Joint scaling of battery discharge and modulation scheme in wireless sensor networks

In wireless sensor networks (WSNs), battery lifetime directly impacts the network lifetime. One of the key considerations in the design of energy consumption modules of a sensor node should be to maximize the energy delivered by the battery; and hence the battery lifetime. In view of the main duty of a sensor node being data collection and transmission and most of the delivered energy consumed in the transceiver module, the battery lifetime is alternatively described as the amount of transmitted data. In this paper, we address the issue of maximizing the amount of transmitted data of node by jointly scaling the battery discharge profile and the modulation scheme. Considering the nonlinearity of battery, we investigate an accurate analytical battery model, which covers two key phenomena affecting the battery lifetime - the capacity effect and the recovery effect. The energy loss of battery is due to the capacity effect, which is related to the rate and profile of discharge current. Thanks to the recovery effect, the lost energy can be partially or even completely recovered, closely depending on the idle duration of node. For energy-efficient transmission, we consider commonly-used orthogonal FSK modulation scheme. By scaling the number of frames of transmitted data at a time and modulation constellation size, the discharge current rate, the discharge current profile and the period of idle operation of node vary and different amount of data is transmitted under the battery lifetime. Numerical analysis and optimization are taken and the results demonstrate that different joint schedules can have significantly different the battery lifetime. For any transmission distance, there always exists an optimal scheme, which make the total bits transmitted in the battery life is maximal.