Mutual Coupling Reduction of ±45° Dual-Polarized Closely Spaced MIMO Antenna by Topology Optimization

A hybrid topology optimization (HTO) method is adopted to reduce the mutual coupling of a ±45° dual-polarized closely spaced MIMO antenna. In order to shorten the optimization time, only the isolation structure region of the two-element MIMO antenna is selected for optimization. More importantly, by exploiting the symmetry of both the MIMO antenna and the isolation structure, only a quarter of the isolation structure needs to be optimized to achieve good antenna performance. In this way, not only the optimization variables, but also the optimization sub-objectives are greatly reduced, and the optimization time is further diminished. The performance of the dual-polarized MIMO antenna with the optimized isolation structure (OIS) is validated by both simulation and measurement. The OIS resonates in both polarizations, thus lowering the mutual coupling between co-polarization ports (
$\vert S_{31}\vert $
and
$\vert S_{42}\vert$
) and between cross-polarization ports (
$\vert S_{41}\vert $
and
$\vert S_{32}\vert$
) simultaneously. With the center distance of only
$0.35\lambda _{0}$
, the measured mutual coupling between different ports are reduced by 6~11 dB within the 10-dB impedance bandwidth (Reflection coefficients < −10 dB) by adding the OIS, and the highest mutual coupling between different ports is reduced from −15 to −21 dB. Besides, the antenna maintains a low profile (
$0.026\lambda _{0}$
), low cross-polarization and excellent radiation pattern performance.