D11.2: Mobility and LBS

 

Network based source of location-information

Network based positioning is based on the fact, that the user of location based services on mobile phones is connected to the mobile communication network (e.g. GSM or UMTS based mobile networks). The network itself is constructed of many (overlapping) network-cells, whose shape is influenced by the environment (buildings, etc) and usually neither hexagonal nor a perfect circle, even though this is the usual way of drawing them (cf. ).

Figure : Cell Based Communication (CBC) and cellular communication networks.

The geographic location of the cell’s base station/transmitter is well-known and can be used as a point of reference. The position of the mobile user can be approximately determined by using cell identity information, the distance and the angel between the mobile user and base stations. Until recently this information was exclusively known by the network operator. Meanwhile Google is aiming to use cell information by own cataloguing initiatives.  

Cell of origin positioning (COO)

The most rudimentary method is the cell of origin (COO) positioning method. Thereby the location of the base station to which the mobile user is connected is considered to be the location of the user. It is more a looking up in the visitor location register than a positioning. The accuracy of the obtained location data depends on the range of the radio cells. The range of the radio cells can vary from 100 meters in urban areas up to 25 kilometres in rural areas, depending on the size of the network’s cell.

 

Time Difference of Arrival positioning (TDOA)

The Time of Difference of Arrival (TDOA) positioning method is based on at least three (synchronised) base stations, which measure the time difference it takes to receive a signal from the mobile user. This information is used to determine the distance between the user and the position relatively to the involved base stations. The location of the user is determined by using advanced triangulation techniques and cross-referencing the distance-information. Multilateration is commonly used in civil and military surveillance applications to accurately locate an aircraft, vehicle or stationary emitter by measuring the time difference of arrival (TDOA) of a signal from the emitter at three or more receiver sites.

Figure : Time Difference of Arrival (TDOC) positioning.

 

Angel of Arrival positioning (AOA)

The angel of arrival (AOA) positioning method seeks to determine a user’s location, based on the angel of the signals sent by user’ s mobile device. This is done by determining the direction of propagation of a radio-frequent wave incident on an antenna array. In order to calculate the AOA, TDOA is used at individual elements of an antenna array. From the resulting delays, the AOA and therefore the direction can be determined. Finally, using multiple base stationas and AOA, the geographical location can be determined. 

Figure : Angel of Arrival positioning (AOA).

 

Enhanced observed time difference positioning (E-OTD)

The enhanced observed time difference method (E-OTD) is an improvement of the TDOC method. It measures the time intervals of the radio signals between a base station and the mobile device and a known fixed spot (called location measurement unit). Three location measurement units are needed to determine the position. The mobile device actively participates in the positioning process in contrast to TDOC. E-OTD only works with mobile devices that include E-OTD technology. 

Figure : Enhanced Observed Time Difference (E-OTD) positioning.

 

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