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Chapter 4 Multiple Access Techniques Contents

• Introduction • Contentionless Multiple Access • Contention Multiple Access • Hanging Multiple Access INTRODUCTION Introduction

• General wireless systems are multi-users systems • Radio resource are limited – Limited bandwidth – Limited number of channels • The radio resource must be shared among multiple users • Multiple Access Technique is defined as a function sharing a (limited) common transmission resource among (distributed) terminals in a network Classification Contentionless Multiple Access (MA)

• Contention-less based: – A logic controller (BS-Base Station or AP-Access Point) is needed to coordinate all transmissions – The controller informs each device when and on which channel it can transmit – Collisions can be avoided entirely – The users transmit in an orderly scheduled manner so every transmission will be a successful one. – The scheduling can take two forms: • Fixed assignment scheduling (Channelization) • Demand assignment scheduling(Non-channelization) Contentionless MA Protocols

• Fixed assignment scheduling – The available channel capacity is divided among the users so that each user is allocated a fixed part of the capacity, independent of its activity. – The division is done in time or frequency. E.g., TDMA, FDMA Contentionless MA Protocols

• Demand assignment scheduling – A user is only allowed to transmit if it is active (if it has information to transmit). Thus, the active (or ready) users transmit in an orderly scheduled manner. • Demand assignment with centralized control, a single entity schedules the transmissions. (Polling Protocol) • Demand Assignment with distributed control, all users are involved in the scheduling process and such a protocol is the token-passing protocol. Contention-based Multiple Access

• Terminals transmit in a decentralized way • No central controller • If several ready users start their transmissions more or less at the same time, all of the transmissions will fail. • The random access protocol should resolve the contention that occurs when several users transmit simultaneously Contention-based Multiple Access

• Example: – ALOHA – Carrier Sense Multiple Access (CSMA) • Standard: – GSM uses the slotted ALOHA in the terminal’s initial access process – IEEE 802.11 uses CSMA/CA based contention access scheme Contention-based Types

• Two types: – Repeated random access protocols – Random access with reservation • Repeated random access protocols – With every transmission there is a possibility of contention – Pure (P)-ALOHA, Slotted (S)-ALOHA, CSMA & its variants Contention-based Types

• Random access with reservation – Only in the first transmission, a terminal does not know how to avoid collisions with other users. – Once it has successfully completed the first transmission, it’s future transmissions will be scheduled in an orderly fashion so that no contention can occur – Implicit: designed without the use of any reservation packet – Explicit: a short reservation packet to request transmission at scheduled times – Ex, Reservation ALOHA (R-ALOHA), packet reservation multiple access (PRMA), Hanging Multiple Access Protocols

• CDMA type (Spread spectrum) protocols – Direct sequence (DS) CDMA – Frequency hopping (FH) CDMA – Time hopping (TH) CDMA • Subcarrier type protocols – Multi-carrier (MC) CDMA – OFDM-FDMA – OFDM-TDMA – OFDMA – Many others Hanging Multiple Access Protocols

• Some protocols, such as CDMA, do not belong to either the contention-less or the contention protocols. • It falls between the two groups. – It is a contention-less protocol where a number of users are allowed to transmit simultaneously without conflict. – However, if the number of simultaneously transmitting users rises above a threshold, contention occurs. Classification Focuses CONTENTION MULTIPLE ACCESS Contention Multiple Access

• ALOHA – Pure (P) ALOHA – Slotted (S)-ALOHA • CSMA (Carrier Sense Multiple Access) – CSMA/CA (CSMA with Collision Avoidance) – CSMA/CA/ACK – CSMA/CA with RTS/CTS ALOHA

• Pure ALOHA – Developed in the 1970s for a packet radio network by Hawaii University – Whenever a terminal (MS) has data, it transmits. – Sender finds out whether transmission was successful or experienced a collision by listening to the broadcast from the destination station. – If there is a collision, sender retransmits after some random time ALOHA

• Slotted ALOHA – Improvement: Time is slotted and a packet can only be transmitted at the beginning of one slot. – Thus, it can reduce the collision duration Pure ALOHA

• Collision mechanism in ALOHA Frames in a Pure ALOHA network Procedure for pure ALOHA protocol Slotted ALOHA

