while the OFDM symbol duration increases by 4x.

69 802.11ax Subcarrier Spacing

802.11ac SUBCARRIER SPACING=312.5 kHz

802.11ax SUBCARRIER SPACING=78.125 kHz 20 MHz / 0.078 = 256 tones

70 Subcarrier spacing

802.11ax standard will operate in both the 2.4 GHz and 5 GHz bands. The specification defines a four times larger FFT, multiplying the number of subcarriers. However, one critical change with 802.11ax is that the subcarrier spacing has been reduced to one fourth the subcarriers spacing of previous 802.11 revisions, preserving the existing channel bandwidths.

The OFDM symbol duration and cyclic prefix also increased 4X, keeping the raw link data rate the same as 802.11ac, but improving efficiency and robustness in indoor/outdoor and mixed environments. Nevertheless, the standard does specify 1024-QAM and smaller cyclic prefix ratios for indoor environment, which will increase the maximum data rate. 71 OFDM symbol duration & subcarriers

72 802.11ax introduces a longer OFDM symbol time of 12.8 μs • 802.11ax introduces a longer OFDM symbol time of 12.8 μs, which is four times longer than the legacy symbol time of 3.2 μs.

• Subcarrier spacing is equal to the reciprocal of the symbol time. As a result of the longer symbol time, the subcarrier size and spacing decreases from 312.5 KHz to 78.125 KHz.

• The narrow subcarrier spacing allows better equalization and therefore enhanced channel robustness. Because of the 78.125 KHz spacing, an OFDMA 20 MHz channel consists of a total of 256 subcarriers (tones).

73 Decoding Issues

• Very low data rates address the problem of decoding signals at long range or in noisy environments because smaller resource units (that is, reduced number of OFDMA sub-carriers) require a lower total energy and still achieve the same Signal-to-Noise Ratio (SNR).

• Although the smallest channel in 802.11ac was 20 MHz, the smallest resource unit in 802.11ax is 2MHz, resulting in a very significant 8-dB reduction in the noise power, and accordingly allowing the required signal power to be 8 dB lower too.

– This situation allows 802.11ax to tolerate 8 dB more noise and achieve a much larger coverage area for low-bit-rate clients (such as IoT telemetry data).

74 Flexible PHY Timing and Guard Interval (GI)

• The flexible PHY timing, including Guard Interval (GI), addresses the problem of multipath fading (for example, outdoor) whereby “echo” energy from one OFDM symbol leaks into the next OFDM symbol, causing Inter-Symbol Interference (ISI).

• It can be shown that this more robust guard interval results in up to twice the throughput in outdoor environments such as those currently served by cellular/LTE technology.

• These two capabilities combined allows Wi-Fi operators to offer compelling cost-efficient Wi-Fi-based solutions competitive with 4G LTE and 5G-NR for the lower-speed IoT space.

75 PHY Timing Options

To support outdoor (for example, metropolitan) or partially outdoor (for example, stadium or hotspot) channels, the guard interval in 802.11ax is extendable from the original 0.8- μs specification of 802.11ac to 1.6 μs or as high as 3.2 μs, depending on the channel type.

when the RF channel is spatially compact (for example, indoor small cells), the Delay Spread (DS) or difference between the shortest and longest path is small (for example, 300 ft) and thus exhibits low delay spread (for example, 300 ns). However, when the RF channel is spatially large (for example, outdoor large cells), then the delay spread is high; for example, one signal component might be LOS but the next may bounce off a far-off building, resulting in a path difference of approximately 1 km (3200 ft) and thus exhibit very high delay spread (3.2 μs). In all OFDMA systems such as 802.11ax and LTE, the OFDMA guard interval must be longer than the delay spread in order to avoid significant decoding errors caused by ISI or the time overlap of one version of a signal upon itself. 76 Longer OFDM Symbol Time, Lower Subcarrier Spacing

802.11c: Less subcarriers More spaced from each other

Cyclic prefix acts as a buffer region where delayed information from the previous symbols can get 802.11ax: More subcarriers stored. The receiver has to exclude samples from the cyclic prefix which got corrupted by the Closer to each other previous symbol when choosing the samples for an OFDM symbol. 77 Technology Building Blocks of 802.11ax

Source: Qualcomm 78 802.11ax PHY • 802.11ax devices are required to support 20, 40, and 80 MHz channels and 2 spatial stream. Several optional features are also defined in 802.11ax: – Using OFDMA – Wider channel bandwidths (80+80 MHz and 160 MHz) – Higher modulation support (optional 1024QAM) – New FFT sizes: 1024, 2048 (256, 512 stays the same as 11ac ) – Carrier spacing of 78.125 kHz – Guarding Interval • 0.8 usec • 1.6 usec • 3.2 usec – 2 or more spatial streams (up to 8) – MU-MIMO (Multi-User MIMO) – The HE PHY provides support for 800 ns, 1600 ns, and 3200 ns guard interval durations. – STBC (Space Time Block Coding) – LDPC (Low Density Parity Check) 79 UL OFDMA and MU-MIMO: Scheduled UL Access for Increased Capacity and Efficiency

80 OFDMA and MU-MIMO

• Multi-user Multiple-Input, Multiple-Output (MU-MIMO) and Orthogonal Frequency-Division Multiple Access (OFDMA) are two of the most significant technical enhancements in 11ax. – Both MU-MIMO and OFDMA are multiuser technologies that enable simultaneous bidirectional communication between an access point (AP) and end users.

MU-MIMO technology remains the same between 11ac and 11ax. When multiple clients are trying to access the medium at the same time, the AP uses RF multipath to send frames to multiple clients at the same time instance. This technology is called MU-MIMO and it utilizes diversity in space.

The difference between downlink MU-MIMO in 11ac and 11ax is that in the latter, the groups of clients are now bigger (up to eight clients in a group) instead of a maximum of four clients in one group. In contrast, MU-MIMO on the uplink is a new feature in 802.11ax.

OFDMA and MU-MIMO are complementary technologies. While OFDMA is ideal for low-bandwidth, small- packet applications such as IoT sensors, MU-MIMO increases capacity and efficiency in high-bandwidth 81 applications like mission-critical voice calls and video streaming. Multiple Access Methods and Advanced Antenna in 802.11ax OFDM OFDM, OFDMA •Orthogonal Frequency Division Multiplexing; Orthogonal Frequency Division Muliple Access

Power •The signal consists of many (from dozens to Frequency thousands) of thin carriers carrying symbols •In OFDMA, the symbols are for multiple users •OFDM provides dense spectral efficiency and robust resistance to fading, with great flexibility of use MIMO MIMO •Multiple Input Multiple Output •An ideal companion to OFDM, MIMO allows exploitation of multiple antennas at the base station and the mobile to effectively multiply the throughput for the base station and users

82 FDMA TDMA

CDMA OFDMA 83 OFDM vs. OFDMA

• Each color represents a burst of user data. In a given period, OFDMA allows users to share the available bandwidth. 84 OFDMA technology complements MU-MIMO

• Orthogonal Frequency-Division Multiple Access (OFDMA) technology, like that used by LTE networks, is a part of 11ax. – It basically divides the channels into smaller segments and allows multiple devices to talk simultaneously, each in their own channel segment, technically called a resource unit (RU). – Though it doesn’t directly increase data rates, it allows devices to better and faster coordinate when they can talk, overall making more efficient use of the channels.

Resource Units (RUs) will function as OFDMA sub-channels When subdividing a 20 MHz channel, The AP can designate 26, 52, 106, and 242 subcarrier Resource Units (RUs), which equates roughly to 2 MHz, 4 MHz, 8 MHz, and 20 MHz channels. The 802.11ax AP dictates how many RUs are used within a 20 MHz channel and different combinations can be used. For example, a Wi-Fi 6 AP could simultaneously communicate with one 802.11ax client using 8 MHz of frequency space while communicate with two other 802.11ax clients using 4 MHz sub-channels. 85 MIMO (Multiple Input/Multiple Output)

• Unlike traditional/old 802.11a/b/g radios, which use single- input and single-output (SISO), 802.11n, 802.11ac and 802.11ax radios use MIMO technology to increase throughput by increasing the number of radio transmit and receive chains (antennas or a group of antennas). – A choice needs to be made between transmit diversity techniques, which increase reliability (decrease probability of error), and spatial multiplexing techniques, which increase rate but not necessarily reliability. – MIMO achieves this by higher spectral efficiency (more bits per second per hertz of bandwidth) and link reliability or diversity (reduced fading). Shannon’s Theory – So, each stream (streams use diverse paths, bouncing off walls, floors, stuff, or just the air) has the potential to increase data throughput OR increase reliability.

86 Shannon Limit

Shannon–Hartley theorem: The limit of reliable data rate of a channel depends on bandwidth and signal-to-noise ratio according to:

Signal power (S) R information rate in bits per second; B channel bandwidth in Hertz; Noise power (N) S total signal power (equivalent to the carrier power C) N total noise power in the bandwidth. Bandwidth (B) f

87 SU-MIMO vs. MU-MIMO

• SU-MIMO – Single User MIMO. This means one client can communicate at a time with the potential of using the multiple streams for faster speed or a more reliable connection.

• MU-MIMO – Allows 1 AP to transmit unique data to multiple stations simultaneously. Allows up to eight simultaneous downlink MU-MIMO clients.

88 Principles of MIMO

A spatial stream is a data set, sent by a transmitting radio chain, which can be mathematically reconstructed by the receiver’s radio chains. In MIMO, each spatial stream is transmitted from a different radio/antenna chain in the same frequency channel as the transmitter.

The receiver receives each stream on each of its identical radio/antenna chains. Since the receiver knows the phase offsets of its own antennas, it can use signal-processing techniques to mathematically reconstruct the original streams. To enhance throughput, each spatial stream contains unique data, and the number of independent spatial streams is therefore limited by whichever WiFi device has the least number of radio chains.

89 Spatial Multiplexing

• Spatial Multiplexing – This is a transmission technique in MIMO wireless communication to transmit independent and separately encoded data. Multiple data streams are transmitted at the same time. They are transmitted on the same channel, but by different antenna. Additional antennas allow for the transmission and reception of multiple simultaneous data streams.

• Streams – Each spatial stream can pack a certain amount of data with 802.11ax wireless. More transmitters and receivers allow the AP to send independent streams of data. Much like adding additional lanes to a road, multiple spatial streams allow the wireless AP to transmit more data simultaneously.

90 MU-MIMO • By adding even more radio chains/antennas, MU-MIMO can control the phased antenna pattern to control both the areas of maximum constructive interference where the signal is the strongest and maximum destructive interference where the signal is the weakest. – With enough antennas and knowledge about the relative positions of all associated client devices, it can create a phased pattern to talk to multiple clients both independently and simultaneously.

91 Beamforming

• Known as Transmit Beamforming, this is a technique with 802.11ax (same as 802.11ac) wireless implemented to improve range and data rate for a given client takes advantage of the multiple transmit antennas available in a multiple-input multiple-output (MIMO) system. – Efficient steering of individual streams in such a system provides overall gain.

92 Beamforming • Beamforming is the ability to adapt the radiation pattern of the antenna array to a scenario. – Beamforming as steering a lobe of power in a direction toward a user. – Relative amplitude and phase shifts are applied to each antenna element to allow for the output signals from the antenna array to coherently add together for a transmit/receive angle and destructively cancel each other out for other signals. – The spatial environment that the array and user are in is not generally considered. This is indeed beamforming but is just one specific implementation of it.

93 Without and With Beamforming

94 Beamforming Benefits and Limitations

• By focusing a signal in a specific direction, beamforming allows you deliver higher signal quality to your receiver which in practice means faster information transfer and fewer errors without needing to boost broadcast power. – As an added benefit, because you aren't broadcasting your signal in directions where it's not needed, beamforming can reduce interference experienced by people trying to pick up other signals. – The limitations of beamforming mostly involve the computing resources it requires; there are many scenarios where the time and power resources required by beamforming calculations end up negating its advantages. – But continuing improvements in processor power and efficiency have made beamforming techniques affordable enough to build into consumer networking equipment.

95 Beamforming and MU-MIMO

• Beamforming is key for the support of multiuser MIMO, or MU-MIMO, which is becoming more popular as 802.11ax routers roll out. – As the name implies, MU-MIMO involves multiple users that can each communicate to multiple antennas on the router. – MU-MIMO uses beamforming to make sure communication from the 802.11ax AP is efficiently targeted to each connected client.

96 Explicit beamforming vs. implicit beamforming

• There are a couple of ways that Wi-Fi beamforming can work. If both the router and the endpoint support 802.11ax-compliant beamforming, they'll begin their communication session with a little "handshake" that helps both parties establish their respective locations and the channel on which they'll communicate; this improves the quality of the connection and is known as explicit beamforming. – A beamforming device can still attempt to target these devices, but without help from the endpoint, it won't be able to zero in as precisely. This is known as implicit beamforming, or sometimes as universal beamforming, because it works in theory with any Wi-Fi device.

• In many routers, implicit beamforming is a feature you can turn on and off. Is enabling implicit beamforming worth it?

97 Multipath environment between antenna array and user

In order to take advantage of the multiple paths, the spatial channel between antenna elements and user terminals needs to be characterized. In literature, this response is generally referred to as channel state information (CSI). This CSI is effectively a collection of the spatial transfer functions between each antenna and each user terminal. This spatial information is gathered in a matrix (H).

98 Massive MIMO

• Massive MIMO can be considered as a form of beamforming in the more general sense but is quite removed from the traditional form. – Massive simply refers to the large number of antennas in the base station antenna array. MIMO refers to the fact that multiple spatially separated users are catered for by the antenna array in the same time and frequency resource. – Massive MIMO also acknowledges that in real-world systems, data transmitted between an antenna and a user terminal and vice versa—undergoes filtering from the surrounding environment. – The signal may be reflected off buildings and other obstacles, and these reflections will have an associated delay, attenuation, and direction of arrival. – There may not even be a direct line of sight between the antenna and the user terminal. 99 Massive MIMO Principles

100 MU-MIMO with 802.11ax works in both the 2.4GHz and 5GHz bands

• Back with 11n and 11ac, SU-MIMO worked in both the 2.4GHz and 5GHz bands, but MU-MIMO with 11ac was only supported in the 5GHz band. • However, with 11ax applying to both bands, we’ll have MU-MIMO (and OFDMA) in both bands as well. – This is one of the biggest improvements to the congested 2.4GHz band we’ve seen in many years. Remember, this band can only support up to three non-overlapping channels at a time, and that’s using the small legacy channel-widths. – MU-MIMO with 11ax could help save this lower band by speeding it up and making it more usable in dense environments.

101 MU-MIMO uses Beamforming

• MU-MIMO uses beamforming, a separate feature of 11ac and 11ax that directs signals toward the intended wireless device(s) instead of randomly in all directions. Since the signal is more efficiently used, the technology helps increase Wi-Fi ranges and speeds. – Although beamforming was optionally available with 11n, most vendors implemented only proprietary versions of it. – Having a standardized version helps beamforming and MU-MIMO in 11ac or 11ax products.

102 User devices require multiple antennas for uplink MU-MIMO

• Like with 11ac, wireless devices aren't required to have multiple antennas to receive MU-MIMO streams from wireless routers and APs. – If the wireless device has only one antenna, it still can receive one MU-MIMO data stream from an AP. – With uplink MU-MIMO, wireless devices are required to have a minimum of two antennas to transmit with MU-MIMO back to the AP or wireless router, even for one stream connections.

• More antennas would allow a device to support more simultaneous data streams (typically one stream per antenna), which would be good for the device's Wi-Fi performance. – However, including multiple antennas in a device requires more power and space, and adds to its cost. It would take eight antennas to take full advantage of the 11ax features.

