Dual-Path Architecture for Energy Harvesting Transmitters with Battery Discharge Constraints

We consider the design of transmission policies for sensor nodes that rely entirely on harvesting energy from the environment. Nodes store the harvested energy in a storage element which has constraints on maximum discharge rate and charging efficiency. Non-zero circuit power is considered and limitations on channel bandwidth and processor clock-rate are incorporated. We assume an additive white Gaussian noise (AWGN) channel and that time is divided into frames, with a fixed number of symbols transmitted in the frame duration. We consider an energy arrival process in which energy arrives at a constant rate within a frame but varies stochastically and independently across frames. In this context, we propose a dual-path architecture for energy flow that is shown to mitigate the performance loss due to the discharge rate constraint. For a given frame, we determine the rate-optimal time sharing ratio between harvesting energy and transmitting data. We also propose three sub-optimal policies, including statistical directional water-filling, for determining the time sharing ratio for a group of frames and compare their performance with an upper bound. We highlight that the discharge rate constraint is an important limitation of the storage element that can potentially hinder the effective use of energy.