Transmit beamforming for SDMA in multi-carrier CDMA downlink on a per subcarrier basis

In this paper, transmit Beamforming (BF) for Space Division Multiple Access (SDMA) is studied in the downlink of wireless cellular systems based on Multi-Carrier Code Division Multiple Access (MC-CDMA). This combination allows a separation of users' signals in space, thanks to BF, and frequency, thanks to CDMA, and thus improves mitigation of multiple access interference. We propose to perform BF on each subcarrier independently to allow any kind of detection algorithms at the mobile terminal. Single user criteria based on maximum ratio transmission and a multi-user criterion maximizing the signal over interference plus noise ratio are studied. We show that beamforming impairs the orthogonality of the user's signals in frequency. However, the advantage of separation in space clearly prevails, especially when low complex detection techniques are used at the mobile terminal. Furthermore, we propose a simple and efficient combination of CDMA and SDMA that significantly increases the system capacity by reallocation of spreading codes.