Shadows of Disparity: Unveiling the Asymmetry of Mutual Coupling in Densely-Packed MIMO

Mutual coupling (MC) refers to the electromagnetic interaction wherein the voltage on one antenna induces excitation currents on its nearby antennas, imposing non-negligible impacts on densely-packed multiple-input-multiple-output (MIMO) arrays. The existing literature often assumes a perfect symmetry in MC effects between the transmitter (Tx) and receiver (Rx). However, such an intuitive symmetry deviates from physical reality and may yield theoretically sound but practically misleading conclusions. This letter systematically unveils the inherent asymmetry of MC for the first time through the following two contributions. Firstly, we demonstrate that the MC matrices are asymmetric at the Tx and Rx based on the Ohm’s and Kirchhoff’s laws. Secondly, we provide quantitative analysis of how the asymmetry of MC matrices exerts asymmetric impacts on MIMO channel capacity. Simulation results are provided to illustrate the asymmetric impacts of MC on capacity. Moreover, the simulation results demonstrate that MC does not always degrade capacity but can even enhance it by up to 36% under specific array orientations. These findings highlight the capacity-enhancement potential through strategically exploiting the asymmetry of MC, such as deliberately orientating the densely-packed MIMO arrays at the Tx and Rx sides in different favorable orientations.