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LBS - Location Based ServicesCristina F. Estorninho and João S. Neto

Abstract—This paper presents a simple introduction to Loca-tion Based Services applications and techniques of implementa-tion in mobile communications. LBS applications in conjunctionwith technologies like Wireless networking (WiFi), cellular tele-phone (GSM), packet radio, radio frequency identifiers (RFID),smart personal object technology (SPOT), global positioningsystems (GPS), and sensor networks allow mobile users to querytheir environment and they allow these applications to monitorand track remote objects. All these applications have strongspatial components – object location, proximity, and connectivityare the central organizing principle of these applications.

Index Terms—LBS, spatial data, GIS, SDB, location tech-niques, GPS, GSM

I. INTRODUCTION

THERE has been an explosion of technologies that enableto communicate with mobile and occasionally-connected

devices and sensors. Technologies like wireless networking(WiFi), cellular telephone (GSM), packet radio, radio fre-quency identifiers (RFID), smart personal object technology(SPOT), global positioning systems (GPS), and sensor net-works are already being implemented in several projectsaround the world, and many completely new communicationinnovations will surely arise in the near future. These tech-nologies enable new applications like allowing mobile users toquery their environment and applications to monitor and trackremote objects. These users can search for nearby services– for example a restaurant, and how to get there from theircurrent location. Emergency services, and taxi dispatchers cansend the closest vehicle to where it is needed. In a similarapproach, monitoring systems can track the flow of goodsand monitor environmental parameters. Services like railroads,airfreight, wholesalers, retailers, and other transportation in-dustries can track dispatched goods from their source totheir final destination. Environmental systems can monitor airquality, noise, stream flow, and other environmental param-eters. Implementing such applications require strong spatialcomponents for object location, proximity, and connectivity.

This document starts by presenting an overview of the widerange of LBS applications that can be developed in severalcommercial areas, namely the various categories of applica-tions and the notions of horizontal and vertical services. Itfollows by explaining the communication model and pointingrelated industry issues for LBS applications so that the tech-nological and economic challenges that have been arisen over

Cristina F. Estorninho is with the Escola Superior de Tecnologia de Setúbalof the Polytechnic Institute of Setubal, 2910-761 Setúbal Portugal (email:042165913@estsetubal.ips.pt)

João S. Neto is with the Escola Superior de Tecnologia de Setúbalof the Polytechnic Institute of Setubal, 2910-761 Setúbal Portugal (email:032165240@estsetubal.ips.pt)

the passed few years can be evidenced. Technological aspects,from the point of view of navigation systems, of how can LBSapplications be supported by effective and efficient retrievaland management of geospatial data are also referenced, namelythe perspective of spatial databases. In the second part ofthis document an approach of data management and servicesis made, referencing the set of services that facilitate thedevelopment and deployment of distributed applications inheterogeneous environments (middleware systems). Notions oflocation techniques are also referenced so that an overview ofhow can the LBS be a possibility with the communicationtechnologies available, as well as how satellite based systems(GPS) and Networked based systems (GSM, UMTS) work toprovide location services to the end-users.

II. WHAT IS LBS

This section provides an overview of the concept of Lo-cation Based Services, the possible application scenarios andissues related to the lack of standardization available at themoment for the usage of LBS.

A. Definition of LBSLocation-based services (LBS) is a recent concept that de-

notes applications integrating geographic location (i.e., spatialcoordinates) with the general notion of services. With the rapiddevelopment of mobile communication, these applicationsrepresent a new challenge both conceptually and technically.In the near future, most of these applications will be part ofeveryday life since computers, personal digital assistants andcell phones are rapidly evolving, providing more processingpower and storage space, within other characteristics essentialto deploy LBS based applications. The concept of LBS can bedefined as services that integrate a mobile device’s location orposition with other information so as to provide added valueto a user [1].

Location Service have been around since the 1970’s withthe worldwide known Global Positions Systems (GPS) developby the United States Department of Defense, but it was onlyin the next decade when the U.S government decided tomake the system’s positioning data freely available to otherindustries around the world that these industries have takenup the opportunity to access position data through GPS andnow use it to enhance their products and services.

