cross-layer wireless bit rate adaptation
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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
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5
10
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0 2000 4000 6000 8000 10000
Time (Milliseconds)
SN
R (
dB
)
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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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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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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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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
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