Non-orthogonal multiple access (NOMA) plays a significant
role in improving bandwidth efficiency through sharing
the spectrum among multiple users. However, the weaker users
in the NOMA system will suffer from interference caused by
stronger users when decoding their own symbols. In this paper,
we design constructive interference precoding for downlink
NOMA to make the interference experienced by weaker users
beneficial and further improve their communication quality.
Specifically, the transmit power optimization problem under
symbol level precoding (SLP) constraints at weaker users is
formulated to reduce the power consumption while guaranteeing
the quality of service of all users and successful successive
interference cancellation (SIC). Afterward, by utilizing some
approximate methods, an iterative algorithm is proposed to
handle the non-convex problem. Simulation results show that
the proposed scheme significantly saves the power consumption
compared to the conventional NOMA. Furthermore, compared
to the classical SLP scheme, the proposed scheme maintains its
feasibility in an overloaded deployment, and shows its superiority
for receivers with a low signal-to-interference-plus-noise
ratio (SINR) target and a relatively large disparity in channel
strengths.

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