The traditional positioning systems, have their locationinformation typically derived by a device and with the help ofa satellite system (i.e., a GPS receiver). However, widespreadinterest in location-based services and the mobile communi-cation technology has really started to boost only in the late1990’s, when a new type of localization technology and newmarket interest in data services was sparked by mobile network

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operators. In late 90’s, mobile networks were widely deployedin Europe as well in other parts of the world, and incomefrom telephony services had proven to be significant to mobileoperators. Yet, even though mobile voice services continue tobe a major revenue generator for mobile operators, they havestarted to look around for means to find new areas for futuregrowth. One major way to reap additional financial benefitsof mobile networks apart from voice is to offer data services,many of which will be location enhanced.

B. Historical PerspectiveThe origins of LBS remotes to 1996 with the launch in

the US of the E911 - Enhanced 911. This service was formobile-network operators to locate emergency callers withprescribed accuracy, so that the operators could deliver acaller’s location to the appropriate emergency response units[9]. The technology of cellular networks was insufficient at thetime to fulfill the accuracy demands of the US government,so the operators made an increased effort to develop thetechnology so that the positioning methods could be improved.

To pay the investment made in the E911 service, commercialLBS’s were launched, mainly based in finder services ondemand that weren’t well accepted by the users, so theoperators phased out these services.

By 2004, operators were offering fleet management, chil-dren and pets tracking services, mainly based in low accuracyposition techniques (Cell-ID technology) [9]. LBS got hissecond wind by the year 2005, when GPS-capable mobiledevices, the Web 2.0 paradigm and 3G broadband wirelessservices entered the market [9].

C. Horizontal and Vertical MarketThe location market is developed around both business and

consumer services and can be grouped into a vertical andhorizontal service sphere. The vertical market is characterizedby users drawn from industry environments where the manage-ment of mobile location information is and has always been anintegral part of the business. As for the horizontal market, thisis characterized by users drawn from industry environmentswhere the use of mobile location information is a new andadded value to existing services.

Table IVERTICAL AND HORIZONTAL MARKETS

D. Application ScenariosThe main usage areas of location services are military

and government industries, emergency services, and the com-mercial sector. As previously mentioned, GPS was the firstknown location system used by the U.S Department of Defense

primarily to serve military purposes. Since it has been freelyavailable worldwide, this technology has encouraged otherindustries to develop their applications.

Emergency services represent a very obvious and reasonableapplication area where the deployment of location technologymakes sense. In many cases, persons calling a so-calledemergency response agency (e.g., police, fire department) areunable to communicate their current location or they simplydo not know it. In Europe, statistics reveal that 50% to 70%of the 80 million ‘‘real’’ EU-wide emergency calls each yearoriginate from mobile phones.As a result, the EU Commissionasked member states to develop national regulations for mobileoperators enforcing the automatic positioning of emergencycalls to the extent technically feasible, which means that unlikesome other continents, European regulators do not enforce thehighest accuracy levels such as GPS for locating emergencycases. Although GPS allows a cell phone to be locatedaccurately, European operators have the right to start out withthe accuracy levels their mobile networks can provide rightnow. So they’ve implemented the so-called Cell-ID technologyfor mobile positioning:

• 100 meters or less accuracy in urban areas• Only up to 3-kilometer accuracy in rural areas

As for commercial services, virtually infinite solutions can beimplemented in this area. The level of accuracy determinesthe usability of the services:

Figure 1. Usability of commercial services

E. Classification of Location Based ApplicationsSeveral analysts and researchers all over the world have

taken several approaches in order to classify LBS applications.A major distinction of services is whether they are person-oriented or device-oriented:

• Person-oriented applications: Applications where a ser-vice is user-based, turning the focus of the applicationsdetermining the position of a person or to use the positionof a person to enhance a service. Usually, the personlocated can control the service. Examples of such appli-cations can be social networking, where the objective isto locate friends or family with the consent of the user.

• Device-oriented applications: Applications that are exter-nal to the user, that may also focus on the position of aperson, but they do not need to. Instead of only a person,an object (e.g., a car) or a group of people (e.g., a fleet)

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could also be located. In device-oriented applications, theperson or object located is usually not controlling theservice. Examples for this kind of applications can becar tracking for theft recovery, where the car is sendinginformation without human intervention.

