Adding diversity to multi-user downlink beamforming by using BER constraints

In this work, we propose a novel structure and criterion for joint transmit diversity and multi-user downlink beamforming. The proposed structure can be seen as an extension of the purely spatial downlink beamforming and is a transformation that, for each user, transforms K real antennas into L virtual antennas, with L les K. Classical transmit diversity is then applied to these virtual antennas. We propose a design criterion to optimize the precoders in order to find the best trade-off between minimizing the interference generated towards the other users and maximizing the channel diversity for the desired user. This criterion is based on the minimization of the total transmit power subjected to a target BER constraint for each user. Applied to a multiple-antenna transmitter, the proposed technique outperforms the purely-spatial downlink beamformer by about 5 dB for a target BER of 10-2 in a simple scenario.