# Design and Analysis on Decoupling Techniques for MIMO Wireless Systems in 5G Applications
## 1 Introduction  
Currently, the field of mobile communication technology is in the transition phase from 4G to 5G technology. The explosive growth of mobile data is driving the users' demands for higher wireless communication services. Communication systems need to achieve timely and fast access to information, higher transmission efficiency, and better system security. As a result, 4G is starting to gradually fail to meet the new communication needs and the shortcomings are coming to the fore. From 2020, 5G took the stage of commercialization, which marked it was gradually becoming the mainstream technology in modern mobile communication. Its frequency band covers sub-6 GHz (450 MHz-6 GHz) and mm-Wave band (24.25-54.6 GHz). The salient features of 5G are high-speed data transfer, and reliable connectivity with zero percent latency [1]. Additionally, the good user's Quality of Experience (QoE) in the 5G era requires excellent antenna diversity and larger channel capacity support. To overcome the signal fading due to multipath effects, diversity systems with two receivers were developed, which was the beginning of the evolution of MIMO [2, 3]. Existing studies [4, 5] have shown that, compared to simple input simple output (SISO) systems with a simple structure, MIMO systems have multiple additional paths, and the multipath effect, delay, and packet loss can be reduced to enhance the communication modes. Moreover, according to [PERSON]'s theorem, the channel capacity depends on the channel bandwidth and signal-noise ratio (SNR). Additionally, MIMO systems have demonstrated the potential for increasing the data throughput multiple times with a fixed bandwidth and transmission power by virtue of the multi-port design [6, 7]. Therefore, the study of MIMO systems is an important part of developing 5G communication systems. MIMO systems applied to 5G devices are a hot research topic in the field of antenna design recently, so many related papers have been published, and they are compared in Table 1. The eight-port MIMO system composed of I-shaped slot antennas was proposed by [PERSON] et al. [8]. To ensure the system isolation of this dual-band MIMO system, four antenna elements are arranged along each side of the ground plane. Each antenna element is kept at a distance of more than 15 mm, and the isolation is better than 11 dB. The tri-band antenna was designed by [PERSON] et al. [9], which consisted of eight inverted F-shaped antennas. The antenna elements were evenly distributed along both sides of the ground plane to obtain isolation higher than 10 dB. In [PERSON] et al. [10], a 12-port MIMO system consisting of an inverted \(\pi\)-shaped antenna was proposed. To ensure the isolation higher than 12 dB, the antenna element spacing was not less than 10 mm. It can be observed from Table 1, that although these antenna systems can meet the practical requirements, their average isolation is only around 11 dB while the average distance between antenna elements is more than 15 mm. This is because the limitation of physical size leads to the mutual coupling effect between antenna elements, which can impair the isolation of MIMO systems. Therefore, it is not a recommended method to obtain satisfactory isolation by just adjusting the antenna element position, and some effective techniques are needed to reduce the mutual coupling. In this paper, our investigation focuses on decoupling techniques for MIMO systems. We review some excellent MIMO decoupling schemes of the last five years and discuss the potential works it faces. These decoupling techniques are divided into two types, including external decoupling techniques and internal decoupling techniques. External decoupling techniques rely on adding additional structures to counteract the mutual coupling, which are further classified into neutralization lines (NL), grounding branch structures, parasitic decoupling elements (PDE), defective ground structures (DGS), metamaterial techniques, etc.