Spatially Decoupling of CP Antennas Based on FSS for 30-GHz MIMO Systems

In this paper, an effective approach for mitigating the near-field coupling between four-port circularly polarized (CP) antennas in a 30-GHz multiple-input, multiple-output (MIMO) system is suggested and investigated. This is obtained by incorporating a two-layer transmission-type frequency selective surface (FSS) superstrate based on planar crossed-dipole metal strips. This paper presents a comparison between the mutual coupling when the patches are radiating in free space and in the presence of the FSS layers. The simulated results, when the FSS layers are applied, show an average of 6-12-dB improvement in the isolation between four adjacent CP-MIMO antennas. In addition, an accurate study is carried out on the insignificant reflections produced by the FSS layers to redirect those and also prevent any interference. The proposed 2 x 2 CP-MIMO antenna along with the superstrate is implemented and tested to validate the simulation results. Experimental results of the coupling and reflection coefficients and axial ratio show an acceptable agreement with the corresponding simulated ones.