The mobile phone as a platform for assisting the independent living ...

4.2.1 Cell-based methodsCell-based methods [29] determine the location of the traveller based on only thevisibility of beacons, without using any distance, angle or signal parametersmeasurements. Localization is based on the knowledge of the limited range of each ofthe beacons, allowing the traveller to be localized to the region of intersection of theranges of all visible beacons. Given the problems with many of the other methods, cellbasedmethods are quite popular for Bluetooth, as well as for RFID and infrared (IR)technologies.An example of how the cell-based localization approach works is shown in Fig.4.Consider five beacons, A, B, C, D, and E, with ranges of varying shapes and sizes. Nowsuppose the traveller is at some location from which beacons B, C, and D are visible,while beacons A and E are not visible. We may conclude that the traveller is located inthe shaded region. Note that we use information on both visibility and non-visibility ofbeacons to determine the region of the traveller. So, if we assume that the location andrange of each beacon in an indoor environment is known, then it is simple to determinethe location of a travelling mobile device based on the visibilities of beacons using themethod illustrated by the example above.Fig. 4: Cell based localization 4An important consideration in cell-based methods is the shape and size of each cell, i.e.,the range of each beacon. In outdoor applications, it is often reasonable to assume cellsof a regular shape, such as an ellipsoid. However, in indoor applications, such anassumption is not realistic due to channelling and other artifacts of buildings and theircontents. For example, it is common for signals to travel along hallways for largedistances, but to attenuate rapidly in a transverse direction due to intervening walls and4 Image courtesy of Sudarshan S. Chawathe, Beacon placement for indoor localization using Bluetooth[29]24