PaperlayerView exact revision on arXiv

Source-linked AI summary

IEEE 802.11ay based mmWave WLANs: Design Challenges and Solutions

Pei Zhou, Kaijun Cheng, Xiao Han, Xuming Fang, Yuguang Fang, Rong He, Yan Long, Yanping Liu

arXiv:1803.07808v1cs.NI

TL;DR

The paper addresses the lack of a comprehensive survey of IEEE 802.11ay while the standard was still under development. It reviews MAC and cross-layer technologies, identifies design challenges, and synthesizes proposed solutions and open research directions for mmWave WLANs.

  • Problem

    IEEE 802.11ay lacked a comprehensive survey, despite the need to address challenges including beamforming limitations, MU-MIMO overhead, and mmWave power consumption.

  • Method

    The paper surveys IEEE 802.11ad technologies and IEEE 802.11ay proposals covering channel access, channel allocation, beamforming, MIMO, power management, and related PHY-MAC designs.

  • Results

    The survey describes IEEE 802.11ay enhancements beyond 802.11ad, including multiple-channel access, SU-MIMO and MU-MIMO beamforming, and more efficient beamforming training mechanisms.

  • Takeaways & Limitations

    The paper provides an up-to-date introduction to IEEE 802.11ay standardization activities and identifies open issues and future research directions for mmWave WLANs.

Abstract

from arXiv · show

Millimeter-wave (mmWave) with large spectrum available is considered as the most promising frequency band for future wireless communications. The IEEE 802.11ad and IEEE 802.11ay operating on 60 GHz mmWave are the two most expected wireless local area network (WLAN) technologies for ultra-high-speed communications. For the IEEE 802.11ay standard still under development, there are plenty of proposals from companies and researchers who are involved with the IEEE 802.11ay task group. In this survey, we conduct a comprehensive review on the medium access control layer (MAC) related issues for the IEEE 802.11ay, some cross-layer between physical layer (PHY) and MAC technologies are also included. We start with MAC related technologies in the IEEE 802.11ad and discuss design challenges on mmWave communications, leading to some MAC related technologies for the IEEE 802.11ay. We then elaborate on important design issues for IEEE 802.11ay. Specifically, we review the channel bonding and aggregation for the IEEE 802.11ay, and point out the major differences between the two technologies. Then, we describe channel access and channel allocation in the IEEE 802.11ay, including spatial sharing and interference mitigation technologies. After that, we present an in-depth survey on beamforming training (BFT), beam tracking, single-user multiple-input-multiple-output (SU-MIMO) beamforming and multi-user multiple-input-multiple-output (MU-MIMO) beamforming. Finally, we discuss some open design issues and future research directions for mmWave WLANs. We hope that this paper provides a good introduction to this exciting research area for future wireless systems.

I. INTRODUCTION

IEEE 802.11ay extends 802.11ad to address growing demand for high-rate, longer-range 60 GHz WLANs. The survey examines its MAC and PHY–MAC design issues, including channel operation, access, allocation, and beamforming.

  • Motivation: 802.11ay targets at least 20 Gbps while maintaining or improving station power efficiency.The standard is presented as an evolution of 802.11ad rather than a revolution.
  • Motivation: Congested low-frequency WLAN bands motivate mmWave, which offers substantial spectrum for broadband applications.The discussion identifies 30–300 GHz as the mmWave range and 60 GHz as a promising unlicensed band.
  • Motivation: 802.11ad supports only one 2.16 GHz channel, limiting multi-channel flexibility and efficiency.802.11ay is introduced partly to overcome this operational constraint.
  • Scope: The survey focuses on MAC-related and PHY–MAC cross-layer technologies for 802.11ay, including channel bonding, allocation, BFT, and MIMO.It also reviews spatial sharing, interference mitigation, beam tracking, and open research issues.
  • 802.11ad baseline: In 802.11ad, a beacon interval contains BHI and DTI phases, with BTI, A-BFT, ATI, CBAPs, and SPs providing distinct access functions.CBAPs use contention-based access, while SPs support contention-free communication between designated stations.

