Research and Centers

Title: Coding Schemes for Asynchronous CDMA Systems

Location: Thursday November 5th from 1-2pm in the Johnson Center, Dewberry Hall North.

Abstract: Novel coding schemes for asynchronous code-division multiple-access (A-CDMA) systems are presented in this talk. The schemes use Wu-Chang N-user multiple-access spreading code; block interleaving and delay-tolerant encoding to support CDMA communications with random delays.

For the noiseless case, each active user generates one of M messages, M= (2T−D−1)/(D+1) , and a (T-D)-stage MLSR encoder encodes the message into a codeword of length T. Given the maximum random delay of D chips, the receiver can decode messages of all active users without ambiguity. The code rate R of the MLSR code is approximately equal to (1 ¨C D / T) - log2 (D + 1) / T.

The talk will discuss the noisy case briefly. For the noisy case, Bose-Caldwell¡¯s extended cyclic codes are used to encode messages. The extended cyclic codes will provide error correction and delay recovery effectively. By using a BCH code of length n as the cyclic code, the extended code can correct up to ¦Ó ¡Ü 𝑛/4 errors and recover maximum random delay of n-1 chips.





Bio: Chang received his Ph.D. in Electrical Engineering from University of Hawaii in 1977. He taught at National Tsing Hua University in Taiwan, University of Massachusetts at Amherst, and George Washington University before joining the Faculty of George Mason University as an associate professor in 1984. During the 1984-1985 academic year, he helped ECE department setting up a well structured graduate program in communications and networking. Chang was co-inventor of Chang-Weldon code for MAAC, Chang-Wolf code for M-ary FSK MAC, Wu-Chang spreading code for synchronous CDMA, Wu-Chang coding schemes for asynchronous CDMA, Echard-Chang classic ¦Ð-rotation LDPC code, Echard-Chang multi-rate extended irregular ¦Ð-rotation LDPC code, Behairy-Chang turbo-like parallel-concatenated LDPC code without interleaver. The Echard-Chang classic ¦Ð-rotation code was proposed to IEEE as advanced coding for 802.11n wireless standards by Nortel in 2004.

Posted on: October 28, 2009