1. Migrating Backoff to the Frequency Domain
No time to countdown:
Migrating Backoff to the Frequency Domain
Souvik Sen
Romit Roy Choudhury, Srihari Nelakuditi
MobiCom 2011

2. Introduction

3. Current WiFi Channel Contention
Conventional WiFi networks >> perform channel contention in time domain
more than 30% reduction in throughput due to backing off
Solution >> migrate the backoff operation to the frequency domain

4. Back2F 802.11 a/g/n PHY adopts OFDM
Pretend OFDM subcarriers as integers
The process is similar to current WiFi
Picks a random value from specified range [0,CW-1] ex) a
Once the channel becomes idle, it begins the backoff operation
Instead of counting down, it transmits a symbol on the a_th sbcarrier
It has two antenna
One for transmit, the other listens to determine which of the subcarriers are active

5. Back2F
Challenges >> Accuracy >> Collision >> Ranking
Contributions >> Time domain -> Frequency domain >> Design an OFDM based system >> Make prototype on the USRP/GNU Radio platform
Challenges
1. Active subcarriers need to be detected accurately
2. Collision among nodes need to be mitigated successfully
3. Since nodes are located in different contention neighborhoods,
the node ranking do not obey any global order
Contributions
We identify an opportunity to migrate protocol operations from the time to the frequency domain. Although we instantiate our ideas through a WiFi based MAC, they may be generalized to other arbitration strategies.
We design an OFDM based system where random backoff is realized by selectively transmitting on a subcarrier. A logical order among senders is enforced in a decentralized manner, for improved channel usage.
We address the challenges behind such a scheme, and prototype it on the USRP/GNU Radio platform. Promising results, in terms of throughput, fairness, and scalability, give us confidence to build a larger system

6. and OFDM

7. 802.11 channel access Chose CW (contention window)
Count down only if the channel is idle (DIFS)
If the node senses a busy channel, frozen
Revived only after the channel is idle
Node completes the countdown first begins transmission
DIFS >> DCF Interframe Space
SIFS >> Short Interframe Space
PIFS >> PCF Interframe Space

8. Channel under-utilization due to 802.11’s backoff

9. OFDM
Benefit >> Its ability to cope with channel adversities; narrowband interference and frequency-selective fading
Back2F >> Back2F node picks a random number and transmits a short signal only on the that subcarrier

10. Architecture and Design

11. Backoff within single collision domain
Basic operation of Back2F

12. Backoff within single collision domain
2nd Round situation

13. Backoff within single collision domain
How about single winner? >> Must go 2nd round
Is it enough 2nd round? >> Two rounds of contention suffice for up to 50 contenders
There is no way to reliably tell between a sole winner and collisions
Because a Back2F node can detect which subcarriers are active, but cannot tell how many nodes transmitted on a given subcarrier

14. Backoff over multiple collision domain
1st domain: AP2 is winner
2nd domain: AP3 is winner

15. Backoff over multiple collision domain

16. Coping with Misdetection due to Fading
False positive >> No one has transmitted on that subcarrier, but a node detects a subcarrier
False negative >> A node fails to detect a subcarrier
False positive does not affect Back2F
>> cause the channel to be wasted for a DIFS duration
Let assume that self subcarrier i fails to detect a legitimate subcarrier f
False negative has no effect when number i is less than f
However, if f is smaller than i in the second round, then it causing a certain collision
It defeats the purpose of backing off

17. Coping with Misdetection due to Fading
To alleviate the impact of false negative
A node can transmit on subcarrier i and ((F/2 + i) mod F)
F is the total number of subcarriers

18. Adjusting backoff values
Pseudo code for Back2F with batched transmissions
2nd round의 가장 큰 backoff 값을 1st round 의 나머지 loser들에서 빼준 값이 새로운 loser node 들의 값이다
Batched transmissions over multiple collision domains
>> Back2F incurs a penalty of DIFS whenever the batch order is not followed

19. Points of Discussion

20. Active subcarrier detection
10 consecutive subcarriers (out of 52)
FFT: 고속 푸리에 변환
DFT와 그 역변환을 빠르게 수행하는 효율적인 알고리즘
N-point FFT
N: the length of your sequence

21. Active subcarrier detection
Higher point FFT reduces leakage
When high point FFT, even in presence of a high self subcarrier, transmissions on adjacent and nearby subcarriers are reliably discerned

22. Active subcarrier detection
259 pt. FFt, the subcarrier adjacent to the self-subcarrier experiences less than 10dB leakage

23. Collision Probability
Collision probability remains less than 2%

24. Implementation and Evaluation

25. USRP/GNURadio Prototype
Transmitter >> 52 subcarriers, converted into a time domain signal using 64pt. IFFT
Listening antenna >> execute 64, 128, 216 pt. FFTs
GNU Radio is a free software toolkit for learning about, building, and deploying software-defined radio systems.
Universal Software Radio Peripheral (USRP)
The USRP product is intended to be a comparatively inexpensive hardware device for software radio.

26. USRP/GNURadio Prototype
Evaluated using traces at 65 locations

27. Subcarrier detection
64pt. FFT
128pt. FFT

28. Subcarrier detection 256pt. FFT
Subcarriers above 14dB can be detected reliably
14dB 또는 그 이상의 SNR에서 97%의 정확도로 adjacent subcarrier들을 검출하였다.

29. False positive/negative
False positive는 앞서 말했듯이 Back2F에 그다지 큰 영향을 끼치지 못하고,
False negative의 경우에는 영향을 미치지만, 14dB 정도에서 확률이 작다.
14dB는 이전의 subcarrier detection에서 보면 14dB 이상이면 97%확률로 검출함

30. Throughput gain with batch

31. Throughput gain with batch
Batching contributes 5% of the gain
The gains with batching is small because basic Back2F
has already reduced the backoff duration substantially

32. Traffic type
Gains are better with Skype traffic due to smaller packet sizes
This is because backoff overheads are fixed, making it proportionally larger for short packets

33. Performance in dense networks
They simulated HD traffic in a single collision domain under varying densities and different bitrates
Back2F provides gains are in the range of 15% to 30%

34. Limitations and Discussion

35. Limitation and Discussion
Robustness of subcarrier based backoff
Collisions due to hidden terminals
Gain over packet aggregation
Interoperability with
Their results are derived from lab experiments, without node/environment mobility Back2F more sensitive to channel fluctuation
They observe that Back2F collisions are solely due to hidden terminals perhaps turn on RTS/CTS (Request-to-send/Clear-to-send)
Aggregation may not be popssible for real time traffic Back2F provides gain with aggregation at higher rates
Back2F wait for longer than DIFS May interoperate but will cause unfairness

36. Conclusion

37. Summary Randomization is an effective method for contention resolution
time domain backoff requires channel to remain idle
Randomization possible in frequency domain by using OFDM subcarriers
Back2F: practical system realizing frequency domain contention