Cellular-Based Positioning Technologies

In this section, we shortly discuss about GSM/UMTS positioning technologies nowadays used by cellular telecommunication companies to enable LBS for cus-tomers and third-party companies.

3.2.1.1 Technology

Two of the most widely used telecommunication systems for mobile telephones are GSM and UMTS protocols [8]. These protocols are based on a network architecture calledcellular. In general, in a cellular architecture, a geographical area is covered by a number ofantennas(or base transceiver stations, BTS) emitting a signal to be received by mobile devices. Each antenna covers an area called cell. In this way, the covered area is partitioned in a number of, possibly overlapping, cells, uniquely identified by the antenna. In urban areas, cells are close to each other and small in area (even micro- or picocells), the diameter can be from 100 m to a kilometer.

In rural areas, the radius of a cell can reach a maximum of 30 km. The presence of a device in a cell is detected by the system periodically to maintain correctness and validity of the location information subsystem. Position data are maintained in GSM/UMTS location databases (called VLR/HLR – visitor/home location reg-ister) [7]. These registers maintain location information within substructure of the

network such as location areas, a group of antennas covering a quite extended area (100–300 km²). The actual location of a device in GSM network is registered at this level. However, since these registers are specialized in routing rather than position-ing, with the advent of LBS in last few years, another class of location register has been introduced in network vendors. These registers are specialized in retrieving and calculating the position of subscribers. Furthermore, the introduction of the serving mobile location center (SMLC) server allows to calculate the location information and an estimation of the achieved accuracy.

3.2.1.2 Data Format

We have different data format to represent location information, depending on which method is used to calculate the position on the network. For example, antenna-based location information is represented by the spatial information as the position of the antenna plus some additional information, such as its orientation, coverage area, and signal power. In some methods, it is possible to use location information derived by two, three, or more antennas, to obtain a more precise and reliable information. In general, data format of the location information should indicate:

– The time of detection that can be represented by a tuple indicating the detection date and time, e.g., dd–mm–aaaa and hh:mm:ss GMT + hh

– A position information represented by a precise location with some geographic reference system (WGS84, UTM32, etc.)

– The identifier of the detected mobile user such as its international mobile subscriber(IMSI) and telephone number (MSISDN)

3.2.1.3 Methods

Various methods can be used to calculate an approximate position of a mobile device. Methods aredevice independentwhen no further requirements are needed on the handset to be localized. Similarly, they arenetwork independentwhen they do not require additional technology to be implemented in the network infrastructure.

Figure 3.2 shows an overview of all accuracy levels of various methods. We can note that the most accurate is GPS method and its accuracy level decreases in urban areas due to high buildings and a number of other obstacles.

In the following, we briefly discuss the most used cellular positioning techniques.

3.2.1.4 Cell Identity

In the cell identity (CI) positioning method, the location of a device is identified by the cell where it is connected. This information is available at the network as well as at the handset. The antenna identifier is converted to a geographic position by means of the existing knowledge residing in the coverage database SMLC. Accuracy

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Fig. 3.2 Position information accuracy (with respect to range of cells)

depends on the cell size and the antenna type (circular or sectorial) and can vary from 100 m to few kilometers. This method can be improved withtiming advance, a measure of the distance between the antenna and the device.

3.2.1.5 Cell Identity and Timing Advance

This method improves CI using measurement reports that contain power level at the handset from the serving cell and cells on the neighbor list. The power level at the handset can be used to estimate distances from device to antenna using simple wave propagation models. Accuracy is slightly better than CI, and depends on the location of antennas and environmental conditions that can affect the signal strength.

3.2.1.6 Enhanced Observed Time Differences

Inenhanced observed time differences(E-OTD) positioning method, the handset measures the arrival time of signals transmitted from three or more antennas. Two specific methods can be implemented depending on the available underlying tech-nology: E-OTD MS assisted and E-OTD MS based. In MS assisted technology, measurements are made by the handset and then transferred to the SMLC that cal-culates, by triangulation, the position of the device. In the MS-based E-OTD, the position calculation function resides at the handset and the calculated position is returned to the SMLC. Accuracy can vary from 50 to 100 m. It is worth noting that, in terms of resources, E-OTD is a very expensive method since it needs some addi-tional and specific equipments to be added to the network. For this reason, it is not widely used by LBS vendors that prefer some cheaper and simpler solution, such as assisted GPS(A-GPS).

3.2.1.7 Assisted GPS

In A-GPS, the handset measures the arrival time of signals transmitted from three or more satellites (satellite-based methods are described in detail in Sect. 3.2.2).

This technology has a quite low impact on the network because it requires only the support at the SMLC level. Positioning performances are better in rural space and poor in urban space where buildings and other obstacles disturb signals from satellites, thus accuracy can vary from 2 m in rural areas to 20 m in urban areas.

This method is quite efficient, in terms of quality/cost ratio, and reliable in terms of quality of given information. At the other side, it has a technological dependence since the handset needs to be GPS compliant to receive signals from satellite.