In the previous chapters we examined some of the implications of the MIMO capacity formula. As we have seen, the statistics of the MIMO capacity are dependent on the statistics of the channel matrix and the average signal-to-noise ratio at the receiver, which we denote by ρ. Both the statistics of H and the value of ρ depend on the propagation characteristics of the channel; thus, an understanding of propagation is important in order to predict and understand the performance of MIMO communication systems. Because MIMO techniques are designed to operate in a scattering environment, we focus on channel phenomena that give rise to scattering and multipath. Without scattering and multipath, the channels between the various combinations of transmit and receive antennas are correlated, which results in poor MIMO performance. In this chapter, the fundamental concepts and terminology of multipath propagation are reviewed.

Phenomenology of multipath channels

In any wireless communications path between a transmitter and a receiver, signals arrive at the receiver through various propagation mechanisms. In general, RF energy propagates between two points in one of two ways: directly or indirectly. Direct propagation refers to transmission of RF energy along a direct path between the transmitter and the receiver that does not involve any reflections, scattering, ducting, or diffractive bending. Direct propagation is called free space propagation and is said to undergo free space attenuation. Indirect propagation, in contrast, involves any one or a combination of the following: reflection, diffraction, scattering, or refraction.