Minimum BER transmit optimization for two-input multiple-output spatial multiplexing

A two-input multiple-output (TIMO) system represents an important special case of multiple-input multiple-output (MIMO) systems and occurs in practical scenarios where there are limitations on cost and/or space to install more antennas, in MIMO with transmit antenna selection which selects two out of multiple transmit antennas and turns MIMO into TIMO, or in cooperative communications with two single-antenna mobiles sharing their antennas. In this paper, minimum bit error rate (MBER) transmit optimization for TIMO spatial multiplexing systems is investigated. Approximate MBER transmit power allocation for a variety of receiver structures is proposed. Transmit beamforming schemes using 4-ary pulse-amplitude-modulation (4-PAM) and quaternary phase-shift-keying (QPSK) pre-mixing are also proposed, which eliminate error floors in ill-conditioned TIMO channels. It is shown both analytically and by numerical simulations that the proposed schemes offer superior performance over existing schemes. Essentially, the proposed transmit optimization provides a simple and efficient means to utilize partial channel state information at the transmitter