Optimum Beam Direction and Width for Directional Antenna Indoor MIMO Systems

Recently, the MIMO (Multiple-Input Multiple-Output) systems are expected as a technology of achieving a high transmission capacity in wireless communication systems. In the MIMO system, SDM (Space Division Multiplexing) method and E-SDM (Eigenbeam SDM) method are proposed [1]. The former is the method transmitting sub-stream with an equal transmit power from all transmitting antennas, and the latter is the method forming orthogonal channel by using SVD (Singular Value Decomposition) of a channel matrix H. The transmission capacity of SDM is not higher than that of E-SDM, however, the transmission performance of SDM method is not degraded by a change of channel characteristics, and it provides a simple system because of not using transmitting weight and its system has already been introduced in wireless LAN [2]. To obtain high performance and low cost MIMO system, we have proposed the introduction of the directional antenna to the base station for the indoor propagation environment in the SDM system [3]. When the directional antenna is used for an actual environment, there are a lot of variety of room shapes, such as, office, meeting room, and etc. Then, the beamwidth and the beam direction of base station antennas should be optimized to maximize the channel capacity depending on the room shape. This paper presents the optimum beam width and the direction of base station antenna for various room shapes using by ray-tracing method to obtain the maximum channel capacity in MIMO system used under indoor propagation environment.