Bandwidth-Efficient Method for Adaptive Bandwidth-Efficient Method for Adaptive Forward Error Correction on Wireless Local Forward Error Correction on Wireless Local

Area NetworkArea Network

Co-Presenters:Co-Presenters:

• David R. Pollard, Graduate Student, David R. Pollard, Graduate Student, Eastern Illinois UniversityEastern Illinois University

• Dr. Ping Liu, Graduate Coordinator, Dr. Ping Liu, Graduate Coordinator, Eastern Illinois UniversityEastern Illinois University

Mobile Collaborative Mobile Collaborative ComputingComputing

Audio QualityAudio Quality

Bandwidth Must Be Bandwidth Must Be OptimizedOptimized

Spread SpectrumSpread Spectrum

Error Detection versus Error Error Detection versus Error CorrectionCorrection

Retransmission of data packets: ARQRetransmission of data packets: ARQ

Correction of data packets without Correction of data packets without retransmission: FECretransmission: FEC

Block Code Block Code RetransmissionsRetransmissions

((ARQ Required for ‘detection’ ARQ Required for ‘detection’ codescodes))

Inadequate for wireless applications.Inadequate for wireless applications.

Bit error rate (BER) high on Bit error rate (BER) high on wireless link.wireless link.

Propagation delay is very long. Propagation delay is very long. (Especially, (Especially, satellites)satellites)

Bandwidth Must Be OptimizedBandwidth Must Be Optimized

Block Error Codeswhen used for ‘correction’

(n-k) block code

k = data bits n = codewords in bits

vcodewords = f(vdatabits) where v is a vector

How much bandwidth is required for block code ‘correction’?

redundancy of the code = (n-k)/k

code rate = k/n

code rate of ½ = twice the bandwidth

How much bandwidth is required for block code ‘correction’? (Another example)

code rate of 2/5 = 2.5 times the bandwidth

if data rate = 1 Mbps then, output from the encoder must be 2.5 Mbps to keep up

Convolutional Codes for Correction

Generates redundant bits continuously.

Error checking and correcting continuously.

Better for collaborative computing and excellent audio quality.

What method provides the best What method provides the best correction and conserves correction and conserves

bandwidth?bandwidth?

Dr. Philip McKinleyDr. Philip McKinley, Pavilion Project, Michigan State , Pavilion Project, Michigan State University:University:

““The best bandwidth-efficient method for FEC on The best bandwidth-efficient method for FEC on

wireless LANs is one which is dynamically wireless LANs is one which is dynamically

adaptive to channel loss behavior.”adaptive to channel loss behavior.”

Redundancy for Correction Determined by a Redundancy for Correction Determined by a Proactive Parameter: AlphaProactive Parameter: Alpha

For each group n: k(1+alpha) sent by For each group n: k(1+alpha) sent by proxyproxy

When a receiver loses < alpha(k), When a receiver loses < alpha(k), recovers locally.recovers locally.

When a receiver loses > alpha(k), NAK When a receiver loses > alpha(k), NAK sent to proxy for k(1+alpha) sent to proxy for k(1+alpha) retransmission.retransmission.

Packets Required by Packets Required by a Specific Receivera Specific Receiver

Alpha IncreaseAlpha Increase

άάincinc is based on observation of NAK is based on observation of NAK

behavior for each group. behavior for each group.

άάinc = inc = = M * L/k = M * L/k

L = requested parity packetsL = requested parity packets

M = small integerM = small integer

Alpha DecreaseAlpha Decrease

άάdecdec prevents high prevents high άά

In the absence of NAKs, In the absence of NAKs, άά is is reduced until one or more NAKs reduced until one or more NAKs are received.are received.

άάincinc resumes resumes

Objective for Objective for άά

For optimal audio signals and most For optimal audio signals and most

efficient use of bandwidth in collaborative efficient use of bandwidth in collaborative

sessions, keep k(1+alpha) slightly higher sessions, keep k(1+alpha) slightly higher

than needed.than needed.

ConclusionConclusion

Unlimited wants for mobile Unlimited wants for mobile collaborative computing.collaborative computing.

Limited bandwidth for excellent Limited bandwidth for excellent quality audio transmissions.quality audio transmissions.

Conclusion Conclusion (continued)(continued)

Error correction using ARQ Error correction using ARQ insufficient for wireless insufficient for wireless communications.communications.

Static forward error correction Static forward error correction requires high usage of bandwidth.requires high usage of bandwidth.

Best Method: Best Method: Spread Spectrum and Adaptive Forward Error Spread Spectrum and Adaptive Forward Error

CorrectionCorrection