New downlink beamforming method for cooperative multiple access point systems

Multiple input multiple output (MIMO) systems represent a very attractive candidate for future high data rate wireless access systems to increase the channel capacity. To obtain a further increase in the channel capacity, a distributed wireless communication system that has multiple access points (APs) and an access controller (AC), was proposed. This system increases not only the peak transmission rate but also the overall transmission rate in the service area. In this paper, we consider a cooperative multiple AP system where APs are synchronized in terms of time, but not in terms of phase. We propose a transmission method for the cooperative multiple AP system that enables the mobile station to obtain a high achievable bit rate regardless of decoding algorithms. The performance of the multiple AP systems is evaluated in comparison to the conventional single AP system. To evaluate the effectiveness of the proposed method in the cooperative multiple AP system, computer simulations are performed using the i.i.d. channel in Rayleigh and Ricean fading environments. Furthermore, we evaluate the performance of the cooperative multiple AP system using the channel measured in an actual indoor environment.