A low complexity adaptive beamformer for OFDM

Because of ease of implementation and robustness to multipath delay spread, orthogonal frequency division multiplexing is an increasing popular technology for ad hoc networks. Unfortunately, OFDM performs poorly in the presence of cochannel interference, which degrades network capacity. In this paper, we develop beamforming techniques which minimize cochannel interference to allow spatial division multiple access. The standard approaches in the literature develop a separate beamformer for each narrowband carrier, with perhaps some grouping based on coherence across frequencies. This approach is both computationally complex and, as shown in this paper, difficult in an outdoor environment with expected Doppler spread. Here, we develop a time domain beamformer with low complexity and more robustness to a changing channel. Performance is demonstrated with multipath angular and Doppler spreads typical of mobile uplinks.