Transmit antenna selection for sum rate maximization in transmit zero-forcing beamforming

MIMO wireless downlinks using transmit zero-forcing beamforming (TZFBF) with M-T transmit antennas can serve up to K = M-T users, each equipped with one antenna. To maximize the channel sum rate, it has been shown that user selection, which is akin to receive antenna selection (RAS) in this case, is required along with waterfilting to find the optimal subset of active receivers S-a, where /S-a/ = 1,...,M-T. To implement TZFBF, channel state information is required at the transmitter (CSIT). When CSIT is available, it is known that additional constraints imposed on the transmitter will reduce the system sum rate. Despite this, transmit antenna selection (TAS) provides a means of increasing the sum rate in some cases when sub-optimal RAS algorithms are used. The mechanism works by assisting the RAS search path to get out of a local maximum., The proposed method requires further RAS to follow any prior TAS process and the restoration of any transmit antennas that were removed. An analysis is provided to give insight to the proposed method. The analysis and scheme are applicable to any sub-optimal RAS algorithm and guidelines on decoupled search strategies are given. The analysis also affirms the statement that given CSIT, TAS does not help improve the sum rate of TZFBF, regardless of the channel condition and signal-to-noise ratio when optimal RAS is already done. This means that joint exhaustive RAS-TAS searches are not needed to achieve the optimal sum rate and instead, only exhaustive RAS or user selection search is needed.