Reducing the fractional rank of interference with space-time-frequency adaptive beamforming

When wideband interference is subject to multi-path, the rank of the interference component sensed at a receive-antenna array increases. In a MIMO communications system, this makes it more challenging to suppress the interference and close the link. In this paper we investigate the use of adaptive filters to suppress interference, by quantifying the impact of multiple time taps and frequency “taps” on the rank of the interference. We show that as the number of taps increases, the relative rank of the interference decreases asymptotically to one over the number of receive antennas. We verify this with measurements of a broadcast interference source.