2) DMG beamforming:

The survey presents 802.11ad beamforming and mmWave design challenges that motivate 802.11ay enhancements. Key concerns include limited beamforming capabilities, bonding trade-offs, and interference-aware multi-channel operation.

  • DMG beamforming: 802.11ad lacks concurrent MU beamforming training and does not support SU-MIMO, MU-MIMO, or hybrid beamforming.The survey identifies A-BFT collisions as another shortcoming in dense-user scenarios.
  • Channel bonding: Wider bonded channels can increase throughput and reduce required SINR, but may reduce range and increase interference and channel competition.The cited challenges arise from higher sensitivity requirements and greater spectrum overlap among WLANs.
  • Channel bonding: MmWave channel bonding requires new MAC and PHY designs, efficient BFT for bonded or aggregated channels, and modified channel-access operations.The proposed changes include coding support, SU-MIMO and MU-MIMO integration, PPDU formats, and spatial reuse.
  • Spatial sharing: Directional transmissions and high path loss create additional opportunities for spatial sharing and require effective interference mitigation.802.11ad and 802.11ay use spatial sharing to schedule non-interfering stations in the same service period.

3) Beamforming training and beam tracking:

The survey reviews beamforming training, beam tracking, blockage recovery, power management, and multi-channel features as central 802.11ay design concerns. These mechanisms must support efficient, robust, and power-conscious directional communication.

  • Beamforming training and beam tracking: 802.11ad beamforming training is fixed and can be time-consuming, motivating lower-complexity and more flexible methods for 802.11ay.Long training delays can increase communication delay and degrade quality of experience.
  • Beamforming training and beam tracking: Beam tracking in 802.11ad searches for a better link only after degradation, so sudden blockage can interrupt the link.The survey therefore identifies efficient and intelligent tracking as an 802.11ay need.
  • Beamformed link blockage: Obstacle blockage may require time-consuming retraining, while relays provide a practical way to find or maintain an alternative link.The survey discusses relays among the solutions for beamformed-link blockage.
  • Power management: Large antenna arrays and multiple active transceivers make power management a serious challenge, especially with dual connectivity and high- and low-frequency cooperation.The survey identifies sleep-mode mechanisms as an open research direction for avoiding unnecessary active operation.
  • 802.11ay enhancements: 802.11ay extends channel operation by allowing access periods on primary and secondary channels, bonded or aggregated communication, and directional allocation.MIMO channel access is also identified as an important supported feature.
  • 802.11ay enhancements: 802.11ay beamforming supports bonded and aggregated channels, polarized BFT, and multi-channel A-BFT to distribute random-access attempts.These mechanisms address collision pressure in dense-user scenarios and support MIMO-related beamforming procedures.

B. Channel bonding

802.11ay extends mmWave WLAN channel use beyond 802.11ad’s single-channel operation through channel bonding, aggregation, and flexible multi-channel allocation. These mechanisms improve spectrum utilization but require revised access, sensing, and interference-mitigation procedures.

