Power and Signature Optimization for Forward Link CDMA with Multiple Antennas

Signature sequences and associated powers are optimized jointly with linear receivers for a multi-user forward-link CDMA system with multiple transmit and receive antennas. The performance criterion is sum capacity over all users. For the model considered, the optimal signatures are sinusoids so that multi-carrier signaling, in which each carrier is assigned to a single user, maximizes the achievable rate. An optimal assignment of carriers to users appears to be difficult to determine in general, but can be efficiently approximated numerically. The asymptotic sum capacity for large number of users and antennas is characterized at high SNRs. The accuracy of these results for moderate SNRs is illustrated through comparisons with numerically optimized power allocations.