Scheduling and Base-Station Cooperation in MIMO Downlink Systems
Licentiate thesis, 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.
room EF, 6th floor, Hörsalsvägen 11, Signals and Systems, Chalmers University of Technology
Opponent: Prof. Erik G. Larsson, Department of Electrical Engineering, Linköping University, Sweden