Determining the location of mobile users is one of the most challenging tasks that must be undertaken in order to enable location-based service. LBS providers currently use different methods to determine locations. Three main families of positioning: Network oriented using some form of triangulation The most common non-GPS solutions for mobile positioning are: Cell of Origin, Time Difference of Arrival, Angle of Arrival and Enhanced Observed Time Difference. All make use of the wireless telecommunications system itself.  Fig. 1 Time Difference of Arrival  Fig. 2 Angle of Arrival Cell of Origin (COO) is the most straightforward solution, and uses the cell identification information within the mobile telephony network to identify the approximate location of the caller. However, this technique is often not very useful because of the low positioning accuracy. In Time Difference of Arrival (TDOA) system (Figure 1) each tower is able to measure the amount of time it takes to receive a phone's signal. They can then translate this information to estimate the distance of the phone from the tower. By cross-referencing this information from other towers in the system, a phone's position is expressed in X and Y coordinates based on longitude and latitude readings. The Angle of Arrival (AOA) (Figure 2) technique seeks to determine the location of a mobile device based on the angle at which signals transmitted from the device arrive at the base station(s). The Enhanced Observed Time Difference (E-OTD) (Figure 3) technique determines the location of a mobile device by using location receivers that are geographically dispersed across a wide area. These so-called Location Measurement Units (LMU) each have an accurate timing source. When it is possible for E-OTD (software-enabled) mobile devices and the LMUs to receive signals from at least three base stations, the time difference of arrival of the signal from each base station at the handset and at the LMU are calculated. The estimated location of the handset is calculated, based on the combination of the differences in time, through a hyperbolic positioning technique. The E-OTD technique offers an accuracy level from 50 to 125m. Global Positioning System based The technique involves standalone GPS receivers and network assisted GPS device (Figure 5). The Global Positioning System (GPS) is a satellite-based navigation system. GPS is a position, velocity and time determination system that is truly global, is able to operate 24 hours a day under all weather conditions, and charges no user fees. GPS offers comparatively high accuracy, when operational conditions are favourable.  Fig. 3 Enhanced Observed Time Difference GPS does, however, have some serious limitations due to the strong attenuation of the satellite signals by buildings, foliage, etc. Therefore GPS does not operate well (or at all) in dense 'urban canyon' areas, or inside buildings. Yet these are often the very areas where demand for location-based services is the highest. However, since GPS offers so many advantages it is considered a 'first choice' solution for many mobile positioning applications, even for the emergency services and the LBS that can be supported by the location determination technology implemented for mobile telephony. In order to overcome the problem of positioning indoors and in urban areas, some GPS receivers have been developed that can operate in weak signal environments and one such is A-GPS.  Fig. 4 GPS Based Positioning Assisted GPS (A-GPS) refers to the GPS positioning technique whereby there is assistance data provided from a special GPS server/base station by the mobile telephony network. A-GPS enables GPS positioning even in urban and indoor areas, where the signal is too weak to be acquired using standard signal tracking procedures within the receiver. A-GPS is basically of two types:
Outside the remit of 2G, 2.5G and 3G cellular networks, exist other families of positioning technologies that are often referred to as 'local positioning', which usually make use of short range networks such as 802.11, Bluetooth, RFID, ultrasound, UWB of IrDA. Furthermore, recently there have been attempts to use positioning technologies based on TV radio signals. Figure 6 shows the level of accuracy by different positioning system . |