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The assumption that no LoS channels exist between wireless accesspoints~(APs) and user equipments~(UEs) becomes questionable in the context ofthe recent developments in the direction of cell free massive multiple inputmultiple output MIMO~(CF-mMIMO) systems. In CF-mMIMO systems, the access pointdensity is assumed to be comparable to, or much larger than the the userdensity, thereby leading to the possibility of existence of LoS links betweenthe UEs and the APs, depending on the local propagation conditions. In thispaper, we compare the rates achievable by CF-mMIMO systems under probabilisticLoS/ NLos channels, with and without acquiring the channel stateinformation~(CSI) of the fast fading components. We show that, undersufficiently large AP densities, statistical beamforming that does not requirethe knowledge about the fast fading components of the channels, performs almostat par with full beamforming, utilizing the information about the fast fadingchannel coefficients, thus potentially avoiding the need for training duringevery frame. We validate our results via detailed Monte Carlo simulations, andalso elaborate the conditions under which statistical beamforming can besuccessfully employed in massive MIMO systems with LoS/ NLoS channels.

How much Training is Needed in Downlink Cell-Free mMIMO under LoS/NLoS channels?