Pre-DFT Combining for Coded OFDM

Multiple receive antennas have been known to improve bit error performance over fading multipath channels by providing spatial diversity. In systems such as orthogonal frequency-division multiplexing (OFDM), this benefit is obtained at the cost of greatly increased system complexity. Pre-discrete Fourier transform (DFT) combining techniques have been proposed by some researchers to obtain multiple antenna gains at a reduced complexity. However, existing works on pre-DFT combining ignore the specific distance properties of the underlying convolutional code, leading to suboptimal solutions. In this paper, we develop a novel technique for pre-DFT combiner design for bit-interleaved coded modulation (BICM)-coded OFDM on Rayleigh block-fading channels. The combiner is designed for the specific code used and outperforms other known pre-DFT combining techniques. Our simulations show about a 2-3-dB improvement in performance over existing techniques.