Positioning based on the use of radio signals has a long history. There are many position estimation techniques for various purposes under different scenarios. Signal strength, angle-of-arrival (AOA), time measurements (TOA, time-of-flight, and time difference of arrival (TDOA)) can all be exploited for the position estimation. Figure 8.1 shows a general positioning system configuration with base stations or sensors and the device to be located (tag). The example below is for a cellular system・ However the concept is generally applicable・