Hetero-Coupled Antenna Arrays for Multi-Band Massive MIMO Systems

Tightly coupled planar massive multiple-input multiple-output (mMIMO) antenna arrays have demonstrated wideband performance, even when employing simple canonical minimum scattering (CMS) antenna elements. This wideband behavior is primarily attributed to the superior signal-to-noise ratio (SNR) observed at lower frequencies. In contrast, loosely coupled arrays exhibit superior SNR at mid-band frequencies. To achieve wideband performance that surpasses both configurations, we propose the use of non-uniformly arranged antenna elements. By employing a circuit-theoretic model to unify the antenna and communication channel domains, together with a genetic algorithm (GA) for placement optimization, we show that non-uniform arrangements yield superior SNR and, consequently, higher system capacity compared to tightly and loosely coupled uniform planar arrays in multi-band mMIMO systems. The optimization is carried out across the reactive near-field (XNF), radiative near-field (RNF), and far-field (FF) regions, where we demonstrate that near-field communication with non-uniform arrays can deliver improved SNR performance even at high frequencies.