Space–time spreading MIMO-CDMA downlink systems using constrained tensor modeling

This paper considers the downlink of a multiple-input multiple-output (MIMO) code-division multiple-access (CDMA) system based on space–time spreading (STS) using multiuser/multicode transmission. A constrained tensor model is proposed for the STS transmitted signal. The new STS model generalizes existing ones by allowing multiuser spatial multiplexing with different code reuse/multiplexing patterns for each user. This model is parametrized by two allocation matrices with variable structure, which allow to control, respectively, the allocation of multiple data streams and spreading codes to the transmit antennas. A methodology is proposed to design these allocation matrices leading to finite sets of feasible STS schemes with guaranteed blind symbol recovery. The construction of feasible STS schemes for 2, 3 and 4 transmit antennas is presented as an illustration of this methodology. A blind receiver based on the alternating least squares algorithm is used for symbol recovery. The bit-error-rate performance of this receiver is tested and compared to some existing STS-based downlink transceivers.