A highly decoupled and compact co-circularly polarized MIMO filtering antenna array system for vehicular communications

This paper presents a compact, single-layer, and highly decoupled four-element multiple-input-multiple-output (MIMO) filtering antenna array system designed for vehicular communications. A novel sequential phase feed network is developed by replacing the λ/4 transformers transmission lines with three-pole hairpin resonator-based band pass filters. The resonators provide filtering characteristics for a 2 × 2 configurations of sequentially rotated corner truncated rectangular patch antennas. The elements of the MIMO filtering antenna array system achieve out-of-band frequency suppression levels exceeding 24 dB in the lower stop band and 36 dB in the upper stop band, demonstrating a sharp filtering response. The MIMO filtering array elements exhibit excellent inter-port isolation, exceeding 34 dB across the entire operating bandwidth of 5.4 GHz to 6.3 GHz. High level of isolation is achieved through two main strategies: first, by rotating the antenna arrays by 120° along the central axis to minimize coupling, and second, by designing a cage to confine the electromagnetic fields. The MIMO filtering array elements have a -10 dB impedance bandwidth of 20%, covering the range from 5.3 GHz to 6.5 GHz, and a 3-dB axial ratio bandwidth of 15.4%, from 5.4 GHz to 6.3 GHz. The elements of the fabricated prototype exhibit a peak realized gain of 11.3 dBic, with an average realized gain of 10.5 dBic. The proposed MIMO filtering antenna array system has the potential to provide the scalability and adaptability needed to support emerging technologies and standards in vehicular communication.This work aligns with the UN Sustainable Development Goals, particularly SDG 9, SDG 11, and SDG 13, by enabling sustainable and intelligent vehicular communications.