• Collision mechanism in slotted ALOHA Frames in a slotted ALOHA network Vulnerable time for slotted ALOHA Pure ALOHA versus Slotted ALOHA Pure ALOHA versus Slotted ALOHA CSMA (Carrier Sense Multiple Access)

• Before transmitting, a terminal senses the channel to see whether there is any carrier or not – If there is a carrier, terminal waits a random backoff time and transmit data (no re-sensing) – If there is no carrier, terminal starts transmission. • Detection delay is the time required for a terminal to sense whether the channel is idle or not . • Propagation delay is how long it takes for a packet to travel from a base station (BS) to a mobile station (MS). CSMA (Carrier Sense Multiple Access)

• Revisions: – CSMA/CD (CSMA with Collision Detection) • Improvement: Stop ongoing transmission if a collision is detected – CSMA/CA (CSMA with Collision Avoidance) • Improvement: Wait a random time and try again when carrier is quiet. If still quiet, then transmit – CSMA/CA with ACK (more reliability) – CSMA/CA with RTS/CTS (to solve hidden terminal problem) Collision Mechanism in CSMA

MS: Mobile Station (trạm di động) Types of CSMA (Access modes) p-persistent CSMA

• Step 1: If the medium is idle, transmit with probability p. • Step 2: If transmission is delayed by one time slot (the probability of this event is 1-p), continue with Step 1 • Step 3: If the medium is busy, continue to listen until medium becomes idle, then go to Step 1 Non-persistent CSMA

• Step 1: If the medium is idle, transmit immediately (same as p=1) • Step 2: If the medium is busy, wait a random amount of time and repeat Step 1 – Random backoff reduces probability of collisions – Waste idle time if the backoff time is too long 1-persistent CSMA

• Step 1: If the medium is idle, transmit immediately • Step 2: If the medium is busy, continue to listen until medium becomes idle, and then transmit immediately – There will always be a collision if two nodes want to retransmit (usually user stops transmission attempts after few tries) Behavior of three persistence methods How to select Probability p?

• Assume that N nodes have a packet to send and the medium is busy – Then, Np is the expected number of nodes that will attempt to transmit once the medium becomes idle • If Np > 1 then a collision is expected to occur – Therefore, network must make sure that Np<=1 to avoid collision, where N is the maximum number of nodes that can be active at a time Throughput Problems in wireless networks

• Decreased signal strength – radio signal attenuates as it propagates through matter (path loss), proportional to the square of the distance • Interference from other sources – standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); motors interfere as well • Multipath propagation – radio signal reflects off objects ground, arriving at destination at slightly different times CSMA/CD Review

• Collisions detected within short time • Colliding transmissions aborted, reducing channel wastage • Collision detection: – easy in wired LANs: measure signal strengths, compare transmitted, received signals – difficult in wireless LANs: received signal strength overwhelmed by local transmission strength CSMA/CD Review

1. NIC receives datagram from 4. If NIC detects another network layer, creates transmission while transmitting, frame aborts and sends jam signal 2. If NIC senses channel idle, 5. After aborting, NIC enters binary starts frame transmission. If (exponential) backoff: NIC senses channel busy, – after mth collision, NIC waits until channel idle, chooses K at random from then transmits. {0,1,2, …, 2m-1}. NIC waits 3. If NIC transmits entire frame K·512 bit times, returns to without detecting another Step 2 transmission, NIC is done – longer backoff interval with with frame ! more collisions CSMA/CD problems in wireless networks

• The sender would apply CS and CD, but the collisions happen at the receiver • A sender may not “hear” the collision, i.e., CD does not work • Furthermore, CS might not work if, e.g., a terminal is “hidden” CSMA/CA

• Carrier sense multiple access with collision avoidance CSMA/CA (mandatory) known as distributed coordination function (DCF) • Improve CSMA performance by not allowing a node’s transmission if another node is transmitting – if station has frame to send it listens to medium – if medium idle, station may transmit – else waits until current transmission completes CSMA/CA

• For collision avoidance, use backoff and RTS/CTS techniques • CSMA/CA includes delays that act as a priority scheme – DIFS: DCF inter-frame space defines how long a channel must be idle before a transmitter attempts transmission – SIFS: short inter-frame space (SIFS < DIFS) defines how long a receiver waits before sending an ACK or other responses CSMA/CA Procedures

