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Recently, the multiuser multiple input multiple output (MU-MIMO) system has attracted much attention as a technology that enhances the total system capacity by generating a virtual MIMO channel between a base station and multiple terminal stations. Although extensive evaluations are needed because the number of system parameters in MU-MIMO is much larger compared to the single user (SU)-MIMO, the performance of MU-MIMO in actual environments is still not well understood. This paper describes the features and effectiveness of a 16times16 multi-user (MU)- MIMO testbed in an actual indoor environment. Moreover, we propose a simple adaptive modulation scheme for MU-MIMO-OFDM transmission that employs a bit interleaver for the frequency and space domains. Finally, we evaluate the frequency efficiency by obtaining the bit error rate of this testbed in an actual indoor environment. We clarify that 16times4times4-user MU-MIMO transmission using the proposed modulation scheme achieves the frequency utilization of 870 Mbps and 1 Gbps (SNR: 31 and 36 dB) with 20 MHz bandwidth, respectively.