A Transmission-Line-Based Decoupling Method for MIMO Antenna Arrays

A transmission-line-based decoupling technique for dual-polarized multiple-input and multiple-output (MIMO) antenna arrays is presented and analyzed. The proposed scheme enables well-canceled coupling for the adjacent elements under co-polarization, without degrading the isolation of the cross-polarized ports. First, a decoupling network based on the presented method for a 2 × 2 MIMO array is provided, along with a comprehensive design procedure. Calculations and simulations are operated to verify decoupling performance. For further verification, a 2 × 2 dual-polarized patch array with the proposed decoupling method is developed. The decoupling network characterizes low profile, compact size, and low insertion loss, which is realized in a single layer. Measurements denote that the isolations between the co-polarized elements are improved from 16 to 20 dB to over 30 dB after decoupling at the center frequency of 2.45 GHz. Subsequently, based on the proposed 2 × 2 decoupling method, a decoupling network for large-scale dual-polarized MIMO arrays is presented. A design example of a 4 × 4 dual-polarized patch antenna array is established. Fullwave simulations indicate that the isolations are enhanced to better than 30 dB with a small insertion loss of less than 0.45 dB and can widely be used for phased array and massive MIMO array systems.

I. Introduction

Seeing that the multiple-input and multiple-output (MIMO) systems are powerful to increase the channel capacity and enhance the spectrum efficiency for wireless communications, the studies on MIMO antennas have attracted growing interests. To maintain the active impedance matching performance of MIMO antennas, one challenge is the coupling suppression among antenna elements. Otherwise, the coupling may lead to dramatic degradations of an adaptive array whose pattern is adjusted automatically to signal environment [1], significant drops of the mean capacity of an MIMO system [2], as well as other negative effects [3]–[5]. Although mutual coupling below −17 dB is sufficient for MIMO-based transmission from the error rate point of view [6], such an isolation level does not mean that the MIMO capacity can be achieved in practice. For instance, the active VSWR would be higher than six with the isolation of 15 dB [3], which can be highly improved if the mutual coupling is reduced to lower than −25 dB [4]. In addition, nonlinear effects would be caused by the strong coupling in the massive MIMO systems [7]. This would lead to significant nonlinear distortions on power amplifiers and decrease the operation bandwidth of the systems. To this end, the mutual coupling between the elements of massive MIMO arrays is preferred to be less than −25 dB or even lower in practice.