## 1 Introduction  
Multiple-input multiple-output [1] technology (MIMO) is a well-established wireless communications technique for sending and receiving multiple data signals simultaneously over the same radio channel. MIMO techniques play a vital role in Wi-Fi communication, including in the 3G, 4G, and 4G LTE networks. Massive MIMO is an important technology in 5G NR, enhancing reliability and data rates in challenging environments [2]. This involves the application of MIMO on a larger scale in order to obtain greater network coverage and capacity. Massive MIMO employs many more transmitting and receiving antennas to increase the transmission gain and the spectral efficiency [3]. Cell-free massive MIMO with a multi-antenna user concept, in which a large number of distributed devices can communicate with the central processing unit (CPU), has gained attention recently for serving a small number of users at the same time-frequency resources block. It remarkably helps in enhancing gain and spectral efficiency [4]. When the multiple antennas are implemented in the MIMO system, the antenna elements are placed physically close to each other, and the effects of their mutual coupling become more pronounced [5, 6]. The mutual coupling between two closely arranged antenna elements occurs due to (i) the flow of a surface current from the port excitation area, (ii) the space radiation, and (iii) the surface waves. Consequently, suppressing the mutual coupling within the miniaturized printed antenna elements becomes a greater challenge in the design of a 5G NR massive MIMO multi-antenna system. The literature offers several techniques for mutual coupling reduction, including incorporation of electromagnetic bandgap (EBG) structures [7; 8; 9], defected ground structures (DGS) [10; 11; 12], decoupling networks [13; 14; 15; 16], a complementary split-ring resonator (CSRR) [17; 18], neutralization lines [19; 20], metamaterials [21; 22; 23], and parasitic or slot elements [24; 25; 26]. The EBG structure can transmit, block, or pass specific electromagnetic (EM) waves. Basically, it acts as a low/high pass, stopband, or a bandpass filter. In [7], the authors propose a novel EBG unit cell and place it in between dual-polarized phased-array antenna elements to reduce mutual coupling. The mutual coupling between two microstrip patch antennas is suppressed using a modified electromagnetic bandgap decoupling structure in [8]. The patch antennas operate at an UWB between 3.1 GHz and 13.5 GHz. The MEBG is employed on the other side of the patch antennas and connected to the ground plane. Likewise, in [9], a uniplanar EBG presents mutual coupling reduction between elements of an UWB MIMO antenna. In [10], UWB MIMO antennas are utilized, and mutual coupling is suppressed by using a defected ground structure. The proposed UWB antenna covers the 2 GHz to 10 GHz band. Introducing arcs on the patch of the monopoles and on the ground plane enhances the performance of an antenna system and increases the mutual impedance among elements. The authors in [11] propose a two-port antenna which comprises a three-layer structure. Rectangular-shape defects in the ground plane are introduced to enhance the isolation between the two antenna ports. In [12], the authors propose compact MIMO antenna systems with improved isolation for sub-6 GHz 5G applications. The isolation is improved by introducing a T-shaped ground stub between the two elements. The antenna system operates at 3.6 GHz. Utilization of decoupling networks is another technique for mutual coupling reduction in MIMO systems [13; 14; 15; 16]. In [13], a two-substrate integrated waveguide (SIW) antenna array is proposed. A decoupling feed network is composed of a two-layer directional coupler which is connected to SIW slot antenna arrays. The results show good performance with coupling being about \(-35\) dB at the center frequency. In [16], a decoupling device referred to as a cascaded power dividing decoupling network (C-PDDN) is proposed.