Performance of an OFDM subcarrier adaptive beamformer

An orthogonal frequency-division multiplexing (OFDM) system transmits an information-bearing signal on several orthogonal subcarriers. Although OFDM is relatively resistant to multipath effects, its performance is affected by cochannel interference. This interference may cover the entire OFDM signal transmission bandwidth, or it might occupy only part of the bandwidth and hence affect a subset of the subcarriers, in this paper, we examine the performance of a subcarrier adaptive beamformer that places nulls on individual subcarriers instead of on the entire OFDM symbol. Each subcarrier of every antenna element is weighted by an adaptive coefficient that is updated by a blind algorithm. Since the beamformer on each subcarrier is adapted independently, it is possible that some beamformers captures an interferer instead of the signal of interest. To avoid this problem, we propose a two-stage system that has a subcarrier adaptive beamformer and a bank of signal cancelers in the first stage, followed by a constrained beamformer in the second stage. We present computer simulations to investigate the behavior of the subcarrier adaptive beamformer as the degree of subcarrier overlap and the angle of arrival (AOA) of the cochannel interference are varied.