Diphase: Characterizing Packet Delay in Multi-Source Energy Harvesting Systems

We consider multi-source energy harvesting communication systems, where the energy harvested from two independent processes is used for the transmission of the data packets. The data packets arrive randomly and wait in a queue for accumulation of sufficient energy and for service completion of previously arrived packets. Thus, the data queue dynamics are influenced jointly by the energy arrival process, the data arrival process, and the data service process. This coupling between the data and energy queues makes an exact system analysis extremely hard, and has led researchers to resort to either computationally intensive numerical solutions or to make simplifying approximations. In this paper, we employ Diphase, a two phase queueing formulation, which decouples the wait stages for the energy arrival process and the service process, to derive closed-form expressions for the average packet delay and the probability of data packet loss due to buffer overflow. These expressions are shown to be exact when the service time is negligible, and robust for a relatively wide range of values of the average service time. We show that these expressions are useful in selecting system design parameters, which maximize the throughput while meeting the required quality of service constraints.