F. Classification of Location Based ServicesAs for the classification of the services that can be imple-

mented for location purposes, two types of application designare being distinguished: push services and pull services:

• Push Services: This kind of services imply that theuser receives information as a result of his whereaboutswithout having to actively request it. The informationmay be sent to the user with prior consent (e.g., asubscription-based terror attack alert system) or withoutprior consent (e.g., an advertising welcome message sentto the user upon entering a new town).

• Pull Services: In contrast to the push services, a useractively uses an application and, requests informationfrom the network. This information may be location-enhanced (e.g., where to find the nearest hotel).

G. LBS Communication ModelTechnologically, the implementation of LBS can be de-

scribed by a three-tier communication model, including apositioning layer, a middleware layer, and an applicationlayer.

Figure 2. LBS Three-tier Communication Model

• Positioning layer: Responsible for calculating the positionof a mobile device or user. It does so with the help ofposition determination equipment (PDE) and geospatialdata held in a geographic information system (GIS).Whilethe PDE calculates where a device is in network terms,the GIS allows it to translate this raw network informationinto geographic information (longitudes and latitudes).The end result of this calculation is then passed on viaa location gateway either directly to an application or toa middleware platform. Originally, the positioning layerwould manage and send location information directly toan application that requests it for service delivery.

• Application layer: Also known simply as client, com-prises all of those services that request location data tointegrate it into their offering.

• Middleware layer: As the LBS application market grows,many network operators have put this layer between thepositioning and application layer, primarily because PDEsits close to the core of a mobile operator’s network,leading to complex and lengthy retrieving of each userdata service. This layer can significantly reduce the com-plexity of service integration because it establishes onesingle connection to the network, and then mitigates andcontrols all location services added in the future, savingoperators and third-party application providers time andcost for application integration [1].

Figure 3. Middleware Model

Simplifying application integration is important for mobileoperators in order to move to a so-called wholesale model forlocation data. The wholesale approach means that operatorsoffer a kind of bulk access to the location of devices. Togive an example of how could this bulk access to locationinformation be used there’s the fleet management serviceswhere company’s offering such services have to buy theinformation of the location of cars from mobile operators.

The problem with this wholesale model is that privacy issuesarise with the offering of location data from operators. Here,location middleware can fulfill another role depending of it’susage in downstream or upstream:

• Downstream: Allows users to manage location accessrights of third-party applications.

• Upstream: Systematically anonymizes location informa-tion revealed.

Thus, the location middleware takes over a similar role as ananonymizing proxy does on the Internet. In this way, manyprivacy concerns are addressed by an operator. Also, users getdirect access to manage their privacy.

III. SPATIAL DATABASES

In various fields there is a need to manage geometric,geographic, or spatial data, which means data related to space.

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The space of interest can be, for example, the two-dimensionalabstraction of the surface of the earth, or parts of it (geographicspace). The term “spatial database system” is associated witha view of a database as containing sets of objects in spacerather than images or pictures of a space. It is considered thatspatial DBMS provide the underlying database technology forgeographic information systems (GIS) and other applications.

A. GIS - Geographic Information SystemsGIS is known to be a technological field incorporating

geographical features with tabular data in order to map,analyze, and assess real-world problems. The key word tothis technology is Geography, which means that the data (orsome portion of the data) is spatial (data that is in some wayreferenced to locations on the earth). Coupled with this datais usually tabular data known as attribute data, that can begenerally defined as additional information about each of thespatial features. An example of this would be schools. Theactual location of the schools is the spatial data. Additionaldata such as the school name, level of education taught, studentcapacity would make up the attribute data. It is the partnershipof these two data types that enables GIS to be such an effectiveproblem solving tool through spatial analysis.

GIS operates on many levels. On the most basic level, GISis used as computer cartography, i.e. mapping. The real powerin GIS is through using spatial and statistical methods toanalyze attribute and geographic information. The end resultof the analysis can be derivative information, interpolatedinformation or prioritized information [1].

