Comparison of pre-FFT and post-FFT processing adaptive arrays for OFDM systems in the presence of co-channel interference

For future high-speed wireless communications using orthogonal frequency division multiplexing (OFDM), two major system requirements emerge: throughput improvement and elimination of rich interference. Adaptive array antennas can potentially suppress interference by using plural antennas and spatial coefficient adaptation. In addition, in the case of no interference, it is predicted that the adaptive array can improve either data throughput or potential coverage. In general, the pre-FFT (fast Fourier transform) domain diversity or beamforming in an OFDM system could be a reasonable solution for easy implementation with low complexity. However, in rich multipath environments for both desired signal and interference, the performance will deteriorate drastically. In contrast, it is possible for the post-FFT array processing to provide high performance even in such severe environments, at the cost of complexity. This paper presents a comparison of antenna array architectures for OFDM system using pre-FFT and post-FFT domain array processing, and clarifies the dependency of the channel property for each array processing. It is found that the performance of the pre-FFT array processing is strongly dependent on the total number of multipaths and interferences. Also, it is shown that it is possible for the post-FFT array processing to reduce the complexity by decreasing the number of antennas and the corresponding analog components because its performance depends on the number of signal sources, furthermore, since the proposed post-FFT array processing can achieve high performance with much lower complexity by using subcarrier clustering, it is a promising candidate for real implementation.