1. A station with a frame to transmit senses the medium. If the medium is idle, it waits to see if the medium remains idle for a time equal to DIFS. If so, the station may transmit immediately. CSMA/CA Procedures

2. If the medium is busy, the station defers transmission and continues to monitor the medium until the current transmission is over. 3. Once the current transmission is over, the station delays another DIFS. – If the medium remains idle for this period, then the station backs off a random amount of time (collision avoidance, multiple time-slot) and again senses the medium. – If the medium is still idle, the station may transmit. – During the backoff time, if the medium becomes busy, the backoff timer is halted and resumes when the medium becomes idle (fairness). CSMA/CA Procedures

4. If the transmission is unsuccessful, which is determined by the absence of an acknowledgement, then it is assumed that a collision has occurred. Hidden terminal problem

• A is transmitting to B • C is sensing the carrier and detects that it is idle • It can not hear A’s transmission • C also transmits and collision occurs at B

• A is hidden from C A’s range C’s range

A B C Hidden terminal Exposed terminal problem

• B is transmitting to A. • C now wants to transmit to D. It senses the existence of carrier signal and defers transmission to D • However, C can actually start transmitting to D while B is transmitting to A since A is out of range of C and C’s signals can not be heard at A • C is exposed to B’s B’s range C’s range transmission

A B C D Exposed terminal Solution: Three way handshake

RTS 1 CTS

DATA 2

3 D

C B

A RTS: Request To Send E CTS: Clear To Send Solution: Three way handshake

• RTS packet contains the length of the proposed data transmission. CTS also contains that info. • Any station overhearing an RTS, it defers all transmissions until some time after the associated CTS packet would have finished. • Any station overhearing a CTS packet defers for the length of the expected data transmission (which is contained in both the RTS and CTS packets). Reliability

• Use acknowledgements (ACKs)  Four-Way Handshake – Sender sends Ready-to-Send (RTS) – Receiver responds with Clear-to-Send (CTS) – Sender sends Data Packet – Receiver acknowledge with ACK • Notes: – RTS and CTS announce the duration of the transfer – Nodes overhearing RTS/CTS keep quiet for that duration – Sender will retransmit RTS if no ACK is received • If ACK was sent out but not received by sender, after receiving new RTS, receiver returns ACK instead of CTS for new RTS Four-Way Handshake

CTS(T)ACK CTS: Clear To Send DATARTS(T) RTS: Request To Send

destination

source Contention-Based Random Access Summary

• ALOHA and Slotted ALOHA – Decentralized, efficiency 16% and 32%, respectively • Carrier Sense Multiple Access (CSMA) – a form of random access used for WLANs – Distributed Coordination Function (DCF) – Theoretical efficiency ~ 60% – Practical efficiency < 50% even for single user systems CSMA/CA (DCF) Summary

• CSMA/CA – Contention-based random access – Collision detection not possible while transmitting • Uses RTS/CTS exchange to avoid hidden terminal problem – Any node overhearing a CTS cannot transmit for the duration of the transfer. – Any node overhearing an RTS cannot transmit for the duration of the transfer (to avoid collision with ACK) • Uses ACK to achieve reliability CONTENTIONLESS MULTIPLE ACCESS Contentionless Multiple Access

• Duplex systems: FDD/TDD/CDD • TDMA – Time Division Multiple Access • FDMA – Frequency Division Multiple Access • CDMA – Code Division Multiple Access – Spread spectrum modulation: DSSS. FHSS Duplexing

• Duplexing facilitates communications in both directions simultaneously: base station to mobile and mobile to base station • Duplexing is done either using frequency or time domain techniques: – Frequency division duplexing (FDD) – Time division duplexing (TDD) – Code division duplexing (CDD) • FDD is suitable for radio communication systems, whereas TDD is more suitable for fixed wireless systems FDD: Frequency Division Duplex

• Forward Channel and Reverse Channel use different frequency bands TDD: Time Division Duplex

