1 IEEE 802.11be – Wi-Fi 7: New Challenges and Opportunities Cailian Deng*, Xuming Fang, Senior Member, IEEE, Xiao Han*, Xianbin Wang, Fellow, IEEE, Li Yan, Student Member, IEEE, Rong He, Yan Long, Member, IEEE, and Yuchen Guo Abstract—With the emergence of 4k/8k video, the throughput access network. According to a recent study from the Wi- requirement of video delivery will keep grow to tens of Gbps. Fi Alliance [1], more than 9 billion Wi-Fi devices, including Other new high-throughput and low-latency video applications personal computers, smartphones, televisions, tablets, sensors, including augmented reality (AR), virtual reality (VR), and online gaming, are also proliferating. Due to the related stringent and so on, are currently in use worldwide. Video traffic is requirements, supporting these applications over wireless local the dominant traffic type over WLAN, and its throughput area network (WLAN) is far beyond the capabilities of the requirement will keep increasing due to the emergence of new WLAN standard – IEEE 802.11ax. To meet these emerging 4k and 8k video whose uncompressed data rate is up to demands, the IEEE 802.11 will release a new amendment stan- 20Gbps. Meanwhile, new ultra-high throughput and stringent dard IEEE 802.11be – Extremely High Throughput (EHT), also known as Wireless-Fidelity (Wi-Fi) 7. This article provides the low-latency applications are also proliferating, such as VR comprehensive survey on the key medium access control (MAC) or AR, gaming (e.g., latency lower than 5ms for online layer techniques and physical layer (PHY) techniques being gaming), telecommuting, online video conference, and cloud discussed in the EHT task group, including the channelization computing. While the recently released IEEE 802.11ax puts and tone plan, multiple resource units (multi-RU) support, more focus on the network performance and user experience 4096 quadrature amplitude modulation (4096-QAM), preamble designs, multiple link operations (e.g., multi-link aggregation of the high-dense deployment scenarios, meeting the above and channel access), multiple input multiple output (MIMO) high-throughput and low-latency requirements is well beyond enhancement, multiple access point (multi-AP) coordination (e.g., the capabilities of IEEE 802.11ax. To meet these new needs, multi-AP joint transmission), enhanced link adaptation and IEEE 802.11 standard organization is going to release a retransmission protocols (e.g., hybrid automatic repeat request new amendment standard IEEE 802.11be EHT beyond IEEE (HARQ)). This survey covers both the critical technologies being discussed in EHT standard and the related latest progresses from 802.11ax, namely Wi-Fi 7. To be consistent with the IEEE worldwide research. Besides, the potential developments beyond 802.11be proposals, we mainly use the terminology “EHT” EHT are discussed to provide some possible future research for IEEE 802.11be in this article. IEEE 802.11 working group directions for WLAN. has established a task group in May 2019 and specified the Index Terms—IEEE 802.11be, EHT, Wi-Fi 7, Multi-link Op- scope of the new generation WLAN, i.e., enabling new PHY eration, Multi-AP Coordination, MIMO Enhancement, HARQ. and MAC modes to support a maximum throughput of at least 30 Gbps, and using carrier frequency operation between 1 and 7.250 GHz while ensuring backward compatibility I. INTRODUCTION and coexistence with legacy IEEE 802.11 devices in 2.4, 5 and 6 GHz bands [2]. To achieve these goals, challenges INCE its adoption in 1990s, WLAN continues its growth of EHT have been identified and some enhanced PHY and in market share over the years and is becoming more S MAC technologies have been explored as potential solutions arXiv:2007.13401v3 [eess.SP] 3 Aug 2020 and more important for providing wireless data services by to overcome these challenges. using Wi-Fi technology. Home, enterprise and hotspots are increasingly dependent on Wi-Fi technology as their main A. PHY Enhancements for EHT Manuscript received XXX XX, 2019; revised XXX XX, XX. The work of C. Deng, X. Fang, L. Yan, and R. He was supported in 1) Providing expanded bandwidth of more than 160 MHz: part by NSFC and High-Speed Rail Joint Foundation under Grant U1834210, Due to the limited and crowded unlicensed spectra in 2.4 Sichuan Provincial Applied Basic Research Plan under Grant 2020YJ0218, and Huawei HIRP Flagship Project under Grant HF2017060002. The work GHz and 5 GHz, the existing 802.11 WLANs (e.g., IEEE of Y. Long was supported in part by NSFC under Grant 61601380. (Corre- 802.11ax [3]) will inevitably suffer from low Quality of sponding author: Xuming Fang.) Service (QoS) when running new emerging applications, such C. Deng, X. Fang, L. Yan, R. He, and Y. Long are with the Key Laboratory of Information Coding and Transmission, Southwest Jiaotong as VR/AR. To fulfill the promise of a maximum throughput University, Chengdu 611756, China (e-mail:[email protected]; of at least 30 Gbps, EHT is envisioned to add new bandwidth [email protected]; [email protected]; [email protected]; modes including contiguous 240 MHz, noncontiguous 160+80 [email protected]). X. Han and Y. Guo are with the WT Laboratory, Huawei, Shenzhen 518129, MHz, contiguous 320 MHz and noncontiguous 160+160 MHz China (e-mail: [email protected]; [email protected]). [4]. Nevertheless, the channelization and tone plan for these X. Wang is with the Department of Electrical and Computer Engineering, new bandwidth modes are still under discussion, such as University of Western Ontario, London, ON N6A 5B9, Canada (e-mail: [email protected]). whether 240 MHz/160+80 MHz is formed by puncturing at *Co-first author. 