## 1 Introduction
It is cascaded between two radiators, to reduce the mutual coupling. It consists of two power dividers, two transmission lines, and one reactive component. The proposed methodology shows that the mutual coupling is suppressed over 20 dB. A CSRR [17; 18] is a periodic or non-periodic structure. It is used to achieve both filtering and isolation improvement. The proposed antenna array of 1 \(\times\) 2 array elements is composed of microstrip patches with a pair of CSRRs placed on the ground plane to reduce the coupling between the two patches. A \(-30\) dB isolation band is achieved at 9.34 GHz to 9.45 GHz. A novel neutralization line with decoupling network (NLDN) is proposed in [19] for wideband multi-element antenna arrays. A decoupling network is established between ports, and a neutralization line is introduced between the antenna arrays. The simulation and measured result show 20 dB isolation between the array elements. An approach based on a fractal isolator using an electromagnetic bandgap structure based on metamaterial for mutual coupling reduction in antenna arrays is proposed in [22]. With this technique, 37, 21, 20, and 31 dB isolation is achieved in the X-, Ku-, K-, and Ka-bands, respectively. In [24], a 4G/5G MIMO antenna is proposed for handphone application. Four antennas are implemented, and the isolation is improved by making slots between them. It gives about 15 dB isolation improvement in the 4G and 5G bands. In [26], mutual coupling is reduced using T-shape parasitic elements between two elements. It cancels the surface current effect and reduces the mutual coupling. The results show that 15 dB mutual coupling is diminished by this method. Mobile communication technology is continuously advancing and involves massive MIMO systems as the novel solutions that need to be implemented in base transceiver systems. As elaborated on above, multi-antenna elements in the transceiver system raise the significant problem of mutual coupling between the closely spaced elements, degrading the overall system performance. This work proposes and investigates a mutual coupling suppression technique which capitalized on some of the aforementioned approaches in an unconventional manner. More specifically, a metamaterial-based periodic FSS is combined with a metallic wall and placed between the two planar monopoles. Furthermore, the isolation is improved by creating a slot element on the common ground plane of the antenna elements. The proposed isolation technique is applied on two different configurations of an MIMO antenna system. In this investigation, first, the FSS unit cell is characterized in CST Microwave studio. The FSS wall, combining six novel unit cells, acts as a bandstop filter with the operating bandwidth covering the target frequency band of 24 GHz to 27 GHz. It is then combined with the metallic strip on the same substrate. Finally, a slot element 1 mm in width is etched on the ground plane of the MIMO system. As a result, an improved isolation is achieved between the closely spaced elements. The originality and the technical contributions of the paper can be summarized as follows: (i) development of a novel frequency-selective surface (FSS) design for bandstop filtering purposes, (ii) development of a novel isolation wall combining the FSS and a metal strip to reduce mutual coupling among closely spaced array elements, (iii) development of a complete isolation enhancement structure consisting of several components not employed before in this combination, leading to excellent performance not reported before, (iv) numerical and experimental demonstration of the applicability of the proposed technique to various MIMO system architectures at mmWave frequencies, (v) demonstration of the superiority of the considered approach over state-of-the-art methods available in the literature.