Influence of Environment Richness on the Increase of MIMO Capacity With Number of Antennas

We study the capacity of MIMO systems with uniform linear arrays in complex media, as the number of antennas increases. The distance between adjacent antennas is kept constant. The transfer matrix describing the channel is expressed in terms of the antennas and the physical characteristics of the environment using direct scattering theory. We prove that the capacity increases logarithmically when the number of antennas in the system belongs to a large interval of intermediate values. We then take the case where the number of antennas is large and we study the effects on the capacity of adding a new scatterer. Using a city model with buildings of different sizes, randomly distributed in space, we show numerically that initially the capacity increases at a nearly linear rate as the number of antennas increases. After a certain threshold the capacity growth becomes logarithmic. We show that the number of antennas where this transition occurs increases as the number of buildings increases.