320/160+160 MHz, repeating the IEEE 802.11ax tone plan or 2 Sub-MAC Sub-MAC PDCP United MAC for Multi-link Aggregation Multi-AP Sounding RLC Explicit Multi-AP Sounding Implicit Multi-AP Sounding Multi-link Channel Access Channel Access Based on Channel Access Based on MAC Multi-AP Transmission One Primary Channel Multiple Primary Channels Multi-link Operation Coordinated OFDMA Coordinated Beamforming (C-OFDMA) (CBF) Temporary Primary Primary Channel Access MIMO Enhancement Channel Access Independently Coordinated Spatial Reuse Joint Transmission (JXT) Multi-AP Coordination (CSR) Multi-link Transmission HARQ Fast Switching Synchronized/ HARQ Granularity/Process/Method Dedicated Between Multi- Asynchronized Other MAC-related Issues Control Link link Multi-link HARQ at A-MPDU Level HARQ at MPDU Level HARQ at Codeword (CW) Level PHY Chase Combining Incremental Punctured CC Enhanced Explicit/Implicit Feedback Channelization and Tone (CC) Redundancy(IR) (PCC) Plan ϕ Only Feedback Time Domain Implicit Multi-RU Support Differential Given ÿs Rotation Channel New Research Directions Enhancement Feedback Variable Angle Quantization EHT Preamble Design and Integrating Low and High- Guaranteed QoS provisioning Preamble Puncturing frequency Bands Based on Machine Learning Multiple Component Finite Two-way Channel Feedback Feedback Sounding Coexistence in the 6 GHz Power Hybrid 4096-QAM Support Band Management Beamforming Interaction or cross-layer design between two protocol layers Fig. 1. Overview of the related technologies covered in this survey and their relationships. defining a new tone plan for 160+160 MHz/320 MHz. Besides, introduced in each generation of WLAN standards, which EHT is supposed to design effective methods to improve the can enable functions including synchronization, automatic gain spectrum utilization of wideband and non-contiguous band- control, time/frequency correction, channel estimation, auto- width. detection to differentiate the version of a physical protocol data 2) Supporting multi-RU assignment to a single user (SU): unit (PPDU) and necessary signaling (e.g., resource allocation In IEEE 802.11ax, each user is only assigned to a specific RU information), etc. According to the PAR [5], the EHT preamble for transmitting or receiving frames, which significantly limits design should ensure backward compatibility and coexistence the flexibility of the spectrum resource scheduling. To solve with legacy PPDUs transmitted on 2.4 GHz, 5 GHz, and 6 GHz this problem and further enhance the spectral efficiency, the bands. Besides, bringing future compatibility to preambles motion of allowing multi-RU assignment to a single user has starting with EHT was proposed in [6]. Since in EHT many been approved in the EHT task group [4]. However, the related new features like multi-RU and MU-MIMO are still being technical details are still pending in EHT, including multi-RU considered, formats and details of the preamble for supporting assignments, multi-RU combinations, coding and interleaving different technologies and scenarios are still pending. By schemes for multi-RU, and signaling designs, and therefore bonding channels in a non-continuous way, the new preamble more efforts in dealing with the relevant multi-RU issues are puncturing in IEEE 802.11ax allows a Wi-Fi device to transmit needed to realize the standardization of multi-RU in EHT. the MU PPDU over the entire bandwidth (e.g., 80 MHz, 3) Introducing 4096-QAM for the peak data rate improve- 80+80 MHz or 160 MHz) except for the punctured preamble ment: The available highest-order modulation scheme of IEEE part. However, due to the absence of the SIG-B field and the 802.11ax is 1024-QAM, where a modulated symbol carries puncturing-related signaling in the preamble in the SU PPDU, 10 bits. To further improve the peak rate, 4096-QAM has the SU PPDU is not allowed to employ preamble puncturing been recommended for EHT to enable a modulated symbol and must be transmitted over the entire available contiguous to carry 12 bits. Therefore, given the same coding rate, EHT bandwidth. Thus, in EHT, there may be a need to improve the can gain a 20% increase in data rate compared to 1024-QAM .