1. 1 Retransmission Repeat: Simple Retransmission Permutation Can Resolve Overlapping Channel Collisions 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)

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

3. 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, SIGMETRICS06] WiFi networks built on digital white space, e.g. WhiteFi [Bahl et al. SIGCOMM09]

4. 4 802.11g overlapping channel collision problem Bob AP a on channel C a Collision! Alice AP b on channel C b Collision! Chuck

5. 5 802.11g overlapping channel collision problem Bob AP a on channel C a More Collision! Alice AP b on channel C b More Collision! Chuck Retransmission

6. 6 6 802.11 background Using 802.11g as an example Each channel has 4 groups of subcarriers: C 1 consists of G 1, G 2, G 3, G 4 ; C 2 consists of G 2, G 3, G 4, G 5 C 1 and C 2 are overlapping adjacent channels; C 1 and C 3 are overlapping non-adjacent channels Bits are assigned to subcarriers E.g. bit sequences A i is assigned to subcarrier G i, i=1,2,3,4 Subcarrier Group G1G1 G2G2 G3G3 G4G4 A1A1 A2A2 A3A3 A4A4

7. 7 7 Remap basic idea: structured permutation Subcarrier Group G1G1 G2G2 G3G3 G4G4 A1A1 A2A2 A3A3 A4A4 Mapping π 1 A4A4 A3A3 A2A2 A1A1 Mapping π 2 A2A2 A1A1 A4A4 A3A3 Mapping π 3 A3A3 A4A4 A1A1 A2A2 Mapping π 4

8. 8 How permutation helps Non-matching collisions on adjacent channels C 1 and C 2 Subcarrier Group G1G1 G2G2 G3G3 G4G4 A1A1 A2A2 A3A3 A4A4 1 st transmission 2 nd transmission A4A4 A3A3 A2A2 A1A1 A2A2 A1A1 A4A4 A3A3 3 rd transmission A3A3 A4A4 A1A1 A2A2 4 th transmission

9. 9 9 How permutation helps (contd) Non-matching collisions on non-adjacent channels C 1 and C 3 Subcarrier Group G1G1 G2G2 G3G3 G4G4 A1A1 A2A2 A3A3 A4A4 1 st transmission 2 nd transmission A4A4 A3A3 A2A2 A1A1

10. 10 Remap basic idea: Matching-collision setting Collision! Alice Bob Collision! AP a on channel C a AP b on channel C b Matching collisions on adjacent channels

11. 11 Remap for matching collisions Matching collisions on adjacent channels C 1 and C 2 A1A1 A2A2 A3A3 A4A4 B5B5 B2B2 B3B3 B4B4 Subcarrier Group G1G1 G2G2 G3G3 G4G4 G5G5 A4A4 A3A3 A2A2 A1A1 B2B2 B5B5 B4B4 B3B3 G1G1 G2G2 G3G3 G4G4 G5G5 Decode A 1 Re-encode A 1 on G 4 Decoded bits: Subtract A 1 A1A1 Decode B 3 Re-encode B 3 on G 3 Subtract B 3 B3B3 Decode A 3 A3A3 Subtract A 3 Re-encode A 3 on G 2 Subtract A 3 Decode B 5 Subtract B 5 B5B5

12. 12 Remap for matching collisions: Decoding graph Decoding graph of collision at adjacent channels C 1 and C 2 A1A1 A1A1 B3B3 B3B3 A3A3 A3A3 B5B5 Re-encode Subtract A4A4 A4A4 B4B4 B4B4 A2A2 A2A2 B2B2 1 st collision 2 nd Collision 1 st collision 2 nd Collision

13. 13 Remap for matching collisions: a time-frequency view collisions at adjacent channels C 1 and C 2 : a time and frequency view PbPb 1 2 A1A1 A2 A2 A3A3 A4A4 S1S1 S2S2 SnSn Time Freq PaPa B5B5 B2 B2 B3B3 B4B4 A4A4 A3 A3 A2A2 A1A1 S1S1 S2S2 SnSn B2B2 B5 B5 B4B4 B3B3 P b P a G1G1 G3G3 G2G2 G5G5 G4G4 G2G2 1 5 9 13 4 10 14 3 7 11 2 6 8 12

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

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

16. 16 Remap Details (contd) 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. 17 Remap Details (contd) Loss of orthogonality Carrier frequency offset Desired symbol and interfering symbol unalignment Desired signal at subcarrier i: Interfering signal at subcarrier i+m: Interference energy: The energy is 19dB lower if m=4; 21dB lower if m=5

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

19. 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

20. 20 Evaluation (contd) 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

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

22. 22 Non-adjacent channel: scatter plot of second experiment Evaluation: non-adjacent channel (contd)

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

24. 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