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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