This paper describes a compact, 4-port C-shaped wide-band MIMO antenna with high isolation for 5 G-NR sub-6 GHz applications. The optimized 4-port MIMO antenna offers a wide impedance bandwidth of 2.6 GHz (3.05 \(-\) 5.65 GHz) and measures 48 \(\times \) 48 \(\times \) 1.2 \(\hbox {mm}^{3}\) (0.55 \(\times \) 0.55 \(\times \) 0.013 \(\lambda ^{3}\) at 3.45 GHz). The design uses an FR-4 substrate with a dielectric constant ( \(\varepsilon _{r}\) ) of 4.5 and a loss tangent of 0.002. A novel optimized C-shaped monopole antenna has been designed and extended to two port planar and orthogonal configurations. It is further optimized and compact into a 4-port MIMO antenna. Spatial diversity and improved isolation have been achieved by positioning the four C-shaped antenna elements orthogonally with mirroring technique. This approach enhances isolation by optimizing intra-element spacing and increasing the edge-to-edge gap. This configuration includes two vertical stubs with a common defected ground structure (DGS) for wide-band performance at lower frequencies and a hexagonal split ring (HSR) as a decoupling element, which enhances the isolation more than 25 dB. The proposed MIMO antenna achieves a diversity gain ( \(\hbox {DG}_{4x4}\) ) above 9.89 dB and an envelope correlation coefficient ( \(\hbox {ECC}_{4x4}\) ) below 0.012. Analysis of the channel capacity loss ( \(\hbox {CCL}_{4x4}\) ), total active reflection coefficient ( \(\hbox {TARC}_{4x4}\) ), radiating efficiency, and mean effective gain ( \(\hbox {MEG}_{4x4}\) ) shows values of < 0.44 bps/Hz, < \(-\) 8.10 dB, 74–86%, and 0.99 dB, respectively. There is a good agreement between simulation and measured results. The proposed antenna having compact size with enhanced isolation and diversity parameters highlights the need for present 5 G-NR sub-6 GHz communication.
