Download - Cross-Layer Wireless Bit Rate Adaptation
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Cross-Layer Wireless Bit Rate Adaptation
Sunghwa Son2011.09.28
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Introduction
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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
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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
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Related Work
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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.
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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?
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Design
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SoftRate: Key InsightChannelQuality
Per-bitConfidences
Estimate BER Detect Interference
Interference-free BER
SoftPHYHints
SoftRate
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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
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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)
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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
5
10
15
20
25
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
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SoftPHY Hints With Weak Signal
0
5
10
15
20
25
30
35
0 100 200 300 400 500 600 700 800
Bit position
So
ftP
HY
Hin
t
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The SoftRate ProtocolData
BERInterference Detection
Interference-free BER
SoftPHY HintsPick rate with max throughput
Receiver
Sender
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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
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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
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When to lower the rate?
10-4 10-310-510-610-7
BER Throughput
18 Mbps
12 Mbps
24 Mbps
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When to increase the rate?
10-4 10-310-510-610-7
BER Throughput
18 Mbps
12 Mbps
24 Mbps
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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
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Implementation
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Evaluation Method
Rate Adaptation (SoftRate, SNR-based, Frame-based)
TCP
Experiments
SoftPHY Traces
ns-3 simulations
Channel Simulator
PHY: GNU Radio + USRP
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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?
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SoftPHY Evaluation
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SoftPHY Hints Predict BER
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SNR vs. BER: Varying Mobil-ity
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Interference detection accu-racy: various bit rate
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Softrate Evaluation
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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
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Is SoftRate close to optimal? (Walking Speed)
0
2
4
6
8
10
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
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SoftRate vs. Frame-based: Walking speed
0
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
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SoftRate vs. SNR-based: Varying Mobility
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Rate selection accuracy
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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
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Conclusion
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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