Linear receivers for the DS-CDMA downlink exploiting orthogonality of spreading sequences

We address the problem of downlink interference rejection in a DS-CDMA system. Periodic orthogonal Walsh-Hadamard sequences spread different users´ symbols followed by scrambling by a symbol aperiodic base-station specific overlay sequence. This corresponds to the downlink of the European UMTS wideband CDMA proposal. The point to point propagation channel from the cell-site to a certain mobile station is the same for all downlink signals (desired user as well as the interference). The composite channel is shorter than a symbol period for some user signals, while other users can have significant ISI owing to a faster transmission rate. In any case, orthogonality of the underlying Walsh-Hadamard sequences is destroyed by multipath propagation, resulting in multiuser interference if a coherent combiner (the RAKE receiver) is employed. We propose linear zero-forcing (ZF) and minimum mean-squared-error (MMSE) receivers which equalize for the estimated channel, thus rendering the user signals orthogonal again. A simple code matched filter subsequently suffices to cancel the multiple access interference (MAI) from intracell users.