Energy Harvesting Communications with Batteries Having Full-Cycle Constraints

Abstract

In energy harvesting (EH) communications, it is customary to use a battery to temporarily store harvested energy prior to using it for communication. In practice, these batteries suffer from degradation in the usable capacity when they are repeatedly charged after being partially discharged and vice versa. The capacity can be recovered by imposing the full-cycle constraint, which says that a battery must be charged only after it is fully discharged and vice versa. Further, practical batteries cannot be charged and discharged simultaneously. With the above constraints, we consider and compare EH communication systems under two cases: (a) the single-battery case and (b) the dual-battery case, in which the transmitters are equipped with a single battery of capacity 2B joules and two batteries, each having capacity of B joules, respectively. Under (a) and (b), our goal is to obtain the long-term average throughputs and throughput regions in a point-to-point (P2P) channel and a multiple access channel (MAC), respectively. For the P2P channel, we derive the optimal solution in the single-battery case, and propose optimal and suboptimal power allocation policies for the dual-battery case, assuming Bernoulli energy arrivals. Based on these policies, we obtain long-term average achievable throughput regions in MACs by jointly allocating rates and powers. From numerical simulations, we find that the optimal throughput in the dual-battery case is significantly higher than that in the single-battery case, although the total storage capacity in both cases is 2B joules.