Antenna mutual coupling effects on correlation, efficiency and Shannon capacity

MIMO (multiple-input-multiple-output) systems make use of multiple antennas at both ends of a communication link to exploit the spatial dimension for increasing the capacity. Correlation of the antenna signals has an affect on the capacity of a MIMO implementation. High correlation levels occur mainly for compact antenna realizations, where the separation of the antenna elements is small and the mutual coupling is strong. The antenna mutual coupling distorts the antenna radiation patterns and the input impedances, and therefore affects the correlation level and the efficiency. The correlation level is decreased by coupling, which would improve the Shannon capacity, but the effiency of the antenna system is also decreased, which leaves the Shannon capacity nearly unchanged as compared to the case when neglecting coupling effects. The Shannon capacity is increased in a interference limited receiving situation and can also be increased in a transmitting situation in a narrow frequency band by using a decoupling network.