Pre-FFT combining with balanced subcarrier performance for STBC-OFDM systems

Recently, pre-fast Fourier transform (pre-FFT) combining has been introduced for space-time block-coded (STBC) orthogonal frequency division multiplexing (OFDM) systems. Despite the complexity reduction, the resultant signal-to-noise ratio (SNR) values of different subcarriers are often not balanced, and it limits the system performance. In this paper, we propose to use the max-min fair criterion for pre-FFT combining in STBC-OFDM systems to solve the problem. Semidefinite relaxation (SDR) is employed to approximate the solution. Simulation results show that the error-rate performance of the proposed pre-FFT combining scheme can be improved as compared with that of the conventional one under various numbers of transmit/receive antennas and multipath channel environments with affordable computational complexity.