Multicarrier orthogonal CDMA signals for quasi-synchronous communication systems

We propose a multi-carrier orthogonal CDMA signaling scheme for a multiple-access communication system, such as the reverse channel of a cellular network, as an alternative to the multiuser interference cancellation approach. The average variance of cross-correlations between sequences is used as a measure for sequence design. We search for sets of sequences that minimize the probability of symbol detection error, given that there is imperfect synchronization among the signals, that is, the signals are quasi-synchronous. Orthogonal sequences based on the Sylvester-type Hadamard matrices (Walsh functions) are shown to provide a signiicant improvement over the case where a Hadamard (orthogonal) matrix is chosen at random. Computer searches suggest that this set of codes is optimal with respect to the above measure. The issue of chip pulse shaping is investigated. Optimal pulses designed to minimize multiple-access interference in quasi-synchronous systems are obtained for various bandwidths and are shown to provide a large improvement over the raised cosine pulses. A multi-carrier signaling scheme is introduced in order to reduce chip level synchronization oosets between the users.