Hybrid NOMA for an Energy Harvesting MAC With Non-Ideal Batteries and Circuit Power

We consider a multiple-access channel (MAC), where transmitters are powered by energy harvesting. They are equipped with batteries having non-ideal charging and discharging characteristics, resulting in a fractional loss of power driven into or drawn from them. Assuming that each user consumes constant power for circuit operation during transmission, we optimize the throughput region, the set of all tuples of the number of bits delivered by the users over a finite duration of time. When circuit powers are zero, it is known that a non-orthogonal multiple access (NOMA) strategy, called Pure-NOMA (P-NOMA), where user transmissions always overlap, achieves all points on the largest throughput region. We show that P-NOMA is no longer optimal with non-zero circuit power and propose a hybrid strategy called H-NOMA that combines P-NOMA with time-division multiple access (TDMA). H-NOMA allocates fixed time windows for single-user and non-orthogonal multi-user transmissions. We maximize the sum-throughput in H-NOMA with non-casual and causal knowledge of the harvested powers and channel power gains. With causal knowledge, we obtain the optimal online policy via dynamic programming and deduce some structural properties and propose a simpler suboptimal online policy that performs significantly better than a naive greedy policy. We numerically show the largest throughput regions of P-NOMA and TDMA are contained within that of H-NOMA.