Definition: A service provided to a subscriber based on the current geographic location of the MS. Location-based services (LBS) provides service providers the means to deliver personalized services to its subscribers
LBS reflects the convergence of multiple technologies:
Internet - Geographic Information System - Mobile devices
Localization
Localization is based on analysis, evaluation and interpretation of appropriate input parameters. Most of them are related to exploitation of physical characteristics being measurable in a direct or indirect way.
From a physical localization point of view, there are three principle techniques to be distinguished:
1. Signal Strength & Network parameters:
The most basic wireless location technology is given by the radio network setup itself. Each base station is assigned a unique identification number, named CellID. The CellID is received by all mobile phones in its coverage area, thus the position of a target is derived from the coordinates of the base station. Signal strength could be used to reduce the target position. Wave propagation is highly affected by several factors, especially in urban areas, thus signal strength is altered and does not provide a reliable parameter for localization. Cell ID accuracy can be further enhanced by including a measure of Timing Advance (TA) in GSM/GPRS networks or Round Trip Time (RTT) in UMTS networks. TA and RTT use time offset information sent from the Base Transceiver Station (BTS) to adjust a mobile handset’s relative transmit time to correctly align the time at which its signal arrives at the BTS. These measurements can be used to determine the distance from the Mobile Station (MS/UE) to the BTS, further reducing the position error.
2. Triangulation/Trilateration:
In trigonometry and elementary geometry, triangulation is the process of finding a distance to a point by calculating the length of one side of a triangle formed by that point and two other reference points, given measurements of sides and angles of the triangle. Such, trigonometric methods are used for position determination. It can be distinguished as
- Distance-based (tri-)lateration (example: Global Positioning System, GPS), For distance-based lateration, the position of an object is computed by measuring its distance from multiple reference points
- Angle- or direction-based (tri-)angulation (example: AOA-Angle of Arrival, TOA-Time of Arrival,AFLT- Advanced Forward Trilateration, EOTD- Enhanced Observed Time Difference)
3. Proximity:
Proximity is based on the determination of the place of an object that is nearby a well-known reference place. Again, one distinguishes three fundamental sub methods:
- Monitoring and matching the terminal location with the database containing stamped locations(with RSSIs from different Base Stations)
- Monitoring of (WLAN Radio) Access Points. Here, it is evaluated whether a terminal is in the range of one or several APs.
Localization Categories
Different localization principles may be applied to gain position information with respect to an object that is to be tracked. Four different categories can be distinguished:
- Network-based: All necessary measurements are performed by the network (by one or several base stations). The measurements usually are sent to a common location centre being part of the core network. This centre takes over the final computation of the terminals’ positions
- Terminal-based: In the terminal-based localization approach, the terminal accounts for position determination.Disadvantages of terminal-based localization obviously are given by increased terminal complexity. Increased challenges with respect to calculation power and equipment lead to the assumption that this method is only partly applicable for legacy terminals.
- Network-assisted: Similar to terminal-based positioning, network-assisted positioning implies that the final calculation of the terminal’s position is taken over by the terminal.The difference is that possible assistance data is sent by the network. This can be done either on request or in a push-manner.
- Terminal-assisted: This too is a hybrid implementation of the other methods like above. The terminal hereby measures reference signals of incoming base stations and provides feedback reports to the network. The final position computation takes place in a central location centre within the network.
Accuracy increased with adoption of Hybrid techniques.
Types of Positioning Techniques:
So we can classify the existing location positioning techniques currently being deployed by wireless operators over the world as:
