In 5G MIMO smartphone antennas, a novel technique combines twin CMs and DGS for good isolation and wideband operation. For lower frequencies, it uses orthogonal CMs resonating from a square-loop slot; for higher frequencies, it uses a shared radiator. As a result, external decoupling structures are no longer necessary. An artificial 8 × 8 MIMO array that fits within the back cover of the smartphone exhibits over 20 dB isolation in the 3.3–3.8 GHz region, which is necessary for effective data transmission. This strategy guarantees 5G smartphone applications operate at peak efficiency.