Bandwidth-Efficient Method for Adaptive Forward Error Correction on Wireless Local Area Network
description
Transcript of Bandwidth-Efficient Method for Adaptive Forward Error Correction on Wireless Local Area Network
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