Decoupling methods of MIMO antenna arrays for 5G applications: a review

Multiple-input multiple-output (MIMO) technique is a key technique for communication in the future. It can effectively enhance channel capacity. For future fifth-generation (5G) terminals, it is still a challenging task to realize desirable isolation within a compact size. To achieve an acceptable isolation level, many decoupling methods have been developed. We review the most recent research on decoupling methods, including the employment of external decoupling structures, orthogonal modes, and reduction of ground effect, and discuss the development trends of the MIMO array in 5G smartphones.