On ergodic rates and optimal array geometry in line-of-sight massive MIMO

We investigate the achievable ergodic rate of Massive multi-input-multi-output (MIMO) system in environments with high mobility and line-of-sight (LoS). A 3-dimensional geometric model with uniform rectangular array at the base station (BS) is used for the investigation. We derive a closed form expression for a lower bound on the uplink ergodic rate taking into account imperfections of the channel state information, number of BS antennas, antenna spacing, and spatial distribution of user terminals. The results show that, in LoS Massive MIMO, when the terminals are spherically uniformly distributed around the BS, the ergodic rate is maximized for antenna spacing equal to integer multiples of one-half wavelength.