Linear Precoding in Cooperative MIMO Cellular Networks with Limited Coordination Clusters

In a cooperative multiple-antenna downlink cellular network, linear precoding techniques are considered to maximize a concave utility function of the user rates. All base stations share channel state information, but each user's message is only routed to the base stations that participate in the user's coordination cluster. In the network, different clusters may overlap. Without base station cooperation, the system is interference-limited. The interference limitation is overcome when the base stations are allowed to jointly encode the user signals, but the capacity-achieving dirty paper coding scheme can be too complex for practical implementation. A new linear precoding technique called soft interference nulling (SIN) is proposed, which performs at least as well as zero-forcing (ZF) beamforming under full network coordination. Unlike ZF, SIN allows the possibility of interference, but over-penalizes any such nonzero interference. The SIN precoder is computed by solving a convex optimization problem. A cellular line network model is considered, and SIN under partial network coordination can outperform full network coordination ZF at moderate SNRs. When the base stations have multiple antennas, it is found that performing multi-user MIMO transmission is more beneficial than inter-cell interference mitigation techniques.