Towards massive-MIMO transmitters: On the effects of deploying increasing antennas in fixed physical space

This paper examines effects of deploying increasing numbers of antennas in fixed physical space. We focus on the resulting performance of large-scale multiple input multiple output (MIMO) transmitters using low complexity closed form precoding techniques. Towards this end, we explore the effects of adding antenna elements in fixed physical spaces, by reducing the antenna spacing. In particular, we investigate the combined effect of reducing the distance between the antenna elements with increasing the number of elements in a fixed transmitter space. This gives rise to two contradicting phenomena: the reduction of spatial diversity due to reducing the separation between antennas and the increase in transmit diversity by increasing the number of elements. To quantify this tradeoff, we investigate densely deployed uniform linear antenna arrays modeled in the CST Microwave Studio. The simulations show the somewhat surprising result that, by reducing the separations between the antennas to significantly less than the transmit wavelength to fit more antennas, improved performance is obtained.