1. Fine-grained Channel Access in Wireless LAN SIGCOMM 2010 Kun Tan, Ji Fang, Yuanyang Zhang,Shouyuan Chen, Lixin Shi, Jiansong Zhang, Yongguang Zhang

2. 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 2

3. Reasons for Low Throughput Efficiency Contention resolution overhead due to CSMA Coarse-grained channel allocation – Whole channel allocated to a single station 3

4. 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! 4

5. An Alternative Approach Fine-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 5

6. 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) 6

7. 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 7

8. 8 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)

9. 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 9

10. 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 10

11. FICA Overview Uplink transmission Downlink transmission similar 11

12. Using carrier sensing Using reference broadcast Symbol Time Misalignement 12

13. PHY Architecture 13 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

14. 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 14

15. 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 15

16. M-RTS, M-CTS 16

17. Frequency Domain Backoff How many subchannels can a node contend for? – n=min(C max, l queue ) 17

18. 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! 18

19. 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. 19

20. Evaluation Simulation Implementation 20

21. Simulation Setup Event-based simulator Only uplink traffic Packet loss only due to collisions Compare against 802.11n – No aggregation – Full aggregation – Mixed traffic 21

22. Simulation Results No Aggregation 22

23. Simulation Results Full Aggregation 23 All nodes saturated, frame size 18KB!

24. Simulation Results Mixed Traffic 24

25. 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 25

26. Evaluation – Time Misalignment With BroadcastingWith Carrier Sensing 26

27. Reliability of PHY Signaling 27

28. Demodulation Performance 28

29. 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 29