• A single frequency channel is used. The channel is divided into time slots. Mobile station and base station transmits on the time slots alternately. CDD: Code Division Duplex Duplex vs. Multiple Access Duplex vs. Multiple Access Models for Multiple Divisions

• A radio signal is a function of frequency, time and code as: s(f,t,c) = s(f,t) c(t) • where s(f,t) is the function of frequency and time and c(t) is the function of code – Use of different frequencies to transmit a signal: FDMA – Distinct time slot: TDMA – Different codes: CDMA FDMA – Frequency Division MA

• FDMA was the initial multiple-access technique for cellular systems • Separates large band into smaller channels. • Each channel has the ability to support user. • Guard bands are used to separate channel preventing co-channel interference Advantages of FDMA

• If channel is not in use, it sits idle • Channel bandwidth is relatively narrow • Simple algorithmically, and from a hardware standpoint • Fairly efficient when the number of stations is small and the traffic is uniformly constant • Capacity increase can be obtained by reducing the information bit rate and using efficient digital code • No need for network timing • No restriction regarding the type of baseband or type of modulation Disadvantages of FDMA

• The presence of guard bands • Requires right RF filtering to minimize adjacent channel interference • Maximum bit rate per channel is fixed • Small inhibiting flexibility in bit rate capability • Does not differ significantly from analog system TDMA

• Entire bandwidth is available to the user for finite period of time. • Users are allotted time slots for a channel allowing sharing of a single channel. • Requires time synchronization. • Each of the user takes turn in transmitting and receiving data in a round robin fashion. TDMA Advantages

• Extended battery life and talk time • More efficient use of spectrum, compared to FDMA • Will accommodate more users in the same spectrum space than an FDMA system TDMA Disadvantages

• Network and spectrum planning are intensive • Multipath interference affects call quality • Dropped calls are possible when users switch in and out of different cells. • Too few users result in idle channels (rural versus urban environment) • Higher costs due to greater equipment sophistication CDMA – Code Division Multiple Access

• CDMA is a spread spectrum technique used to increase spectrum efficiency. • Different spread-spectrum codes that are orthogonal to each other are selected and assigned to each user, and multiple users share the same frequency Structure of a CDMA system CDMA: Walsh code

• CDMA requires synchronization among the users, since the waveforms are orthogonal only if they are aligned in time. • An important set of orthogonal codes is the Walsh set • Walsh codes are used in direct sequence spread spectrum (DSSS) systems (such as IS-95, CDMA2000, etc.). • They are also used in frequency hopping spread spectrum (FHSS) systems to select the target frequency for the next hop CDMA: Walsh code

• A Walsh-encoded signal appears as random noise to a CDMA-capable mobile terminal, unless that terminal uses the same code as the one used to encode the incoming signal. • The CDMA system requires orthogonal codes for channel selection. • Since CDMA systems use the same frequency bands for all transmissions, only a different code can be used to select a channel. CDMA: Walsh code

• Walsh codes are generated using the Hadamard

matrix with H1 = [0], where H1 is a 1x1 matrix and is an order 1. The Hadamard matrix is built by

• For example, the Hadamard matrix of order 2 and 4 will be: CDMA: Walsh code

• From the corresponding Hadamard matrix, the Walsh codes are given by the rows. • We usually map the binary data to polar form so we can use real number arithmetic when computing correlations. • So 0s are mapped to 1s and 1s are mapped to -1s. CDMA: Walsh code

• This means the kth row of the H(n) Hadamard matrix is:

• The basic set of Walsh codes is the set of four patterns 0000, 0101, 0011, 0110 CDMA: Walsh code CDMA: Walsh code Near-Far Problem

Received signal levels at the BS from different MSes are different due to the difference in the path lengths Interference in CDMA Advantages of CDMA

• Many users of CDMA use the same frequency, TDD or FDD may be used • Multipath fading may be substantially reduced because of large signal bandwidth • No absolute limit on the number of users • Easy addition of more users • Impossible for hackers to decipher the code sent • Better signal quality • No sense of handoff when changing cells Disadvantages of CDMA

• As the number of users increases, the overall quality of service decreases • Self-jamming • Near- Far- problem arises HANGING MULTIPLE ACCESS Important Points

