AutoMAC : Rateless Wireless Concurrent Medium Access

AutoMAC : Rateless Wireless Concurrent Medium Access

Aditya Gudipati, Stephanie Pereira, Sachin KattiStanford University

Conventional Schemes : Avoid Interference at all costs !

Alice Bob

Conventional Schemes : Avoid Interference at all costs !

• Interference Avoidance– RTS/CTS– CSMA with exponential backoff

• Interference handling– ZigZag

RTSCTS

AliceRTSAlice Bob

CTSAlice

CTSAlice

Our Approach : Encourage and Exploit Interference !

Alice Bob

Alice Decoder

Bob DecoderSubtract

Our Approach : Encourage and Exploit Interference !

Alice Decoder

Bob DecoderSubtract

Alice Channel DecoderSignal = PNoise = Bob’s Power + N = P + NThroughput =

Bob Channel DecoderSignal = PNoise = NThroughput =

Received Power from Alice at AP = PReceived Power from Bob at AP = P

Our Approach vs Conventional Scheme

• Conventional Scheme

• Our Approach

• {Our Approach > Conventional Scheme}

Even on Downlink, Conventional Schemes avoid Interference

Alice Bob

P1 P2

P2 > P1

Our Approach on Downlink alsoExploits Interference !

Alice Bob

P1 P2

P2 > P1

Our Approach on Downlink also exploits Interference !

Received Power from AP at Alice = P1

Received Power from AP at Bob = P2

Alice Decoder

ALICE

P2 > P1

Alice Decoder

Bob DecoderSubtract

BOB

Our Approach on Downlink also exploits Interference !

Decode Bob’s PacketSignal = 0.5*P2

Noise = NThroughput =

Received Power from AP at Alice = P1

Received Power from AP at Bob = P2

BOB

P2 > P1

Decode Alice’s PacketSignal = 0.5*P1

Noise = Bob’s Power + N = 0.5*P1 + NThroughput =

ALICE

Alice Decoder

Alice Decoder

Bob DecoderSubtract

Our Approach vs Conventional Scheme

• Conventional Scheme

• Our Approach

• {Our Approach > Conventional Scheme} if P2 > P1

– Capacity function concave with power

AutoMAC Contributions

• Practical system for exploiting interference• Rateless encoding scheme– Upto 3 successful concurrent transmissions

• Novel MAC protocol – Leverage gains due to Interference

• Implemented on USRP2s and evaluated– 60% gain on Uplink– 35% gain on Downlink

Challenge 1 : Rate Adaptation

• Weak Channel for Alice– Alice SNR = – Bob SNR =

• More redundancy in Alice’s transmission– Need Rate adaptation

Alice Decoder

Bob DecoderSubtract


5 10 15 200

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Thresholding behaviour of channel codes

16-QAM 1/2coderate16-QAM 2/3coderate64-QAM 1/2coderate64-QAM 2/3coderate

SNR (in dB)

Deco

ding

Pro

babi

lity

Increasing Rates


• Alice needs to adapt its rate• Alice needs to figure out– Own SNR at the AP– Who is interfering?– How strong is the Interference ?

• Can we avoid this ?

Alice Decoder

Bob DecoderSubtract

Solution 1 : Rateless Codes

• Orthogonal to choice of rateless code• Strider [Sigcomm’11]– Encoder generates stream of transmissions– Receiver ACKs once decoded– SNR determines # transmissions– # transmissions determines rate

Solution 1 : Rateless Codes

5 7 9 11 13 15 17 190

1

2

3

4

5

6

7

8

9

Average Transmissions needed for Strider to decode

SNR (in dB)

Tran

smis

sion

s

Challenge 2 : Wasted Transmissions

• Weak Channel for Alice– More transmissions needed for Alice Decoder

• Strong Channel for Bob– Fewer transmissions needed for Bob Decoder

• Bob can’t be decoded before Alice !– Wastage

Alice Decoder

Bob DecoderSubtract

Challenge 2 : Wasted Transmissions

A1 + B1 A1 + B1 A1 + B1 A1 + B1 A1 + B1 A1 + B1

A1

Noisy B1

Noisy B1

Noisy B1

Noisy B1

Noisy B1

Noisy B1

B1

3 transmissions wasted !!

Alice Decoder

Bob DecoderSubtract

Alice Decoder needs 6 txBob Decoder needs 3 tx

Solution 2 : Speculative ACKing

A1 + B1 A1 + B1 A1 + B1 A1 + B2 A1 + B2 A1 + B2

A1

Noisy B1

Noisy B1

Noisy B1

Noisy B2

Noisy B2

Noisy B2

B1 No wastage !! B2

ACK

Alice Decoder

Bob DecoderSubtract

Alice Decoder needs 6 txBob Decoder needs 3 tx

MAC Protocol

• AP driven MAC– Frequency Domain Backoff [Sen et al Mobicom’11]

Contention Ad

Alice

Charlie

Bob

David

MAC Protocol


1-Alice 2 - Bob

Alice

Charlie

Bob

David

Freq

MAC Protocol


Alice

Charlie

Bob

David

Pre

Data

Data

Pre Data

Pre Pre

Evaluation

• Implement PHY layer on USRP2s– GNURadio platform with RawOFDM– OFDM with 64 subcarriers – External Clock to Synchronize USRP2s (Jackson)

• Upto 3 successful concurrent transmissions• Compared to omniscient rate adaptation – 30% gain on Uplink , 35% gain on Downlink

Uplink CDF

1.5 2 2.5 3 3.5 4 4.5 5 5.5 60

0.1

0.2

0.3

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0.5

0.6

0.7

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1

CDF of throughput on uplink

Omniscient AutoMAC

Throughput (b/s/Hz)

Cum

ulat

ive

Frac

tion

30% median throughput gain

Downlink CDF

0 1 2 3 4 5 60

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

CDF of throughput on downlink

Omniscient AutoMAC

Throughput (b/s/Hz)

Cum

ulat

ive

Frac

tion

35% median throughput gain

Downlink gains depend on Relative SNRs

-1 1 3 5 7 9 11 13 151

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

Downlink gains from AutoMAC as a function of Relative SNR

Relative SNR (dB)

Rel

ativ

e do

wnl

ink

gain

s of

Aut

oMA

C o

ver

Om

nisc

ient

Interference Cancellation isn’t Perfect !

10 12 14 16 18 20 22 240

1

2

3

4

5

6

Drop in weaker signal SNR after cancellation

Stronger signal SNR (dB)

Drop

in w

eake

r sig

nal S

NR (d

B)

Simulations

• Simulate MAC layer– Dense Network (8 contending nodes)– Fully Loaded

• 60% uplink gain over 802.11 MAC– Improved PHY layer– Efficient channel utilization at MAC layer

Conclusion

• Exploit interference instead of avoiding it• Novel PHY & MAC protocol– Rateless encoding scheme enables SIC– AP driven MAC coordinates interferers

• Implemented on USRP2s and evaluated• Future Work– Other applications of SIC , eg. MIMO systems

AutoMAC : Rateless Wireless Concurrent Medium Access

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