Comprehensive Study on Structural and Metamaterial Based Isolation Enhancement Techniques in 5G MIMO Antennas

The advent of 5G and emerging 6G technologies has driven the adoption of millimeter-wave (mmWave) frequencies and compact multiple-input multiple-output (MIMO) antenna systems to support high data throughput and spectral efficiency. However, densely packed antenna elements in mmWave MIMO systems suffer from significant mutual coupling, adversely affecting parameters such as envelope correlation coefficient (ECC), diversity gain, and channel capacity. This paper presents a comprehensive review of advanced isolation enhancement techniques designed to mitigate mutual coupling in compact 5 G MIMO antennas. These techniques are classified into structural and metamaterial-based methods. Structural techniques include slotted ground planes, decoupling stubs, defected ground structures (DGS), and electromagnetic bandgap (EBG) structures, while metamaterial methods explore the use of split ring resonators (SRRs), complementary SRRs, artificial magnetic conductors (AMCs), metasurfaces, and hybrid approaches. Comparative analyses based on metrics such as isolation level, ECC, diversity gain, and antenna size demonstrate the effectiveness and trade-offs of each approach. This study provides valuable insights into current design strategies and identifies key challenges and future directions for achieving high isolation in next-generation wireless systems.