Typically, GIS is integrated by several components:• Hardware: Equipment needed to support the many activ-

ities of GIS, such as data collection and data analysis.The workstation, which runs the GIS software and isthe attachment point for ancillary equipment, it’s themain component Data collection requires a digitizer forconversion of hard copy data to digital data and a GPSdata logger to collect data in the field. With the adventof web-enabled GIS, web servers have also become animportant piece of equipment for GIS.

• Software: The GIS application package is essential forcreating, editing and analyzing spatial and attribute data,therefore this package contain a myriad of GIS functionsinherent to it. Extensions or add-ons are software thatextends the capabilities of the GIS software package.Component GIS software is the opposite of applicationsoftware. Component GIS seeks to build software applica-tions that meet a specific purpose and thus are limited intheir spatial analysis capabilities. Utilities are stand-aloneprograms that perform a specific function. For example,a file format utility that converts from on type of GISfile to another. There is also web GIS software that helpsserve data through Internet browsers.

• Data: The heart of any GIS. Data is divided in twoprimary types that are used in GIS: a geodatabase isa database that is in some way referenced to locationson the earth and geodatabases are grouped into twodifferent types: vector and raster. Vector data is spatialdata represented as points, lines and polygons, and as

for raster data is cell-based data such as aerial imageryand digital elevation models. Together with this data isusually data known as attribute data, generally defined asadditional information about each spatial feature housedin tabular format. Documentation of GIS datasets isknown as metadata, which contains such information asthe coordinate system, when the data was created, whenit was last updated, who created it and how to contactthem and definitions for any of the code attribute data.

• Trained personnel: Well-trained people knowledgeable inspatial analysis and skilled in using GIS software areessential to the GIS process.

Figure 4. GIS Components

IV. LOCATION TECHNIQUES

Systems that determine the location of a mobile objects canbe divided into two categories [8]:

• Tracking: when a sensor network determines the location.The object to track has to be equipped with a specific tagor badge that allows the sensor network to acquire it’sposition. The location information is first available in thesensor network. If the mobile object needs it’s locationdata, the sensor network has to transfer this informationto it by wireless communication.

• Positioning: when a system of transmitters or beaconssends out radio, infrared, or ultrasound signals. Thelocation is directly available at the mobile system anddoes not have to be transferred wirelessly. In addition,location information is not readable for other users, thusthe positioning system does not have to consider privacyissues.

Systems that use tracking as well as positioning are basedon the following various basic techniques, often used incombination:

• Cell of Origin (COO): Technique used if the positioningsystem has a cellular structure. Wireless transmittingtechnologies have a restricted range (i.e., a radiated signalis available only in a certain area around the cell). Ifthe cell has a certain identification, it can be used todetermine a location.

• Time of Arrival (TOA), Time Difference of Arrival(TDOA): Electromagnetic signals move at a very highspeed (light speed - approximately 300,000 km/s), the

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corresponding runtimes are very short. If a nearly con-stant light speed is assumed, the time difference betweensending and receiving a signal to compute the spatialdistance of transmitter and receiver can be used. A similarprinciple can be used with ultrasound. The signals take alonger time, thus measurement is simpler, but ultrasoundcan only reach low distances. If the time differencebetween two signals is measured, the term TDOA is used.In GSM networks, the term Enhanced Observed TimeDifference (E-OTD) is often used instead of TDOA.

• Angle of Arrival (AOA): Using antennas with directioncharacteristics, the direction of arrival of a certain signalcan be found. Given two or more directions from fixedpositions to the same object, the location of the object canbe computed. Because it is too difficult to constantly turnan antenna for measuring, receivers use a set of antennasthat are lined up with a certain angle difference in alldirections.

• Measuring the signal strength: The intensity of elec-tromagnetic signals decreases even in vacuum with thesquare of the distance from their source. Given a spe-cific signal strength, the distance to the sender can becomputed. Unfortunately, obstacles such as walls or treesadditionally reduce the signal strength, thus this methodis inaccurate.

