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The MIMO antenna is one of the essential elements for the implementation of the new fifth-generation (5 G) technologies. The design of the MIMO antenna goes through several steps; the important one is the design of the single-element antenna that resonates at the desired frequency band(s). Its design becomes more difficult if it is a multi-band antenna. In addition, determining the dimensions of multi-band antennas by simulation or mathematical models is both more complicated, difficult, and time-consuming. In this research, different approaches based on artificial neural networks (ANN), radial basis function neural networks (RBFNN), and adaptive neuro-fuzzy inference systems (ANFIS) are used to predict the dimensions of a dual-band circular patch antenna with defected ground structure (DGS) for mm-wave 5 G applications, and then the performances of the three models are compared. To determine the training and the testing data, 280 antennas with different patch values and added slots in the ground plane are simulated. The resonant frequencies of the dual-band circular patch antenna and the height of the substrate are used as the input vector for the ANN, RBFNN, and ANFIS models. Two hundred twenty simulated antennas were used for training, and the remaining 60 data points were used to test the ANN, RBFNN, and ANFIS models. The performance of these models is compared in the training and testing process using some statistical criteria such as MSE, MAE, and RMSE. The proposed dual-band circular patch single-element antenna has a compact size of 5 × 6 × 0.8 m m 3 and exhibits good performance, such as broad band (3.8 and 3 GHz), high gain (6 and 7.1 dB), and high efficiency (99 % and 95 % ) in both operating bands. After studying the single-element antenna and in order to improve its performance, we proposed a 4 × 4 MIMO antenna with a total volume of 17 × 17 × 0.8 m m 3 that was designed by using four copies of the single-element antenna arranged orthogonally on the same low-loss Rogers Duroid 5880 substrate. The suggested 4 × 4 MIMO antenna achieves a high level of isolation (more than - 30 dB in both operating bands), which is obtained by inserting a cross-shaped decoupling structure among elements on the upper and lower sides of the substrate. Moreover, the proposed MIMO antenna has a good gain of 6.4 and 7.5 dB in both bands and a high radiation efficiency of 99.5 % and 99 % at 28 and 38 GHz, respectively. Furthermore, the diversity performance of the MIMO antenna is evaluated using a variety of crucial indicators, all of which greatly surpass realistic norms. The envelope correlation coefficient (ECC) is less than 0.005, the diversity gain (DG) is greater than 9.99 dB, the total active reflection coefficient (TARC) is below 10 dB, and the channel capacity loss (CCL) is no longer than 0.4 bits/s/Hz. In addition, the proposed dual band circular patch single element antenna and the 4 × 4 MIMO antenna are fabricated, and the measured results are in good agreement with the simulated results using HFSS and CST software. [ABSTRACT FROM AUTHOR]