Scheduling and Base-Station Cooperation in MIMO Downlink Systems

Licentiatavhandling, 2009

Multiple input multiple output (MIMO) techniques have been shown to improve the capacity and link reliability of wireless communication without a need to extra power and bandwidth. In multi-user (MU) MIMO networks, a linear increase of sum capacity in the number of transmit antennas can be achieved by using linear precoding combined with efficient scheduling algorithms. However, the promising capacity gain of MU-MIMO techniques, when deployed in a multicell environment, is severely degraded due to other-cell interference (OCI). This is especially important near the cell boundaries. Furthermore, most of these techniques assume that perfect channel state information (CSI) is available at the transmitter, which is very difficult to obtain. In order to increase the capacity of a cellular system, base-station (BS) cooperation can be used to jointly transmit/receive to/from one or multiple users in the downlink/uplink. This thesis investigates the design and performance of spectrally-efficient MU-MIMO downlink systems with BS cooperation under more practical scheduling algorithms with a limited feedback. Three contributions are included in the thesis. In Paper A the performance of a distributed antenna system (DAS) under time-varying frequency-selective fading based on a realistic channel model is investigated. A new cellular layout is obtained by shifting the hexagonal cellular layout in the conventional system and without a need for additional BS towers. The results show that, with the same total transmit power and bandwidth, DAS can reduce the OCI in a multi-cell environment and improve the outage capacity especially near the cell boundary. In Paper B a low-complexity space-frequency scheduling algorithm for the downlink of a cluster-based MU-MIMO with BS cooperation is presented. In the proposed algorithm, users are grouped based on the large-scale CSI from surrounding BSs. As the cooperation is done adaptively among BSs, there is no need for perfect CSI from all BSs, which leads to a limited backbone overhead as well as to reduced computational complexity. In Paper C a low-complexity algorithm for selecting users and their corresponding number of data streams, denoted as user transmission mode (UTM), is proposed for the downlink of a MU-MIMO system. The selection is only based on the average received signal-to-noise ratio (SNR) from the base station (BS) for each user, which reduces the amount of feedback for scheduling. Analytical average throughput approximations are derived for each user at different UTMs. Simulation results demonstrate that the proposed algorithm provides performance close to dirty paper coding (DPC) with considerably reduced feedback.