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1.2.3. Positioning

If we do not consider the manual input of the position as a location method a general classification of positioning methods can be done into two groups: The first group is called network-based positioning. Here a tracking and evaluation of the user location is done by using the base station network (see Figure A below). Therefore the mobile device sends either a signal or is sensed by the network. The second positioning group is called terminal-based positioning. Here, the location is calculated by the user device itself from signals received from base stations. The most famous example for a terminal-based system is the use of the Global Positioning System (GPS). The base stations for the GPS system are the GPS-satellites (see Figure B below). Finally a third group of positioning techniques emerges from combination of network and terminal positioning techniques.

Figure A: network positioningFigure A: network positioning device positioningdevice positioning

The basic principle for the calculation of the user position, valid for all groups, is that base stations have a known position at all times. Usually distances and/or angles from the sender to the receiver can be measured. The following basic techniques are often used for positioning, mostly in combination:

  1. Cell of origin (COO), Cell-ID, location signature, location beacons: The cell of origin or Cell-ID is usually the identifier of the nearest base station, e.g. a mobile phone antenna. With this technique the position is known in a defined circle or cell around the base stations known position. In the context of mobile phone networks COO is usually termed Cell-ID. But COO positioning is also possible with other wireless networks such as WLAN or Bluetooth. Beacons, e.g. infrared, ultrasound or termRFID, are used mostly indoors. Here beacons have an identifier id or transmit their exact position to the mobile device which is in reach.
  2. Time of Arrival (TOA): As electromagnetic signals move with light speed. Knowing the speed and the time difference between sending and receiving the distance can be computed. Light speed is approx 300’000km/s thus the runtimes are very short and exact timers are needed. The same principle can also be used for slower signals like ultrasound.
  3. Time Difference of Arrival (TDOA), Enhanced Observed Time Difference (E-OTD): these techniques do also compute the distance by measuring the runtime, but in difference they use therefore the time difference between the signals of usually three different base stations. Thus having signals from different neighbouring base stations the position can be triangulated. In the case of TDOA the calculation of the position is done by the network provider, in the case of E-ODT it's done in the mobile device.
  4. Angle of Arrival (AOA), Direction of Arrival (DOA): by using antennas with direction characteristics the angle of arrival in the mobile device can be detected. Because of a moving mobile device this is not very exact. Another possibility is that many base stations have segment antennas (usually 2-4) which divide the circumcircle of the base station in segments of 90, 120 or 180 degrees.
Time Difference of Arrival PositioningTime Difference of Arrival Positioning Angle of Arrival PositioningAngle of Arrival Positioning

The currently two most common position technologies are the already mentioned GPS and the position evaluation using the Cell-ID from the nearest base transceiver station, a network method. Whereas GPS delivers a very accurate position (accuracy up to 5m) does the Cell-ID deliver a very coarse position (accuracy between 100m to km). Especially GPS is (currently) a outdoor positioning method. To obtain indoor positions with high accuracy, as needed for instance in museums or shopping malls, localisation methods based on WLAN, Bluetooth or infrared technologies should be applied.

The Figure shows a number of positioning methods with their accuracy and their applicability to indoor and outdoor user activities. As a rule over the thumb one can say on one hand that network positioning is useful for LBSs where precision is not critical. Here, the Figure below shows the usually lower positioning accuracy of network methods. On the other hand the terminal based positioning is to recommend for LBSs where precision is important: e.g. dispatch, driving directions or billing (Lopez 2004).

Positioning methods,      accuracy and application. (AGPS: Assisted GPS, AOA: Angle of Arrival, TOA: Time      of Arrival, E-OTD: Enhanced Observed Time Difference)Positioning methods, accuracy and application. (AGPS: Assisted GPS, AOA: Angle of Arrival, TOA: Time of Arrival, E-OTD: Enhanced Observed Time Difference)

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