  • Channel bonding: 802.11ay targets at least 20 Gbps through wider bandwidth from channel bonding and downlink MU-MIMO.Channel bonding provides higher throughput, while DL MU-MIMO distributes capacity to multiple STAs simultaneously.
  • Channel bonding: 802.11ay supports up to six 2.16 GHz channels, including overlapped bonded channels, with primary-channel requirements for bandwidths of 4.32 GHz or wider.EDMG channelization defines the available channel combinations and preserves a primary channel for wider allocations.
  • Channel bonding: Channel bonding merges contiguous channels into one wideband channel, whereas channel aggregation combines contiguous or non-contiguous channels separated by spacing.The two approaches also differ in physical frame format.
  • Channel access: The PCP/AP can schedule SPs and CBAPs across multiple or bonded channels, while transmissions cannot exceed the equivalent bandwidth of four 2.16 GHz channels.BTI and ATI remain on the primary channel, whereas A-BFT also occupies an adjacent secondary channel.
  • Channel access: Frequency multiple access and alternative channel access schemes aim to improve utilization when primary-channel congestion limits spectral-resource use.These schemes support flexible allocations over channels and bonded channels, but non-primary allocations require careful sensing.
  • Channel access: Secondary-only allocation lets 802.11ay devices use an idle secondary channel even when the primary channel is occupied by a legacy allocation.When the allocation excludes the primary channel, full physical and virtual carrier sensing helps prevent hidden-node collisions and supports NAV setting.

C. MIMO channel access

802.11ay MIMO channel access supports simultaneous spatial streams and concurrent spatial sharing, while requiring sensing and reservation procedures that cover the beams used for transmission. The section also addresses fairness, measurement efficiency, and inter-PBSS interference.

  • MIMO channel access: MIMO channel access enables multiple streams between multi-antenna STAs, improving spatial reuse and robustness against blockage of one stream.The robustness benefit follows from avoiding outage when an individual stream is blocked.
  • MIMO channel access: MIMO access uses RTS/DMG CTS exchange and requires devices to remain able to receive SISO frames while MIMO access is pending.Two proposed access options both require SISO reception.
  • MIMO channel access: MIMO sensing can use a shared backoff timer or multiple beam-associated timers, allowing access when the beam conditions satisfy the corresponding clear-channel rules.The multiple-timer method permits access when one timer reaches zero and the remaining timers are not suspended.
  • MIMO channel access: An alternative requires energy-detection CCA to cover all MIMO beams and permits access only after the beams remain clear for PIFS.This option is recommended because of lower complexity and power consumption.
  • Spatial sharing: SU-MIMO spatial sharing requires concurrent measurements using multiple receive-antenna configurations matched to multiple streams.Measurement results can be reported individually for detail or averaged for shorter reports.
  • Spatial sharing: Inter-PBSS SPSH mitigation exchanges coexisting-link information and applies mutual avoidance to improve throughput among co-channel PBSSs.The method addresses inter-PBSS spatial sharing within a cluster, a case identified as underexplored.

V. EFFICIENT BEAMFORMING TRAINING AND BEAM TRACKING

802.11ay retains the basic 802.11ad beamforming structure while adding more flexible training procedures, including receive training during BTI and scalable multi-STA training. These changes reduce training overhead and time while supporting refined transmit and receive beam configuration.

  • Beamforming training: SLS is mandatory and trains both transmit-sector directions, while optional BRP trains receive sectors and refines antenna weight vectors.SLS includes initiator sweep, responder sweep, feedback, and acknowledgment subphases; BRP includes setup, MID, BC, and refinement transactions.
  • Beam refinement: BRP pairs transmit and receive sectors and explores additional transmit and receive antenna weight vectors through request-response exchanges and TRN units.TRN-T units train transmit sectors, while TRN-R units train receive sectors.
  • Beamforming in BTI: Receive beamforming during BTI appends TRN-R units to DMG Beacon frames, allowing receive-sector training while the beacon is received.This trains responder receive sectors during the beacon interval rather than leaving all receive training to later phases.
  • Beamforming in BTI: Receive and transmit training in BTI reduces BFT time and improves training efficiency.The cited procedure shifts responder receive-sector training into the BTI phase.
  • Scalable BFT: A scalable method uses antenna-pattern reciprocity to reduce BFT overhead and train one initiator with multiple STAs simultaneously.Responders derive results from receive training and therefore do not need responder transmit sector sweeps.
  • Beamforming in BTI: The BFT time scales with the initiator’s transmit-sector count multiplied by the maximum responder receive-sector count, independently of the number of responders.Multiple responders can receive the training frames simultaneously.

