Downlink MIMO Systems Using Cooperation Among Base Stations in a Slow Fading Channel

To increase the achievable sum rate of downlink MIMO (Multiple Input Multiple Output) systems, the cooperation among Base Stations (BS) is investigated in a slow fading channel. Three levels of BS coordination are considered: full coordination, partial coordination, and no coordination. This paper assumes that the downlink MIMO channel state information (CSI) is sent to the corresponding BS through a feedback link with some delay. In a fully coordinated system, the CSI at each BS needs to be collected at the central coordinator, and this process may result in significant additional delay. In this paper, the achievable throughput of each coordination level is evaluated and compared including the effects of the delay in CSI. A quasi-static fading is considered where the channel is static within one codeword, but it gradually changes over time based on the well-known Jake´s model. Also, dirty paper coding (DPC) is applied, which is a transmission method shown to achieve the sum capacity of MIMO broadcast channels (BC). The results show that under certain delay conditions, the partially-coordinated system outperforms the fully-coordinated system. In addition, this paper presents a new partial-coordination method that provides higher throughput than the conventional partial-coordinated system.