# Design and Analysis on Decoupling Techniques for MIMO Wireless Systems in 5G Applications
## 6 Conclusions and Discussion  
Numerous investigations regarding isolation enhancement have been proposed. However, the properties and similarities between these decoupling techniques are in lack a specific analysis. In this review paper, a methodical and comprehensive summary about a variety of decoupling designs is presented. Some background knowledge about the mutual coupling effect is briefly reviewed in the former part of the article to provide a preparation for the technical analysis. Then, the state of the art used for coupling reduction is discussed in detail. Along with the discussed methods above, several additional effects brought by the adoption of decoupling techniques are concluded. Hence, the potential benefits and weaknesses of these techniques are revealed visually in an attempt to provide ideas for the future design of MIMO wireless systems in the field of 5G applications. Although much work has been done to improve the isolation of MIMO antenna systems, the decoupling design of MIMO systems that combine high isolation and compact size still presents new challenges, and the potential work is as follows:  
1. Hybrid decoupling structures  
To solve coupling effects, several decoupling techniques were proposed. The various decoupling techniques mentioned above have different characteristics. Some are effective in suppressing the transmission of space waves, and some play a role in cutting off the flow of surface waves. However, the origin of the mutual coupling effect is not due to a single reason, which is the consequence of the combined action of multiple sources of radiation elements interference. Thus, the hybrid decoupling schemes that cooperate with decoupling techniques aimed at a wide range of interference paths can be considered for further isolation enhancement research. However, in the process of adopting hybrid deco upling schemes, it is hard to guarantee the stability of system complexity and power consumption. Consequently, it is prospective to try hybrid decoupling structures to attain decoupling breakthroughs while maintaining favorable MIMO performance. 2. Low complexity  
It is a barrier that constructive complexity limits the practical application of decoupling techniques. To cover a wider bandwidth and enable high-isolation, coupling reduction designs are no longer satisfied with the basic forms of the decoupling structures, expanding to multiple branches or arranging decoupling elements into arrays. Nevertheless, the more decoupling structures integrated with the MIMO systems, the more complicated the device structures will be. Over the long term, it is not advisable to sacrifice physical space for a good degree of isolation. Other breakthroughs can be found, such as the use of new substrate materials. It was pointed out that the substrate dielectric constant and thickness also affect mutual coupling [74]. Besides the traditional ways in structure to suppress interactions, materials with low dielectric constants can be exploited to compensate for the spatial limitations on decoupling. In addition, it is notable for us that the application of MIMO in 5G portable devices. Modern portable devices aim for a large screen size percentage, metallic bezel carrying antenna system, and unreserved ground clearance. That means requirements of narrow metal bezel effect and extremely small volume of antenna elements should be considered, like literature [59; 60; 61]. Thus, keeping a balance between physical dimensions and decoupling performances of MIMO systems deserves investigating. 3. Systematic optimization procedure  
Viewing conventional coupling reduction methods, most of these solutions strongly rely on the experience that engineers already had. Both the selection of the optimal decoupling point and the parameter adjustment of the decoupling structure have a lack general optimization procedures for the MIMO system. Nevertheless, shortcomings have been improving. For instance, the application of the CMA technique in decoupled design is mentioned above.