Large Scale multiple input multiple output (MIMO) systems have recently emerged as a promising technology for 5G communications. While they have been shown to offer significant performance benefits in theoretical studies, the large scale MIMO transmitters will have to be deployed in the limited physical space of today's base stations (BSs). Accordingly, this paper examines effects of deploying increasing numbers of antennas in fxed physical space, by reducing the antenna spacing. We focus on the resulting performance of large-scale MIMO transmitters using low complexity closed form precoding techniques. In particular, we investigate the combined effect of reducing the distance between the antenna elements with increasing the number of elements in a fxed 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 antenna arrays modelled by detailed electromagnetic simulation. Our results show the somewhat surprising result that, by reducing the separations between the antennas to signifcantly less than the transmit wavelength to ft more antennas, the resulting system performance improves.
- mutual coupling
- transmit correlation