TABLE OF CONTENTS Perhaps the most basic location technique is that of binary proximity involving a simple decision of whether two nodes are within reception range of each other. A set of references nodes are placed in the environment in some non-overlapping (or nearly non-overlapping) manner. Either the reference nodes periodically emit beacons, or the unknown node transmits a beacon when it needs to be localized. If reference nodes emit beacons, these include their location IDs. The unknown node must then determine which node it is closest to, and this provides a coarse-grained localization. Alternatively, if the unknown node emits a beacon, the reference node that hears the beacon uses its own location to determine the location of the unknown node.
An excellent example of proximity detection as a means for localization is the Active Badge location system [221] meant for an indoor office environment. This system consists of small badge cards (about 5 square centimeters in size and less than a centimeter thick) sending unique beacon signals once every 15 seconds with a 6 meter range. The active badges, in conjunction with a wired sensor network that provides coverage throughout a building, provide room-level location resolution. A much larger application of localization, using binary proximity detection, is with passive radio frequency identification (RFID) tags, which can be detected by readers within a similar short range [52]. Today there are a large number of inventory-tracking applications envisioned for RFIDs. A key difference...
