Improving Loss Resilience with Multi-Radio Diversity in Wireless Networks

Allen Miu, Hari BalakrishnanMIT Computer Science and Artificial Intelligence

Laboratory

C. Emre KoksalComputer 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, etc…

Inconsistent and poor performance

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Current solutions are inefficient for recovering losses Coding (e.g., FEC)

Hard on highly variable channels Retransmission

Wasteful: outage durations can be long (tens to hundreds of milliseconds)

Bit-rate adaptation Hard on variable channels Slows down other clients

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May use only one path Uses only one communication path

Today’s wireless LAN (e.g., 802.11)

AP1

Internet

5

May use only one path Allow multiple APs to simultaneously receive transmissions from a single transmitter

Today’s wireless LAN (e.g., 802.11)

AP1

AP2

Multi-Radio Diversity (MRD) – Uplink

Internet

MRDC

10%

20%

Loss independence simultaneous loss = 2%

6

Multi-Radio Diversity (MRD) –Downlink

Internet

AP1

AP2

MRDC

Allow multiple client radios to simultaneously receive transmissions from a single transmitter

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)

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Individual FLR > Simultaneous FLR

FLR(R1 ∩ R2)

FL

R

y = x

R1R2R1*R2

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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

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2. Select bit combination at unmatched bit locations, check CRC

Patterns CRC Ok

Bit-by-bit frame combining

1100 0000 R1

1101 1010 R2

0001 1010

1. Locate bits withunmatched value

1100 0000 --1100 0010 X1100 1000 X1100 1010 O Corrected frame

TX: 1100 1010

Problem: Exponential # of CRC checks in # of unmatched bits.

Combine failure

1

1

0

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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

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1.0Failure decreases with NB and burst size

Burst error length parameter

Pro

bab

ility

of

failu

re

NB = 2

NB = 4NB = 6

NB = 16

…

0.8

0.6

0.4

0.2

00 10 20 30 40 50

Frame size = 1500B

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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

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Request-for-acknowledgment (RFA) for efficient feedback

link

IP

link

MRDC

link

MRD MRDDATA

ACK

DATA RFA

MRD-ACK

DATAIP

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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.

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Experimental setup

~20 m

R1R2

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)

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MRD improves throughput

2.3x Improvement

R1 R2 MRD

Th

rou

gh

pu

t (M

bp

s)

8.25 Mbps

18.7 Mbps

Each color shows a different trial

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MRD maintains high bit-rate

Frame recovery data(% of total losses at R1)via R2 42.3%frame combining 7.3%Total 49.6%

Selected bit rate (Mbps)

Fra

ctio

n of

tra

nsm

itted

fr

ames

60

9 12 18 24 36 48 56

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Delay Analysis

One way delay (10x s)

Fra

ctio

n of

del

iver

ed

pack

ets

10-310-4

010-2 10-1 1

User space implementation caused high delay

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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)

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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