Fine-grained Channel Access in Wireless LAN

SIGCOMM 2010

Kun Tan, Ji Fang, Yuanyang Zhang,Shouyuan Chen, Lixin Shi, Jiansong

Zhang, Yongguang Zhang

Trends in 802.11 WLANs• PHY data rate increases– 802.11n up to 600Mbps– 802.11ac/ad up to >1Gbps

• Data throughput efficiency degrades with PHY data rate

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Reasons for Low Throughput Efficiency• Contention resolution overhead due to CSMA• Coarse-grained channel allocation– Whole channel allocated to a single station

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Possible solutions

• Reduce overhead– Infeasible, physical laws/technology

• Increase useful channel time – frame aggregation– OK, used in 802.11n but– Practical limitations: 80% efficiency at 300Mbps

requires frame size of 23KB!

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An Alternative ApproachFine-Grained channel Access

• Divide channel into smaller subchannels

• Multiple users contend for and use subchannels simultaneously – Based on traffic demands

• Amortize MAC coordination, increase channel efficiency

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Challenges• Need to avoid interference between

neighbor subchannels

• Traditional approach: guard bands– High overhead

• OFDM – Orthogonal Frequency Division Multiplexing– “Eliminates” need for guard bands– Requires tight synchronization (100s of nsec)

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OFDM – High Level Overview• Divides spectrum into

many small, partially overlapping subcarriers

• Subcarrier frequencies “orthogonal” to each other

• OFDM system with FFT size N– N subcarriers, each with

bandwidth B/N

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OFDM as multi-access technology• Different stations assigned different subcarriers

in the same channel–WiMAX, LTE

• Symbol timing alignment is critical

• Requires tight synch with cellular BS– Use of guard times, CP (cyclic prefic)

– 802.11: CP-to-symbol length ratio 1:4 (0.8μs to 3.2μs)

OFDM-based Channel Access in WLANs• Challenge 1: Coordinate random

access among multiple stations– Cannot use cellular-type synchronization– Need a new OFDM architecure for

distributed coordination

• Challenge 2: Longer symbol length to maintain 1:4 CP-to-symbol length ratio–Makes backoff mechanism inefficient– Need new MAC contention mechanism,

new backoff scheme

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Paper Contributions• Design and implementation of FICA– Cross-layer architecture based on OFDM– Enables fine-grained subchannel random

access in WLANs

• Two key techniques– New PHY architecture based on OFDM– Novel frequency domain contention

method

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FICA Overview• Uplink transmission

• Downlink transmission similar

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• Using carrier sensing

• Using reference broadcast

Symbol Time Misalignement

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PHY Architecture

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• Each 802.11 channel (20Mhz) divided into 1.33Mhz subchannels– 14 + guardband

• Each subchannel divided into 17 subcarriers– 16 + pilot

• Data is transmitted over all 16 subcarriers

Frequency Domain Contention

• Allocate K subcarriers per subchannel– Contention band

• Each node contending for a subchannel picks randomly a subcarrier and sends a ‘1’ in M-RTS

• AP arbitrates contention and sends winning subcarriers in M-CTS

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Issues in Frequency Domain Contention

• What if 2 nodes choose the same subcarrier?– Collision– No transmission

• How large should K be?– K=16 (initial backoff value in 802.11)

• Who is returning M-CTS?– Only potential receivers– Allocate 40 subcarriers, hash receiver’s ID into

0..39, set appropriate subcarrier

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M-RTS, M-CTS

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Frequency Domain Backoff• How many subchannels can a node contend

for?– n=min(Cmax, lqueue)

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Downlink Transmission• AP can transmit simultaneously to many clients

– Different subchannels per client, has to contend for each subchannel

• Two-way traffic– FICA uses no backoff, AP and station can send M-RTS

simultaneously

• Solution: use different DIFS to prioritize transmissions– Fixed DIFS to all stations, 2 DIFS to AP– If AP uses short DIFS, use long DIFS next time– If AP receives M-RTS, use short DIFS next time– Fair interleaving of uplink-downlink, not among all

stations!

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Multiple Domains – Hidden Terminals

• Hidden terminals– Collisions may cause M-RTS/M-CTS loss– Random backoff after M-CTS loss

• Multiple domains– Nodes may receive inconsistent M-CTS from

different nodes– Node only allowed to transmit if wins contention

in all domains it participates.

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Evaluation• Simulation

• Implementation

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Simulation Setup• Event-based simulator• Only uplink traffic• Packet loss only due to collisions• Compare against 802.11n– No aggregation– Full aggregation–Mixed traffic

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Simulation Results No Aggregation

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Simulation Results Full Aggregation

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• All nodes saturated, frame size 18KB!

Simulation Results Mixed Traffic

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Implementation• Sora platform [NSDI ‘09]– Fully programmable software radio

platform

• Implementation cannot run in real time– Takes too long to transfer PHY frames

from CPU to RCB (Even though Sora is the fastest platform available)

– Have to prestore all PHY frames in RCB

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Evaluation – Time Misalignment

With Broadcasting With Carrier Sensing

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Reliability of PHY Signaling

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Demodulation Performance

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Conclusion• Trend in 802.11 WLANs– Throughput efficiency decreases as data rate

increases

• Fundamental reason– Entire wide-band channel allocated to one node

• FICA– Cross-layer design to enable fine-grained

subchannel random access – New PHY arhitecture based on OFDM– New frequency domain backoff scheme

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