A compact 16-port multiband antenna array for massive Multiple-Input Multiple-Output (MIMO) applications in future 5G smartphones is proposed. The multi-antenna system consists of eight pairs of compact antenna arrays. In order to reserve the local space of the 2G/3G/4G antennas, the eight pairs of antenna arrays are printed on both sides of the smartphone. Each antenna array pair consists of two compact gap-coupled antennas, arranged on the upper and lower sides of the system board. The upper antenna operates in LTE band 46(5 150-5 925 MHz), and lower antenna covers LTE band 42/43(3 400- 3 800 MHz). Test results show that the antenna array has good impedance matching and isolation performance. In addition, this paper also studies and analyzes some characteristic parameters of MIMO. Finally, the effects of hand and head for antenna are studied. The results of simulation and measurement show that the antenna array has good radiation characteristics and MIMO performance.