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Techniques that are based on the presence of the mobile subject in the vicinity of a sensor (with a finite range and analysis capabilities) are referred to as proximity‐based localization approaches. The proximity of the mobile subject can be detected via physical contact or by monitoring a physical quantity in the vicinity of the sensor, such as a magnetic field. When a mobile subject is detected by a single sensor, it is considered to be collocated with it. Several proximity‐based localization techniques have been implemented, involving IR, RFID, and cell identification (Cell‐ID).

One of the first IR‐based proximity indoor positioning systems was the Active Badge system [102] designed at AT&T Cambridge in the 1990s. By estimating the location of active badges worn by people in the building, the Active Badge system was able to locate persons in a building. The active badges would transmit a globally unique IR signal every 15 s (with a battery life of 6 months to a year). In each room, one or more sensors were fixed and detected the IR signal sent by an active badge. Using the measured location of the people in the building, the system was able to track the employees, their location (room numbers), and the nearest telephone to reach them. The accuracy of the system was driven by the operating range of the IR sender, which was 6 m. The proximity system based on wearable IR emitters was small, lightweight, and easy to carry; however, the network of fixed sensors deployed across the building had a substantial cost associated with it. Moreover, the update rate of 15 s is too large for real‐time localization (navigation).

An RFID system consists of RFID readers (scanners) and RFID tags. The RFID reader is able to read the data from RFID tags that are either passive or active. Passive RFID tags rely on inductive coupling and operate without a battery, reflecting the RF signal transmitted to them from a reader and adding information by modulating the reflected signal. Inductive coupling allows the passive tags to receive sufficient energy in the form of RF waves from the nearby RFID reader to perform signal modulation, to transfer their unique serial ID (or other information) back to the reader. But the range of passive RFID tags is very limited (1–2 m), and the cost of the readers is relatively high. Active RFID tags are small transceivers that can actively transmit their ID (or other additional data) in response to an interrogation. Systems based on active RFID use smaller antennas and have a much longer range (tens of meters). LANDMARC [103] utilizes active RFID‐based fixed location reference tags for proximity‐based indoor location calibration.

The Cell‐ID (or Cell‐of‐Origin) method is based on the principle of capturing the ID of an anchor node that is generating the RF signal with the highest RSSI, and then identifying the mobile subject’s position as having the same coordinates as the anchor node. For example, mobile cellular networks can identify the approximate position of a mobile handset by knowing which cell site the device is using at a given time. Wi‐Fi APs can also be used to obtain the ID of the AP with the highest RSSI and perform localization with respect to that AP. In general, the localization accuracy when using cell‐ID is quite low (50–200 m), depending on the cell size (or Wi‐Fi coverage). The accuracy is often higher in densely covered areas (e.g. urban places) and much lower in rural environments [104].