Low Mutual Coupling Design for 5G MIMO Antennas Using Multi-Feed Technology and Its Application on Metal-Rimmed Mobile Phones

In this paper, an effective method for developing a multi-feed multiple-input multiple-output (MIMO) antenna system using the metal rims of a mobile phone without any structural interruption on the long edge frames is proposed. Instead of traditionally designing MIMO antennas with multiple similar antenna structures on the mobile phone frame or its ground plane, the MIMO antennas in this work are realized by reasonably constructing feeding points and grounding points on the entire edge frame. The MIMO antenna system established by such multi-feed technique is capable to achieve much simpler structural configuration, excellent isolation and matching performance while multiple antennas are connected. A planar eight-port MIMO antenna system operating in the 5G frequency band of 3.4~3.6 GHz is investigated initially, and subsequently applied to the curved metal-rims which comply with the industry design of a modern flagship mobile phone. The prototypes are fabricated and measured, evaluation results show that isolations among all ports are better than 15 dB with reflection coefficients below −10 dB within the band of 3.4~3.7 GHz and −6 dB within the band of 3.3~3.8 GHz. In addition, more than 57% total efficiency and envelope correlation coefficient (ECC) of less than 0.3 are obtained. The satisfactory performance reveals the proposed design of multi-feed MIMO antennas can be a promising candidate for 5G metal-rimmed mobile phone applications.