Isolation Enhancement of Wide-Band MIMO Array Antennas Utilizing Resistive Loading

In this paper, we propose the use of resistor-loaded paired parallel-coupled resonators (PCRs) for mutual coupling reduction and isolation enhancement in wideband MIMO array antennas. It was shown that the isolation enhancement using a conventional PCR structure was drastically degraded in the middle of the frequency band. By placing an optimally designed resistor between the PCRs, we can improve the isolation considerably, compared with the simple PCR structure. Moreover, it could also improve the diversity parameters, such as envelope correlation coefficient (ECC), diversity gain (DG), and channel capacity loss (CCL). The validity of the proposed resistor-loaded PCR technique was verified with ultra-wideband array antennas with 1 and 14 patch array configurations, achieving a 0.125λ0 center-to-center spacing between narrowly spaced patches and isolation better than 25 dB within the range 3-9-GHz without any degradation in its frequency response, radiation patterns, or diversity parameters. The presented results confirm that the proposed technique is suitable in various MIMO systems applicable for wireless local area networks (WLANs), long-term evolution (LTE), and possibly fifth-generation (5G) communication due to the compact size and improved isolation performance.