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.
Linear receivers for the DS-CDMA downlink exploiting orthogonality of spreading sequences
ASILOMAR 1998, 32nd IEEE Annual Asilomar Conference on Signals, Systems and Computers, November 2-5, 1998, Pacific Grove, USA
Systèmes de Communication
© 1998 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.
PERMALINK : https://www.eurecom.fr/publication/195