MU-MIMO enables even more efficient beamforming, because it allows the router to constantly keep tabs on the relative locations of various devices, enabling the beamforming technology to optimize the antennas accordingly. This is especially important in environments where there are multiple mobile devices vying for Wi-Fi. With MU- 103 MIMO and beamforming, you have Wi-Fi that cuts through the clutter and is always giving you the best connection. 802.11ax builds upon OFDM using OFDMA

•OFDM is a very powerful transmission technique. •It is based on the principle of transmitting simultaneously many narrow-band orthogonal frequencies, often also called OFDM subcarriers or subcarriers. •The number of subcarriers is often noted N. •These frequencies are orthogonal to each other which (in theory) eliminates the interference between channels. •Each frequency channel is modulated with a possibly different digital modulation (usually the same in the first simple versions). •The frequency bandwidth associated with each of these channels is then much smaller than if the total bandwidth was occupied by a single modulation. • This is known as the Single Carrier (SC). •A data symbol time is N times longer, with OFDM providing a much better multipath resistance.

Having a smaller frequency bandwidth for each channel is equivalent to greater time periods and then better resistance to multipath propagation (with regard to the SC). Better resistance to multipath and the fact that the carriers are orthogonal allows a high spectral efficiency. OFDM is often presented as the best performing transmission technique used for wireless systems.

104 OFDM characteristics from 802.11ac to 802.11ax

The OFDM symbol is the basic building-block of a Wi-Fi transmission. It is a small segment in time of the modulated waveform of a subcarrier, carrying information: the more variants of a symbol are available, the more information (binary bits) it can carry. The fundamental characteristics: fast Fourier transform (FFT) size, subcarrier spacing and OFDM symbol duration are linked, given a fixed channel width. In 802.11ax, the subcarrier spacing is reduced by a factor of 4x while the OFDM symbol duration increases by 4x. 105 OFDMA: How it works

106 OFDMA (Orthogonal Frequency Division Multiple Access) Preamble Preamble Preamble Preamble Preamble Preamble … OFDM DL Data UL BA DL Data UL BA DL Data UL BA (STA 1) (STA1) (STA 2) (STA2) (STA3) (STA3)

SIFS Contention SIFS Contention SIFS t

f DL Data (STA 1) UL BA (STA1) Preamble Preamble Preamble

OFDMA DL Data (STA 2) MU-BAR UL BA (STA2) …

DL Data (STA 3) UL BA (STA3)

SIFS SIFS t – Issue: MAC efficiency drops as STA density increases and when short packets are transmitted (increase in contention, collision, IFS, preambles) – Aggregation in 11n combines short packets in TIME from a single user, DL MU-MIMO in 11ac combines different users SPATIALLY, OFDMA combines different users together in FREQUENCY – OFDMA does NOT increase the maximum PHY rate – Downlink OFDMA: AP groups users to maximize downlink transmission efficiency – Uplink OFDMA: Users are grouped together and transmit in sync to AP to maximize uplink transmission efficiency – Transmit power can be adjusted per resource unit (RU) in either UL or DL to improve SINR for specific users 107 Subcarriers

• OFDM divides a channel into subcarriers through a mathematical function known as an Inverse Fast Fourier Transform (IFFT). The spacing of the subcarriers is orthogonal, so they don’t interfere with one another despite the lack of guard bands between them.

• This creates signal nulls in the adjacent subcarrier frequencies and prevents intercarrier interference (ICI).

• An OFDM 20 MHz 802.11n/ac channel consists of 64 312.5 kHz subcarriers. 802.11ax introduces a longer OFDM symbol time of 12.8 microseconds, which is four times the legacy symbol time of 3.2 microseconds. As a result of the longer symbol time, the subcarrier size and spacing decreases from 312.5 kHz to 78.125 kHz.

• The narrower subcarrier spacing allows better equalization and enhanced channel robustness. Because of the m78.125 kHz spacing, an OFDMA 20 MHz channel consists of a total of 256 subcarriers.

108 STA near the 802.11ax AP: OFDMA

Legacy/Enhanced UE (OFDMA DL)

Time → T-2T-1 T T+1 T+2 T+3

109 Broadcom MAX (OFDMA Video)

110 Example of 802.11ax Solutions

• Broadcom MAX • Aerohive – Aerohive_Datasheet_AP650X – Aerohive_Datasheet_AP650 – Aerohive_Datasheet_AP650X – Ruckus R730: Indoor 802.11ax 8x8:8 Wi-Fi Access Point with Multi-gigabit Backhaul

111 Broadcom MAX

112 OFDM. Only 1 client can use the channel bandwidth at a time Subcarriers CHANNEL WIDTH

Time

CLIENT ONE CLIENT THREE CLIENT FIVE CLIENT TWO CLIENT FOUR CLIENT SIX

113 OFDMA technology across several different clients Subcarriers

Time

CLIENT ONE CLIENT THREE CLIENT FIVE

CLIENT TWO CLIENT CLIENT SIX FOUR The fundamental aspect of OFDMA: the use of OFDM technology to multiplex traffic by allocating specific patterns of sub-carriers in the time-frequency space to different users.

In addition to data traffic, control channels and reference symbols can be interspersed. Control channels carry information on the network and cell while reference symbols assist in determining the propagation channel response. 114 The downlink and uplink physical layer of 802.11ax is based on OFDMA. Why OFDMA selected for 802.1ax

• Too many short data frames (MAC data, management and control frames) • 70-80% of frames are under 256 B (observation of office environment, homes and stadiums) • <5% of frames over 1KB\50-60% control frames – Low duty cycle – Overall system throughput efficiencies – Short packets can be aggregated – MAC inefficiency – Larger preamble overhead

115 Comparison of subcarrier spacing and OFDM symbol length for 802.11n/ac vs. 802.11ax

116 Powerful OFDMA and MU-MIMO: Complementary in 802.11ax

Source: Qualcomm 117 Common PHY Parameters

PHY parameters BW All BSSs either all at 2.4GHz, or all at 5GHz [20MHz BSS at 2.4GHz, or 80 MHz BSS at 5GHz] Data Preamble Type [2.4GHz, 11n; 5GHz, 11ac] STA TX Power 15 dBm per antenna AP TX Power 20 dBm per antenna P2P TX Power 15 dBm per antenna AP Number of TX antennas All APs with [2] or all with 4 antennas AP Number of RX antennas All APs with [2] or all with 4 antennas STA Number of TX antennas All STAs with [1] or all with 2 antennas STA Number of RX antennas All HEW STAs with [1] or all with 2 antennas AP antenna gain +0dBi STA antenna gain -2dBi Noise Figure 7dB Distance-based Path Loss Computed on the basis of 3-D distance, with a minimum 3-D distance of 1 meter. Formulas shall be evaluated with carrier frequency equal to 2.4GHz for channels within the 2.4 GHz band, and with carrier frequency equal to 5GHz for channels within the 5 GHz band. 118 Common MAC Parameters

MAC parameters Access protocol [EDCA with default parameters] parameters Aggregation [A-MPDU / max aggregation size / BA window size, No A-MSDU, with immediate BA] Max number of retries Max retries: 10

RTS/CTS Threshold [no RTS/CTS]

119 REVIEW: OFDMA vs. OFDM

Source: Ruckus wireless 120 REVIEW: OFDMA is Based on LTE for Efficient Access

Source: Qualcomm 121 Module Summary

• In this module, we discussed Key 802.11ax technologies and building blocks more in depth.

122 Which is better, OFDMA or MU-MIMO?

• Most industry experts believe that OFDMA will be the most relevant technology that 802.11ax offers. – Downlink MU-MIMO was introduced with Wave-2 802.11ac access points, however, real-world implementation of MU-MIMO for indoor environments is rare: • There are not so many MU-MIMO capable clients in the current marketplace and the technology is not used in the enterprise widely. • MU-MIMO requires spatial diversity, therefore the physical distance between the clients is necessary. Most modern-day enterprise deployments of Wi-Fi involve a high density of users that is not conducive for MU-MIMO conditions. – Because MU-MIMO requires spatial diversity, a sizable distance between the clients and the AP is necessary. Most modern-day enterprise deployments of Wi-Fi involve a high density of users that is not conducive for MU-MIMO conditions. – MU-MIMO requires transmit beamforming (TXBF) which requires sounding frames. The sounding frames add excessive overhead, especially when the bulk of data frames are small. – MU-MIMO would only be a favorable option in very low density, high bandwidth environments. 123 h N H i g E f f i c i e n c y W L A

Overview of Spatial Frequency Reuse/BSS coloring, Power Savings, Target Wake Time/TWT, Trigger-based Random

Access, MAC Enhancement 124 REVIEW: OFDM vs. OFDMA

802.11ax access point uses OFDMA technology to partition a channel into smaller sub-channels called resource units (RUs) so that simultaneous multiple-user transmissions can occur.

In OFDM In OFDMA (802.11ax): (802.11a/g/n/ac): The whole channel is The whole channel allocated to many is allocated to 1 users at the time depending on the user at the time; The AP mandates the RU allocation of a 20 conditions ; highly MHz for multiple clients for both downlink and very inefficient efficient 125 uplink OFDMA. REVIEW: OFDMA compared with single-user OFDM

126 REVIEW: OFDMA in 802.11ax

– OFDMA can maximize the resource utilization and multiplexing flexibility (both frequency and time domain can be multiplexed) – OFDMA Tone Plan • 1024 FFT in 80 MHz (4x symbol rate vs. 802.11ac)

• Guard Interval: 0.8 us, 1.6 us, and 3.2 us to cover both indoor and KEYSIGHT CONFIDENTIAL outdoor operation • One user can allocated from Min. 26 tone to Max. 996 tone Smallest subcarrier is 26 subcarrier or 2 MHz There are 9 available 26-sub-channels in 20 MHz channel Traditional OFDM only Sub-channel refers to Resource Unit (RU) multiplex in frequency domain

OFDMA can multiplex in both frequency and time domain

127 Smallest subcarrier is 26 subcarrier or 2 MHz There are 9 available 26-sub-channels in 20 MHz channel Sub-channel refers to Resource Unit (RU)

128 Possible RUs

• RU-26 or 2 MHz • RU-106 • RU-242 • RU-484 • RU-996

129 RU Allocation at the PHY Layer

The HE-SIG-B field is used to communicate RU assignments to clients.

RU allocation information is communicated to clients at both the PHY and MAC layers. 130 RU allocation using Trigger Frame (MAC)

• The trigger frame allocates specific RUs to three client stations for simultaneous uplink transmission within a 20 MHZ OFDMA channel.

• Clients STA-1 and STA-2 are each assigned to a 52-tone RU, whereas client STA-3 is assigned to a 106-tone RU.

Power-saving with Target Wake Time An 802.11ax AP can negotiate with the participating STAs the use of the Target Wake Time (TWT) function to define a specific time or set of times for individual stations to access the medium. The STAs and the AP exchange information that includes an expected activity duration. This way the AP controls the level of contention and overlap among STAs needing access to the medium. 802.11ax STAs may use TWT to reduce energy consumption, entering a sleep state until their TWT arrives. Furthermore, an AP can additionally devise schedules and deliver TWT values to STAs without individual TWT agreements between them. The standard calls this procedure Broadcast TWT operation 131 Detailed PHY service specifications

• Basic service and options

• PHY-SAP inter-(sub)layer service primitives

• PHY SAP service primitives parameters

132 PHY SAP Detailed Service Specification

• PHY-DATA.request • PHY-DATA.indication • PHY-CCARESET.request • PHY-CCA.indication • PHY-TRIGGER.request • PHY-TRIGGER.confirm

133 Parameters for 11ax PHY Structure

• Basic Parameter Set of 20 MHz for 11ax PHY Parameters Values

Bandwidth 20 MHz (Other BWs need to be extended) Subcarrier frequency spacing 78.125 kHz (256 FFT), 312.5 kHz (64 FFT) IDFT/DFT period 12.8 us (256 FFT), 3.2 us (64 FFT) Guard interval duration 0.8us, Other CPs to be considered Number of frequency segments/subbands 4 or 8 (256 FFT), 1 (64 FFT) • Need to determine the subcarrier allocation parameters • The number of subcarriers for guard band in 256 FFT for 20 MHz BW • The total number of used subcarriers for data and pilots • The number of subcarriers per subband

Smallest subcarrier is 26 subcarrier or 2 MHz There are 9 available 26-sub-channels in 20 MHz channel 134 Sub-channel refers to Resource Unit (RU) How does 802.11ax go so fast?

• Peak wireless speed is the product of four factors: channel bandwidth, constellation density, number of spatial streams, and per-symbol overhead.

• 802.11ax pushes on constellation density by adding 1024 QAM but more significantly improves the per-symbol overhead with flexible PHY timing parameters.

• First, going from 256 QAM to 1024 QAM increases peak rates by 10/8 = 1.25 times.

• Being closer together, the constellation points are more sensitive to noise, so 1024 QAM helps most at shorter range.

135 Legacy Channel description attributes

Subcarriers Capacity relative to Capacity relative to 20 PHY standard Subcarrier range Pilot subcarriers (total/data) 802.11a/g MHz 802.11ac

–26 to –1, +1 to 52 total, 48 802.11a/g ±7, ±21 x1.0 n/a +26 usable(8% pilots)

802.11n/802.11ac, –28 to –1, +1 to 56 total, 52 ±7, ±21 x1.1 x1.0 20 MHz +28 usable(7% pilots)

802.11n/802.11ac, –58 to –2, +2 to 114 total, 108 ±11, ±25, ±53 x2.3 x2.1 40 MHz +58 usable(5% pilots)

–122 to –2, +2 to ±11, ±39, ±75, 242 total, 234 802.11ac, 80 MHz x4.9 x4.5 +122 ±103 usable(3% pilots)

–250 to –130, – ±25, ±53, ±89, 802.11ac, 160 126 to –6, +6 to 484 total, 468 ±117, ±139, ±167, x9.75 x9.0 MHz +126, +130 to usable(3% pilots) ±203, ±231 +250

136 Symbol Rate • Second, going from a fixed symbol duration (Ts) of 3.2 microseconds (µs) and only two Guard Intervals (GI) of 400 or 800 ns to a longer Ts (12.8 µs) and three guard-interval options (0.8, 1.6, or 3.2 µs) allows both higher speed and, when needed, more reliability.

– Mathematically, the Ts-to (GI + Ts) ratio determines the peak time-domain efficiency, which for 11ac was up to 3.2 µs/(3.2 µs + 400 ns) or 88.9 percent, whereas with 802.11ax we can achieve up to 12.8/(12.8 + 0.8) = 94-percent efficiency for a peak throughput gain of 5.9 percent, and yet with much greater multipath robustness.

• In addition, the 802.11ax tone plan is denser with 980 data tones (OFDMA sub-carriers) per 13.6 µs (Ts + minimum GI) over 80 MHz, whereas 802.11ac has 234 data tones (OFDM sub-carriers) per 3.6 µs in the same 80 MHz.

• This increased tone density results in an additional peak throughput gain of 10 percent with respect to 802.11ac in the same spectrum (since (980/13.6)/(234/3.6) = 1.1).

137 Preamble Updates

The preamble is used for synchronization between transmitting and receiving radios and consists of two parts: legacy and high efficiency (HE).

The legacy preamble is easily decodable by legacy stations (STAs) and is included for backward compatibility. The HE preamble components are used to communicate information between 802.11ax radios about OFDMA, MU-MIMO, BSS coloring, and more.

138 HE PPDU Formats

139 Preambles and Training Sequences HE-SIG-A field, which contains information about the packet to follow, including whether it is downlink or uplink, BSS color, modulation MCS rate, bandwidth and spatial stream information, and remaining time in the transmit opportunity. This field has different content for single-user, multi-user and trigger-based frames, and is repeated in the ‘extended range mode’ of 802.11ax.

