  This paper proposes and optimizes a cooperative non-orthogonal
multiple-access (Co-NOMA) scheme in the context of multicell visible light
communications (VLC) networks, as a means to mitigate inter-cell interference
in Co-NOMA-enabled systems. Consider a network with multiple VLC access points
(APs), where each AP serves two users using light intensity. In each cell, the
weak user (the cell edge user) can be served either directly by the VLC AP, or
through the strong user that can decode the weak user's message and forward it
through the radio-frequency (RF) link. The paper then considers the problem of
maximizing the network throughput under quality-of-service (QoS) constraints by
allocating the powers of the users' messages and APs' transmit powers, and
determining the serving links of each weak user (i.e., VLC or hybrid VLC/RF).
The paper solves such a non-convex problem by first finding closed form
solutions of the joint users' powers and link selection for a fixed AP power
allocation. The APs' transmit powers are then iteratively solved in an outer
loop using the golden section method. Simulation results show how the proposed
solution and scheme improve the system sum-rate and fairness as compared to
conventional non-orthogonal multiple-access (NOMA) schemes.
