Interference suppression and diversity exploitation for multiantenna CDMA with ultra-low complexity receivers

We consider transmitter precoding for interference suppression and diversity exploitation for multiantenna, multipath code division multiple access (CDMA). The receivers are constrained to matched filter detection without channel state information (CSI) or receiver-based multiuser detection. We first develop precoders for flat-fading scenarios where perfect downlink channel information is available at the transmitter. We show that full transmit antenna diversity is achievable, even for nonorthogonal spreading codes, although we suffer an SNR loss that is a simple function of the spreading code crosscorrelations. We also develop precoders for cases in which the transmitter has partial channel information, which is modeled as knowledge of conditional channel correlation matrices, and we apply them to Jakes´ fading model. Finally, we show that the precoding approach for flat fading can be modified for multipath fading to fully exploit multipath diversity when used in conjunction with prerake diversity combining. We show, in summary, that transmitter precoding offers a reasonable alternative to receiver-based multiuser signal processing when minimizing computational complexity at the mobile unit is a priority.