1 retransmission repeat: simple retransmission permutation can resolve overlapping channel...
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Retransmission ≠ Repeat: Simple Retransmission Permutation Can
Resolve Overlapping Channel Collisions
Li (Erran) LiBell 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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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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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]
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802.11g overlapping channel collision problem
Bob
APa on channel Ca
Collision!
Alice
APb on channel Cb
Collision!
Chuck
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802.11g overlapping channel collision problem
Bob
APa on channel Ca
More Collision!
Alice
APb on channel Cb
More Collision!
Chuck
Retransmission
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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
A1 A2 A3 A4
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Remap basic idea: structured permutation
Subcarrier Group
G1 G2 G3 G4
A1 A2 A3 A4Mapping π1
A4 A3 A2 A1Mapping π2
A2 A1 A4 A3Mapping π3
A3 A4 A1 A2Mapping π4
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How permutation helps
Non-matching collisions on adjacent channels C1 and C2
Subcarrier Group
G1 G2 G3 G4
A1 A2 A3 A41st transmission
2nd transmission A4 A3 A2 A1
A2 A1 A4 A33rd transmission
A3 A4 A1 A24th transmission
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How permutation helps (cont’d)
Non-matching collisions on non-adjacent channels C1 and C3
Subcarrier Group
G1 G2 G3 G4
A1 A2 A3 A41st transmission
2nd transmission A4 A3 A2 A1
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Remap basic idea: Matching-collision setting
Collision!
Alice Bob
Collision!
APa on channel Ca
APb on channel Cb
Matching collisions on adjacent channels
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Remap for matching collisions
Matching collisions on adjacent channels C1 and C2
A1 A2 A3 A4
B5B2 B3 B4
Subcarrier Group
G1 G2 G3 G4
G5A4 A3 A2 A1
B2B5 B4 B3
G1 G2 G3 G4
G5
Decode A1 Re-encode A1 on G4
Decoded bits:
Subtract A1Subtract A1
A1
Decode B3 Re-encode B3 on G3
Subtract B3 Subtract B3
B3
Decode A3
A3
Subtract A3 Re-encode A3 on G2
Subtract A3
Decode B5 Subtract B5
B5
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Remap for matching collisions: Decoding graph
Decoding graph of collision at adjacent channels C1 and C2
A1A1
B3B3
A3A3
B5
Re-encode
Subtract
A4A4
B4B4
A2A2
B2
1st collision 2nd Collision 1st collision 2nd Collision
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Remap for matching collisions: a time-frequency view
collisions at adjacent channels C1 and C2 : a time and frequency view
Pb
∆1∆2
A1 A2
A3
A4
S1 S2Sn
Time
Freq
Pa
B5
B2
B3
B4
A4 A3
A2
A1
S1 S2Sn
B2
B5
B4
B3
P′b
P′a
G1
G3
G2
G5
G4
G2
1
59
13
410
14
3
711
2
68
12
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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.
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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
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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.
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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:
Interference energy:
The energy is 19dB lower if m=4;
21dB lower if m=5
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Remap Details (cont’d)
Techniques dealing with loss of orthogonality Iterative interference cancellation Exploiting uneven interference of subcarriers
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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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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
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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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Non-adjacent channel: scatter plot of second experiment
Evaluation: non-adjacent channel (cont’d)
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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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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