  We outline a general framework to use coordinated multi-point (CoMP)
transmission technology in downlink multi-cell non-orthogonal multiple access
(NOMA) systems considering distributed power allocation at each cell. In this
framework, CoMP transmission is used for users experiencing strong
receive-signals from multiple cells while each cell adopts NOMA for resource
allocation to its active users. After a brief review of the working principles
of different CoMP schemes, we identify their applicability and necessary
conditions for their use in downlink multi-cell NOMA system. After that, we
discuss different network scenarios with different spatial distributions of
users and study the applicability of CoMP schemes in these network scenarios.
To the end, a numerical performance evaluation is carried out for the proposed
CoMP-NOMA systems and the results are compared with those for conventional
orthogonal multiple access (OMA)-based CoMP systems. The numerical results
quantify the spectral efficiency gain of the proposed CoMP-NOMA models over
CoMP-OMA. Finally, we conclude this article by identifying the potential major
challenges in implementing CoMP-NOMA in future cellular systems.
