The MIMO antenna systems have become the cornerstones of the current wireless communication standards, offering the ability to greatly increase the spectral efficiency, the data rates, and the reliability of the links. Implementation of MIMO antennas on the smartphone, however, presents severe challenges because of the small size of the devices, especially the mutual coupling between the tightly placed antenna elements. Mutual coupling results in dire isolation degradation, a higher envelope correlation coefficient (ECC), radiation pattern distortion, and a negative effect on diversity performance, and thus a lower system capacity. This is a review article, which gives a detailed analysis of the mutual coupling reduction methods that can be used in smartphone MIMO antennas. Geometrical arrangement, neutralization line, defective ground structure, electromagnetic bandgap structure, metamaterial structure, and combinations of these methods are critically reviewed in terms of their design mechanisms, their ability to be implemented and their performance. Recent developments that include reconfigurable decoupling structures, AI-assisted optimization and proximity to the user are also addressed to emphasize new directions. According to the literature reviewed, the geometrical decoupling techniques reach the level of -10 dB to -15 dB of isolation, whereas neutralization line methods can reach − 15 dB to -22 dB of isolation with an ECC value usually less than 0.05. DGS and EBG designs show a higher level of suppression of the surface currents, which suppresses the isolation up to -20 dB to -25 dB, and the reported ECC values are usually less than 0.03. Greater performance is further obtained by metamaterial and hybrid methods, such that the isolation is better than − 25 dB in some designs, as ECC values reach to about 0.01, and with reasonable radiation efficiency and bandwidth. The topical publications in each category of techniques are summarized in comparative tables, which allows researchers to realize the design trade-offs and viable constraints in smartphone settings. Future research avenues are suggested to ease compact, efficient, and high-performing smartphone MIMO antennas aligned with 5G and upcoming 6G requirements. This article serves as a resource for antenna engineers, system designers, and researchers involved in developing next-generation mobile communication devices.
Shoaib et al. (Sat,) studied this question.