The paper presents a Multiple-input- multiple-output (MIMO) antenna with four ports, designed on an 80 × 80 mm2 substrate. The antenna utilizes an Eighth-mode substrate integrated waveguide (EMSIW) circular cavity-backed structure, with narrow half-wavelength linear slots to suppress mutual coupling between the elements. This design provides a minimum isolation of 27 dB over the operational frequency band. The antennas are arranged at 90° angles to each other, are linearly polarized, and offer polarization diversity, further improving isolation. This orthogonal design creates overall system performance by lowering the potential of signal deterioration owing to multipath propagation. The Envelope Correlation Coefficient (ECC) is within the acceptable threshold across the operating band, confirming better diversity performance. The antenna operates at 3.5 GHz, with a gain of 1.39 dBi, making it suitable for 5G use cases, particularly in small-cell deployments for modern wireless communication systems where space and performance are major constraints.