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Improving Loss Resilience with Multi-Radio Diversity in Wireless Networks

This paper explores using multi-radio diversity to combat inconsistent wireless loss issues. It presents strategies for frame recovery, acknowledges challenges, and proposes MRD-aware rate adaptation. Experimental evaluations show improved throughput by up to 2.3x.

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Improving Loss Resilience with Multi-Radio Diversity in Wireless Networks

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  1. Improving Loss Resilience with Multi-Radio Diversity in Wireless Networks Allen Miu, Hari Balakrishnan MIT Computer Science and Artificial Intelligence Laboratory C. Emre Koksal Computer and Communication Sciences, EPFL

  2. The problem • New wireless applications demand high performance • But: wireless channels are loss-prone • Interference • Noise • Attenuation • Multi-path • Mobility  Inconsistent and poor performance

  3. Main Idea of This Paper • Given that losses are often path-dependent, location-dependent, but statistically independent between different receiving radios, multiple radios that receive versions of the same frame may work together to correctly receover a frame.

  4. Internet Today’s wireless LAN (e.g., 802.11) May use only one path • Uses only one communication path AP1

  5. MRDC Internet AP2 Multi-Radio Diversity (MRD) – Uplink Today’s wireless LAN (e.g., 802.11) May use only one path • Allow multiple APs to simultaneously receive transmissions from a single transmitter 10% AP1 20% Loss independence  simultaneous loss = 2%

  6. MRDC Internet Multi-Radio Diversity (MRD) –Downlink • Allow multiple client radios to simultaneously receive transmissions from a single transmitter AP1 AP2

  7. Are losses independent among receivers? • Broadcast 802.11 experiment at fixed bit-rate: 6 simultaneous receivers and 1 transmitter • Compute loss rates for the 15 receiver-pair (R1, R2) combinations • Frame loss rate FLR(R1), FLR(R2) vs. simultaneous frame loss rate FLR(R1 ∩ R2)

  8. Individual FLR > Simultaneous FLR y = x FLR R1 R2 R1*R2 FLR(R1 ∩ R2)

  9. Challenges in developing MRD • How to correct simultaneous frame errors? • Frame combining • How to handle retransmissions in MRD? • Request-for-acknowledgment protocol • How to adapt bit rates in MRD? • MRD-aware rate adaptation

  10. Patterns CRC Ok 1100 0000 1 R1 1100 0000 -- 1100 0010 X 1101 1010 R2 1 1100 1000 X 0001 1010 1100 1010 0 1. Locate bits with unmatched value 2. Select bit combination at unmatched bit locations, check CRC O Corrected frame Bit-by-bit frame combining TX: 1100 1010 Combine failure Problem: Exponential # of CRC checks in # of unmatched bits.

  11. Block-based frame combining • Observation: bit errors occur in bursts • Divide frame into NB blocks (e.g., NB = 6) • Attempt recombination with all possible block patterns until CRC passes • # of checks upper bounded by 2NB • Failure rate increases with NB

  12. Failure decreases with NB and burst size 1.0 Frame size = 1500B Probability of failure 0.8 0.6 NB = 2 0.4 NB = 4 0.2 NB = 6 … NB = 16 0 10 20 30 40 50 0 Burst error length parameter

  13. Flawed retransmission schemes • Conventional link-layer ACKs do not work • Final status known only to MRDC • Two levels of ACKs are redundant • Cannot disable link-layer ACKs

  14. DATA RFA DATA DATA MRD-ACK ACK Request-for-acknowledgment (RFA) for efficient feedback IP IP MRD MRD link link link MRDC

  15. MRD-aware rate adaptation • Standard rate adaptation does not work • Reacts only to link-layer losses from 1 receiver • Uses sub-optimal bit-rates • MRD-aware rate adaptation • Reacts to losses at the MRD-layer Implication: First use multiple paths, then adapt bit rates.

  16. Experimental setup ~20 m R2 R1 L • 802.11a/b/g implementation in Linux (MADWiFi) • L transmits 100,000 1,500B UDP packets w/ 7 retries • 802.11a @ auto bit rate (6, 9, 12, 18, 24, 36, 48, 54) • L is in motion at walking speed, > 1 minute per trial • Variants: R1, R2, MRD (5 trials each)

  17. 18.7 Mbps 2.3x Improvement 8.25 Mbps MRD improves throughput Throughput (Mbps) R1 R2 MRD Each color shows a different trial

  18. MRD maintains high bit-rate Fraction of transmitted frames Frame recovery data (% of total losses at R1) via R2 42.3% frame combining 7.3% Total 49.6% 0 6 9 12 18 24 36 48 56 Selected bit rate (Mbps)

  19. Delay Analysis Fraction of delivered packets User space implementation caused high delay 0 10-3 10-2 10-1 1 10-4 One way delay (10x s)

  20. Related work • Physical layer spatial diversity techniques • Antenna diversity • 802.16 MIMO/802.11n • Macro-diversity in CDMA networks • Retransmission with memory [Sindhu ’77] • Opportunistic forwarding [Biswas ‘05][Jain ’05] • Bit rate selection (AARF, RBAR, MiSer, OAR, Sample Rate)

  21. Summary • Design of Multi-Radio Diversity WLAN • Block-based frame combining • Request-for-acknowledgment protocol • MRD-aware rate adaptation • Analysis of block-based frame combining • Experimental evaluation • MRD reduces losses by 50% and improves throughput by up to 2.3x

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