C. Beamforming in A-BFT

802.11ay improves A-BFT beamforming by expanding channel and slot resources and randomizing retry behavior to reduce collisions in dense scenarios. These mechanisms address A-BFT’s role as a channel-access and beamforming bottleneck.

  • Multi-channel A-BFT assigns corresponding A-BFT configurations across primary and secondary channels, avoiding collisions when STAs select the same slot on different channels.
  • EDMG STAs randomly select retry-limit and backoff values, reducing synchronized collisions when many STAs join the BSS simultaneously.
  • SA-BFT: SA-BFT extends legacy A-BFT with additional EDMG slots while preserving backward compatibility through beacon timing fields.
  • An alternative inserts additional A-BFT slots before legacy slots and adjusts the beacon Duration field so legacy STAs remain compatible.
  • A-BFT is a channel-access and beamforming bottleneck because dense-user collisions can delay failed STAs until the next beacon interval, affecting QoE.

D. Beamforming in DTI

802.11ay introduces several DTI beamforming enhancements, including simultaneous polarized-sector training, BRP-based sector sweeps, multi-resolution training, and TRN-assisted training for bonded or aggregated channels. These proposals target faster or more flexible beam acquisition and refinement.

  • Multi-BF simultaneously steers sectors across horizontal and vertical polarization domains to reduce beamforming-training time in LOS scenarios.Half of the BFT frames can use horizontal polarization and the remainder vertical polarization separately.
  • The BRP TXSS protocol exchanges BRP frames and feedback while sweeping initiator and responder antennas, supporting both transmit- and receive-side sector sweeps.
  • Multi-resolution BFT allows either endpoint to request a resolution level, trading beamforming efficiency against beam accuracy without sweeping every beam.
  • BRP frames can append TRN units for efficient beam refinement on bonded and aggregated channels, avoiding a complete SLS or BRP phase when LOS characteristics make it unnecessary.

F. Hybrid beamforming

The section combines hybrid beamforming with mechanisms for asymmetric links and beam tracking. It describes analog and digital beamforming roles, directional-link establishment, and TRN-based recovery when beam quality or channel availability changes.

  • Hybrid beamforming: Hybrid beamforming combines analog beamforming for link gain with digital beamforming for spatial multiplexing, using effective channel information.The effective channel is formed from the physical channel and the transmitter and receiver analog beamformers.
  • Beamforming for asymmetric links: The asymmetric-link mechanism uses TRN-R-assisted receive training, directional SSW transmissions, ordered sector listening, and Sector ACK frames to establish directional transmissions.
  • Beam tracking: When beam quality falls below a specified threshold without link blockage, appending TRN units to data frames enables beam tracking instead of repeating BFT.
  • Beam tracking: Multi-channel beam tracking lets an STA try other allocated channels when its operating channel is blocked and no usable beam is found.

I. Beamformed link robustness

802.11ay robustness mechanisms address blockage, mobility, and multi-stream operation through beam tracking, alternative links, relaying, and SU-MIMO training. The section also describes how SU-MIMO phases configure and train multiple transmit and receive sectors and antennas.

  • Beam tracking: Beam tracking seeks alternate beam pairs during blockage, while double-link and cooperative schemes use alternative or unblocked links to restore transmission.
  • Relaying: Relay links provide backup for LOS transmissions, but existing relay operations have fixed source-relay and relay-destination channel times and lack multi-hop and efficient bidirectional support.Decode-and-forward relays can extend coverage and address blockage but reduce effective throughput.
  • SU-MIMO: SU-MIMO beamforming configures transmit and receive antenna settings for simultaneous multiple spatial streams between capable initiators and responders.
  • SU-MIMO: The SU-MIMO SISO phase collects mutual sector-sweep feedback, while the MIMO phase trains sector and antenna combinations through setup, initiator training, responder training, and feedback.
  • SU-MIMO: SU-MIMO BF setup communicates the dialog token, requested sector-combination counts, time-domain feedback requirements, and TRN subfield counts.