The HE-SIG-B field is only included for multi-user packets. It has information common to all recipients, and other fields that are user- specific, so its length depends on the number of users receiving the transmission. When OFDMA is used, the HE-SIG-B client-specific fields are sent concurrently in each sub-channel used for the subsequent packet transmission. More on this later. The HE-STF training field allows receivers to synchronize to the timing and frequency of the incoming frame before decoding the packet body, while the HE-LTF is important for channel estimation, enabling beamforming and MIMO spatial diversity. The 802.11ax frame starts with the ‘legacy’ preamble for backwards-compatibility: these fields have been used since before 802.11n and allow older devices to recognize there is an 802.11 frame on the air. This allows the CSMA/CA protocol to continue functioning in the presence of 802.11ax transmissions. The next field, RL-SIG, would be the beginning of the frame body in older protocols like 802.11g. It identifies the frame to follow as 802.11ax rather than pre-802.11n. The ‘legacy’ preamble and RL-SIG field are transmitted in parallel in all 20-MHz sub-channels used for subsequent transmissions, for backwards- compatibility. 140 Preambles and Training Sequences Fields

• The subsequent fields are used for 802.11ax purposes (‘HE’ is ‘High Efficiency’, the IEEE 802.11 name for 802.11ax) and use a mix of symbol formats, with ‘legacy’ modulation used for low-rate fields and for backwards compatibility, while other fields use the new, close subcarrier spacing and longer OFDMA symbol of 802.11ax. – First is the HE-SIG-A field, which contains information about the packet to follow, including whether it is downlink or uplink, BSS color, modulation MCS rate, bandwidth and spatial stream information, and remaining time in the transmit opportunity. This field has different content for single-user, multi-user and trigger-based frames, and is repeated in the ‘extended range mode’ of 802.11ax. – The HE-SIG-B field is only included for multi-user packets. It has information common to all recipients, and other fields that are user-specific, so its length depends on the number of users receiving the transmission. When OFDMA is used, the HE-SIG-B client-specific fields are sent concurrently in each sub- channel used for the subsequent packet transmission. More on this later. – The HE-STF training field allows receivers to synchronize to the timing and frequency of the incoming frame before decoding the packet body, while the HE-LTF is important for channel estimation, enabling beamforming and MIMO spatial diversity.

141 Packet tail: padding, tail bits and packet extension

• The new structures and applications of 802.11ax mean some new fields are added to the end of the packet.

Padding may be added after the packet payload. It is required when OFDMA is used and the frame, as built by the transmitter, is not quite long enough to fill the negotiated transmit opportunity. The calculations to determine optimal bandwidth utilization are performed by the AP, and it varies the sub- channel, MCS rate and transmit power for the frames grouped in a transmission to ensure that all transmissions start and end simultaneously. This is important because the other devices on the channel, including pre-802.11ax devices, must see signals at a certain power level filling the channel in order for their CSMA/CA contention mechanisms to work correctly. Padding can be included in the forward error correction (FEC) calculation, or added after the calculation. 142 Packet tail: padding, tail bits and packet extension • If the AP is doing a good job (assuming the system is operating at capacity), very little padding will be used. If it has a ‘short’ frame, it can always reduce the MCS rate to improve the transmission’s error rate, driving a longer duration. – Tail bits may be added after the data field. They are only necessary when BCC error correction is used, not for LDPC. – This field existed prior to 802.11ax. (Binary convolutional codes (BCC) were used in early 802.11 standards for error correction. As data-rates increased, the BCC decoder became complex, and now higher data-rates use low- density parity check (LDPC) coding, a lower-complexity alternative.)

143 Packet Extension

• The packet extension field may be added at the end of the frame. – It is used to allow extra time for the receiver to process the frame’s contents before responding with a frame of its own, recognizing for the first time in 802.11 that some chips may move certain functions to slower software layers rather than fast-calculating hardware. – A client requiring extra time to process received frames must signal its requirements to the AP: the allowed values for packet extension are 0, 4, 8, 12 or 16 usec.

144 New 802.11ax PHY Headers

• Added to all 802.11 frames is a physical (PHY) header that contains a preamble and other information used for initial setup of communications between two radios. The 802.11ax amendment also defines four new PHY headers to support high efficiency (HE) radio transmission, as follows:

• HE SU

• HE MU

• HE ER SU

• HE TB

145 HE SU and HE MU

• HE SU: The high efficiency single-user PHY header is used for single-user transmissions.

• HE MU: The high efficiency multi-user PHY header is used for transmissions to one or more users. – This format information is used for both multi-user multiple-input multiple-output (MU-MIMO) and multi-user orthogonal frequency domain multiple access (MU-OFDMA), as well as resource unit (RU) allocation.

146 HE ER SU and HE TB

• HE ER SU: The high efficiency extended-range single-user format is intended for a single user. Portions of this PHY header are boosted by 3 dB to enhance outdoor communications and range.

• HE TB: The high efficiency trigger-based format is for a transmission that is a response to a trigger frame. In other words, this PPDU format is used for uplink communications.

147 Non-DMG non-S1G STA MAC architecture

DMG to specify a directional multi-gigabit PHY implementation. S1G to specify a sub 1 GHz PHY implementation.

148 non-DMG non-S1G STA

• In a non-DMG non-S1G STA: – The MAC provides the PCF, HCF, and MCF, and TUA services using the services of the DCF. – The PCF is optionally present in nonmesh STAs and absent otherwise. – The HCF is present in QoS STAs and absent otherwise. – he MCF is present in mesh STAs and absent otherwise. – The TUA is present in non-AP HE STAs and absent otherwise.

149 Triggered Uplink Access (TUA)

• A non-AP HE STA also implements trigger- based UL access methods. • Triggered UL access (TUA) is used when an HE AP triggers one or more non-AP HE STAs to transmit HE TB PPDUs simultaneously. – The optional UORA additionally allows a non-AP HE STA to access one of a number of resource units designated for random access by the HE AP.

150 MAC Control Frames

• RTS frame • CTS frame • PS-Poll frame • CF-End frame • BlockAckReq frame • BlockAck frame • VHT/HE NDP Announcement frame • Trigger frame

151 MAC Management Frames

• Beacon frame • Association Request frame • Association Response frame • Reassociation Request frame • Reassociation Response frame • Probe Request frame • Probe Response frame

152 Management and Extension frame body components

• QoS Info field • Operating Mode field • TWT Information field • HE MIMO Control field • HE Compressed Beamforming Report field • HE MU Exclusive Beamforming Report field • HE CQI-only Report field

153 OFDMA Subcarriers

• Just like with OFDM, there are three types of subcarriers for OFDMA, as follows:

– Data Subcarriers: These subcarriers will use the same modulation and coding schemes (MCSs) as 802.11ac and two new MCSs with the addition of 1024-QAM.

– Pilot Subcarriers: The pilot subcarriers do not carry modulated data; however, they are used for synchronization purposes between the receiver and transmitter.

– Unused Subcarriers: The remaining unused subcarriers are mainly used as guard carriers or null subcarriers against interference from adjacent channels or sub-channels.

• These tones are grouped into smaller sub-channels, known as resource units (RUs). By subdividing the channel, parallel transmissions of small frames to multiple users can happen simultaneously. The data and pilot subcarriers within each resource unit are both adjacent and contiguous within an OFDMA channel

154 The OFDM symbol is the basic building-block of a Wi-Fi transmission. It is a small segment in time of the modulated waveform of a OFDMA DL subcarrier, carrying information: the more variants of a symbol are available, the more information (binary bits) it can carry.

t 155 The fundamental characteristics: fast Fourier transform (FFT) size, OFDMA UL subcarrier spacing and OFDM symbol duration are linked, given a fixed channel width. In 802.11ax, the subcarrier spacing is reduced by a factor of 4x while the OFDM symbol duration increases by 4x.

156 DL OFDMA Operation

• The general downlink OFDMA operation is as follows:

– 1. The access point decides how many STAs and the size of each resource unit in this TXOP, and indicates it in a field in the preamble of the PPDU. – 2. The access point transmits downlink data to multiple STAs in their allocated resource unit (MU-PPDU). – 3. The access point requests block acknowledgement from all STAs (MU-BAR). – 4. STAs send block ACKs back to access point (M-BA).

157 UL OFDMA Operation

• The general uplink OFDMA operation is as follows: • 1. The access point decides which STAs need to be asked for data and how many resource units will be allocated to each • 2. The access point requests or polls data from STA with a trigger (HE Trigger) • 3. STAs respond with data (uplink MU-PPDU) • 4. The access point responds with an ACK (M-BA).

• Unlike 802.11ac, the 802.11ax access point is in control of the downlink and uplink resource-unit allocation on a per-PPDU basis, which can be seen as a form of access-point scheduling (in the frequency and spatial domains).

158 OFDMA subdivides a channel into smaller frequency allocations, called resource units (RUs)

• Both OFDM and OFDMA divided a channel into subcarriers through a mathematical function known as an inverse fast Fourier transform (IFFT). The spacing of the subcarriers is orthogonal, so they will not interfere with one another despite the lack of guard bands between them. – This creates signal nulls in the adjacent subcarrier frequencies, thus preventing inter-carrier interference (ICI).

• What are some of the key differences between OFDM and OFDMA?

• A 20 MHz 802.11n/ac channel consists of 64 subcarriers. • Fifty-two of the subcarriers are used to carry modulated data; four of the subcarriers function as pilot carriers; and eight of the subcarriers serve as guard bands. – OFDM are sometimes also referred to as OFDM tones. In this blog series, we will use both terms interchangeably. Each OFDM subcarrier is 312.5 KHz.

159 RU Allocation

At the Physical layer, RU allocation information can be found in the HE-SIG-B field of the PHY header of an 802.11 trigger frame. HE-SIG-B field consists of two sub-fields: the common field and user-specific 160 field. Smallest subcarrier is 26 subcarrier or 2 MHz There are 9 available 26-sub-channels in 20 MHz channel RU Allocation Sub-channel refers to Resource Unit (RU) • RU allocation information is communicated to clients at both the PHY and MAC layers. – At the Physical layer, RU allocation information can be found in the HE-SIG-B field of the PHY header of an 802.11 trigger frame. – HE-SIG-B field consists of two sub-fields: the common field and user-specific field. – A sub-field of the common field is used to indicate how a channel is partitioned into various RUs. – For example, a 20 MHz channel might be subdivided into one 106-tone RU and four 26-tone RUs. – The user-specific field comprises multiple user fields that are used to communicate which users are assigned to each individual RU. 161 RU Allocation at the PHY Layer

The HE-SIG-B field is used to communicate RU assignments to clients.

RU allocation information is communicated to clients at both the PHY and MAC layers. 162 RU allocation using Trigger Frame (MAC)

• The trigger frame allocates specific RUs to three client stations for simultaneous uplink transmission within a 20 MHZ OFDMA channel.

• Clients STA-1 and STA-2 are each assigned to a 52-tone RU, whereas client STA-3 is assigned to a 106-tone RU.

164 Subdividing Wi-Fi channels using various Resource Unit sizes

RU type CBW20 CBW40 CBW80 CBW160 and CBW80+80 26-subcarrier RU 9 18 37 74

52-subcarrier RU 4 8 16 32

106-subcarrier RU 2 4 8 16

242-subcarrier RU 1-SU/MU-MIMO 2 4 8

484-subcarrier RU N/A 1-SU/MU-MIMO 2 4

996-subcarrier RU N/A N/A 1-SU/MU-MIMO 2

2x996 subcarrier N/A N/A N/A 1-SU/MU-MIMO RU

165 Resource Units and Wide Channels

166 OFDMA Resource Units – 20 MHz channel

An OFDMA channel consists of a total of 256 subcarriers. These subcarriers can be grouped into smaller subchannels know as resource units (RUs).

When subdividing a 20 MHz channel, an 802.11ax AP can designate 26, 52, 106, and 242 subcarrier RUs, which roughly equates to 2 MHz, 4 MHz, 8 MHz, and 20 MHz channels, respectively.

167 OFDMA RU

• OFDMA subdivides a Wi-Fi channel into smaller frequency allocations, called resource units (RUs), thereby enabling an access point (AP) to synchronize communication (uplink and downlink) with multiple individual clients assigned to specific RUs.

• By subdividing the channel, small frames (such as streaming video) can be simultaneously transmitted to multiple users in parallel.

• The simultaneous transmission cuts down on excessive overhead at the medium access control (MAC) sublayer, as well as medium contention overhead.

– The AP can allocate the whole channel to a single user or partition it to serve multiple users simultaneously, based on client traffic needs.

168 Resource Units (RUs)

A resource unit (RU) is the smallest unit of resources that can be allocated to a user

169 OFDMA and Resource Unit Allocation

• The ability to allocate a resource unit, a set of contiguous OFDMA sub-carriers (“tones”), to each client or station (STA) in the same PPDU is unique to 802.11ax within the 802.11 family.

– With the smallest resource unit being 26 tones (2 MHz) and the largest being 2 x 996 tones (160 MHz), there is a large degree of flexibility to balance aggregate (average) performance and peak throughput.

– At the same time, 802.11ax supports multiuser MIMO and can allocate 1 to 8 Spatial Streams (SS) to each STA.

170 OFDMA Resource Dimensions

171 How Does an 802.11ax AP Allocate OFDMA Resource Units?

• 802.11ax access point uses OFDMA technology to partition a channel into smaller sub-channels called resource units (RUs) so that simultaneous multiple-user transmissions can occur. – The AP mandates the RU allocation of a 20 MHz for multiple clients for both downlink and uplink OFDMA.

172 Sub-channel Allocation for OFDMA

As in prior generations of OFDM, not all subcarriers in a channel can be used for data. Some subcarriers are unused for guard-band purposes, so as not to interfere with transmissions in adjacent channels, or between sub-channels. Others are used for DC or pilot tones, to provide a frequency reference and allow accurate demodulation of the signals. 173 Sub-channel Allocation for OFDMA

174 REVIEW: Usable sub-channels, subcarriers and data-rates for OFDMA

Usable sub-channels, subcarriers and data-rates for OFDMA

The table lists the menu of options for RU-N (e.g. RU-26) sub-channels in OFDMA. These RU’s can move around but only in certain configurations as specified in 802.11ax.

175 OFDMA Resource Unit Allocation Examples

8 OFDMA assignments in 80MHz BSS 16 OFDMA assignments in 80MHz BSS

RU assignments can vary packet to packet

176 OFDMA Resource Unit Allocation Examples

177 Example of OFDMA Allocation

178 Fixed Position of Building Blocks

• The proposed resource units are at fixed positions (as shown below) – RUs are building blocks for the scheduler to assign them to different users

179 802.11ax system may multiplex the channel using different RU sizes

Note that the smallest division of the channel accommodates up to 9 users for every 20MHz of bandwidth. 180 Subdividing Wi-Fi channels using various Resource Unit sizes

181 Tone Allocation- 20 MHz

•26-tone with 2 pilots, 52-tone with 4 pilot and 106-tone with 4 pilots and with 7 DC Nulls and •(6,5) guard tones, and at locations shown in the Figure •An OFDMA PPDU can carry a mix of different tone unit sizes within each 242 tone unit boundary

182 20 MHz BSS: Example 1 • Eight interlaced null subcarriers are illustrated by black arrows: – Exact location of leftover tones is open for discussions

20 MHz OFDMA building blocks: •26-tone with 2 pilots, 52-tone with 4 pilot and 106-tone with 4 pilots and with 7 DC Nulls and (6,5) guard tones •An OFDMA PPDU can carry a mix of different tone unit sizes within each 242 tone unit boundary

Usable tones

26 tone RUs

52 tone RUs + one 26-tone

102 data tones plus 4 pilots RUs (picture shows 108-tone) + one 26-tone

242 tone RU (242 non-OFDMA)

183 20 MHz BSS: Example 2

• Two null subcarriers are located in between pair of 26-tone units: – Exact location of leftover tones is open for discussions

Usable tones

26 tone RUs

52 tone RUs + one 26-tone

102 data tones plus TBD pilots RUs (picture shows 108-tone) + one 26-tone

242 tone RU (242 non-OFDMA)

184 Tone Allocation – 40 MHz

• 26-tone with 2 pilots, 52-tone with 4 pilots, 106-tone with 4 pilots and 242-tone with 8 pilots and with 5 DC Nulls and (12,11) guard tones.

