  This work considers the problem of dynamic power allocation in the downlink
of multi-cell networks, where each cell utilizes non-orthogonal multiple access
(NOMA)-based resource allocation. Also, coordinated multi-point (CoMP)
transmission is utilized among multiple cells to serve users experiencing
severe inter-cell interference (ICI). More specifically, we consider a two-tier
heterogeneous network (HetNet) consisting of a high-power macro cell underlaid
with multiple low-power small cells each of which uses the same resource block.
Under this {\em CoMP-NOMA framework}, CoMP transmission is applied to a user
experiencing high channel gain with multiple base stations (BSs)/cells, while
NOMA is utilized to schedule CoMP and non-CoMP users over the same transmission
resources, i.e., time, spectrum and space. Different CoMP-NOMA models are
discussed, but focus is primarily on the joint transmission CoMP-NOMA
(JT-CoMP-NOMA) model. For the JT-CoMP-NOMA model, an optimal joint power
allocation problem is formulated and the solution is derived for each CoMP-set
consisting of multiple cooperating BSs (i.e., CoMP BSs). To overcome the
substantial computational complexity of the joint power optimization approach,
we propose a distributed power optimization problem at each cooperating BS
whose optimal solution is independent of the solution of other coordinating
BSs. The validity of the distributed solution for the joint power optimization
problem is provided and numerical performance evaluation is carried out for the
proposed CoMP-NOMA models including JT-CoMP-NOMA and coordinated scheduling
CoMP-NOMA (CS-CoMP-NOMA). The obtained results reveal significant gains in
spectral and energy efficiency in comparison with conventional CoMP-orthogonal
multiple access (CoMP-OMA) systems.
