Revisiting Effectiveness of Energy Conserving Opportunistic Transmission Schemes in Energy Harvesting Wireless Sensor Networks

Opportunistic transmission schemes improve the lifetime of conventional wireless sensor networks (WSNs) by reducing the number of transmissions. However, this comes at the expense of performance since fewer measurements are available. We show that in energy harvesting (EH) WSNs, in which the sensor nodes harvest energy from the environment, this trade-off is fundamentally different. For a general model in which the nodes experience independent and non-identical fading and the EH process at a node is stationary and ergodic, we present lower bounds on the mean squared error (MSE) for two important classes of channel-based opportunistic transmission schemes, namely, censoring and ordered transmissions. For the latter, we present two novel variants that arise depending on whether the energy in the battery of an EH sensor node is accounted for before ordering or not. For censoring, the lower bound leads to an insightful and explicit characterization of the optimum censoring threshold for each node. For ordered transmissions, it helps determine the optimal number of nodes that should be selected to transmit. We find that the ordered transmission schemes can outperform the censoring scheme. We also propose a hybrid scheme that combines the best features of the above schemes for EH WSNs.