185 40 MHz BSS (37 subchannels)

• Duplicated 20MHz assignment • In case of 52-tone and 108-tone resource units, there are additional 26-tone units that each is located in the middle

Usable tones

26 tone RUs

52 tone and 26-tone RUs

102 data tones plus TBD pilots RUs (picture shows 108-tone) + 26-tone RUs

242 tone RU

2x242 tone RU (484 non-OFDMA)

186 Tone Allocation – 80 MHz

• Define 80 MHz OFDMA building blocks as follows: •26-tone with 2 pilots, 52-tone with 4 pilots, 106-tone with 4 pilots, 242-tone with 8 pilots and 484-tone with 16 pilots and with 7 DC Nulls and (12,11) guard tones

187 80 MHz BSS

188 80 MHz BSS (37 subchannels)

• Duplication of 40MHz + one 26 central • The OFDMA assignment of resource units to different users are completely aligned with 242- boundary

Usable tones

26 tone RUs

52 tone RUs and 26-tone

102 data tones plus TBD pilots RUs (picture shows 108-tone) + 52 tone RUs and 26-tone

242 tone RUs and 26-tone

2x242 tone RU and 26-tone

Non-OFDMA 996 tone

189 Example 1: 16 OFDMA assignments in 80MHz BSS • The proposed resource units at fixed positions are used as building blocks for the scheduler to assign them to different users

190 Example 2: 8 OFDMA assignments in 80MHz BSS • The proposed resource units at fixed positions are used as building blocks for the scheduler to assign them to different users

191 802.11ax OFDMA Operation

• Although 802.11ax does not formally specify time-based scheduling similar to licensed- spectrum LTE, one can imagine advanced queuing or QoS techniques being used to achieve similar results as cellular because the basic framework is already in place and a pure 802.11ax network would have excellent spectrum and interference management capabilities.

192 Uplink OFDMA Transmission Even within an OFDMA packet body, it is very important that the transmitter maintains frequency accuracy, transmitter linearity and other parameters to avoid causing interference to transmissions in adjacent RU’s: implementation of OFDMA is more complex than the simple diagrams above would indicate.

OFDMA works in the uplink direction much as for the downlink, except that the client devices transmit and the access point receives. The difficult functions are for the access point to calculate the best grouping of clients, and then to signal when each should transmit, and on which sub-channel. More on that later. Also, synchronization of preamble symbols in the uplink direction is complex because each preamble is transmitted across a full 20 MHz channel.

This was an implementation decision in the IEEE, and it requires that all preamble waveforms are synchronized in time, frequency and amplitude when received at the AP’s antennas. This has driven a number of new requirements for Wi-Fi devices including calibrating signal strength measurements, local oscillator requirements and others, which may be useful in other areas. 193 Control for Multi-user Downlink Modes

802.11 ax includes two multi-user modes: MU-MIMO, which exploits diversity in space, and OFDMA in the frequency dimension. Both modes allow simultaneous bi-directional communication between an AP and multiple client devices, and 802.11ax provides common control mechanisms.

Downlink and uplink are different: the former has no prior signaling, the AP just starts to transmit in appropriate modes, and receivers synchronize as the packet arrives. But multi- user uplink traffic requires a special ‘trigger’ frame where the AP allocates MU-MIMO groups and OFDMA Resource Units to its clients, and informs them of the allocation, and this in turn requires that the AP polls clients for their uplink traffic requirements.

194 Control for Multi-user Downlink Modes

BQRP Bandwidth Query Report Poll BRP Beamforming Report Poll BSRP Buffer Status Report Poll

Multi-user frames follow a simple format, no trigger or signaling frames are necessary. However, it becomes more difficult to manage acknowledgements, as these are uplink transmissions and, in multi-user mode, require coordination and a trigger frame from the AP.

Options for the trigger frame are BlockAck request (MU-BAR), buffer status report (BSRP), bandwidth query report (BQRP) and uplink multi-user response scheduling control fields in the basic packet preamble. 195 MU-MIMO Process

• The overall process for MU-MIMO is as follows: – The AP broadcasts a sounding frame – Each MU-MIMO compatible client device transmits back matrix data to the access point – The AP computes the relative position of each associated MU-MIMO compatible client device – The AP selects a group of client devices for simultaneous communication – The AP computes the necessary phase offsets for each data stream to each client in the group and transmits all of the data streams in the client group – The AP sends a BlockAckRequest to each client device in the group separately to get confirmation as to whether the client device received the data – The AP receives a BlockAck from each client device in the group that successfully received data

196 Control for multi-user downlink modes

BlockAck’s can be used in conjunction with downlink MU modes, allowing a set of MU BlockAck’s to be deferred to the end of a group of downlink data frames, transmitted in the same transmit opportunity up to the TXOP limit of 4.096 msec. This minimizes the overhead from contention and multiple Ack’s. (802.11ac introduced downlink multi-user MIMO, but had no uplink multi-user mode, so recipients of a downlink MU transmission had to ack one after the other, wasting time on the air. The 802.11ax approach is an improvement on 802.11ac.) 197 Control for Multi-user Uplink Modes

Uplink multi-user control The uplink is more complicated than the downlink, as the AP first has to discover what traffic clients are ready to transmit. Following this, it must calculate the optimum allocation of MU-MIMO groups and OFDMA RU’s, then signal the allocation information to its clients and synchronize them to transmit simultaneously. 198 Control for Multi-user Uplink Modes

The trigger frame format is shown above. It contains the following information. • The length of the uplink transmission window • Which client devices are to transmit • Which OFDMA RU’s are to be used by each client device • How many spatial streams are to be used by each client device • Which MCS modulation level is to be used by each client device • Whether functions like STBC are to be used on the uplink • Required signal strength at the AP for the client’s transmission (this is calculated by the client using the AP’s transmit power level, the client’s receive RSSI level and an 199 assumption of channel reciprocity) RU Allocation information: communicated to clients at both the PHY and MAC layers

• At the Physical layer, RU allocation information can be found in the HE-SIG-B field of the PHY header of an 802.11 trigger frame.

• The HE-SIG-B field is used to communicate RU assignments to clients.

• The HE-SIG-B field consists of two sub-fields: the common field and user-specific field. A sub-field of the common field is used to indicate how a channel is partitioned into various RUs.

• For example, a 20 MHz channel might be subdivided into one 106- tone RU and four 26-tone RUs. The user-specific field comprises multiple user fields that are used to communicate which users are assigned to each individual RU.

200 RU Allocation at the PHY Layer

The HE-SIG-B field is used to communicate RU assignments to clients.

RU allocation information is communicated to clients at both the PHY and MAC layers. 201 Coordinating Uplink Multi-user Operation

To coordinate uplink MU-MIMO or uplink OFDMA transmissions the AP sends a trigger frame to all users. This frame indicates the number of spatial streams and/or the OFDMA allocations (frequency and RU sizes) of each user.

It also contains power control information, such that individual users can increase or reduce their transmitted power, in an effort to equalize the power that the AP receives from all uplink users and improve reception of frames from nodes farther away. The AP also instructs all users when to start and stop transmitting.

The AP sends a multi-user uplink trigger frame that indicates to all users the exact moment at which they all start transmitting, and the exact duration of their frame, to ensure that they all finish transmitting simultaneously as well. Once the AP receives the frames from all users, it sends them back a block ACK to finish the operation. 202 Trigger Frames

• This is a very versatile frame because it can be concatenated with several other functions, as listed below. – Basic Trigger frame: This has no extra functions. It specified how and when client devices should respond. – Beamforming Report Poll (BRP): This solicits beamforming reports from client devices. User Info fi specify how the beamforming report is formatted. There is no Common field in this frame. – Multi-user BlockAck Request (MU-BAR): This trigger frame requests a BlockAck from multiple client devices simultaneously. User Info fi specify the frames that are to be Ack’d. – Multi-user Request To Send (MU-RTS): This trigger frame is used to clear the air before a transmission, in the same way as single-user RTS-CTS. – Buffer Status Report Poll (BSRP): This trigger frame allows the AP to find what traffic client devices have queued to transmit, allowing the AP to schedule uplink traffic – Bandwidth Query Report Poll (BQRP): This trigger frame requests client devices to report on the occupancy of 20 MHz RF channels, allowing the AP to control uplink channel use efficiently – Group Cast with Retries multi-user BlockAck Request (GCR MU-BAR): This intimidating frame is used when the AP is building a multicast group and solicits a BlockAck from each member of the group. 203 AP Scheduling for Downlink Multi-user Modes

AP scheduling for 802.11ax multi-user modes With the addition of OFDMA and uplink MU-MIMO, the AP must perform additional functions not required in previous generations of 802.11. Scheduling of the downlink and uplink traffic becomes critical for optimal performance in a heavily-loaded system.

204 Downlink-Uplink multi-user Cascading Frame Exchanges

The most efficient multi-user scheme available to the AP cascades uplink and downlink sets of multi-user frames, multiplexed in space with MU-MIMO. The downlink packets include Ack’s and triggers, and the uplink are trigger- based frames which also carry Ack’s, and all are controlled and orchestrated by the AP.

New multi-user modes in 802.11ax, along with efficient scheduling by the AP, allow a Wi-Fi system to act in a near-cellular (TDD + TDM/TDMA + OFDMA) fashion. The AP can schedule consecutive multi-user transmit opportunities for uplink and downlink and, with appropriate traffic, the per-packet overhead associated with prior 802.11 protocols becomes so small as to nearly disappear. Meanwhile the scaling of client numbers and granularity of OFDMA bandwidth assignment allows a very broad range of client-density and traffic scenarios to be accommodated.

205 Example of RU allocation using Trigger Frame

• The trigger frame allocates specific RUs to three client stations for simultaneous uplink transmission within a 20 MHZ OFDMA channel.

• Clients STA-1 and STA-2 are each assigned to a 52-tone RU, whereas client STA-3 is assigned to a 106-tone RU.

RU allocation information is delivered in the user information field in the body of a trigger frame.

The table highlights all the possible RUs within a 20 MHz channel and the subcarrier range for each RU.

Each specific RU is defined by a unique combination of 7 bits within the user information field of the trigger frame, known as the RU allocation bits. How RU allocation information is communicated at the MAC layer?

207 Example using Trigger Frames

• In this example, the trigger frame allocates specific RUs to three client stations for simultaneous uplink transmission within a 20 MHZ OFDMA channel. – Clients STA-1 and STA-2 are each assigned to a 52-tone RU, whereas client STA-3 is assigned to a 106-tone RU.

208 Example using Trigger Frames

Clients STA-1 and STA-2 are each assigned to a 52-tone RU, whereas client

STA-3 is assigned to a 106-tone RU. 209 Trigger Frames

• A trigger frame for example is used to allocate RUs to 802.11ax clients. – Trigger frames are also used to query 802.11ax clients about buffered data and about the quality of service (QoS) category of data intended for uplink OFDMA transmissions.

• 802.11ax clients respond with buffer status reports which assists the AP in allocating RUSs for synchronized uplink transmissions.

• The rules of medium contention still apply. – The AP still has to compete for a transmission opportunity (TXOP) against legacy 802.11 stations.

• Once the AP has a TXOP, the A is then in control of up to nine 802.11ax client stations for either downlink or uplink transmissions within a 20 MHz channel. – The number of RUs used can vary on a per TXOP basis.

210 Trigger Frames

• A Trigger frame allocates resources for and solicits one or more HE TB PPD transmissions.

• The Trigger frame also carries other information required by the responding STA to send an HE TB PPDU.

The RA field of the Trigger frame is the address of the recipient STA(s).

The TA field is the address of the STA transmitting the Trigger frame if the Trigger frame is addressed to STAs that belong to a single BSS. The TA field is the address of the transmitted BSSID if the Trigger frame is addressed to STAs from at least two different BSSs of the multiple BSSID set.

211 Trigger Frame Format in 802.11ax

RU allocation information is delivered in the user information field in the body of a trigger frame.

212 Common Info Field: Trigger Frame

213 Trigger Type Subfield Encoding

Trigger Type subfield value Trigger frame variant 0 Basic 1 Beamforming Report Poll (BFRP) 2 MU-BAR (Multi-user block ack request) 3 MU-RTS 4 Buffer Status Report Poll (BSRP) 5 GCR MU-BAR 6 Bandwidth Query Report Poll (BQRP) 7 NDP Feedback Report Poll (NFRP) 8-15 Reserved

214 Frame Aggregation MPDU/MSDU sizes

Data unit 802.11a 802.11n 802.11ac MSDU 2304 2304 2304

A-MSDU - 7935 According to max MPDU size MPDU According to According to max 11454 max MSDU size A-MSDU size PSDU 4095 65535 4692480 PPDU According to According to max According to max max PSDU size PSDU size PSDU size

Theoretical studies have shown that combined aggregation improves performance over either technique used alone. However, most practical implementations to date concentrate on A-MPDU, which performs well in the presence of errors due to its selective retransmission ability. A-MPDU is obligatory for 802.11ac.

215 MAC Aggregation

The value of MAC aggregation lies in more efficient use of the air, for higher throughput and more capacity. This stems from two effects. A-MSDU aggregation requires a full MAC header only on the first packet of the sequence, reducing header overhead. This is a significant effect, but eliminating per-packet contention is bigger: with both A-MSDU and A-MPDU aggregation, the transmitter is able to negotiate a transmit opportunity covering many packets, greatly reducing contention overhead. Packet aggregation is not changed for 802.11ax, but it still plays a significant part in optimizing network capacity. It works in conjunction with MU-MIMO, and with OFDMA: for all the OFDMA illustrations in this paper, packets within sub-channels will often be aggregated packets.

216 Services example

BA: BlockAck BAR: BlockAck request 217 Resource Scheduling: Significantly Improves

Battery Life Existing power-save modes are supplemented with new mechanisms allowing longer sleep intervals and scheduled wake times.

Also, for IoT devices, a 20MHz-channel-only mode is introduced, allowing for simpler, less powerful chips that support only that mode.

Source: Qualcomm 218 Sleeping Modes

• Deep sleep power state of a wireless module is defined as a sleep state with the wireless radio turned off, i.e., RF, baseband and MAC processors are all switched off. – The only power consumed by the wireless module is leakage power.

• Shallow sleep power state of a wireless module is defined as a sleep state with baseband and MAC processors turned on, but RF is switched off.

219 Sleeping Modes: Power Transition Parameters

Power Transition parameters Average Power Consumption State Transitions Transition Time (ms) (mW)

Transmit Listen TTL=0.01ms 75mW Receive Listen 0.001ms 55mW

Listen Transmit TLT = 0.01ms PLT = 100mW

Transmit Shallow Sleep TTS=0.01ms PTS = 15mW

Receive Shallow Sleep TRS=0.2ms PRS = 15mW Listen Shallow Sleep T =0.2ms LS PLS = 5mW Shallow Sleep Listen 0.5 ms (TSL)

Listen Deep Sleep TLD=1ms PDS = 5mW

220 1024-QAM Modulation

221 1024 QAM

• The introduction of 1024 QAM into 802.11ax was achieved by pairing it with 3/4 and 5/6 coding rates to create two new Modulation and Coding Schemes (MCS) 10 and 11*.

– The raw speed gain over the 802.11ac 256 QAM is 10/8 or 25 percent, making 802.11ax the first commercial wireless technology capable of gigabit speeds with a single antenna.

*802.11ax also introduces two new MCSs: MCS-10 and MCS-11, which will most likely be optional. 1024-QAM can only be used with 242- subcarrier resource units (RUs) or larger.

This means that at least a full 20 MHz channel will be needed for 1024- QAM.

222 Parameters for HE-MCSs

• HE-MCSs for 26-tone RU • HE-MCSs for 52-tone RU • HE-MCSs for 106-tone RU • HE-MCSs for 242-tone RU and non-OFDMA 20 MHz • HE-MCSs for 484-tone RU and non-OFDMA 40 MHz • HE-MCSs for 996-tone RU and non-OFDMA 80 MHz

223 64- and 256- QAM

224 256- and 1024-QAM

While the effect of 1024 QAM on overall throughput is expected to be greater for smaller, denser cells (<2500 ft2) than for larger cells (>5000 ft), the peak speeds of 4.8 Gbps will enable new capabilities such as immersive enterprise-grade virtual reality using wireless headsets (HMD), a very favorable outcome.

