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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)

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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

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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]

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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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.

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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

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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.

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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

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

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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

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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

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

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

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

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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

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