Duty-Cycle Optimization in Unslotted 802.15.4 Wireless Sensor Networks

We present a novel approach for minimizing the energy consumption of medium access control (MAC) protocols developed for duty-cycled wireless sensor networks (WSN) for the unslotted IEEE 802.15.4 standard while guaranteeing delay and reliability constraints. The main challenge in this optimization is the random access associated with the existing IEEE 802.15.4 hardware and MAC specification that prevents controlling the exact transmission time of the packets. Data traffic, network topology, MAC, and the key parameters of duty cycles (sleep and wake time) determine the amount of random access, which in turn determines delay, reliability and energy consumption. We formulate and solve an optimization problem where the objective function is the total energy consumption in transmit, receive, listen and sleep states, subject to constraints of delay and reliability of the packet delivery and the decision variables are the sleep and wake time of the receivers. The optimal solution can be easily implemented on existing IEEE 802.15.4 hardware platforms, by storing light look-up tables in the receiver nodes. Numerical results show that the protocol outperforms significantly existing solutions.