Cross-Layer Wireless Bit Rate Adaptation

Sunghwa Son2011.09.28

Introduction

Time-varying wireless chan-nel

Large-scale attenua-tion• Due to changing dis-

tance

Small-scale fading• Due to multipath

Interference• Unpredictable

0

5

10

15

20

25

30

0 2000 4000 6000 8000 10000

Time (Milliseconds)

SN

R (

dB

)

0

5

10

15

20

25

1500 1550 1600 1650 1700 1750

Time (Milliseconds)

SN

R (

dB

)

Bit errors, frame losses

10 s

250 ms

Online Bit Rate Adaptation Varying modulation & coding

(redundancy) Better channel Higher rate Huge gains possible (802.11g has 1-54

Mbps)

Estimate channel accurately Responsive

Do not react to interference

Requirements

Related Work

Existing Algorithms

• RRAA, Wong et. al., 2006.• SampleRate, Bicket, 2005.

SNR-basedFrame-based

Estimate frame loss rate at each bit rate

Data

ACK

Data

Lookup table SNR best rate

SNR using preamble

• RBAR, Holland et. al., 2001.

• CHARM, Judd et. al., 2008.

Problems With Existing Algo-rithms

ChannelQuality

SNR-based

Frame-based X

“SNR bit rate” table specific to environment

Reacts to interferenceToo slow

Indoor

Outdoor

Which table to use when?

Design

SoftRate: Key InsightChannelQuality

Per-bitConfidences

Estimate BER Detect Interference

Interference-free BER

SoftPHYHints

SoftRate

SoftRate: Contributions

• Adapts to channel accurately and quickly

• Robust to collision losses

• Feedback: interference-free BER from SoftPHY

hints

• 2X gain over frame-based and SNR-based

Computing SoftPHY Hints

SoftPHYHints

Encoder Modulator

Bits Coded bits Symbols

TX

Demodulator Decoder

Symbols Coded Bits Bits

RX Soft OutputDecoder

Error vectors(rcvd symbol – closest valid symbol)

BER from SoftPHY Hints

logPr(correctly decoded)

Pr(incorrectly decoded)

1-pplog=s

p1

1 + es

Soft Output Viterbi(or) BCJR decoder

decoded bits

=

SoftPHY hint of a bit

Probability of bit error

BER = Average p over all bits in the packet

Log Likelihood Ratio

For linear block or convolutional code

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0 500 1000 1500 2000 2500 3000 3500 4000

Bit position

So

ftP

HY

Hin

t

Computing Interference-free BER

Compute Interference-free BER

Interfer-ence

SoftPHY Hints With Weak Signal

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5

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0 100 200 300 400 500 600 700 800

Bit position

So

ftP

HY

Hin

t

The SoftRate ProtocolData

BERInterference Detection

Interference-free BER

SoftPHY HintsPick rate with max throughput

Receiver

Sender

Rate Selection at the Sender

10-4 10-310-510-610-7

BER Throughput

18 Mbps

12 Mbps

24 Mbps

Adjacent rates have an order of magnitude difference in BER

When is the current rate opti-mal?

10-4 10-310-510-610-7

BER Throughput

18 Mbps

12 Mbps

24 MbpsOptimality range for 18 Mbps

When to lower the rate?

10-4 10-310-510-610-7

BER Throughput

18 Mbps

12 Mbps

24 Mbps

When to increase the rate?

10-4 10-310-510-610-7

BER Throughput

18 Mbps

12 Mbps

24 Mbps

The SoftRate ProtocolData

Interference Detection

Interference-free BER

SoftPHY Hints

Receiver

SenderPrecompute optimality ranges If BER below optimality range,

increase rate. If above range, decrease rate. Otherwise, continue at current rate.

BER

Implementation

Evaluation Method

Rate Adaptation (SoftRate, SNR-based, Frame-based)

TCP

Experiments

SoftPHY Traces

ns-3 simulations

Channel Simulator

PHY: GNU Radio + USRP

Evaluation Questions

SoftPHY

• Can SoftPHY hints estimate channel BER?

• Can SoftPHY hints identify interference?

SoftRate

• Gains of SoftRate in mobile channels?

• SoftRate robust to interference?

SoftPHY Evaluation

SoftPHY Hints Predict BER

SNR vs. BER: Varying Mobil-ity

Interference detection accu-racy: various bit rate

Softrate Evaluation

SoftRate Evaluation in Mobile Channels

• Compare with– StaticBest: omniscient—best for each

pkt– SNR-based: RBAR and CHARM– Frame-based: RRAA and SampleRate

Traces

TCP

Clients

AP

WiredLAN

Is SoftRate close to optimal? (Walking Speed)

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12

1 2 3 4 5

Number of clients

Ag

gre

gat

e T

hro

ug

hp

ut

StaticBest

SoftRate

Within 10% of the optimal

SoftRate vs. Frame-based: Walking speed

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2

4

6

8

10

12

1 2 3 4 5

Number of clients

Ag

gre

gat

e T

hro

ug

hp

ut

SoftRate

RRAA

SampleRate

Up to 2X over best frame-based algorithm

SoftRate vs. SNR-based: Varying Mobility

Rate selection accuracy

Evaluation Answers

SoftPHY

• Can SoftPHY hints estimate channel BER?

• Can SoftPHY hints identify interference?

SoftRate

• Gains of SoftRate in mobile channels?

• SoftRate robust to interference?

~80% of the time

Almost always

YES

2X - 4X

Conclusion

SoftRate: Summary

• Accurate, responsive, robust to collision losses

• Feedback: interference-free BER from SoftPHY

hints

• 2X over frame-based, 4X over untrained SNR-

based

Looking ahead

• BER computation from SoftPHY hints useful for

other cross-layer protocols