TABLE OF CONTENTS Chapters 3, 4, and 5 described distance measurement and location methods that are based on propagation time the time for electromagnetic radiation to propagate from a transmitter to a receiver. In this chapter, methods of relating distance to received signal strength (RSS) are discussed. On the average, signal strength at a receiver decreases as distance from the transmitter increases. If the relationship between signal strength and distance is known, analytically or empirically, the distance between two terminals can be determined. When several base stations and a target are involved, triangularization can be applied to determine the target's location.
RSS has several advantages over the TOF methods. It can be implemented on an existing wireless communications system with little or no hardware changes. All that's needed is the ability to read a RSSI (received signal strength indicator) output that is provided on virtually all receivers, and to interpret the reading using dedicated location estimation software. The modulation method, data rate, and system timing precision are not relevant. Coordination or synchronization between the initiator and the responder for distance measurement are not required. Thus location capability can be added to a wireless system for very low incremental cost.
On the other hand, there are specific problems in implementing location awareness with the RSS method. Because of large variations of signal strength due to interference and multipath on the radio channel, location accuracy is generally less than what can be achieved using TOF methods. Propagation is location/environment...