C. MU-MIMO beamforming

MU-MIMO beamforming in 802.11ay uses staged training and feedback to select configurations that limit multiuser interference, while hybrid designs address the high overhead of RF and digital beam determination.

  • MU-MIMO BFT establishes an antenna configuration for EDMG MU PPDUs while minimizing mutual interference among responders’ data streams.
  • MU-MIMO SISO phase: The SISO phase trains initiator–responder sectors, collects sector IDs and SNRs, and estimates whether each responder will experience small multiuser interference.
  • MU-MIMO MIMO phase: The MIMO phase includes setup, training, feedback, and selection subphases, with setup frames identifying the MU group, responders, dialog token, and feedback requirements.
  • MU-MIMO MIMO phase: The initiator may select TX-sector subsets and exclude responders whose expected multiuser interference is negligible, reducing MU-MIMO BFT time.
  • Hybrid beamforming: Increasing STA counts and codebook sizes causes RF and digital beam-search overhead to grow exponentially, motivating low-complexity, low-overhead hybrid beamforming.
  • Hybrid beamforming: A three-stage hybrid design selects RF beams from BFT, obtains quantized effective-channel feedback from STAs, and then calculates digital beamforming.
  • DL MU-MIMO training: UL training for DL MU-MIMO reduces training overhead and improves beam accuracy, but requires more precise calibration because UL/DL channel reciprocity is unavailable.

VII. OPEN RESEARCH ISSUES AND FUTURE WORK

The paper identifies future WLAN research directions spanning cooperation, connectivity, power management, uplink MU-MIMO, beam management, and distributed resource allocation for mmWave systems.

  • Inter-AP cooperation: Inter-AP cooperation could prepare handoff information when an STA’s serving link degrades or becomes blocked by obstacles.
  • Connectivity: HF/LF cooperation and dual connectivity can maintain simultaneous links across mmWave and sub-6 GHz or multiple APs while dynamically managing WLAN traffic.
  • Power management: Power consumption is a serious issue when large antenna arrays and multiple active transceivers maintain dual or cooperating links, motivating sleep-mode operation.
  • UL MU-MIMO: Future mmWave WLANs must address timing and frequency offsets between users when adopting beamspace UL MU-MIMO.
  • Beam management: Group-based beam indication and switching can reduce signaling overhead and support beam recovery through group switching after blockage.
  • Resource management: Distributed WLAN resource management remains difficult because all STAs operate distributively and interference limits performance.
  • Survey scope: The survey presents 802.11ay channel access and SU-MIMO/MU-MIMO beamforming technologies while identifying open issues before standard finalization.

APPENDIX A

Appendix A lists acronyms used throughout the paper, covering beamforming, channel access, WLAN architecture, signal processing, and performance terminology.

  • Beamforming procedures: BFT-related acronyms include Association Beamforming Training, Initiator/Responder SU-MIMO BF Training, and Initiator/Responder Transmit Sector Sweep.
  • Channel access: Channel and access terminology includes Alternative Primary Channel, Beacon Transmission Interval, Contention-Based Access Period, and Carrier Sense Multiple Access with Collision.
  • Signal processing and quality: PHY and signal-quality terms include Digital-to-Analog/Analog-to-Digital, Digital Baseband Channel Tracking, Modulation and Coding Scheme, and Signal-to-Interference-Plus-Noise-Ratio.
  • Beamforming and antenna systems: MU-MIMO denotes downlink multi-user multiple-input multiple-output, while MIMO denotes multiple input multiple output.
  • WLAN architecture: The appendix also defines WLAN architecture and protocol terms including Directional Multi-Gigabit, Network Allocation Vector, Personal Basic Service Set, and Wireless Local Area Network.
Loading 1803.07808v1…