Efficient Beamforming in Downlink Cooperative Cellular Networks with Soft Interference Nulling

A simple line network model is proposed to study the downlink cellular network. 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) under full network coordination. Unlike ZF, SIN allows the possibility of interference, but over-penalizes any non-zero interference. The SIN beamforming precoder is computed by solving a convex optimization problem, and the formulation is extended for multiple- antenna channels. SIN can be applied when only a limited number of base stations cooperate; it is shown that SIN under partial coordination can outperform full-coordination ZF at moderate SNRs.