Sensor networks using free-space optical communication have been proposed for several applications, including environmental monitoring, machine maintenance, and area surveillance [3]. Such systems usually consist of many distributed autonomous sensor nodes and one or more interrogating transceivers. Typically, instructions or requests are sent from a central transceiver to sensor nodes, using a modulated laser signal (downlink). In response, information is sent from the sensor nodes back to the central transceiver, using either active or passive transmission techniques (uplink). To implement active uplinks, each sensor node is equipped with a modulated laser. In contrast, to implement passive uplinks, the central transceiver illuminates a collection of sensor nodes with a single laser. The sensor nodes are equipped with reflective modulators, allowing them to transmit back to the central transceiver without supplying any optical power. As an example, the communication architecture for Smart Dust [3,6], which uses passive uplinks [3], is shown in Figure 7.3. A modulated laser sends the downlink signals to the sensor nodes. Each sensor node employs a CCR [3] as a passive transmitter. By mechanically misaligning one mirror of the CCR, the sensor node can transmit an on–off keyed signal to the central transceiver. While only one sensor node is shown in Figure 7.3, typically, there are several sensor nodes in the camera field of view ...

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