Adapting radio transmit power in wireless body area sensor networks

Emerging body-wearable devices for continuous health monitoring are severely energy constrained and yet required to offer high communication reliability under fluctuating channel conditions. This paper investigates the dynamic adaptation of radio transmit power as a means of addressing this challenge. Our contributions are three-fold: we present empirical evidence that wireless link quality in body area networks changes rapidly when patients move; fixed radio transmit power therefore leads to either high loss (when link quality is bad), or wasted energy (when link quality is good). This motivates dynamic transmit power control, and our second contribution characterises the off-line optimal transmit power control that minimises energy usage subject to lower-bounds on reliability. Though not suited to practical implementation, the optimal gives insight into the feasibility of adaptive power control for body area networks, and provides a benchmark against which practical strategies can be compared. Our third contribution is to develop simple and practical on-line schemes that trade-off reliability for energy savings by changing transmit power based on feedback information from the receiver. Our schemes offer on average 9--25% savings in energy compared to using maximum transmit power, with little sacrifice in reliability, and demonstrate adaptive transmission power control as an effective technique for extending the lifetime of wireless body area sensor networks.