Low Power Wake-Up in Wireless Sensor Networks using Free Space Optical Communications

In many Wireless Sensor Network applications the desired deployment lifetime is specified in months or years. To meet very low power requirements duty cycled medium access control protocols incur increased latency in communication. In this paper, we present the use of a secondary, very low power, wake-up channel, using Free Space Optical (FSO) transmission. The FSO channel is used to send small wake-up signals, after the wake-up is received successfully the higher power radio channel is used to transfer data packets more efficiently. The FSO receiver can be powered continuously with a consumption of 317 mu W. The performance of the FSO transceiver has been evaluated for deployments of networks in buildings. A reliable communication range of fifteen metres can be achieved with a transmitter power consumption of 16.5 mW, comparable to a WSN radio. The performance of the FSO wake-up system is compared with duty cycled radio communications and an order of magnitude improvement in latency is shown.