Systems of Wireless Localization

Positioning and LBS

Location-based services (LBS) are services that have location in the core of their functionality. LBSs function in a specific manner when the location of their user or related objects is determined. In their most basic form, LBSs are based on the three pillars of geoinformation, location, and communication (Werner, 2014, p. 5). LBSs are a recently developed field of informatics and up until a few decades ago they were defined as tools that add a functionality to a well-established service rather than a separate entity. Key element to a LBS is the location and an answer to the positioning problem: determination of the location and its level of certainty.

Positioning refers to a group of tasks that eventually give the answer to the positioning problem. The medium for the solution of the problem could be found in the geographic signal space (Do, 2008). Radio signals are good candidates since they are ubiquitous and have a large variety of characteristics.

Geographic signal space, adapted from (Do, 2008)
Wireless localization

Location-based services operating in real time must run over an underlying real-time location system (RTLS). These systems are responsible for providing the LBS with location data in real time. RTLS are named after their purpose rather than their design; it is thus possible to integrate a number of technologies in one flexible system design that can be implemented in a number of industries. There is an ongoing discussion on whether the combination of RFID and GPS can be considered as a RTLS (Roberti, 2013). However, it can definitely accompany an RTLS in occasions where it is not feasible to use GPS systems. In wireless sensor networks, for example, where location data is of foremost importance it is not always possible to integrate a GPS receiver into a node for a number of reasons, mainly high costs and high energy consumption for a device that normally operates for an extended period of time (Wang & Xu, 2010). The focus has therefore shifted to GPS-free localization methods, where each node can estimate its location by calculating its distance from other nodes in relatively close proximity (Kenarangui, 2010).

Classification of wireless localization systems. The diagram classifies according to granularity (yellow), positioning method (green), ranging technique (blue) and measurement (white)

Localization systems can be classified as either coarse-grained or fine-grained. Coarse-grained systems operate under the assumption that the location of an object that interacts with a beacon is somewhere within the range of the beacon. On the other hand, fine-grained systems utilize techniques such as signal strength, angle, signal phase, or signal delay to pinpoint the exact distance between the agent and the beacon. Of course, these descriptions set the stage for advanced positioning techniques and for systems that exploit more than one methods to achieve even more precise estimations.

In a more complex system, fine-grained localization, and not mere beaconing, can be used as input for a coarse-grained system; this hybrid method is referred to as fingerprinting and is commonly implemented in indoor positioning. The end result, however, is still a coarse-grained-class result.

The location of the agent is determined by position fixing processes that include analysis of the data provided by the positioning system. The distinct process which is to be followed for the calculations can be one of the following:

  • Lateration
  • Hyperbolic lateration
  • Angulation
  • Proximity detection
  • Inertial navigation
  • Fingerprinting

In RFID-positioning implementations, lateration and fingerprinting are popular methods. This could be attributed to their simplicity and low cost, since antennas able to measure angle of arrival are more expensive. In lateration, the spatial location of an object or a person is described by a set of coordinates indicating its distance (Cartesian coordinates) or the angle and distance (Polar coordinates) from a point of reference.

[to be continued…]