• What is the main Difference between OFDM & OFDMA? • What is SC-FDE? • What is the main Difference between SC-FDE & SC-FDMA? • Compare SC-FDMA & OFDMA? • Where are SC-FDMA and OFDMA used today? Hanging Multiple Access Protocols

• CDMA type (Spread spectrum) protocols – Direct sequence (DS) CDMA – Frequency hopping (FH) CDMA – Time hopping (TH) CDMA • Subcarrier type protocols – Multi-carrier CDMA – OFDM-FDMA – OFDM-TDMA – OFDMA – Many others Focuses Hanging Multiple Access

• OFDMA (Orthogonal FDMA) – OFDM – Multi-user OFDM – OFDMA • SC-FDMA (Single Carrier FDMA) – SC-FDE – SC- FDMA • OFDMA vs. SC-FDMA Summary of Multiple Access Schemes

• Contentionless (Channel partitioning) MA protocols – Share channel efficiently at high load – Inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node! • Contention (Random access) MA protocols – Efficient at low load: single node can fully utilize channel – High load: collision overhead • “Hanging” protocols look for best of both worlds! Channel partitioning Multiple Access Channel partitioning Multiple Access 4G Multiple Access Schemes • OFDMA (Orthogonal FDMA) – Used in major standards • DL & UL for IEEE 802.16e/m • DL for LTE and LTE-A • Proposed for LTE-A UL – Based on the OFDM modulation technique • Very effective in combating the adverse effects of multipath • Intrinsic scalability in the frequency domain • Relatively simple to incorporate advanced antenna technologies • Flexible platform for implementing a single frequency network (SFN) • Flexible resource allocation for multimedia applications 4G Multiple Access Schemes

• SC-FDMA (Single Carrier FDMA) – Based on single carrier with frequency domain equalization (SC-FDE) – Used in UL for LTE OFDM Overview

• OFDM is a digital multi-carrier modulation scheme, which uses a large number of closely- spaced orthogonal sub-carriers. • Each sub-carrier is modulated with a conventional modulation scheme (such as QPSK, 16QAM, 64QAM) at a low symbol rate similar to conventional single-carrier modulation schemes in the same bandwidth. OFDM Overview

• Frequency-Time Relation of an OFDM Signal OFDM Overview

• Advantages of OFDM technologies – Higher spectral efficiency in real-life time dispersive channels – More robust – less multi-path interference – Easy to integrate MIMO technologies – Simpler receiver to cope with real-life time dispersive channels  lower cost OFDM for Multi-User Communications

• Find a suitable multiple access scheme that maps the user data to a modulation block ! OFDM Multiple Access Schemes OFDM-TDMA

• Every user allocates all subcarriers in a certain number of time slots (OFDM symbols) in each OFDM modulation block OFDM-TDMA

• Advantages – No multiple access interferences (MAI) – Incoherent or coherent modulation – Adaptation to channel characteristics – High coding gain due to diversity – Robust against estimation errors – No MAI in case of synchronisation errors – Easy implementation • Disadvantages – Performance of ”normal“ OFDM system OFDM-FDMA

• Every user transmits on a certain number of OFDM subcarriers during all time slots of the OFDM modulation block OFDM-FDMA

• Advantages – No multiple access interference – Incoherent or coherent modulation – Adaptation to channel characteristics – Select good subcarriers – Bit-loading on selected subcarriers – Robust against estimation errors • Disadvantages – Stronger requirements on carrier frequency synchronisation between users in the uplink OFDM-CDMA

• Every user transmits on all OFDM subcarriers during all OFDM symbols of an OFDM modulation block using an orthogonal code (e.g. Walsh- Hadamard). OFDM-CDMA

• Advantages – Processing gain due to frequency diversity – Robust against interferences • Disadvantages – Multiple access interferences – Only coherent modulation possible – No adaptation to channel characteristics Time-Frequency Block

• To allow the utilization of subcarrier by different users, define a time-frequency modulation block consisting of b subcarriers in a OFDM symbols OFDMA Concept

• OFDM can be used as a multi-access scheme, where the available sub-carriers may be divided into several groups of sub-carriers called sub- channels. • Different sub-channels may be allocated to different users as a multiple access mechanism. This type of multi access scheme is called Orthogonal Frequency Division Multiple Access (OFDMA). OFDMA Concept

