An efficient channel coding scheme for direct sequence CDMA systems

The author introduces an efficient modulation/coding scheme for direct-sequence (DS) spread-spectrum (SS) code division multiple access (CDMA) networks. The transmitter is allowed to choose between multiple spreading sequences, thus adding redundancy to the transmitted bit stream as well as providing multi-user interference rejection. The multiple code sequences assigned to each user are taken to be cyclic shifts of a single PN (pseudonoise) sequence. The resulting signal set is a multidimensional biorthogonal constellation. A simple receiver is presented for demodulating this signal set, and the performance of the system in the presence of multiple users is given. This uncoded system is found to have performance which is about equivalent to a two-dimensional signaling scheme with a significant amount of coding. A class of trellis codes is then given which gives an additional 3-dB coding gain (asymptotic). The system is found to give a 1.2-dB improvement as compared to previously reported results with about the same complexity