Optimal space-time block codes for CDMA systems

Space-time block codes have been well-addressed for single-user systems in recent works, the counterpart for CDMA systems will be discussed in this paper. A general system model is presented, the Chernoff bound for quasi-static channel is derived. Based on the Chernoff bound, performance criteria are obtained: diversity gain and coding grain. The coding gain is shown to be the same coding gain for a single-user system plus the effect of spreading, which essentially provides more coding gain than a narrowband system. Optimal 2-by-2 space-time block codes for the BPSK constellation are provided, and their performance as a function of correlation is investigated. It is observed that unitary codes do not necessarily achieve best performance. To this end, a non-unitary code for and BPSK constellation is supplied, which outperforms unitary codes for both spread and single-user systems. Performance analysis via numerical integration is discussed