ISM band has been approved for spread spectrum communication in radio LAN in Japan since December 1992. This frequency band extends from 2.474 GHz to 2.5 GHz with 26 MHz bandwide. In an indoor environment, the maximum observed delay spread is 100-200 ns in a room, it is too short to generate a selective fading, thus flat fading conditions are often observed. Serve as an alleviation, we propose a new system of multi-antenna in base station (BS) and single antenna in mobile station (MS). In this system, MS should have a simple structure for its small size and energy-saving.