• OFDMA is essentially a hybrid of FDMA and TDMA. • OFDMA = OFDM + FDMA + TDMA • Users are dynamically assigned sub-carriers (FDMA) in different time slots (TDMA). • OFDMA allows Time + Freq DMA ⇒ 2D Scheduling • OFDMA is a flexible multiple access technique that can accommodate many users with widely varying applications, data rates and QoS requirements. OFDMA Concept OFDMA Main Steps

• The input information bits corresponding to each user are modulated to symbols (complex numbers). The generated symbols are assumed to be in the frequency domain. • The symbols are then mapped to a distinct set of subcarriers. • The IDFT block converts the symbols into the time domain, which are then transmitted though the channel after adding the cyclic prefix. OFDMA Main Steps

• Cyclic prefix (CP) – The CP is typically a repetition of the last samples of data portion of the block that is appended to the beginning of the data payload. – The CP prevents inter-block interference and makes the channel appear circular and permits low- complexity frequency domain equalization. – Can completely eliminate ISI as long as the CP duration is longer than the channel delay spread. OFDMA Main Steps

• The IDFT operation can be viewed as each symbol modulating one subcarrier and transmitting the subcarriers in parallel OFDM vs. OFDMA

• OFDMA is a multi-user version of the popular OFDM digital modulation scheme. • Like OFDM, OFDMA employs multiple closely spaced sub-carriers, but the sub-carriers are divided into groups of sub-carriers. Each group is named a sub-channel. • The sub-carriers that form a sub-channel need not be adjacent OFDM vs. OFDMA OFDM vs. OFDMA

• OFDM – All carriers are transmitted in parallel – Only one user is supported at the same time • OFDMA – Divides the carrier space into many groups – Many users can be supported at the same time OFDM vs. OFDMA OFDM vs. OFDMA OFDMA Flexibility

• With OFDMA the user allocation is flexible – Can change from frame to frame – Multiple allocations for several applications • Allocation changes – In WiMAX every 5 ms – In LTE every 1 ms OFDMA Advantages

• Scalability – Support a wide range of bandwidth • Robustness to Multipath – The number of multi-path components does not limit the performance of the system as long as all these multi-paths are within the cyclic prefix window. • Downlink Multiplexing • Uplink Multiple Access • MIMO (Multiple Input Multiple Output) Benefits OFDMA Challenges

• Since the subcarrier spacing is typically quite small (e.g. 10.94kHz for 802.16e), OFDMA is more sensitive to freqency offset and phase noise. • OFDMA is a type of multi-carrier modulation, the time domain waveform is Gaussian like with high peak to average power ratio (PAPR). High PAPR imposes more strigent requirement on transmit power amplifer. SC-FDMA (Single Carrier FDMA)

• SC-FDE • SC- FDMA SC-FDE

• A variation of OFDM is SC-FDE (Single Carrier modulation with Frequency Domain Equalization), a modulation scheme that contains all the same blocks but moves the IFFT from the transmitter to the receiver OFDM Communication System FDE Communication System SC-FDE

• Because of the single carrier modulation at the transmitter, SC-FDE does not have the high PAPR disadvantage as OFDM. – Peak-to-Average Power Ratio • Input to OFDM is in frequency domain and in SC- FDE is in time domain. Single-carrier FDMA Concept

• Single-carrier FDMA (SC-FDMA) is a frequency- division multiple access scheme. • SC-FDMA is the multi-user version of SC-FDE. • Like other multiple access schemes (TDMA, FDMA, CDMA, OFDMA), it deals with the assignment of multiple users to a shared communication resource. Single-carrier FDMA Concept

• The distinguishing feature of SC-FDMA is that it leads to a single-carrier transmit signal, in contrast to OFDMA which is a multi-carrier transmission scheme. • The main benefit of SC-FDMA , compared to a multi-carrier transmission scheme such as OFDMA, is reduced variations in the instantaneous transmit power, implying the possibility for increased power-amplifier efficiency. Single-carrier FDMA Concept

