28 GHz Circular Polarized MIMO Antenna System With Polarization Diversity for Millimeter-Wave Applications

A four-port dual circularly polarized (CP) MIMO antenna system is proposed for millimeter-wave (mm-wave) applications, especially 5G mobile communications. The MIMO antenna system is constructed from a compact-size CP antenna designed at 28 GHz. The CP antenna has some defects in the ground plane that have the effect of increasing the operating bandwidth and improving the axial ratio. The antenna patch and the defected ground are symmetric about the 45° line to ensure good circular polarization. A microstrip line divided into three sections is used to feed the antenna. The feed line sections are used to match the input impedance to 50 Ω at 28 GHz. The design evolution is described starting with a square patch, which is modified through the design phases to reach the final radiating patch. Numerical simulations of the single-element CP antenna are performed using the CST microwave studio simulator to ensure the antenna performance at 28 GHz regarding the circular polarization, impedance matching, and radiation pattern. MIMO system is constructed using four elements of the designed CP antenna. Two of the antennas produce right-hand circular polarized waves, and the other two produce left-hand circular polarized waves. The proposed MIMO configuration is appropriate to satisfy the polarization diversity scheme. A practical model for the single and MIMO antenna is fabricated and subjected to experimental verification through measurements. A good consent is found between the numerical results and the experimental measurements for both the single antenna and the MIMO system. The CP antenna in the proposed MIMO system has a return loss of less than$-20\text{dB}$ and an axial ratio of less than $0.4\text{dB}$. Finally, a comparison between the proposed CP antenna and MIMO system with similar designs found in the literature is performed.