The cost of this high speed is 50-percent tighter constellation points, resulting in approximately a 6-dB higher SNR requirement. However, unlike 802.11ac, 802.11ax is designed to support 8 x Tx and 8 x Rx antennas, facilitating greater transmit beamforming and Maximal-Ratio-Combining (MRC) gains to offset this deficit. From a Wi-Fi deployment perspective, designers should consider these peak speeds in terms of the required network capacity. 225 1024 QAM – 25% increase in PHY data rate

1024 QAM – 25% increase in PHY data rate

11ac – 256 QAM 8 bits per symbol

TX EVM MCS9 = -32 dB Min Sens MCS9 (20 MHz, 80 MHz) = -57, -51 dBm

226 MIMO Mode of Operation

• Single User (SU) – Normal mode – whole bandwidth allocated to one user at the time – Both DL/UL

• Multiuser (MU) – Multiple users multiplexed together – Initiated by AP – Both DL/UL

227 802.11ax vs. 802.11ax

11ax 11ac

Data Mode Gain Data rate Mode rate (Mbps) (Mbps) Min 0.375 1SS, MCS0, DCM, 6.5 1SS, MCS0, 20 MHz 26-tone

Max, 143.4*NSS 1024-QAM, r=5/6, 65% 86.7*NSS 256-QAM, r=3/4 (256-QAM, r=5/6 20 13.6 usec symbol only valid for NSS=3,6), 3.6 usec MHz symbol

Max, 286.8*NSS 1024-QAM, r=5/6, 43% 200*NSS 256-QAM, r=5/6, 3.6 usec symbol 40 13.6 usec symbol MHz Max, 600.4*NSS 1024-QAM, r=5/6, 39% 433.3*NSS 256-QAM, r=5/6, 3.6 usec symbol 80 13.6 usec symbol MHz Max, 160 600.4* 1024-QAM, r=5/6, 39% 433.3*2*NSS 256-QAM, r=5/6, 3.6 usec symbol MHz 2* NSS 13.6 usec symbol

NSS = 1…8 for both 11ac and 11ax 228 Multi-User MIMO

– 802.11ac introduced DL MU-MIMO, but we’re experiencing the following issues: – Many client devices are single antenna, and many two antenna clients switch to single stream mode for DL MU-MIMO for protection against interference – With 4 antenna AP, gains compared to Single User aremodest – Even if we built an 8 antenna AP, groupings are limited to 4 users – Channel sounding responses from the users are transmitted serially in time resulting in high overhead – TCP/IP on downlink with TCP ACK on uplink is impaired with no UL MU enhancement – UL MU-MIMO was initially considered in 11ac, but not included due to implementation concerns – 802.11ax MU-MIMO enhancements – UL MU-MIMO – Sounding frames, data frames, etc can be grouped among multiple users to reduce overhead and increase uplink response time – Groups expanded to eight users for both DL and UL – Now even with devices in single stream mode, MU-MIMO throughput can be doubled or tripled over single user operation

229 MU-MIMO DL

Reduces the collision domain, similar to an Ethernet Switch

• Spatial multiplexing • Same frequency spectrum for all users • Number of users is limited by the AP's number of transmit antennas

230 MO-MIMO UL

231 Multi-user operation by OFDMA and Space-division multiple access (SDMA)

• OFDMA, where multiple-access is achieved by assigning subsets of subcarriers to different users, allowing simultaneous data transmission by several users, and each group of subcarriers is denoted as a resource unit (RU). • The RUs can be allocated to STAs depending on their channel conditions and service requirements, and an OFDMA system can potentially allocate different transmit powers to different allocations.

• MU-MIMO. Both downlink and uplink MU- MIMO transmissions are supported on portions of the PPDU bandwidth (on resource units greater than or equal to 106 tones) and in an MU-MIMO resource unit, there is support for up to eight users with up to four space-time streams per user with the total number of space-time streams not exceeding eight. • OFDMA+MU-MIMO, Potentially can be •RU Size: A Resource Block (RB) is a time- and used. frequency resource that occupies subcarriers. •Resource unit (RU) size could be 26, 52, 106, 242, 484, 996, or 2*996 sub carriers. 232 SU-MIMO, MU-MIMO, Co-MIMO

• Single-User MIMO allows the single user to gain throughput by having multiple essentially independent paths for data • Multi-User MIMO allows multiple users on the reverse link to transmit simultaneously to the eNB, increasing system capacity • Cooperative MIMO allows a user to receive its signal from multiple eNBs in combination, increasing reliability and throughput

233 Beamforming and MU-MIMO

Diversity and multiplexing methods (1)  Cyclic Shift Diversity (CSD) One spatial stream drives multiple antennas. Received signal minima are avoided by giving each transmit antenna’s signal a large phase shift relative to the others.  Space Time Block Coding (STBC) Cyclic Shift Diversity A number of transmit antennas is used (CSD) to transmit a known sequence of variants of the original OFDM symbol (e.g. Alamouti 21). It can be used for conveying multiple data streams. Requires channel state information (CSI) at the receiver. 21, 42, 63, 84 STBC modes are specified for 802.11ac. Space Time Block Coding (STBC) 234 Beamforming and MU-MIMO

Diversity and multiplexing methods (2)  Beamforming Beamforming is based on the principle of weighting antenna signals (in amplitude and phase) and steer the beam towards a specific RX antenna. It requires CSI at the transmitter.

Simple  Single User MIMO (beamforming) beamforming The transmitter is notified by the receiver about the channel matrix H (the gains and phases of each combination of TX and RX antennas) and precodes the transmission with a proper matrix Q to maximize SNR at the receiver. 802.11ac specifies a maximum of up to 8 streams (vs. 4 streams for SU- 802.11n). MIMO

235 Beamforming and MU-MIMO

Diversity and multiplexing methods (3)

 Multi User (MU) MIMO (beamforming) Generalization of SU-MIMO where multiple users receive information simultaneously (Space Division Multiple Access - SDMA). This is introduced for first time in 802.11ac, which further specifies:  Support for up to 8 spatial streams per AP in both SU and MU-MIMO.  No more than 4 spatial streams per client in a MU transmission.  All SS have identical MCS for a specific STA

236 Beamforming and MU-MIMO

Beamforming sounding and feedback (1)  Beamforming success depends on the channel status feedback that the beamformer receives from the beamformee in order to form the proper steering matrix Q.

 In 802.11ac this feedback is provided after sounding by the beamformee in a “request-response” fashion (explicit feedback).

237 Beamforming and MU-MIMO

Beamforming sounding and feedback (2)  All beamformees have to provide explicit feedback to the beamformer. Ideally, the beamformer should provide high gain to the direction of each user and very low gain to the directions of the other users (null-steering)

238 Beamforming and MU-MIMO

239 Transmit Beamforming in 802.11

• Technique to ensure that transmitted signal couples into wireless channel with maximum gain – Realizes array gain + diversity gain

• Channel knowledge required at transmitter

• Support for both explicit and implicit beamforming

240 802.11 Explicit Beamforming

Sounding Feedback Steered packet

Beamformee

Beamformer • Beamforming based on explicit knowledge of the forward channel – Channel is sounded via Null Data Packet (NDP)

• Steering matrix feedback compressed via Given’s rotation 241 UL MU-MIMO or UL-OFDMA Transmission

Frequency/Spatial Domain

time 242 Downlink multi-user MIMO Transmission

243 MU-MIMO Application

• MU-MIMO is only possible where propagation characteristics allow the AP to identify that a transmission optimized for one client or group of clients will not be heard at a significant signal strength by another client, and vice versa.

– These are the conditions that allow it to build separate data frames for each client group, and transmit them simultaneously.

244 Sounding: Beamforming explicit feedback

In order to identify candidates for MU-MIMO, the AP performs sounding operations, sending null frames from all its antennas to clients, which then return responses with matrices of the measured receive levels for each AP-antenna to client-antenna pair. Sounding is used for beamforming as well as MIMO. Multi-user sounding in 802.11ac could be time consuming because the beamforming report matrix can be large, and the client devices had to stagger their responses to avoid interference: the new 802.11ax multi-user control protocol makes it much more efficient with 245 simultaneous responses. Downlink Sounding

246 Basic Frame Exchange Sequence for UL MU transmissions – New Trigger control frame – Specifies the length of the UL window – Specifies the users that may send during the UL window – Allocates resources for the UL-MU PPDUs: –RU allocation Trigger frame Acknowledg –Spatial stream allocation AP e frame –MCS to be used by the user UL MU PPDU – Supports transmission time, frequency, sampling symbol clock, and power pre- STA1 UL MU PPDU correction by the participating users STA2

– UL MU transmission may be OFDMA or MU- STA3 UL MU PPDU

MIMO STA4 Frequency/ UL MU PPDU domain Spatial – Acknowledgement frame can be – DL MU transmission with individually addressed BlockAck frames – New “Multi-STA BlockAck” frame contained in Legacy frame or HE MU PPDU – Trigger frame can be used as a Beamforming Report Poll, MU-BAR, MU-RTS, Buffer Status Report Poll, Bandwidth Query Report Poll…

247 Downlink MU Performance

248 Uplink MU Performance

249 Beamforming and MU-MIMO

Beamforming sounding and feedback (3)

• NDP announcement includes the STAs information that have to reply while NDP (Null Data Packet) is a packet with no data. • Each beamformee creates a feedback matrix based on the received power and phase shifts between each pair of TX-RX antennas for each subcarrier. The matrix is compressed and send back to the beamformer. • The size of the matrix may vary from some hundreds of bytes up to several KB 250 Example of MU PPDU exchange and NAV setting

251 DL MU Operation

A-MPDU

STA ACK STA

STA

• A trigger frame may be present in the DL MU PPDU enabling the recipient to send it ACK/BA. • Multi-TID A-MPDU is supported.

252 UL MU Operation

Trigger Frame M-STA ACK AP

STA STA

STA UL Transmissions • The specification defines a new Trigger frame sent by the AP to trigger UL transmissions by selected STAs. • A trigger frame schedule can be set by using a variant of target wake up time (TWT) introduced in 11ah. • The specification defines a new ACK type, multi-STA ACK enabling the AP to acknowledge transmissions from multiple STAs

253 MU Acknowledgement

Acknowledgment is performed separately for each STA. Two methods are specified with respect to TXOP. A Block Acknowledgment Request (BAR) precedes the BA.

254 MU Acknowledgement

Acknowledgment is performed separately for each STA. Two methods are specified with respect to TXOP. A Block Acknowledgment Request (BAR) precedes the BA.

255 UL MU Operation

Trigger Frame M-STA ACK AP

STA STA

256 Improved spatial reuse with MU-MIMO

• The radio channel is used for omnidirectional transmissions. When the AP transmits, the radio energy is received by both the laptop and the smartphone, and the channel may be used to communicate with only one of the devices at any point in time. One of the reasons why high-density networks are built on small coverage areas is that the same radio channel can be reused multiple times, and each AP in a 257 dense network can transmit on the channel independently. MU-MIMO and OFDMA Support Multi-user support: MU-MIMO and OFDMA – Increase network efficiency by multiplexing users in both frequency and space

Freq

PHY OFDMA Header Sub- Band STA#10 • 4 STAs STA#35 • 52 Subcarriers each STA#26

20 MHz • Option 1: No MIMO STA#54 • Option 2: Each 2 SS SS 1,2 STA #3 SS 3,4,5 STA #8 20 MHz

Time • STA #3, #8 and #19 SS 6 STA #19 • Using MIMO features • No OFDMA

MU-MIMO Space Frames are transmitted employing either OFDMA, MU-MIMO or a mixture of both

258 Limitations

• Experience with 802.11ac MU-MIMO in real-world deployments revealed some limitations. – For instance, it was not always possible to form usable groups, and even with a 4-antenna AP, gains over single-user mode were sometimes modest: in 802.11ax, the larger MU-MIMO groups (increased from 4 to 8 clients) will allow considerable improvement.

• 802.11ax can accommodate large numbers of client devices by grouping clients and dealing with groups sequentially. The example shows grouping of clients for beamforming reports, but the concept is also extended to other packet types.

• And any link- or transport-level protocol like TCP/IP that includes acks will gain from the improved downlink performance but may still be bottlenecked by the uplink: this will be solved when uplink MU-MIMO is added in 802.11ax.

259 MU-MIMO and OFDMA Support

• 4 STAs • 52 Subcarriers each • Option 1: No MIMO

PHY OFDMA • Option 2: Each 2 SS Header Sub- STA#10 Band

STA#35 STA#26

0MHz 20 STA#54

SS 1,2 STA #3

SS 3,4,5 STA #8

260 20 MHz Channel Configuration

• Three transmission modes are compared for the multi-user downlink scenario: 1. OFDMA - each of the four users is assigned a separate resource unit (RU) and the transmission is beamformed. 2. MU-MIMO - all four users share the full band. 3. Mixed MU-MIMO and OFDMA - Two users share a single RU in a MU-MIMO configuration, and the remaining two users are assigned a single RU each.

261 Example of Channel Configuration

OFDMA Sub- Band STA#1 • 4 STAs STA#2 • Different RU • Option 2: Each 1 SS STA#3

STA#4

262 Downlink and uplink gains with respect to STA

For example, with only 4 STAs, the 802.11ax downlink throughput (with large 1500B packets) is only 10 percent higher than 802.11ac but the uplink throughput is 2.2 times that of 802.11ac (or 120-percent gain). In general, the more clients and access point serves in each TXOP or channel-access, the more efficiency over 802,11ac the access point achieves, especially with small packets such as from voice, video, or TCP ACKs. 263 REVIEW: 802.11ax Selected Rates (Short GI)

264 REVIEW: 802.11ax Selected Rates (Short GI)

265 REVIEW: Usable sub-channels, subcarriers and data-rates for OFDMA

Usable sub-channels, subcarriers and data-rates for OFDMA

The table lists the menu of options for RU-N (e.g. RU-26) sub-channels in OFDMA. These RU’s can move around but only in certain configurations as specified in 802.11ax.

266 Overlapping Basic Service Set (OBSS)

For example, if AP-1 on channel 6 hears the preamble transmission of a nearby AP (AP-2), also transmitting on channel 6, AP-1 will defer and can’t transmit at the same time. Likewise, all the clients associated to AP-1 must also defer transmission if they hear the preamble transmission of AP-2. If a client associated to AP-2 is transmitting on channel 36, it is possible that AP-1 (and any clients associated to AP-1) will hear the PHY preamble of the client and must defer any transmissions. 267 BSS (Basic Service Set) Coloring

802.11ax was also tasked with addressing the OBSS challenge by improving spatial reuse, which is often referred to as BSS coloring.

BSS coloring is a mechanism, originally introduced in 802.11ah, to address medium contention overhead due to OBSS by assigning a different “color” — a number between 0 and 7 that is added to the PHY header of the 802.11ax frame — to each BSS in an environment

268 Rational Behind BSS Coloring

269 Rational behind “BSS Color Codes”

• As well as the multi-user OFDMA support, 11ax introduces another feature that increases the efficiency of high-density network deployments where many access points are operating in a limited area. – In this situation, access point Basic Service Sets (BSSs) can overlap when using the same channel, leading to wireless contention and interference problems. – The 11ax standard implements a “color code” for each BSS that is transmitted in the signal preamble, enabling clients to detect when transmissions are from an overlapping BSS. – In an enterprise network, the ability to detect each BSS color code enables clients and access points to set specific signal-detection thresholds and transmit power levels, which provides better management of contention and interference. – The result is an improvement in overall network performance and a more efficient use of spectrum resources.