• The system configuration in an SC-FDMA system is similar to OFDMA with the addition of a DFT and an IDFT block. • SC-FDMA is based on the same principle as SC- FDE, the only difference is SC-FDMA is for multiple users, whereas SC-FDE is a single-user modulation scheme. SC-FDMA Components • DFT , IDFT: required to generate the single carrier FDMA signal. • Serial to Parallel Convertor (S-to-P): modulated symbols are converted into parallel symbols and organized into blocks. • K-Point IDFT: Converts the mapped subcarriers to time domain for efficient computations of IDFT K>N. • N-Point DFT (Discrete Fourier Transform): Converts time domain single carrier blocks into N discrete frequency tones. • Subcarrier Mapping: Controls the frequency allocation, and maps N-discrete frequency tones to subcarriers for transmission. SC-FDMA Components

• Parallel to Serial Convertor (P-to-S): The time domain subcarriers are converted back from parallel to serial. • Add Cyclic Prefix: CP is added to avoid ISI. The length of CP is larger than the channel delay spread in order to avoid ISI at the receiver. • Digital to Analog Convertor (DAC): Converts the digital signal to analog signal and upconvert (convert set of values to higher set of values) to RF for transmission over the channel. Explaining SC-FDMA Explaining SC-FDMA

• Transmitter – Converts the input information bit stream into a parallel bit stream – Groups the bits into sets of m bits – The sets are mapped to M-ary symbols where • A block of K modulation symbols from some modulation alphabet, e.g. QPSK, 16-QAM or 64- QAM, is first applied to a size- K DFT. – The DFT block operates on chunks of symbols, each chunk containing K symbols Explaining SC-FDMA

• A K point DFT operation transforms the time domain symbols into the frequency domain. • Next, the transmitter maps the outputs of the DFT block to N orthogonal subcarriers where N > K and where the unused inputs of the IDFT are set to zero. • In a system with L user terminals, if all the terminals transmit K symbols per block, then N = K×L. Explaining SC-FDMA • The output of the IDFT will be a signal with ‘single-carrier’ properties, i.e. a signal with low power variations, and with a bandwidth that depends on K. • If the DFT size K equals to the IDFT size N, the cascaded DFT and IDFT blocks would completely cancel out each other. • Typically, the inverse-DFT size N is selected as for some integer n to allow for the IDFT to be implemented by means of computationally efficient radix-2 IFFT. Explaining SC-FDMA

• After subcarrier mapping, an N point Inverse DFT (IDFT) operation is performed to generate a time domain signal. • The transmitter then adds the Cyclic Prefix (CP), containing the last part of the block of symbols, to the start of the block in order to prevent against Inter Block Interference (IBI). • Finally, after passing through the transmission filter for pulse shaping, the signal is transmitted. Subcarrier Mapping

• Controls the frequency allocation, and maps N- discrete frequency tones to subcarriers for transmission. • Two types of SM in a SC-FDMA system – Localized (LFDMA): the M outputs of the DFT block from a particular terminal are mapped to a chunk of K adjacent subcarriers – Distributed (DFDMA): the symbols are mapped to subcarriers which are equally spaced across a particular part of the (or the entire) bandwidth. • Interleaved SC-FDMA (IFDMA) is a special case of DFDMA, where the chunk of K subcarriers occupy the entire bandwidth with a spacing of L − 1 subcarriers (where L is the number of users) Subcarrier Mapping

• In both of the subcarrier allocation methods, the transmitterassigns zero amplitudes to the remaining N−K unused subcarriers. Subcarrier Mapping - Example

• For K = 4 symbols per block, N = 12 subcarriers, and L = 3 user terminals. • The input time domain symbols from user terminal L1 are x1, x2, x3 and x4, and X1, X2, X3 and X4 represent the outputs of the DFT blocks Subcarrier Mapping - Example OFDMA VS. SC-FDMA Comparing OFDMA and SC-FDMA

• QPSK example using M=4 subcarriers PAR vs constellation analysis Multipath protection

• Multipath protection with short data symbols – The subcarriers of each SC-FDMA symbol are not the same across frequency as shown in earlier graphs but have their own fixed amplitude & phase for the SC- FDMA symbol duration. – The sum of M time-invariant subcarriers represents the M time-varying data symbols. • When the CP is inserted, multipath protection is achieved despite the modulating data symbols being much shorter. Multipath protection Signal generation and reception