• Processing video data: Using video cameras, significantpatterns in a video data stream can be acquired todetermine the user’s location. If users wear badges withconspicuous labels, they can be detected in video images.For this, positioning systems use techniques from imageprocessing to detect and interpret image data. In prin-ciple, video positioning systems are based on the AOAtechnique: a specific pixel in an image represents a certainangle relative to the camera’s optical axis; however,video data can transport color information, which can beused to transfer additional information (e.g., the user’sidentification).

A. Triangulation, Trilateration, and Traversing• Triangulation: (Figure 5) needs two fixed positions (p1

and p2). From each position, the angle to the locationu is measured. Geometrically speaking, u is obtained iftwo lines are intersected. With the help of trigonometricfunctions, the coordinates of u can be calculated.

Figure 5. Triangulation

• Trilateration: (Figure 6) also needs two fixed positions,but uses two distances to the unknown location. The

location u is obtained if two circles are intersected.Usually, there exist two intersection points, thus thereis the need to eliminate one point with the help ofadditional information. In contrast to triangulation, trilat-eration leads to nonlinear equation systems, which haveno closed solution for 3D positioning.

Figure 6. Trilateration

• Traversing: (Figure 7) uses several distance–angle pairs.It starts with a known point p1 and the distance anddirection to another point p2 is measured. After a fewsteps, the unknown point u is obtained. Note that inprinciple a single step from a known point to the unknownpoint could be used.

Figure 7. Trilateration

V. GPS - GLOBAL POSITIONING SYSTEM

As previously referenced, the usage of satellites for position-ing goes back to the 1960’s. The advantages of such method,among others, are:

• Positioning can theoretically be carried out around theglobe.

• Environmental conditions, such as the weather, have norelevant influence on the positioning process.

• Acquired positions have highly precise rates.

As disadvantages there are:• Considerable costs with the launch and maintenance of

the satellites.• The positioning it’s only possible if the user receives

a certain number of satellites. Particularly, positioninginside buildings is not possible.

A. Basic Principles of Satellite NavigationTo determine a position with the help of satellites, the

exact positions of the satellites and the exact distances to thesatellites are required. With this information, the positioningof an object is restricted to the spherical surfaces around each

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satellite. At least three satellites are needed to determine theobjects location in a tri-dimensional space [4].

Figure 8. Positioning with satellites

Satellites move around in space on fixed orbits, thus amobile object can easily compute it’s exact position at anytime. A list of all working satellites and their orbits it’scommonly named an almanac, and it’s frequently downloadedto the mobile receiver. It is also updated when satellites areshut down or new satellites start to operate in new orbits.

B. Global Positioning Systems (GPS)The GPS system is divided into three segments [7] (Figure

9):

Figure 9. GPS Segments

• User segment: contains the devices of the mobile userslike GPS receivers, which are constantly subject to minia-turization and price reduction. They are often the size ofa mobile telephone. GPS receivers can be plug-in cardsor separate devices with a serial interface connection.

• Space segment: consists of the satellites that move aroundin space. Every satellite weighs between 1.5 and 2 tonsand has an autonomous energy supply with solar cells.The central computer of the typical satellite has a 16-MHz CPU. They were programmed in Ada, a structured,statically typed, imperative, wide-spectrum, and object-oriented high-level computer programming language, ex-tended from Pascal and other languages. The operatingsystem of a satellite normally consists of approximately25,000 lines of code.

• Control segment: necessary for administration of thesatellites as well as for correction of the satellite internaldata (system time and orbits). Several monitor stationspermanently receive the satellite signals. They have aprecisely known, fixed position and atomic clocks thatare synchronized with the system time; thus, the monitorstations can calculate the correction data. They are passed

on to the Master Control Station (MCS), which is locatedin Colorado Springs, Colorado.

To be able to achieve global coverage from the equator tothe poles, 24 satellites move on six different orbits with foursatellites per orbit (Figure 9). Every satellite orbits the earthat the distance of approximately 20,200 km. A satellite needs12 hours for a complete orbit. They move in a way that atleast five and at most 11 satellites are mostly visible over thehorizon from every point on the earth’s surface. The numberof satellites that can actually be received can be lower becauseof shadowing by buildings or landscape formations. A satellitehas an expected lifetime of 7.5 years. In order for the GPS toremain operable after satellite failures, more than 24 satellitesare in orbit. The number was sometimes increased up to 28.Currently, an operator needs 60 days to launch a new satelliteinto orbit after the failure of a satellite. It is planned for reasonsof cost to shorten the time for launching to 10 days. With thischange, the number of satellites could be reduced to 25.

