Cross-Layer Wireless Bit Rate Adaptation Sunghwa Son 2011.09.28 Introduction Time-varying wireless channel Large-scale attenuation SNR (dB)

30 Due to changing distance Interference Unpredictable Bit errors, frame losses 15 10 5

0 2000 4000 10 s 6000 8000 10000 1700

1750 Time (Milliseconds) 25 SNR (dB) Due to multipath 20 0 Small-scale fading

25 20 15 10 5 0 1500 1550 1600 1650

250 ms Time (Milliseconds) Online Bit Rate Adaptation Varying modulation & coding (redundancy) Better channel Higher rate Huge gains possible (802.11g has 1-54 Mbps) Requirements Estimate channel accurately Responsive

Do not react to interference Related Work 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 Problems With Existing Algorith ms Indoor Channel Quality Outdoor SNR-based SNR bit rate table specific to environment

Which table to use when? X rame-based Too slow Reacts to interference Design SoftRate: Key Insight Channel Quality

SoftPHY Hints Per-bit Confidences Estimate BER SoftRate Detect Interference Interference-free BER 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 TX Encoder Bits RX Modulator Demodulator

Symbols Symbols Coded bits Soft Output Decoder Decoder Coded Bits Error vectors (rcvd symbol closest valid symbol)

Bits SoftPHY Hints BER from SoftPHY Hints Soft Output Viterbi (or) BCJR decoder decoded bits log For linear block or convolutional code 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 Computing Interference-free B ER 25 Compute Interference-free BER SoftPHY Hint 20 15 10

Interferenc e 5 0 0 500 1000 1500 2000

2500 Bit position 3000 3500 4000 SoftPHY Hints With Weak Sig nal 35 SoftPHY Hint

30 25 20 15 10 5 0 0 100 200 300

400 Bit position 500 600 700 800 The SoftRate Protocol Data Receiver Interference-free BER

BER Sender Pick rate with max throughput Interference Detection SoftPHY Hints Rate Selection at the Sender BER 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 Throughput When is the current rate optim al? BER 24 Mbps Optimality range for 18 Mbps 18 Mbps

12 Mbps 10-7 10-6 10-5 10-4 10-3 Throughput When to lower the rate? BER 24 Mbps 18 Mbps 12 Mbps 10-7 10-6 10-5 10-4 10-3

Throughput When to increase the rate? BER 24 Mbps 18 Mbps 12 Mbps 10-7 10-6 10-5 10-4 10-3 Throughput

The SoftRate Protocol Data Receiver Interference-free BER BER Sender Precompute optimality ranges If BER below optimality range, increase rate. If above range, decrease rate. Otherwise, continue at current rate. Interference Detection

SoftPHY Hints Implementation Evaluation Method TCP ns-3 simulations Rate Adaptation (SoftRate, SNR-based, Frame-based) SoftPHY Traces PHY: GNU Radio + USRP Experiments

Channel Simulator 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 Mobilit y

Interference detection accur acy: various bit rate Softrate Evaluation SoftRate Evaluation in Mobile Channels Compare with StaticBest: omniscientbest for each p kt SNR-based: RBAR and CHARM Frame-based: RRAA and SampleRate TCP AP Clients

Wired LAN Traces Is SoftRate close to optimal? (Walking Speed) Aggregate Throughput 12 10 8 StaticBest 6

SoftRate 4 2 0 1 2 3 Number of clients Within 10% of the optimal 4

5 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

SoftRate vs. SNR-based: Varying Mobility Rate selection accuracy Evaluation Answers SoftPHY YES Can SoftPHY hints estimate channel BER? ~80% of Can SoftPHY hints identify interference? the time SoftRate 2X - 4X

Gains of SoftRate in mobile channels? SoftRate robust to interference? Almost always Conclusion SoftRate: Summary Accurate, responsive, robust to collision losses Feedback: interference-free BER from SoftPHY hin ts 2X over frame-based, 4X over untrained SNR-bas ed Looking ahead