In 5G mobile communication systems, superb performances, namely large system capacity, low latency and massive connection are expected. A massive multiple input multiple output (MIMO)antenna has attracted a great attraction to become a key technology to meet such 5G requirements. In this paper, we report development results of an active phased array antenna (APAA)employing a highly integrated GaAs RF frontend module at 28 GHz-band. The transmission trial of 16-beam spatial-multiplexing using the APAA with 256 antenna elements is also reported. A spectral efficiency of 63.7 bps/Hz and a downlink transmission rate of 25.5 Gbps were successfully achieved.