A Compact Fractal-Based Super-Wideband mmWave MIMO Antenna for 5G NR and 6G Services

This paper presents a compact fractal-based super-wideband multiple-input multiple-output (MIMO) antenna for millimeter-wave (mmWave) 5G new radio (NR) and prospective 6G applications. The MIMO system comprises four Koch fractal monopole elements integrated with a modified shared ground plane. By adopting the second Koch iteration, the antenna achieves enhanced impedance bandwidth and stable radiation behavior compared with lower-order iterations. The elements are arranged in a polarization-diversity configuration within a 30 × 30 mm2 footprint on a 0.8 mm-thick Rogers RO4835 substrate (εr = 3.5, δ = 0.0025). The proposed design provides an impedance bandwidth exceeding 14 GHz over 26.5–41 GHz, covering key bands at 28, 32, 38, and 40 GHz, while maintaining high inter-element isolation (around 30 dB over the operating range). The optimized ground modification enables a fully connected common ground and suppresses mutual coupling without additional decoupling structures. The antenna achieves 4–6 dBi realized gain with radiation efficiency exceeding 95%. MIMO performance metrics, including the envelope correlation coefficient (ECC), mean effective gain (MEG), and diversity gain (DG), confirm excellent diversity characteristics. The antenna is further evaluated under bending, demonstrating stable matching and isolation for conformal and wearable scenarios, and the concept is extendable to a non-planar 12-port configuration within the same footprint. Measured results agree well with simulations, validating the proposed design for wideband mmWave 5G/6G devices.