A virtual MIMO channel representation and applications

A key to maximal exploitation of MIMO (multiple-input multiple-output) systems is a fundamental understanding of the interaction between the underlying complex physical scattering environment and the space-time signal space. In time- and frequency-selective MIMO (space-time) channels, this interaction happens in time, frequency and space. We present a four-dimensional Karhunen-Loeve-like virtual representation for spacetime channels that captures such interaction and exposes the intrinsic degrees of freedom in the channel. The virtual representation is a Fourier series dictated by the finite array apertures, signaling duration and bandwidth and corresponds to a uniform, fixed sampling of the angle-delay-Doppler scattering space. It provides a much-needed connection between the two existing (extreme) modeling approaches - idealized statistical models and detailed physical (ray tracing) models. In particular, it yields a simple geometric interpretation of the effects of physical scattering on channel statistics and capacity. We discuss various insights into the structure of space-time channels afforded by the virtual representation as well its application in capacity assessment, spatial multiplexing and space-time code design.