Bandwidth-Efficient Method for Adaptive Forward Error Correction on Wireless Local Area Network  Co-Presenters: David R. Pollard, Graduate Student, Eastern Illinois UniversityDavid R. Pollard, Graduate Student, Eastern Illinois University Dr. Ping Liu, Graduate Coordinator, Eastern Illinois UniversityDr. Ping Liu, Graduate Coordinator, Eastern Illinois University

Mobile Collaborative Computing

Audio Quality

Bandwidth Must Be Optimized

Spread Spectrum

Error Detection versus Error Correction  Retransmission of data packets: ARQ  Correction of data packets without retransmission: FEC

Block Code Retransmissions ( ARQ Required for ‘detection’ codes ) Inadequate for wireless applications. Inadequate for wireless applications. Bit error rate (BER) high on wireless link. Propagation delay is very long. (Especially, satellites)

Bandwidth Must Be Optimized

Block Error Codes when used for ‘correction’ (n-k) block code k = data bitsn = codewords in bits v codewords = f(v databits ) 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 correction and conserves bandwidth?  Dr. Philip McKinley, Pavilion Project, Michigan State University:  “The best bandwidth-efficient method for FEC on wireless LANs is one which is dynamically adaptive to channel loss behavior.”

Redundancy for Correction Determined by a Proactive Parameter: Alpha  For each group n: k(1+alpha) sent by proxy  When a receiver loses < alpha(k), recovers locally.  When a receiver loses > alpha(k), NAK sent to proxy for k(1+alpha) retransmission.

Packets Required by a Specific Receiver

Alpha Increase ά inc is based on observation of NAK behavior for each group. ά inc = = M * L/k L = requested parity packets M = small integer

Alpha Decrease  ά dec prevents high ά  In the absence of NAKs, ά is reduced until one or more NAKs are received.  ά inc resumes

Objective for ά For optimal audio signals and most efficient use of bandwidth in collaborative sessions, keep k(1+alpha) slightly higher than needed.

Conclusion Unlimited wants for mobile collaborative computing. Limited bandwidth for excellent quality audio transmissions.

Conclusion (continued) Error correction using ARQ insufficient for wireless communications. Static forward error correction requires high usage of bandwidth.

Best Method: Spread Spectrum and Adaptive Forward Error Correction