Compact and low mutual coupling 4 × 4 wideband MIMO antenna design for 5G millimeter-wave applications

This paper presents a wideband 4 × 4 Multiple-Input-Multiple-Output (MIMO) antenna system operating within the 5G millimeter-wave (mmWave) Frequency Range 2 (FR2). The design targets enhanced mobile broadband (eMBB) applications and features four orthogonally arranged Greek cross-shaped slot antenna (GCSA) elements fed by microstrip lines to effectively reduce mutual coupling and ensure high isolation. Circular edges on the radiating elements are introduced to enhance the impedance bandwidth, while a strategically placed quadrilateral slot in the ground plane optimizes the radiation characteristics. The antenna is fabricated on a Rogers RT/Duroid 4003C substrate (ε_r = 3.55, thickness = 0.8 mm), resulting in a compact 40 × 40 × 0.8 mm³ structure. The proposed MIMO system achieves a measured impedance bandwidth of approximately 8 GHz, spanning 24 to 32 GHz at |S₁₁|≤ - 10 dB, with a peak gain of 6 dBi at 27.5 GHz. The antenna exhibits directional radiation patterns perpendicular to the MIMO plane, yielding mutual coupling levels of - 25 dB and - 20 dB at 25 GHz and 29 GHz, respectively. Additionally, the design demonstrates excellent MIMO performance with an Envelope Correlation Coefficient (ECC) < 0.001 and Diversity Gain (DG) ~ 10 dB across the 23.93-31.18 GHz operational band. A comprehensive evaluation of scattering parameters, surface currents, radiation patterns, specific absorption rate (SAR), and diversity metrics confirms the robustness of the proposed design. Prototype measurements closely match the simulated results, validating the antenna's suitability for 5G millimeter-wave communication systems.