Compact wideband circularly polarized antenna and its self-decoupled MIMO configuration for 28-GHz band targeting 5G/6G millimeter-wave networks

Millimeter-wave communication has become a cornerstone of emerging 5G and future 6G systems due to its ability to support multi-gigabit data rates, ultra-low latency, and dense device connectivity. These performance requirements enforced constraints that are not limited to antenna size, bandwidth, polarization, and multiple-input-multiple-output (MIMO) configuration. Addressing these challenges, this work proposes a compact circularly polarized antenna optimized for broadband operation around the 28-GHz band. The design employs simple yet effective techniques, utilizing a ring-shaped radiator excited through a via-fed central patch which enables strong impedance matching and stable circular polarization. A three-stage geometric evolution is adopted to achieve enhanced bandwidth and improved return loss, where the introduction of an annular slot and arc-shaped perturbation plays a key role in generating the required CP mode. The final single-element achieves impedance bandwidth of 25.5–30.55 GHz, with an axial-ratio bandwidth ranges 26.4–28.4 GHz. An equivalent circuit model is developed using ADS, where the simulated results closely match the electromagnetic response obtained for the CST. To enhance spatial diversity and satisfy mm-wave MIMO requirements, the design is extended onto four-port self-decoupled configuration. The MIMO array maintains high isolation across the operational bandwidth without relying on additional parasitic decoupling elements. A comparison with recent literature verifies the compact size of antenna in standalone and MIMO configurations, along with high performance parameters in both cases. These results demonstrate the suitability of the antenna for compact 28-GHz terminals, present, and future 5G/6G wireless devices.