Comparison of practical feedback algorithms for multiuser MIMO
IEEE Transactions on Communications
Packet error probability of a transmit beamforming system with imperfect feedback
IEEE Transactions on Signal Processing
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In this paper, the performance of a closed-loop transmit beamforming system employing the ";unconventional"; technique of analog channel state information (CSI) feedback is studied. The use of analog or unquantized symbols for CSI feedback leads to a low feedback rate of only one symbol per transmit antenna per frame, and does not result in an unrecoverable distortion as in the traditional digital CSI feedback approach. The BER of differential BPSK and 8PSK in the downlink of the proposed system is evaluated via computer simulation. The results show that analog feedback can perform substantially better than digital feedback under some operating conditions. Another contribution of the paper is the use of position dependent modulation (PDM) to counter the varying beamforming quality across the data frame. It was shown that PDM can substantially increase the downlink throughput (relative to optimal fixed modulation) at given target BERs.