It is experimentally demonstrated that 20 multi‐input multi‐output (MIMO) antennas operated in 3.5‐GHz band (3400∼3600 MHz) with acceptable antenna performance and low envelope correlation coefficients (ECCs < 0.1) can be embedded in the smartphone for fifth‐generation (5G) or future mobile communications. Additionally, in a 20 × 12 MIMO system with 12 transmitted MIMO streams, it is shown that the 20‐antenna smartphone as a receiver can achieve 5.5‐Gbps data throughput with 100‐MHz bandwidth (3450∼3550 MHz). That is, the smartphone can have a spectral efficiency of 55 bps/Hz. With the user's hand holding the 20‐antenna smartphone, about 5.3‐Gbps data throughput in the 20 × 12 MIMO operation can still be obtained, indicating that stable multi‐Gbps smartphone is promising to obtain. Details of the 20‐antenna smartphone for MIMO operation are described. Experimental results of the 20‐antenna smartphone in the 20 × 12 MIMO operation including the user's hand presence scenario are also presented and discussed.