MIMO-CDMA systems: Signature and beamformer design with various levels of feedback

We investigate the signature and beamformer design problem for the uplink of multiple-input multiple-output code-division multiple-access (MIMO-CDMA) systems with the sum capacity and the systemwide mean-square error (MSE) as performance metrics. We first construct iterative algorithms to find the jointly optimum temporal signatures and transmit beamformers under the assumption of perfect temporal signature and transmit beamforming feedback. Next, motivated by the need to reduce the amount of feedback, we present a low-complexity sequential orthogonal temporal signature assignment algorithm for given transmit beamformers. Our approach is based on minimizing the difference between the performance of the MIMO-CDMA system and the described upper bounds at each signature assignment step. We next note that the transmit beamformers can be shaped depending on the channel state information (CSI) available at the transmitter. We investigate the cases of various levels of available feedback resulting in different beamformer structures and present a joint orthogonal temporal signature assignment and beamforming algorithm. We observe that as the available feedback level is increased, the performance of the joint signature and beamformer assignment algorithm approaches the performance upper bounds. In particular, we observe that a substantial performance gain is obtained when the individual channel state information is available at the transmitter side.