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