On the Complexity of Feedback Generation in MIMO Beamforming and Diversity Schemes

Recently, dierent schemes have been introduced that improve the bit error rate of single user MISO transmissions through transmit diversity or transmitter-sided beamforming. This is particularly interesting in the downlink of cellular systems, where it is feasible to employ multiple transmit antennas at the base stations, but only few receive antennas at the mobile terminals. In this paper, we observe both classical beamforming schemes, where a beamforming vector is chosen to optimize the SINR at the receiver, and Coherent Alamouti schemes, which extend the classical rate one Alamouti STBC to multiples of two transmit antennas. In both cases, full diversity and an additional gain can be achieved, if a certain extent of channel knowledge is present at the transmitter, which usually has to be supplied by the receiver through feedback. In our work, we evaluate the performance and feedback requirements of the dierent schemes, while putting a special emphasis on the non-negligible computational complexity required at the receiver to produce optimal feedback. We introduce a novel beamforming scheme that can significantly reduce this complexity while achieving a comparable performance, and provide a comprehensive summary on the choice of an optimal transmission scheme for dierent applications.