Coded CDMA systems with and without MMSE multiuser equalizer

We study the performance of coded symbol-synchronous CDMA systems on AWGN channels with and without linear minimum mean square error (MMSE) receiver. Two systems, one with the MMSE receiver and one without, are considered. In the first system, each user uses a fixed chip pattern as the signature sequence to modulate all its coded symbols and the receiver consists of a MMSE receiver followed by the soft-decision Viterbi decoder. In the second system, each user modulate its coded symbol using a random signature sequence and a single-user matched filter is used before the soft-decision Viterbi decoder. We find that using the MMSE receiver does not always lead to performance improvement because of the constraint to use the same signature sequence for every coded symbol. Our numerical examples show that it is only advantages to use the MMSE receiver when the total processing gain is large enough. For systems with small processing gain, time-varying randomized signature sequence provides better averaging and it outweighs the gains from the MMSE receiver. We also find that whether the MMSE receiver should be used or not is not very sensitive to the bandwidth efficiency of the system