  We study a wireless-powered uplink communication system with non-orthogonal
multiple access (NOMA), consisting of one base station and multiple energy
harvesting users. More specifically, we focus on the individual data rate
optimization and fairness improvement and we show that the formulated problems
can be optimally and efficiently solved by either linear programming or convex
optimization. In the provided analysis, two types of decoding order strategies
are considered, namely fixed decoding order and time- sharing. Furthermore, we
propose an efficient greedy algorithm, which is suitable for the practical
implementation of the time-sharing strategy. Simulation results illustrate that
the proposed scheme outperforms the baseline orthogonal multiple access scheme.
More specifically, it is shown that NOMA offers a considerable improvement in
throughput, fairness, and energy efficiency. Also, the dependence among system
throughput, minimum individual data rate, and harvested energy is revealed, as
well as an interesting trade-off between rates and energy efficiency. Finally,
the convergence speed of the proposed greedy algorithm is evaluated, and it is
shown that the required number of iterations is linear with respect to the
number of users.
