mmWave communication typically resorts to beamforming in order to overcome the unfavorable propagation conditions in the high frequency bands. The directional transmission established using beamforming is beneficial also in terms of reducing the effective channel impulse response (CIR) and, correspondingly, the required guard periods in a block transmission scheme. While this effect has been investigated in the literature for the single stream case, in this paper we analyse the shortening of the effective multiple-input, multiple-output (MIMO) channel impulse response in a mmWave multistream transmission. It is shown that even though the complete effective MIMO channel impulse response might be relatively long, smart time delaying of streams sent over different physical paths can exploit the sparsity of the mmWave channel in the temporal-angular domain, and result in significant CIR shortening. In this way, guard period reduction can be achieved, with the overhead approaching that of the single stream case.
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