  This paper investigates the optimal resource allocation of a downlink
non-orthogonal multiple access (NOMA) system consisting of one base station and
multiple users. Unlike existing short-term NOMA designs that focused on the
resource allocation for only the current transmission timeslot, we aim to
maximize a long-term network utility by jointly optimizing the data rate
control at the network layer and the power allocation among multiple users at
the physical layer, subject to practical constraints on both the short-term and
long-term power consumptions. To solve this problem, we leverage the
recently-developed Lyapunov optimization framework to convert the original
long-term optimization problem into a series of online rate control and power
allocation problems in each timeslot. The power allocation problem, however, is
shown to be non-convex in nature and thus cannot be solved with a standard
method. However, we explore two structures of the optimal solution and develop
a dynamic programming based power allocation algorithm, which can derive a
globally optimal solution, with a polynomial computational complexity.
Extensive simulation results are provided to evaluate the performance of the
proposed joint rate control and power allocation framework for NOMA systems,
which demonstrate that the proposed NOMA design can significantly outperform
multiple benchmark schemes, including orthogonal multiple access (OMA) schemes
with optimal power allocation and NOMA schemes with non-optimal power
allocation, in terms of average throughput and data delay.
