Correlation and capacity of MIMO systems and mutual coupling, radiation efficiency, and diversity gain of their antennas: simulations and measurements in a reverberation chamber

TLDR

MIMO systems attain maximum capacity, but correlation between channel signals—primarily caused by mutual coupling in antenna arrays—reduces this capacity, and mutual coupling also diminishes diversity gain. The study explains how MIMO and diversity antennas with mutual coupling can be analyzed using classical embedded element patterns computed by standard computer codes. It further describes measuring radiation efficiency, diversity gain, correlation, and channel capacity in a reverberation chamber. In the investigated MIMO system, mutual coupling simultaneously increases correlation (raising capacity) and decreases radiation efficiency (lowering capacity), leading to an overall capacity reduction that is confirmed by measurements agreeing with simulations.

Abstract

MIMO systems are characterized by their maximum available capacity, which is reduced if there is correlation between the signals on different channels. The correlation is primarily caused by mutual coupling between the elements of the antenna arrays on both the receiving and transmitting sides. Similarly, diversity antennas can be characterized by a diversity gain that also is affected by mutual coupling between the antennas. We explain how such MIMO and diversity antennas with mutual coupling can be analyzed by classical embedded element patterns that can be computed by standard computer codes. In the MIMO example under investigation, the mutual coupling reduces both correlation, which increases the capacity, and radiation efficiency, which decreases it, and the combined effect is a net capacity reduction. We also explain how the radiation efficiency, diversity gain, correlation, and channel capacity can be measured in a reverberation chamber. The measurements show good agreement with simulations.