Searching for a Lion in the Desert Acquisition Algorithms for Networks of Sensors

Networks of sensors are a promising emerging technology for high accuracy real-time data gathering in applications such as pollution monitoring, home security, surveillance, industrial control, etc. One of the possible communication modalities for sensor networks is optical wireless communication. A notable advantage of this method is the facility of passive communication where a base station illuminates an area containing sensor nodes equipped with retroreflecting mirrors. Thus, sensors can communicate with the base station, with minimal energy expenditure on the part of the sensors, by modulating the retroreflected light. Moreover, the location of the sensors can be mapped, with no energy expenditure by the sensors, using only the retroreflected light received at the base station. In this paper we analyze and compare different acquisition algorithms for networks of sensors using passive optical wireless communication. The goal of the acquisition algorithm is to locate sensors in a given area. We present a hexagonal search that is based on the binary search principle in order to locate a single sensor node. We use varying-diameter beams of constant power density to delineate the possible locations of a sensor node and reject areas where there are no sensors. The trade-off between the scan time and the achievable signal-to-noise ratio (SNR) (and consequent probability of detection) is investigated. Upon locating a sensor node, the base station can query and interrogate the node regarding its data. We compare the acquisition times for the binary search with a classical grid search that employs a small single size beam-spot.