Equivalent Circuit Based Calculation of Signal Correlation in Lossy MIMO Antennas

Correlation coefficient of received signals across a pair of antennas is a key performance indicator for multiple-input-multiple-output (MIMO) systems. For multipath environments with uniform 3-D angular power spectrum, the signal correlation between two antennas can be exactly calculated from their 3-D radiation patterns. When radiation patterns are unavailable, a simplified approach that only requires the antennas´ scattering parameters can be used instead. However, the simpler method assumes lossless antennas and thus only works well for antennas with high radiation efficiencies. To take into account the antenna loss, the idea of equivalent circuit approximation is used in this paper to analytically separate the lossy components (resistance or conductance) from the lossy antenna arrays, using known scattering parameters and radiation efficiencies. The simplified method using S parameters can then be applied to obtain the correlation coefficient of the equivalent lossless antennas. The effectiveness of the method has been verified on antennas operating at a single mode, such as dipole or patch at its lowest resonant frequency. Good results were also obtained for the measured case of a dual-antenna mobile terminal, consisting of a monopole and a PIFA.