Retransmission ≠ Repeat: Simple Retransmission Permutation Can Resolve Overlapping Channel Collisions

1. Li (Erran) Li Bell Labs, Alcatel-Lucent Joint work with: Junliang Liu(MSR,Beijing), Kun Tan(MSR, Beijing), Harish Viswanathan (Bell Labs), Yang Richard Yang (Yale) Retransmission ≠ Repeat: Simple Retransmission Permutation Can Resolve Overlapping Channel Collisions

2. Talk Outline Wireless networks with overlapping channels 802.11g overlapping channel collision problem Remap basic idea Remap details Evaluation Conclusion and future work 2

3. Wireless networks with overlapping channels • Chaotically deployed WiFi networks • Each user chooses its own channel • Planned WiFi networks • Due to shortage of orthogonal channels, partially overlapped channels are beneficial [Misra et al, SIGMETRICS’06] • WiFi networks built on digital white space, e.g. WhiteFi [Bahl et al. SIGCOMM’09]

4. 802.11g overlapping channel collision problem Collision! APa on channel Ca Alice Bob Collision! APb on channel Cb Chuck

5. 802.11g overlapping channel collision problem • Retransmission More Collision! APa on channel Ca Alice Bob More Collision! APb on channel Cb Chuck

6. 802.11 background Using 802.11g as an example Each channel has 4 groups of subcarriers: C1 consists of G1, G2, G3, G4; C2 consists of G2, G3, G4, G5 C1 and C2 are overlapping adjacent channels; C1 and C3 are overlapping non-adjacent channels Bits are assigned to subcarriers E.g. bit sequences Ai is assigned to subcarrier Gi, i=1,2,3,4 Subcarrier Group G1 G2 G3 G4 A4 A2 A3 A1 6

7. Remap basic idea: structured permutation A1 A3 A2 A4 Mapping π2 A3 A1 A4 A2 Mapping π3 A2 A4 Mapping π4 A1 A3 Subcarrier Group G1 G2 G3 G4 Mapping π1 A4 A2 A3 A1 7

8. How permutation helps Non-matching collisions on adjacent channels C1 and C2 A1 A3 A2 A4 2nd transmission 3rd transmission A3 A1 A4 A2 4th transmission A2 A4 A1 A3 Subcarrier Group G1 G2 G3 G4 1st transmission A4 A2 A3 A1

9. How permutation helps (cont’d) Non-matching collisions on non-adjacent channels C1 and C3 A1 A3 A2 A4 2nd transmission Subcarrier Group G1 G2 G3 G4 1st transmission A4 A2 A3 A1 9

10. Remap basic idea: Matching-collision setting • Matching collisions on adjacent channels Collision! APa on channel Ca Bob Alice Collision! APb on channel Cb

11. Remap for matching collisions Matching collisions on adjacent channels C1 and C2 A3 A3 A1 A1 B3 B5 B3 Subcarrier Group G1 G2 G3 G4 G1 G2 G3 G4 A2 A4 G5 A4 A2 G5 B2 B4 B4 B2 B5 Subtract A1 Decode A1 Subtract A1 Re-encode A1 on G4 Subtract B3 Decode B3 Re-encode B3 on G3 Subtract B3 Subtract A3 Re-encode A3 on G2 Decode A3 Subtract A3 Subtract B5 Decode B5 Decoded bits: A1 B3 A3 B5

12. Remap for matching collisions: Decoding graph Decoding graph of collision at adjacent channels C1 and C2 2nd Collision 1st collision 2nd Collision 1st collision A1 A4 A1 A4 Re-encode B3 B3 B4 B4 Subtract A3 A2 A3 A2 B2 B5 12

13. Remap for matching collisions: a time-frequency view collisions at adjacent channels C1 and C2 : a time and frequency view 5 7 11 10 9 8 6 14 13 12 1 2 3 P′a 4 Pa Time Sn S1 S2 Sn S2 S1 A4 G1 A1 A3 A2 G2 Freq A2 G3 A3 A1 A4 G4 ∆2 B5 B2 G2 ∆1 B4 B3 B3 B4 B2 B5 G5 P′b Pb

14. Remap for matching collisions Theorem on a pair of matching collisions: Assume that Alice and Bob use different permutations for the two transmissions, Alice’s AP and Bob’s AP can each decode both packets despite collisions.

15. Remap Details Detecting collision Correlation to detect collision Energy detection to determine which group’s energy has no change before and after the correlation peak Detecting matching collision Correlating subcarrier group Gi and its remapped subcarriers Detecting modulation Cannot decode PLCP header of Bob’s packet Solution: raw sample subtraction for the first pass 15

16. Remap Details (cont’d) Loss of orthogonality Carrier frequency offset Desired symbol and interfering symbol unalignment Desired signal at subcarrier i: Interfering signal at subcarrier i+m: Aligned interference symbols on non-adjacent subcarriers have zero Interference energy.

17. Remap Details (cont’d) Loss of orthogonality Carrier frequency offset Desired symbol and interfering symbol unalignment Desired signal at subcarrier i: • Interference energy: Interfering signal at subcarrier i+m: • The energy is 19dB lower if m=4; 21dB lower if m=5

18. Remap Details (cont’d) Techniques dealing with loss of orthogonality Iterative interference cancellation Exploiting uneven interference of subcarriers 18

19. Evaluation • Experimental setup for non-matching collisions: • Use MSA Sora software-radio platform for 802.11g • Fix Alice at channel 3 • For adjacent-channel collision test, Bob (the interferer) is at channel 4; for non-adjacent channel collision test, Bob is at channel 5 • For each packet, Alice transmits the original and 3 remapped versions • Alice and Bob continuously transmit for 100 ms; data collected is called a dump • 100 dumps are performed 19

20. Evaluation (cont’d) • Segment samples in each dump into groups of 4 packets each • Decode each group, success if pass CRC check • Performance metric • Normalized throughput: actual number of decoded packets divided by the ideal number of decoded packets 20

21. Evaluation: non-adjacent channel • SNR measured at receiver tuned to channel 3 • Almost no packets can be decoded using successive interference cancellation 21

22. Evaluation: non-adjacent channel (cont’d) • Non-adjacent channel: scatter plot of second experiment 22

23. Evaluation: Adjacent Channel • SNR measured at receiver tuned to channel 3 • Almost no packets can be decoded using successive interference cancellation 23

24. Conclusion and future work • Generalize Remap to other channel structures • Investigate techniques that deal with loss-of-orthogonality issue • Evaluate how well matching collision detection and decoding work • Extend Remap to dynamic spectrum access networks 24