Spatial limits to mutual information scaling in multi-antenna systems

Previous results have shown significant capacity gains by employing multiple antennas at both transmitter and receiver, however, due to physical size restraints (particularly at the receiver) these may not be obtained. In this paper, we consider the capacity behaviour of multi-antenna systems when the receiver sampling is constrained to a finite region of space. By characterizing the wavefield generated at the receiver due to transmitted signals and the scattering environment, a theoretically derived sampling threshold is shown to exist, at which the capacity growth is reduced from linear to logarithmic with increasing number of sampling outputs. Furthermore, this threshold is shown to be linearly dependent on the receiver region radius within which the sampling is constrained, and is independent of the sampling characteristics such as the antenna properties, array geometry, and/or array signal processing.