270 BSS Color Codes

• A basic service set (BSS) is the cornerstone topology of any 802.11 network. The communicating devices that make up a BSS consist of one AP radio with one or more client stations. • 802.11ax radios can differentiate between BSSs using a BSS color identifier when other radios transmit on the same channel. If the color is the same, this is an intra-BSS frame transmission. In other words, the transmitting radio belongs to the same BSS as the receiver. • If the detected frame has a different BSS color from its own, then the STA considers that frame as an inter-BSS frame from an overlapping BSS. • An 802.11ax AP can change its BSS color if it detects an OBSS using the same color. The duplicate color detection of an OBSS is also referred to as a color collision. An associated 802.11ax client may send a color collision report to its associated AP if the client detects a color collision. If AP-1 cannot hear AP-2, however, an associated client to AP-1 can hear the OBSS with the same color and can then send a color collision report. The client station’s autonomous report will include BSS color information of all OBSSs that the client is 271 able to detect. BSS Coloring

BSS coloring detects a color bit in the PHY header of an 802.11ax frame transmission. This means that legacy 802.11a/b/g/n clients will not be able to interpret the color bits because they use a different PHY header format. When an 802.11ax radio is listening to the medium and hears the PHY header of an 802.11ax frame sent by another 802.11ax radio, the listening radio will check the BSS color bit of the transmitting radio. Channel access is dependent on the color detected:

If the color bit is the same, then the frame is considered an intra-BSS transmission and the listening radio will defer.

If the color bit is different, then the frame is considered an inter-BSS transmission from an OBSS and the listening radio treats the medium as busy only for the time it took to determine the color bit was different.

272 BSS Coloring Decision Making

• BSS coloring decision making: – Station detect RF energy. – Clear channel assessment suggests whether energy threshold is above -82 or below. – If RSSI greater than -82 dBm and station checks whether it can demodulate traffic? – If yes, then it will read frame header to see color of the frame. – If its same color, then its means frame is from intra-BSS and will have to go through normal CSMA/CA process. – If color is not same as its own BSS, then it’s an inter-BSS frame. – At this point there is another threshold check, it checks if signal strength of frame is above -62 or below. – If signal strength is greater then -62 its mean its too close and medium will be considered as busy. – If signal strength is lower than -62 then station will not contend for this transmission and will continue transmitting.

273 BSS Coloring Decision Making

274 802.11ax Enhancements and Design Considerations • Several other 802.11ax enhancements including target wake time (TWT).

• Target wake time (TWT) is a power-saving mechanism originally defined in the 802.11ah-2016 amendment.

• A TWT is a negotiated agreement, based on expected traffic activity between the access point (AP) and Wi-Fi clients, to specify a scheduled target wakeup time for clients in power-save (PS) mode.

– In addition to the power-saving benefits, the negotiated TWTs allow an AP to manage client activity by scheduling client stations to operate at different times and therefore minimize

contention between the clients. 275 Rational behind Target-Wakeup Time (TWT)

• To address device power management issues the 11ax standard includes a mechanism for extended sleep states that reduce power consumption. – An 11ax access point allows clients to request a specific Target-Wakeup Time (TWT) to transmit or receive frames, rather than rely on periodic beacons. – This enables client devices to have much longer sleep states without having to wake up to receive beacons, resulting in significant power savings. – In addition, the client TWTs can be scheduled and controlled by the access point to both manage contention in the network as well as accommodate delay-sensitive traffic.

276 Target Wake Time (TWT): From IEEE 802.11ah to 802.11ax • 802.11ax deployed in dense device environments will support higher service-level agreements (SLAs) to more concurrently connected users and devices with more diverse usage profiles.

– This is made possible by a range of technologies that optimize spectral efficiency, increase throughput and reduce power consumption.

– These include Target Wake Time (TWT), OFDMA and MU-MIMO, Uplink MU-MIMO, sub-carrier spacing and MAC/PHY enhancements. 277 Target Wake Time (TWT) in 802.11ax

• Target Wake Time enables devices to determine when and how frequently they will wake up to send or receive data.

• Essentially, this allows 802.11ax access points to effectively increase device sleep time and significantly conserve battery life, a feature that is particularly important for the IoT.

– In addition to saving power on the client device side, Target Wake Time enables wireless access points and devices to negotiate and define specific times to access the medium.

– This helps optimize spectral efficiency by reducing contention and overlap between users.

278 Target Wake Time (TWT)

–Target Wake Time (TWT) is a power saving mechanism in 802.11ah, negotiated Power Consumption Profiles between a STA and its AP,

which allows the STA to Access delay Lookup + Access Sleep sleep for periods of time, Wake delay and wake up in pre- Beacon ULBA DLBA SM: Sleep Mode scheduled (target) times to LM: Listen Mode exchange information with SM LM RM LM/RM TM RM LM/RM RM TM SM RM: Receive Mode its AP TM: Transmit Mode • Baseline PS-POLL Slot delay –802.11ah TWT Wake Sleep mechanism modified to support triggered- based Beacon UL BA DL BA uplink transmissions SM LM RM ?M TM RM RM TM SM • Beacon-based access Sleep –New Broadcast TWT Wake operation added in UL BA DL BA 802.11ax to support non- SM LM TM RM RM TM SM AP STAs that have not negotiated any implicit • TWT-based access agreement with HE AP

279 Target wake time (TWT) is a power-saving mechanism • Target wake time (TWT) is a power-saving mechanism originally defined in the 802.11ah-2016 amendment. – A TWT is a negotiated agreement, based on expected traffic activity between the access point (AP) and Wi-Fi clients, to specify a scheduled target wakeup time for clients in power- save (PS) mode. – In addition to the power-saving benefits, the negotiated TWTs allow an AP to manage client activity by scheduling client stations to operate at different times and therefore minimize contention between the clients. – A TWT reduces the required amount of time that a client in PS mode needs to be awake. – This allows the client to sleep longer and reduces energy consumption.

280 TWT vs. Legacy Client Powersaving

• As opposed to legacy client powersaving mechanisms, which require sleeping client devices to wake-up in microsecond intervals, TWT could theoretically allow client devices to sleep for hours. TWT is thus an ideal powersaving method for mobile devices and Internet of Things (IoT) devices that need to conserve battery life.

• It remains to be seen if IoT device manufacturers will take advantage of 802.11ax radios in their IoT devices as opposed to other communication technologies such as Bluetooth Mesh, Thread, and Zigbee.

– TWT setup frames are used between the AP and the client to negotiate a scheduled TWT. For each 802.11ax client there can be as many as 8 separate negotiated scheduled wake-up agreements for different types of application traffic. 802.11ax has also extended TWT functionality to include a non-negotiated TWT capability.

• An AP can create wake-up schedules and deliver TWT values to nthe 802.11ax clients via a broadcast TWT procedure.

281 TWT (Target Wake Time) for Max Power Efficiency

• 802.11ax introduces a technology called “target wake time” to improve wake and sleep efficiency on smartphones and other mobile devices. This technology will make a significant improvement in battery life.

• TARGET WAKE TIME (TWT) FOR MAX POWER EFFICIENCY – 802.11ax implements TWT, which allows the Wi-Fi radio in battery-powered devices such as phones to go to sleep when not exchanging data. This lowers power consumption and saves device battery.

– With TWT, 802.11ax capable routers and mobile devices can negotiate the sleep cycle according to the data traffic, so that they only wake up when it is their turn to communicate, allowing them to preserve the battery.

– Also with TWT, devices can be programmed to wake up at the same time to take advantage of OFDMA so that they can communicate at the same time. The result is a well-synchronized data flow that allows all devices to connect simultaneously and based on their needs.

– This improves the user experience as video, voice, data, and IoT traffic is proportioned and prioritized effectively. 282 Role of TWT

• A TWT reduces the required amount of time that a client in PS mode needs to be awake. This allows the client to sleep longer and reduces energy consumption. As opposed to legacy client powersaving mechanisms, which require sleeping client devices to wake-up in microsecond intervals, TWT could theoretically allow • client devices to sleep for hours.

– TWT is thus an ideal powersaving method for mobile devices and Internet of Things (IoT) devices that need to conserve battery life.

283 Transmit power control in multi-user mode

• Multi-user modes in 802.11ax allow much more control over transmit power levels, and most of the control lies at the AP. – This should be useful for clients’ battery life, and for limiting co-channel interference as, for example, clients currently tend to transmit at maximum power even though APs may be on reduced power in a dense multi-AP deployment, increasing the interference radius.

• As a result of the sounding procedure, an AP learns how its clients are receiving its signals, which allows it to estimate the path loss and RF channel conditions.

• Thus it can adjust its transmit power to target a particular signal level at the client, or more often, a signal-to-noise-and-interference (SINR) level. – Since MCS and error rates are related to SINR, it can choose to optimize by reducing the error level, or increasing the MCS and/or transmit power to increase data rates and reduce time on the air.

284 Limitations

• One interesting possibility is to increase the power transmitted in certain OFDMA RU’s, while reducing that used in others. – This is interesting because it opens an opportunity for ‘water- filling’, a technique to allocate resources to the most-effective recipient, but also allows the AP to transmit above the allowed power levels (EIRP) for certain sub-carriers, while reducing power on others.

– So long as the overall EIRP on a 20 MHz channel is within limits, this configuration would be allowed by regulation. • With the new multi-user signaling mechanisms, the AP can now control the client’s transmit characteristics.

285 TWT Setup Frames

• TWT setup frames are used between the AP and the client to negotiate a scheduled TWT. – For each 802.11ax client there can be as many as 8 separate negotiated scheduled wake-up agreements for different types of application traffic. 802.11ax has also extended TWT functionality to include a non- negotiated TWT capability.

• An AP can create wake-up schedules and deliver TWT values t the 802.11ax clients via a broadcast TWT procedure.

286 287 Power-save options before 802.11ax

One of the goals of the 802.11ax project is to improve performance by a factor of 4x while keeping power requirements unchanged or improved. With the emerging IoT market, power-save mechanisms at the other end of the performance scale were also a particular focus. Several power-save mechanisms already exist in prior 802.11 standards: these remain, and are supplemented with a new mechanism, ‘target wait time’ (TWT).

TWT was introduced in 802.11ah, the amendment for low-power, long-range IoT transmission; but since 802.11ah chips and devices have not been widely adopted by the market, it is new to users of Wi-Fi equipment.) TWT is particularly useful for battery-powered devices that communicate infrequently.

288 Power-save mechanisms in 802.11ax

• The existing ‘legacy PS’ mechanism has been in use since 802.11b, the first widely- used Wi-Fi standard. Clients can sleep between AP beacons, or multiples of beacons, waking when they have data to transmit (they can transmit at any time, the AP does not sleep) and for beacons containing the delivery traffic information map (DTIM), a bit-map indicating that the AP has downlink data buffered for transmission to particular clients. – If the DTIM bit is set for a client, it can retrieve its data by sending a trigger frame to the AP immediately after the beacon. PS is an effective mechanism but only allows clients to sleep for a small number of beacon intervals, usually clients must wake several times per second to read the DTIM.

• As explicit voice-over-Wi-Fi support was added with 802.11e, the IEEE recognized that voice-capable devices required a new power save mechanism, as voice packets are transmitted at short time intervals, typically 20 msec. Unscheduled automatic power-save delivery (U-APSD) allows a client to sleep at intervals within a beacon period. As in PS, the AP buffers downlink traffic until the client wakes and requests it. With symmetrical traffic like voice, the client can often send and receive frames in the same waking interval.

289 TWT power-save options in 802.11ax

290 The new TWT mechanism in 802.11ax allows more flexible, long-term and even multi-client sleeping arrangements

• First, a negotiation between the client and AP sets up an agreed schedule for the client to wake and communicate. The schedule is often periodic, with a long, multi-beacon interval (minutes, perhaps hours or days) between activities. – When its designated time arrives, the client wakes, awaits a polling trigger frame from the AP (required in multi-user mode) and exchanges data, subsequently returning to the sleep state. – Since the AP negotiates separately with each client, it can group or separate scheduled transmissions in order to achieve best traffic efficiency or to accommodate traffic requirements from other clients.

291 Example of Target Wake Time Broadcast operation

A significant benefit of TWT is that it can also be used as an uplink scheduling method akin to UL-OFDMA. That is, because TWT effectively puts clients to sleep with a predetermined wake-up time (based on their request), deterministic transmission times and hence uplink scheduling is possible. The access point can use this ability to both reduce contention (more distributed channel usage) and address delay sensitivity of applications.

292 Example of Target Wake Time Broadcast operation

In prior generations of 802.11, low-power devices such as mobile phones were accommodated with Unscheduled Automatic Power Save Delivery (U-APSD) or Wi-Fi Multi Media Power-Save (WMM-PS). A client in this mode can have the access point buffer transmissions to it instead of sending it immediately. Instead, the access point signaled availability of data in periodic beacons through a Traffic Indication Message (TIM), which allows the client to keep its radio receiver off (saving power) and waking-up only periodically to receive beacons (generally a multiple of every 102.4 ms). However, this strict adherence to beacons limits the potential energy-saving potential for IoT devices that don’t require regular channel access like a mobile phone yet must always be ready to receive a phone call. 293 Other Power Saving Features

–Receive Operating Mode indication is a procedure to dynamically adapt the number of active receive chains and channel width for reception of the subsequent PPDUs, by using a field in the MAC header of a Data frame, –rather than Operating Mode Notification management frame exchange (e.g. 11ac) –Minimal impact on channel efficiency –Transmit Operating Mode indication is a procedure for client devices to dynamically adapt their transmit capability: –Channel width & maximum number of spatial streams –SU vs UL MU operation –Use of BSS Color field and UL/DL flag in preamble to enable intra PPDU power saving

294 h N H i g E f f i c i e n c y W L A

Overview of 802.11az and 802.11ay 295 The Wi-Fi CERTIFIED Location™

• Wi-Fi Alliance’s new interoperability certification program to enable Wi-Fi for indoor location. – The Wi-Fi CERTIFIED Location™ feature is based on the Fine Timing Protocol from IEEE 802.11-2016 and delivers metre-level accuracy for indoor device location.

– This feature enables the use of Wi-Fi for use cases such as indoor navigation, asset tracking and network management.

296 IEEE 802.11az

• Next Generation Positioning (NGP), a study group was formed in January 2015 to address the needs of a “Station to identify its absolute and relative position to another station or stations it’s either associated or unassociated with.” – The goals of the group would be to define modifications to the MAC and PHY layers that enable “determination of absolute and relative position with better accuracy with respect to the Fine Timing Measurement (MTM) protocol executing on the same PHY-type, while reducing existing wireless medium use and power consumption, and is scalable to dense deployments.” The current estimate on approval of this standard is March 2021.

297 Fine Time Measurement (FTM) Protocol

298 Previous methods for Wi-Fi Location

• Up to now, indoor location using Wi-Fi has so far mainly relied on signal strength measurements, to either provide an estimate of distance by measuring a reduction in signal strength or matching a pattern of received signal strengths to measure patterns at known points. – However, signal strength measurements are quite variable, limiting the inherent accuracy of these methods. In order to get a good accuracy, a site survey needs to be carried out to measure the signal strengths at each location. This is a very time-consuming process and needs to be repeated every time the equipment is changed.

• Measuring location using Wi-Fi – how it works • Wi-Fi signals travel through the air at a predictable rate – the speed of light. Therefore, the time between a transmission leaving an access point (AP) or station (STA) and arriving at another AP or STA can be converted to distance by multiplying by the speed of light. – This is called a ‘time-of-flight measurement’.

299 Measuring location using Wi-Fi – how it works

Frame in Flight

Access Point STA Transmit at T=0 Receive at T=22.5 nsec

Finding distance by measuring the speed of the light

300 Limited Accuracy enabled by RSSI/path-loss based Location Technologies

• Facing the limited positioning accuracy enabled by RSSI/path-loss based location technologies and the limited scalability of fingerprint-based systems, industry vendors began seeking alternative WLAN-based positioning technologies, which will enable to achieve higher positioning accuracy. • Taking advantage of the high bandwidth supported by the WLAN systems (ranging between 20-160 MHz), the approach pursued was geolocation based on time- delay estimation. – Though the early releases of the 802.11 standard included means for time delay estimation, the timing resolution enabled by these mechanisms was in the microseconds level- too coarse for any practical indoor positioning applications.

