The assumption that no LoS channels exist between wireless access points~(APs) and user equipments~(UEs) becomes questionable in the context of the recent developments in the direction of cell free massive multiple input multiple output MIMO~(CF-mMIMO) systems. In CF-mMIMO systems, the access point density is assumed to be comparable to, or much larger than the the user density, thereby leading to the possibility of existence of LoS links between the UEs and the APs, depending on the local propagation conditions. In this paper, we compare the rates achievable by CF-mMIMO systems under probabilistic LoS/ NLos channels, with and without acquiring the channel state information~(CSI) of the fast fading components. We show that, under sufficiently large AP densities, statistical beamforming that does not require the knowledge about the fast fading components of the channels, performs almost at par with full beamforming, utilizing the information about the fast fading channel coefficients, thus potentially avoiding the need for training during every frame. We validate our results via detailed Monte Carlo simulations, and also elaborate the conditions under which statistical beamforming can be successfully employed in massive MIMO systems with LoS/ NLoS channels.

翻译：在无小区大规模多输入多输出（CF-mMIMO）系统的最新进展背景下，无线接入点（AP）与用户设备（UE）之间不存在视距（LoS）信道的假设变得值得商榷。在CF-mMIMO系统中，接入点密度假定与用户密度相当或远大于用户密度，因此根据本地传播条件，UE与AP之间可能存在LoS链路。本文比较了在概率LoS/NLoS信道下，是否获取快衰落分量的信道状态信息（CSI）时，CF-mMIMO系统可实现的速率。我们证明，在接入点密度足够大的条件下，无需了解信道快衰落分量的统计波束成形，其性能几乎与利用快衰落信道系数信息的全波束成形相当，从而可能避免每帧都需要进行信道训练。通过详细的蒙特卡洛仿真验证了结果，并阐述了在LoS/NLoS信道下的大规模MIMO系统中成功应用统计波束成形的条件。