A typical 5G multiple-input and multiple-output (MIMO) system must combine a high number of antennas at both the transmitter and receiver to realize spatial multiplexing capability. In this paper, a wideband 16- element indoor base station (BS) antenna array that can cover 3.3–6.0 GHz is proposed for 5G applications. A π-shaped monopole antenna is designed to cover the Lower band (LTE bands 42/43–N77–N78), the intermediate band (N79), and the higher band (LTE 46). The antenna elements are arranged in a limited space printed on a substrate take the Hexakaidecagon Polygon shape. As the antenna elements are arranged with good isolation, achieving good polarization diversity. The proposed BS array is simulated, fabricated, and tested. The typical results, S-parameters, antenna efficiency, and radiation patterns are investigated. Moreover, to validate the MIMO performances, a very low envelope correlation coefficient (ECC) below 0.02, high antenna efficiency of about 82–93.2% are achieved. The calculated ergodic channel capacity of the 16 × 16 MIMO system reached up to 85 bps/Hz. The proposed antenna array was compared to some other 5G MIMO indoor base station antennas.