Indoor environments are complex because the propagation of electromagnetic signals may be influenced by the
existence of obstacles like walls, equipment, and even human beings, which causes multi-path effects. There are
various different strategies to perform Indoor Localization using RFID [24] and they may be classified based on the
strategy used to compute location, using various types of signal measurements (Received Signal Strength (RSS),
Angle Of Arrival (AOA) and Time Of Arrival (TOA)). When used this criterion to classify RFID Indoor
Localization Systems, they fall into these different categories: Proximity, Triangulation and Scene Analysis.
Proximity is the easiest approach. When the object we intend to locate enters in the radio range of a Reference
Point their location is assumed to be the same. Typically when using this approach the Reference Point is a RFID
Reader and the object to locate is carrying a RFID tag. Triangulation approach, on the other hand, uses the geometric
properties of triangles to determine the location of an object and has two variants: lateration and angulation.
Lateration estimates the location of an object by measuring its distance from multiple reference points. Angulation
locates an object by computing angles relative to multiple reference points. Finally Scene Analysis uses a completely
different approach. It can be divided into two phases: an offline phase and an online phase. In the first phase, called
the offline phase, information concerning the localization area is collected and stored in a fingerprinting map. Then
in the online phase, online measurements are taken and compared with those previously observed in the offline
phase in order to infer the object position. Due to the very limited capabilities of RFID tags the Proximity approach
is widely used, however it may present low accuracy.
RFID Localization can also be classified depending on the different roles assumed by RFID tags and readers.
When using Reader localization scheme, the RFID reader is attached to the object that is intended to locate and
RFID tags are scattered in the localization area (typically the floor) [25]. In the Tag localization scheme, the opposite
is proposed: the object to locate carries the RFID tag and the Reference Points are implemented by RFID readers
installed in the localization area [26]. Both Reader and Tag localization may present the same accuracy, however