Based on the characteristic mode theory, a 16-port multiple-input multiple-output (MIMO) cube antenna design for an indoor environment is presented. First, a high-isolation 4-port MIMO antenna unit operating at 5.150-5.875 GHz is designed by exciting different modes of a rectangular plate simultaneously, based on a modal analysis using the characteristic mode theory. An artificial magnetic conductor (AMC) surface is introduced to replace the reflector of the original antenna, which greatly reduces the profile of the antenna;it is also used to surround the antenna to form a 16-port (4×4) MIMO cube antenna. This antenna fulfils the multi-port and multi-polarization requirement within a small space. The simulation and experimental results show that the antenna operates with a port reflection coefficient S_ii < -10 dB and port isolation |S_ij| > 20 dB in the 5.150-5.875 GHz band.