Although a significant amount of research attention has been devoted to microphone-array beamforming, the performance of all the developed algorithms in practical acoustic environments is still far from meeting our expectation. So further research efforts on this topic are indispensable. In this paper, we treat a microphone array as a multiple-input multiple-output (MIMO) system and develop a general framework for analyzing performance of beamforming algorithms based on the acoustic MIMO channel impulse responses. Under this framework, we study the bounds for the length of beamforming filter, which in turn shows the performance bounds of beamforming in terms of speech dereverberation and interference suppression. We also discuss the intrinsic relationships among different classical beamforming techniques and explain, from the channel condition point of view, what the prerequisites have to be fulfilled in order for those techniques to work.
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