Comparison of antenna array techniques for the downlink of multi-carrier CDMA systems

The paper deals with transmit pre-filtering using antenna arrays in the downlink of wireless cellular systems based on multi-carrier code division multiple access (MC-CDMA). In addition to the signal separation in frequency obtained by spreading codes, the base station exploits the spatial separation of users to improve interference mitigation, which enables the use of low-complexity mobile receivers. We propose transmit filtering techniques optimizing the signal separation in space only, or jointly in space and frequency, according to single- and multi-user criteria. These techniques assume channel knowledge at transmission, and are particularly destined for time division duplex systems, where we may benefit from channel reciprocity between up- and downlink. We compare the performance of the proposed techniques in terms of interference reduction and robustness to time variant channels. Simulation results emphasize the benefit of transmit filtering optimized jointly in space and frequency, which allows a striking increase on the user capacity thanks to space division multiple access.