Energy Harvesting Communications With Batteries Having Cycle Constraints

Practical energy harvesting (EH) based communication systems typically use a battery to temporarily store the harvested energy prior to its use for communication. The battery capacity can quickly degrade with time if it is subject to repeated shallow charge-discharge cycles. This motivates the <italic>cycle constraint</italic> which mandates that a battery must be charged only after it is sufficiently discharged and vice versa. We consider a Bernoulli energy arrival model, and a half-duplex battery constraint. In this context, we study EH communication systems with: (a) a <italic>single battery</italic> with capacity <inline-formula> <tex-math notation="LaTeX">$2{B}$ </tex-math></inline-formula> units and (b) <italic>dual batteries</italic>, each having capacity of <inline-formula> <tex-math notation="LaTeX">${B}$ </tex-math></inline-formula> units. The aim is to obtain the best possible long-term average throughputs in point-to-point (P2P) channels and multiple access channels (MAC). For the P2P channel, we obtain an analytical optimal solution in the single battery case, and propose optimal and suboptimal power allocation policies for the dual battery case. We extend these policies to obtain 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 can be more than twice of that in the single battery case, although the total energy storage capacity in both cases is <inline-formula> <tex-math notation="LaTeX">$2{B}$ </tex-math></inline-formula> units.