• High-accuracy positioning in a dense multipath environment imposed several hardware design changes in the existing WLAN chipsets, in order to increase the timing resolution from the microseconds level to the nanosecond level (or even sub-nanosecond level).

– The solution that was endorsed by the IEEE TM group, was a novel time-delay based ranging protocol called “fine-timing measurement” (FTM).

301 802.11mc

WiFi Fine Time Measurement (FTM) Round Trip Time (RTT) for indoor location. The time a WiFi signal takes to travel from a smartphone (“station” - STA) to a WiFi access point (AP) is, of course, proportional to the distance between them (about 3.3 nanoseconds per meter). Since the internal clocks in the smartphone and the access points are not synchronized, a one-way time measurement cannot be based on differences between timestamps at the two ends.

Fortunately, the difference in timestamps when the signal travels in the reverse direction is affected in the opposite way by the clock offset. As a result, the round trip time (RTT) can be obtained without having to know the clock offsets - by simple addition and subtraction of four times: RTT = (t4-t1 + t2-t3).

302 FTM Protocol Overview

303 Finding distance by measuring the “Round-trip time”

• The challenge with ‘time-of-flight’ is difficulty in getting both the devices in the measurement synchronized to the same clock reference; within a nanosecond or so.

• This problem is normally avoided by making a ‘round-trip-time’ measurement.

Frame in Flight

Ack in Flight Access Point STA Transmit at T=0 Turnaround time at Received at T=1045 nsec T=1000 nsec

Finding distance by measuring the Round-trip Time

304 Wi-Fi Location protocol: Fine Time Measurement (FTM)

1 Frame in Flight

Ack in Flight 2 Access Point STA

Protocol Frame 3 Transmit at T=5000 nsec (t ) with t0 and t4 0 Receive at T=5022 nsec (t1) Receive at T=6045 nsec (t ) 4 Transmit at T=6022 nsec (t3)

AP sends timestamps to STA STA now has 4 accurate timestamps

305 The Round Trip Time Measurements

• The round trip time measurements are not perfectly accurate, being subject to various types of measurement noise, RF interference as well as the positions and motions of objects in the environment. • Repeated measurements improve the quality a bit:

306 Indoor positioning using time of flight with respect to WiFi access points

The animation above shows floor plans of three levels of a building with six “responders” (WiFi access points advertising IEEE 802.11mc) shown in green, and the estimates of positions computed by an Android smartphone shown in red, as it is carried upstairs and around floors. The position is recovered in 3-D and then quantized to floor level for plotting. The floor plan is only for visualization and not used in the computation of the position of the phone. As a result, the red dot may at times appear to pass through a wall or even appear outside the building.

The final position accuracy is about a meter or two (aside from the occasional outlier), while the raw distance measurements have a 10% to 90% CDF (cumulative distribution function) range of between 0.2 and 0.8 meter depending on circumstances (see Measurement Error). The “dilution of precision” (noise gain), depends on the positions of the responders, and varies throughout the volume. Optimal placement of responders is an open problem (see FTM RTT Placement), as is the best way to deal with the measurement errors.

307 Wi-Fi Location: Ranging with RTT

You can use the Wi-Fi location functionality provided by the Wi-Fi RTT (Round-Trip-Time) API to measure the distance to nearby RTT-capable Wi-Fi access points and peer Wi-Fi Aware devices. If you measure the distance to three or more access points, you can use a multilateration algorithm to estimate the device position that best fits those measurements. The result is typically accurate within 1-2 meters.

With this accuracy, you can develop fine-grained location-based services, such as indoor navigation, disambiguated voice control (for example, "Turn on this light"), and location-based information (for example, "Are there special offers for this product?").

The requesting device doesn't need to connect to the access points to measure distance with Wi-Fi RTT. To maintain privacy, only the requesting device is able to determine the distance to the access point; the access points do not have this information. Wi-Fi RTT operations are unlimited for foreground apps but are throttled for background apps.

Wi-Fi RTT and the related Fine-Time-Measurement (FTM) capabilities are specified by the IEEE 802.11mc standard. Wi-Fi RTT requires the precise time measurement provided by FTM because it calculates the distance between two devices by measuring the time a packet takes to make a round trip between the devices and multiplying that time by the speed of light. https://developer.android.com/guide/topics/connectivity/wifi-rtt

308 Animation Wi-Fi Location: Ranging with RTT

309 Wi-Fi Location protocol: Fine Time Measurement (FTM)

310 FTM Protocol

• The FTM protocol enables a WLAN station to measure its distance with respect to time another station1.

• The range measurement is based on high-resolution, time delay estimation, which also accounts for the latency imposed by the chipset hardware.

• The hardware-imposed latency (e.g. the receive/transmit filters’ group-delay and other hardware latencies), is measured and pre-calibrated by the chipset in order to reach the required timing resolution.

• Obtaining an accurate time delay estimate in a dense-multipath environment is typically implemented using some super-resolution method , which are applied to the estimated channel response.

The FTM protocol has first appeared in the 2016 release of the IEEE802.11 standard (formerly known as IEEE802.11REVmc). 311 FTM Protocol Message Flow Example

Initiating STA Responding STA

FTM is a point-to-point (P2P), single-user protocol, which includes an exchange of multiple message frames between an initiating WLAN station (STA) and a responding STA. The initiating STA attempts to measure its range with respect to the responding station (e.g., WLAN AP or a dedicated FTM responder).

The time of flight (ToF) between the two stations is calculated using:

RTT = (t4-t1 + t2-t3)

where t1 denotes the time of departure (ToD) measured by responding station, and t4 denotes the time of arrival (ToA), which is estimated by the responding station. The values of t1 and t4 are reported back to the initiating station2 after the completion of the FTM measurement phase.

312 WiFi Fine Time Measurement (FTM) Round Trip Time (RTT) for indoor location • The time a WiFi signal takes to travel from a smartphone (“station” - STA) to a WiFi access point (AP) is, of course, proportional to the distance between them (about 3.3 nanoseconds per meter). – Since the internal clocks in the smartphone and the access points are not synchronized, a one-way time measurement cannot be based on differences between timestamps at the two ends.

– Fortunately, the difference in timestamps when the signal travels in the reverse direction is affected in the opposite way by the clock offset. As a result, the round trip time (RTT) can be obtained without having to know the clock offsets - by simple addition and subtraction of four times: RTT = (t4-t1 + t2-t3).

313 The time of flight (ToF)

• The time of flight (ToF) between the two stations is calculated using:

• where t1 denotes the time of departure (ToD) measured by responding station, and t4 denotes the time of arrival (ToA), which is estimated by the responding station.

• The values of t1 and t4 are reported back to the initiating station2 after the completion of the FTM measurement phase.

– Notice that FTM only enables to measure the range between two stations. – Obtaining a position estimate based on multiple range measurements is out of the standard scope. However, the standard does define mechanisms for the responding stations to provide their location information (such as, absolute or relative position coordinates, floor level etc.), in an information element (IE), called location configuration information (LCI). The LCI of the responding stations may be used by the initiating station to estimate its absolute or relative position.

314 FTM as a P2P Protocol

• The initiating station combines these parameters along with its own estimated ToA, t2, and measured ToD, t3 values, to obtain a range estimate w.r.t. the responding station3.

• Being a P2P, single-user protocol, the FTM protocol is limited in scenarios where an extremely large number of users are requesting positioning services simultaneously.

• Provided no FTM message transactions are lost on the way due to temporal channel interruptions, the initiating station should be able to obtain a range estimate w.r.t. the responding station within about 30ms. Hence, obtaining its position, which involves ranges estimation towards 3 additional stations, should ideally take about 100-120ms. This implies that each FTM responder may be able to serve about 30 client stations per second.

315 Collaborative time of arrival” (CToA) protocol

• Clearly, with more and more navigating stations attempting to execute FTM sessions, the collision likelihood increases, which effectively decreases the number of stations that can be serviced.

• Consider for example, large stadiums hosting rock concerts or major sports events. In such occasions it is easy to imagine tens of thousands of users navigating throughout the stadium area using location-based services. Servicing all these users might require to deploy a network of thousands of • “Collaborative time of arrival” (CToA) protocol, can provide a more cost-effective solution for such use cases.

316 Security Issue of FTM

STA1 STA2 (Responding FTM Request (Initiating STA) STA) • The FTM frames and Ack frames and protection Ack – A malicious device can transmit fake Ack frames, which pretend to be the Ack frames from the Initiating STA, to the Responding STA. – Because an Ack is transmitted within an SIFS, the FTM_m payload contains t1_m [t1_(m-1), t4_(m-1)] authentic Ack arrives in the middle of the fake Ack t2_m reception at the Responding STA. An implementation Fake Ack uses implementation specific criteria to decide whether t4_m’ t3_m to declare a collision (and abandon the t4_m capturing) Authentic Ack or to decode either the Fake Ack or the Authentic Ack. t4_m

FTM_(m+1) payload contains [t1_m, t4_m’] • Using the implementation specific criteria, when t1_(m+1) t2_(m+1) the fake Ack is decoded successfully, the

Fake Ack Responding STA obtains the wrong t4_m and t4_(m+1)’ Authentic Ack t3_(m+1) includes it in the payload of the subsequent FTM t4_(m+1) frame, which causes the Initiating STA to derive the wrong RTT.

317 Example of Hardware support for Fine Timing Measurement

• The accuracy of Wi-Fi Location depends on the accuracy of the timestamp (1ns of uncertainty is equivalent to 30 cm of distance). • Thus, it is very important to have accurate time stamps. – Highly accurate time stamps are crucial to maintain the accuracy of Wi-Fi location measurements.

High-resolution timers record the WLAN packet arrival and departure times with very low uncertainty. A 48-bit counter with high resolution operates at 320 MHz. The snapshot of the counter can be taken either by configuring a trigger register (in software) or using hardware.

318 Example of Hardware support for Fine Timing Measurement

319 Multipath Problem

320 Summary

• In practice the ‘round-trip-time’ is several orders of magnitude greater than the ‘time-of-flight’ and varies over time, making the raw measurement inaccurate. • If the STA can also measure accurate timestamps, it can determine the time it took to turn around the frame with an accuracy of several nanoseconds. • If the protocol enables timestamps to be transmitted between devices, all four timestamps can be collected in one device and the calculation can proceed. • The distance calculation depends on all four timestamps (or two time differences) being in one place. One device has to send its timestamps to the other, enabling that second device to make the calculation. • The IEEE 802.11-2016 now includes a Fine Time Measurement (FTM) protocol for WiFi ranging, and several WiFi chipsets offer hardware support albeit without fully functional open software. This paper introduces an open platform for experimenting with fine time measurements and a general, repeatable, and accurate measurement framework for evaluating time-based ranging systems.

321 h N H i g E f f i c i e n c y W L A

Appendix: Additional Topics

322 802.11ax AP Examples: Netgear

• AX12-NIGHTHAWK AX12 12-STREAM WI-FI 6 ROUTER • MODEL: RAX120

• AX6000-NIGHTHAWK AX8 8-STREAM WI-FI 6 ROUTER • MODEL: RAX80

• https://www.netgear.com/compare.aspx 323 NIGHTHAWK AX12 12-STREAM WI-FI 6 ROUTER - Model: RAX120

1. Up to 6Gbps Speeds—1200+4800 Mbps with 12-stream connectivity. Source: Netgear 2. 8x8 MU-MIMO—Go faster by simultaneously streaming to multiple devices. 3. 1024-QAM – 25% increased data efficiency and faster speeds than a 256-QAM router. 4. 4X Better Performance than an AC Router—12 Stream WiFi with up to 1200+4800 Mbps† for ultra fast wireless speeds. 5. More WiFi for More Devices—OFDMA allows efﬁcient data transmission up to 8 devices at the same time. 6. Up to 8 Simultaneous WiFi Streams—8-stream MU-MIMO‡ enables up to four (4) 2x2 devices to stream content at the same time. 7. Maximized Range—Eight (8) antennas extend wireless range coverage indoors and out. 8. Supports all current WiFi devices—Backward compatible with 802.11a/b/g/n/ac client devices. 9. Supports WPA3 - The latest WiFi security protocol that offers stronger encryptions, password protection and added security benefits to your home network. 324 NIGHTHAWK AX8 8-STREAM WI-FI 6 ROUTER- Model: RAX80

1. 802.11ax Dual Band WiFi (AX6000) Source: Netgear 2. 2.4GHz AX: 4x4 (Tx/Rx) 1024 QAM 40MHz, up to 1.2Gbps 3. 5GHz AX: 4x4 (Tx/Rx) 1024 QAM 160MHz, up to 4.8Gbps 4. Uplink & Downlink OFDMA—Improved capacity and efficiency 5. 1024-QAM—25% data efficiency and faster speeds than a 256-QAM router 6. Backwards compatible with 802.11a/b/g/n/ac WiFi 7. Standards-based WiFi Security (802.11i, 128-bit AES encryption with PSK)

325 Additional AP Reviews

• Aerohive AP650 – Appendix A: Aerohive_Datasheet_AP650

• Aerohive AP650X – Appendix B: Aerohive_Datasheet_AP650X

326 Comparison

• RAX120 – 12-Stream WiFi - Faster and more reliable network speeds to every connected device – OFDMA —Improved capacity and efﬁciency – 1024-QAM—25% data efﬁciency and faster speeds than a 256-QAM router – AX Optimized Processor —Powerful 64bit quad-core 2.2GHz processor – 8 High-Performance Antennas— Longer range and coverage – MU-MIMO-Simultaneous streaming of data to multiple devices – Supports WPA3 - The latest WiFi security protocol – Beamforming+— Boosts speed, reliability and range of WiFi Connections for 2.4 and 5GHz

• RAX80 – Uplink & Downlink OFDMA—Improved capacity and efficiency – 1024-QAM—25% data efficiency and faster speeds than a 256-QAM router – 160MHz channel support – MU-MIMO-Simultaneous streaming of data to multiple devices – Longer range with 4 high-performance antennas

327 Question and Answer

328 2/26/2020 328 Appendix A: Principles of Beamforming

329 Beamforming is the process of trying to concentrate the EM energy to particular directions. An alternative method of transmission is to focus energy toward a receiver, a process called beamforming

330 Beamforming

• Beamforming increases the performance of wireless networks at medium ranges. • At short ranges, the signal power is high enough that the SNR will support the maximum data rate. • At long ranges, beamforming does not offer a substantial gain over an omnidirectional antenna, and data rates will be identical to non- beamformed transmissions. Beamforming works by improving what is called the rate over range—at a given distance from the AP, a client device will have better performance. 331 A Single Antenna

332 How an Antenna’s Radiation Adds Up

Zero current at each end • An antenna is just a passive conductor each tiny carrying RF current imaginary “slice” of the antenna – RF power causes the current flow does its share of radiating – Current flowing radiates electromagnetic fields

TX Maximum current RX – Electromagnetic fields cause at the middle current in receiving antennas • The effect of the total antenna is the sum of what every tiny “slice” of the Current induced in receiving antenna antenna is doing is vector sum of contribution of every – Radiation of a tiny “slice” is tiny “slice” of radiating antenna proportional to its length times the magnitude of the current in it, at the phase of the current

Width of band denotes current magnitude

333 Different Radiation In Different Directions

Minimum Radiation: contributions • Each “slice” of the antenna produces a out of phase, cancel definite amount of radiation at a specific phase angle • Strength of signal received varies, depending on direction of departure from radiating antenna – In some directions, the components Maximum add up in phase to a strong signal level Radiation: TX contributions – In other directions, due to the different in phase, reinforce distances the various components must travel to reach the receiver, they are out of phase and cancel, leaving a much weaker signal • An antenna’s directivity is the same for transmission & reception

Minimum Radiation: contributions out of phase, cancel

334 Phased Array Systems

335 Multiple Antennas

336 Beam Steering

337 Receive Antenna: Delays

338 DL MU-MIMO with OFDMA

Cyclic shift diversity (CSD) per STS insertion

339 Beamforming Range Effects

340 Beamformer and Beamformee

• Any device that shapes its transmitted frames is called a beamformer, and a receiver of such frames is called a beamformee.

