Opportunistic Cooperation in Low Duty Cycle Wireless Sensor Networks

Energy efficient asynchronous duty cycle MAC protocols are crucial to the success of wireless sensor networks (WSNs). In asynchronous protocols, each node operates its active/sleep schedule independently and enjoys a very low duty cycle when there is no traffic. However, when a node has data to send, it has to keep active and wait until the receiver wakes up due to the lack of schedule knowledge of the receiver. In low duty cycle networks, where nodes wake up infrequently, a sender usually suffers a long period of waiting, which consumes more energy than transmitting data packet itself. In this paper, we propose a new asynchronous MAC protocol, called OCMAC, which decreases the waiting time of sender by exploring opportunistic cooperation among senders. In OC-MAC, neighboring active senders are permitted to exchange data with each other aggressively when waiting for receivers to wake up. After delegating data to another sender, a sender can go to sleep before its receiver wakes up. Though this cooperation works only when there are multiple neighboring active senders, itself almost incurs no additional overhead. Simulation results and measurements on a testbed have shown that OC-MAC helps decrease idle listening, collision and end-to-end delay further.