Cross-layer Wireless
Bit Rate Adaptation
Mythili Vutukuru, Hari Balakrishnan, Kyle Jamieson (MIT CSAIL)
ACM SIGCOMM 2009
2009. 9. 16
Presenter: Soyoung Park
Slides provided courtesy of Mythili Vutukuru
Contents
 Bit rate adaptation
 Problems with existing algorithms
 SoftRate
• SoftPHY Hints
• Computing Interference-free BER
• Rate selection
 Evaluation
 Conclusion
2/25
Bit Rate Adaptation
 Varying modulation & coding, redundancy
 Huge gains possible
Requirements
Estimate channel
accurately
Responsive
Do not react
to interference
3/25
Existing Algorithms
Frame-based
 RRAA, Wong et. al., 2006.
 SampleRate, Bicket, 2005.
SNR-based
 RBAR, Holland et. al., 2001.
 CHARM, Judd et. al., 2008.
Data
Data
ACK
Estimate frame loss rate at
each bit rate
SNR using preamble
Lookup table
SNR  best rate
4/25
Problems With Existing Algorithms
Indoor
Channel
Quality
Outdoor
SNR-based
“SNR  bit rate” table specific to environment
Which table to use when?
X
Frame-based
Too slow
Reacts to interference
5/25
SoftRate: Key Insight
Channel
Quality
SoftPHY
Hints
Per-bit
Confidences
Estimate BER
SoftRate
Detect Interference
Interference-free BER
6/25
Computing SoftPHY Hints
TX
Bits
Encoder
Modulator
Coded bits
RX
Demodulator
Symbols
Symbols
Soft Output
Decoder
Decoder
Coded Bits
Error vectors
(received symbol – closest valid symbol)
Bits
SoftPHY
Hints
7/25
BER from SoftPHY Hints
Soft Output Viterbi
(or) BCJR decoder
decoded bits
log
Pr(correctly decoded)
Pr(incorrectly decoded)
Log Likelihood Ratio
SoftPHY hint of a bit
s =
Probability of bit error
p =
1-p
log p
1
1 + es
BER = Average p over all bits in the packet
8/25
SoftPHY Hints With Weak Signal
35
SoftPHY Hint
30
25
20
15
10
5
0
0
100
200
300
400
500
600
700
800
Bit position
9/25
Computing Interference-free BER
25
Compute Interference-free BER
SoftPHY Hint
20
15
10
Interference
5
0
0
500
1000
1500
2000
2500
3000
3500
4000
Bit position
10/25
The SoftRate Protocol
Data
Receiver
Interference-free BER
BER
Interference Detection
Sender
Pick rate with max throughput
SoftPHY Hints
11/25
Rate Selection at the Sender
BER
Throughput
24 Mbps
18 Mbps
10-7 10-6 10-5 10-4 10-3
12 Mbps
Adjacent rates have an order of
magnitude difference in BER
12/25
When is the current rate optimal?
BER
Throughput
24 Mbps
Optimality range for 18 Mbps
18 Mbps
10-7 10-6 10-5 10-4 10-3
12 Mbps
13/25
When to lower the rate?
BER
Throughput
24 Mbps
18 Mbps
10-7 10-6 10-5 10-4 10-3
12 Mbps
14/25
When to increase the rate?
BER
Throughput
24 Mbps
18 Mbps
10-7 10-6 10-5 10-4 10-3
12 Mbps
15/25
The SoftRate Protocol
Data
Receiver
Interference-free BER
BER
Interference Detection
Sender
Precompute optimality ranges
 If BER below optimality range,
increase rate.
 If above range, decrease rate.
 Otherwise, continue at current rate.
SoftPHY Hints
16/25
Evaluation Method
ns-3 simulations
SoftPHY Traces
PHY: GNU Radio + USRP
Experiments
Channel Simulator
17/25
Evaluation Topology
Testbed topology
Simulation topology
18/25
Evaluation Questions
SoftPHY
 Can SoftPHY hints estimate channel BER?
 Can SoftPHY hints identify interference? ~80%
SoftRate
 Gains of SoftRate in mobile channels?
 SoftRate robust to interference?
Almost always
19/25
SoftPHY Hints Predict BER
0.001
0.01
0.1
1
Actual BER
1
0.1
Expt, Walking
Sim, Walking
0.01
Sim, Train
0.001
BER Estimate from SoftPHY
20/25
Is SoftRate close to optimal?
(Walking Speed)
Aggregate Throughput
12
10
8
StaticBest
6
SoftRate
4
2
0
1
2
3
4
5
Number of clients
Within 10% of the optimal
21/25
SoftRate vs. Frame-based:
(Walking speed)
Aggregate Throughput
12
10
8
SoftRate
6
RRAA
SampleRate
4
2
0
1
2
3
4
5
Number of clients
Up to 2X over best frame-based algorithm
22/25
1
0.8
0.6
SoftRate
RBAR (Untrained)
0.4
0.2
0
1ms
(Walking speed)
500us
200us
Increasing
mobility
Channel
Coherence
Time
Normalized Throughput
SoftRate vs. SNR-based:
Varying Mobility
100us
(Train speed)
4X over untrained SNR-based algorithm
23/25
SNR vs. BER: Varying Mobility
0
5
10
15
Actual BER
1
0.1
Expt, Walking
Sim, Walking
Sim, Train
0.01
0.001
SNR (dB)
SNRBER and SNRbest rate
specific to operating environment
24/25
SoftRate: Summary
 Accurate, responsive, robust to collision losses
 Feedback: interference-free BER from SoftPHY hints
 2X over frame-based, 4X over untrained SNR-based
25/25