– 802.11ax defines new terms for the sender and receiver of beamformed frames because in a single exchange it is possible to have only one initiator and one responder, but a station may be both a beamformer and a beamformee

341 Beamforming Terminology and Process

Beamformer Beamformee

342 Beamforming terminology and process

• To steer transmissions in a particular direction, a beamformer will subtly alter what is transmitted by each array.

• All antennas transmit at the same time. As a result, the total transmission radiates in each direction equally.

• Gains from beamforming are variable and depend on the radio environment, the sophistication of the antenna array on both sides of the link, the relative motion of both sides of the link, and many other factors. – A reasonable expectation would be that beamforming can result in a gain of anywhere between 2 to 5 dB, with the best results coming for mid-range transmissions.

– At short ranges, transmissions are already at the maximum data rate and there will not be any gain in speed. At long ranges, the beamforming gain is not sufficient to add speed.

343 Using multiple antennas to steer transmissions

344 Traditional WiFi vs. 802.11ax

Traditional WiFi 802.11ax Beamforming Technology

345 Beamforming in 802.11ax

346 Transmit Power Limitations and Beamforming

• 802.11ac operates subject to regulations regarding transmit power. When MIMO was first introduced, regulators imposed a limit based on the array gain. • MIMO systems improve performance by analyzing signals across multiple antennas, which offers a signal-processing gain that is equivalent in concept to simply using a larger antenna. – The array gain is related to the number of antennas in the array and is defined as 10*log(N), where N is the number of antennas in the array. – For a two-antenna system the array gain is 3 dB, and for a three-antenna system the array gain is 4.8 dB. 347 Effective Radiated Power (ERP) Rules

• Regulatory rules are typically a cap on effective radiated power (ERP), and ERP includes the array gain in both the US and Europe. Because ERP is the sum of the power from the Wi-Fi radio itself plus the antenna gain, in practice, regulations impose a lower limit on MIMO systems. – For a two-antenna MIMO array limited to 20 dBm ERP, the maximum input power to the array will be 17 dBm because the ERP includes the array gain: 17 dBm conducted power + 3 dB antenna gain = 20 dBm ERP. – European regulations have always required that the array gain adjustment be used only in “correlated” transmission methods such as beamforming; • This may limit the practical advantage of transmit beamforming systems, at least until regulatory rules are changed again.

348 Null Data Packet (NDP) Beamforming in 802.11ax

• One of the biggest changes between 802.11n and 802.11ac is that beamforming has been dramatically simplified. Proprietary beamforming technologies had existed prior to 802.11n, but it was only in 802.11n that a standard for beamforming was introduced. In the 802.11n specification, multiple beamforming methods were described.

• Before using beamforming, both sides of the link had to agree on one method they shared, but due to the complexity of implementing multiple methods, many product vendors chose not to implement any. To avoid a repeat with 802.11ax, engineers writing the specification settled on just one method of beamforming, called null data packet (NDP) sounding.

• The second major change in beamforming with 802.11ac has not yet been realized, but it has the potential to dramatically change how much data wireless networks can support. .

349 Null Data Packet (NDP) Beamforming in 802.11ax

350 Channel measurement (sounding) procedures

• Beamforming depends on channel calibration procedures, called channel sounding in the 802.11ac standard, to determine how to radiate energy in a preferred direction. • Within the multi-carrier OFDM channel used by 802.11ac, there may be a strong frequency-dependent response that requires limiting data rates over the channel. Alternatively, between two 802.11ac devices, a particular frequency may respond much more strongly to one path than another. • Beamforming enables the endpoints at either side of a link to get maximum performance by taking advantage of channels that have strong performance while avoiding paths and carriers that have weak performance. • Mathematically, the ability to steer energy is represented by the steering matrix, which is given the letter Q in 802.11ac. • Matrices are used to represent steering information because they are an excellent tool for representing the frequency response from each transmission chain in the array over each transmission stream. Matrix operations allow the spatial mapper to alter the signal to be transmitted for each OFDM subcarrier over each path to the receiver in one operation. Naturally, after applying the steering matrix to the data for transmission, it will leave the antenna array in a decidedly non- omnidirectional pattern.

351 Channel Sounding

• Channel sounding consists of three major steps: – The beamformer begins the process by transmitting a Null Data Packet Announcement frame, which is used to gain control of the channel and identify beamformees. Beamformees will respond to the NDP Announcement, while all other stations will simply defer channel access until the sounding sequence is complete. – The beamformer follows the NDP Announcement with a null data packet. The value of an NDP is that the receiver can analyze the OFDM training fields to calculate the channel response, and therefore the steering matrix. For multi-user transmissions, multiple NDPs may be transmitted. – The beamformee analyzes the training fields in the received NDP and calculates a feedback matrix. The feedback matrix, referred to by the letter V in the 802.11ac specification, enables the beamformer to calculate the steering matrix. – The beamformer receives the feedback matrix and calculates the steering matrix to direct transmissions toward the beamformee. – With the steering matrix in hand, the beamformer can then transmit frames biased in a particular direction.Without beamforming, energy is radiated in all directions more or less equally. Along any direction away from the beamformer, the signal level will be roughly comparable (assuming an ideal omnidirectional antenna). If the transmitter applies a steering matrix, however, the array will send energy in a way that prefers one path. On the preferred path, transmissions from the array will reinforce each other and become stronger, and on other paths, the transmissions from the array will interfere with each other and become weaker. In effect, the combination of the steering matrix and the channel determines whether a signal becomes stronger or weaker.

352 Effects of Steering Matrix

353 Channel sounding procedures

• Channel sounding procedures do have a cost in airtime because the sounding exchange must complete before a beamformed transmission can be sent.

• If the speed gain from transmitting a beamformed frame is not sufficient to offset the airtime consumed by the sounding exchange, the overall speed will be slower. – Roughly speaking, a sounding exchange requires 500 microseconds.

• Once the effect of contention is added into the mix, a rough guideline is that the sounding procedure requires about 0.5% to 1% of available airtime, which can add up to a substantial fraction of available capacity on networks with high numbers of clients.

354 The Feedback Matrix

• The key to beamforming is calculating the steering matrix Q for the channel between the beamformer and the beamformee.

• The steering matrix can potentially have quite large dimensions because it represents the channel behavior between each of the transmitters in the beamformer’s array and each of the receivers in the beamformee’s array.

• Rather than transmitting a steering matrix, the beamformee calculates a feedback matrix and compresses it so that it can be represented by a smaller frame and thus take up less airtime. Compression of the beamforming matrix is accomplished by using matrix operations to send a representative set of values that can be used to reconstitute the matrix instead of sending the raw matrix itself.

355 Feedback Matrix Calculation

• To calculate the feedback matrix, the beamformee runs through the following procedure: – Calculating the feedback matrix can only begin after receiving the NDP from the beamformer. Once the NDP is received, each OFDM subcarrier is processed independently in its own matrix that describes the performance of the subcarrier between each transmitter antenna element and each receiver antenna element. The contents of the matrix are based on the received power and phase shifts between each pair of antennas. – The feedback matrix is transformed by a matrix multiplication operation called a Givens rotation, which depends on parameters called “angles.” Rather than transmitting the full feedback matrix, the beamformee calculates the angles based on the matrix rotation. The 802.11ac standard specifies the order in which these angles are transmitted so that the beamformer can receive a long string of bits and appropriately delimit each angle. – Having calculated the angles, the beamformee assembles them into the compressed feedback form and returns them to the beamformer. Only one set of angles is required to summarize the radio link performance for all of the OFDM subcarriers, though naturally, the set of angles can be quite large with wider channels. – The beamformer receives the feedback matrix and uses it to calculate the steering matrix for transmissions to the beamformee. – One feedback matrix is sent by each beamformee. In single-user beamforming, there is one feedback matrix from the beamformee and one steering matrix used. In multi-user beamforming, each beamformee sends a feedback matrix and the beamformer must maintain a steering matrix for each client.

356 Parameters of the feedback matrix V

Number of subcarriers Per-subcarrier angle count Angle field size

Single-user: 6 bits or 10 20 MHz channel: 52 subcarriers 2x2: 2 angles/subcarrier bits/angle

Multi-user: 12 bits or 16 40 MHz channel: 108 subcarriers 3x3: 6 angles/subcarrier bits/angle

80 MHz channel: 234 subcarriers 4x4: 12 angles/subcarrier

160 MHz channel: 486 6x6: 30 angles/subcarrier subcarriers

8x8: 56 angles/subcarrier

357 Size of the feedback matrix

• To estimate the size of the feedback matrix, multiply the results of each of the following items:

– Single-user 2x2 MIMO @ 20 MHz, low resolution: 78-byte report 52 subcarriers x 2 angles/subcarrier x 6 bits/angle = 624 bits or 78 bytes. This is the smallest steering matrix available in 802.11ac.

– Single-user 3x3 MIMO @ 80 MHz, high resolution: 1.7 kB report 234 subcarriers x 6 angles/subcarrier x 10 bits/angle = 14,040 bits or 1.7 kB. This will be a typical steering matrix for a single-user MIMO system released in the first wave of 802.11ac.

– Single-user 4x4 MIMO @ 80 MHz, high resolution: 3.4 kB report This is the same as the previous example, but it adds an additional transmitter and receiver. In a 4x4 system there are more degrees of freedom, which is why there are more angles required per subcarrier.

– Multi-user 8x8 MIMO @ 80 MHz, high resolution: 53 kB report 486 subcarriers x 56 angles/subcarrier x 16 bits/angle = 435,456 bits or 53 kB. Large sets of angles can group subcarriers together in order to reduce the report size and help it fit into a frame. In practice, a multi-user report with 80 MHz channels would group subcarriers to reduce the report size.

358 Single-User (SU) Beamforming

• Single-user beamforming is readily understandable because its purpose is to shape a transmission from a single transmitter to a single receiver.

• The beamformer sends a null data packet, which is a frame with a known fixed format. – By analyzing the received NDP frame, the beamformee calculates a feedback matrix that is sent in a reply frame. Beamformees do not send a steering matrix directly because the beamforming sounding protocol needs to enable multiple-user MIMO, as described in the next section.

359 Single-user channel calibration procedure

360 Channel Calibration for Single-User Beamforming

• The channel calibration procedure is carried out as a single operation, in which the beamformer and beamformee cooperatively measure the channel to provide the raw data needed to calculate the steering matrix.

• The sounding procedure does not transmit the steering matrix directly, but instead works to exchange all the information necessary for the beamformer to calculate its own steering matrix.

361 NDP Announcement frame

• The channel sounding process begins when the beamformer transmits a Null Data Packet Announcement frame, which is a control frame. • The entire channel sounding process is carried out in one burst, so the duration set in an NDP Announcement corresponds to the length of the full exchange of three frames. In single-user MIMO beamforming, the NDP Announcement frame relays the size of the feedback matrix by identifying the number of columns in the feedback matrix. • The main purpose of the NDP Announcement frame is to carry a single STA Info field for the intended beamformee. The STA Info field is two bytes long and consists of three fields: – AID12 (12 least significant bits of the intended beamformee’s association ID) Upon association to an 802.11 access point, client devices are assigned an association ID. The least significant 12 bits of the beamformee’s association ID are included in this field. When a client device acts as a beamformer, this field is set to 0 because the AP does not have an association ID. – Feedback Type In a single-user NDP Announcement frame, this field is always 0. – Nc Index This index describes the number of columns in the feedback matrix, with one column for each spatial stream. As a three-bit field it can take on eight values, which matches the eight streams supported by 802.11ac. This field is set to the number of spatial streams minus one. 362 NDP Announcement frame format (single-user)

363 NDP frame

• Upon transmission of the NDP Announcement frame, the beamformer next transmits a Null Data Packet frame. • The reason for the name “null data packet” should be obvious in looking at the frame;

• Channel sounding can be carried out by analyzing the received training symbols in the PLCP header, so no MAC data is required in an NDP. – Within an NDP there is one VHT Long Training Field (VHT- LTF) for each spatial stream used in transmission, and hence in the beamformed data transmission.

364 NDP format

365 Feedback Matrix response

• Following receipt of the NDP, the beamformee responds with a feedback matrix.

• The feedback matrix tells the beamformer how the training symbols in the NDP were received, and therefore how the beamformer should steer the frame to the beamformee.

• The Action frame header indicates that the frame contains a feedback matrix.

366 Compressed Beamforming Action frame (single-user)

367 More on MU-MIMO Capability

Beamforming and MU-MIMO/SU-MIMO SU-MIMO uses beamforming to improve the signal strength and achieve higher bitrates to a single client. MU- MIMO uses beamforming to direct energy to one client and steer that energy away from other clients addressed by the MU-MIMO transmission. MU-MIMO will also do the same energy direction for any subsequent clients that connect. MU-MIMO in 802.11ax sends to multiple receivers: •An access point with four antennas sends one stream each to three smartphones, all at the same time. •The access point must beamform one space-time stream to each receiver and simultaneously null-steer that space-time stream to the two other receivers.

368 802.11ax MU-MIMO Capabilities Advertisements: AP

• 802.11 frames detail how MU-MIMO is negotiated between the AP and the client. A similar technique is used to negotiate SU- MIMO as well. • AP – The MU Beam-former flag will be set by the AP in these following frames: – Probe Response – Beacon

369 370 802.11ax MU-MIMO Capabilities Advertisements: Client • Client • The MU Beam-formee flag will be set by the client in these following frames:

– Directed Probe Request – Association Response

371 372 Transmit Beamforming (TxBF) and MU-MIMO

• Many of the 802.11ax features are enhancements of existing features in the 802.11ac standard, while others add important new capabilities to Wi-Fi technology.

• A key improvement – Single User Transmit Beamforming (SU-TxBF) – and the new feature in 802.11ac/ax MU-MIMO or MU-TxBF.

373 Standardized Closed Loop TxB

• Beamforming is a technique that focuses the AP transmit energy of the MIMO spatial stream towards the targeted station (STA) or client device. Using channel estimation carefully to introduce a small difference in the phase and amplitude in the transmission (precoding) allows the AP to focus the transmitted signal in the direction of the receiving STA. The signal produced is strong enough to use more aggressive modulation techniques (which support higher data rates) at greater distance. • 802.11n defined several methods of beamforming, however, none of them mandated for certification. As a result, chipset vendors implemented various non-interoperable techniques. The lack of a single, consistent method prevented this feature of 802.11n from providing the intended range enhancements across end-products and kept it from becoming mainstream. • The 802.11ac/ax specification defines a single closed-loop method for transmit beamforming. In this method, the AP transmits a special sounding signal to all STAs, which then estimates the channel and reports their channel feedback back to the AP. This feedback from STAs is standardized so consumers can be sure any standards-based beamforming devices (APs and STAs) interoperates with all other 802.11ac compliant products. 374 MU-MIMO or MU-TxBF

• The most significant advancement in 802.11ac/ax is MU-MIMO technology, which provides a dramatic breakthrough in the performance and flexibility available to WLAN users. – In SU-MIMO, a device transmits multiple spatial streams at once, but only to one device at a time. – MU-MIMO allows multiple spatial streams to be assigned to different clients simultaneously, increasing the total throughput and capacity of the WLAN system.

– MU-MIMO builds upon the transmit beamforming capabilities to establish up to four simultaneous directional Radio Frequency (RF) links.

375 MU-MIMO operation

• In MU-MIMO operation, an AP uses enhanced beamforming techniques to maximize transmission in the desired client direction while simultaneously minimizing transmission in the direction of undesired clients through null steering. – Known as spatial reuse, this technique provides each of the four users with its own dedicated full-bandwidth channel in much the same way cellular phone networks use small cell nodes and spectrum reuse techniques to increase system capacity.

• While MU-MIMO benefits nearly every scenario, it is most advantageous to mobile devices (smartphones, tablets, laptops, etc.) using one or two streams that typically are allocated only a relatively small portion of the network capacity.

376 Question and Answer

377 2/26/2020 377