In this paper, a compact MIMO antenna array is designed with a defected ground structure (DGS) using spatial diversity techniques on a Rogers RT/Duroid 5880 substrate. The dimensions of the proposed antenna are 1.4$\lambda_{0}$ $\times$ 2.8$\lambda_{0}$$\times$0.073 $\lambda_{0}$ (15 mm$\times$30 mm$\times$0.79 mm), where $\lambda_{0}$ is the free-space wavelength operating at 28 GHz. The antenna array with DGS has enhanced the gain and isolation of the antenna. The gain of the proposed antenna array is 10.3 dBi with 97.01% radiation efficiency at 28 GHz resonant frequency. The -10 dB impedance bandwidth lies between 26.95-29.50 GHz, which covers the important mmWave frequency bands such as the n257 at 26-29.5 GHz, the n258 at 24.25?27.5 GHz, and the n261 at 27.5?28.35 GHz.In this paper, the MIMO performance metrics are calculated, such as isolation, envelope correlation coefficient, diversity gain, and channel capacity loss. The ECC value observe is 1.29 $\times$ 10 $^{-7}$ at 28 GHz. The channel capacity loss for MIMO antennas is below 0.4 bps/Hz in the operating band, and the isolation is ($|S_{21}|$ = 45.24 dB) at 28 GHz and ($|S_{21}|$ $\ge$ 28 dB) in the complete operating band All the calculated parameters satisfy the standards of the ITU. The proposed MIMO antenna is a good candidate for a 5G wireless communication system.