Energy Harvesting WSNs with Adaptive Modulation: Inter-delivery-aware Scheduling Algorithms

In this paper, we deal with the regularity of status updates in a monitoring system. Specifically, we consider a system consisting of independent energy harvesting nodes with adaptive modulation capabilities that transmit status updates to a non energy harvesting sink over a fading channel. Due to the randomness of the energy arrival and the channel time variations, a node may have difficulties maintaining regular status updates. Hence, the objective of this work is to design scheduling algorithms that minimize the number of violations of inter-delivery time over a finite time horizon. An inter-delivery violation event occurs when the time duration between two consecutive status updates exceeds a given time limit. We focus on online modulation and power adaptation policies where the transmitting sensor node adjusts the M-ary modulation level and transmission power based on both the channel state and the battery level. Specifically, we propose both deterministic and randomized algorithms to efficiently solve the considered scheduling problem for the onenode system. Deterministic solutions were extended to the multi-node system. The numerical results show that the proposed algorithms realize significant gain in terms of violations events compared to the benchmark fixed modulation solutions.