C. GPS ServicesTo determine a position with the help of GPS a registration

it’s not needed, since the GPS signals are free of charge.The mechanism is based on one-way communication of thesatellites to the users. Two GPS services exist [1]:

• Precise Positioning Service (PPS): service that allowspositioning with a precision of 22 m in the horizontaland 27.7 m in the vertical. Over a period of 24 hours,95% of the measuring is within the given precision. PPS(formerly called P-Code or Precision Code) is encryptedand can only be decoded by the armed forces of theUnited States and members of the North Atlantic TreatyOrganization (NATO). This service is not accessible tocivilian users.

• Standard Positioning Service (SPS): Formerly called C/A-Code or Coarse/Acquisition Code, this service is availablefor civilian users. Until April 30, 2000, it had a precisionof 100 m in the horizontal and 156 m in the vertical.

The satellites send out a continuous signal with approximately20 W, and they use two frequencies: L1 (1575.42 MHz) forPPS and SPS, and L2 (1227.6 MHz) exclusively for PPS.Because all satellites send signals at the same frequencies,a receiver must be able to assign the signals to the respectivesatellites. GPS uses Code Division Multiple Access (CDMA)for this purpose: every satellite uses a unique code calledthe Pseudo Random Noise (PRN). The receiver knows allof the codes and can filter out the corresponding sequencefrom the superimposed signals of all satellites. The PRN’sdo not disturb themselves mutually (they are designed to beorthogonal). With the help of the satellite signal, the receivercan measure the time difference of the involved clocks andcompute the pseudo range. As a second function, the signaltransfers data with a data rate of 50 bits/s. These data containthe position of the satellite, the system time, and the orbits ofother satellites.

The GPS system is subject to the following distortingeffects, which influence the precision [6]:

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Table IIGPS PRECISION

• Clock errors: Although the clocks in the satellites workperfectly, they can cause an error of 1.5 m in the positioncalculation.

• Orbit fluctuations: The satellites do not move around per-fectly in their calculated orbits, as the gravitational forcesof the sun and moon disturb them. Such fluctuations cancause errors up to 2.5 m.

• Atmosphere disturbances: Atmospheric pressure andweather conditions affect the signal spreading and causeerrors around values of 0.5 m.

• Ionosphere disturbances: The loaded particles of theionosphere disrupt the signal spreading and cause errorsup to 5.0 m.

• Multipath error: Reflected signals in the environment ofthe receiver cause errors of around 0.6 m.

Besides these effects, the SPS signal was artificially distorteduntil the year 2000 to prevent very precise measuring ofpositions. This mechanism, called Selective Availability (SA),randomly dithered the time sent by the satellites. In addition,the orbit information was distorted. Through this system, anexact positioning was no longer possible. The background ofSA was that the U.S. army did not want to enable too exactpositioning for other forces. SA was switched off on May 1,2000 for economic reasons. SPS now provides a precisionof 25 m in the horizontal and 43 m in the vertical (with95%). Table 1 summarizes the precisions of the different GPSservices. Future developments are planned to improve theprecision of SPS, especially to correct ionospheric distortion.

VI. NETWORKED BASED POSITIONING

The development of positioning systems is often a sig-nificant investment. To reduce the costs, existing wirelessnetworks can be used for positioning services. Particularlycellular networks are suitable for this purpose because thecell identification already transports a rough location (COO).Additional mechanisms such as runtime measurement (TOA)or angle measurement (AOA) allow a more exact delimitationof the position. At the present time, two of the main meansof transport for positioning systems are GSM and UMTS.These architectures will only be successful if they providea portfolio of attractive services. There will not be a single“killer application” since people need variety to be satisfiedby a single application [4].

A. GSM - Global System for Mobile CommunicationsCellular phone networks are highly available, cover a large

geographic area, and reach a high number of mobile users.Cellular phone infrastructures are often viewed as the mostpromising platforms for LBS. In 2003, more than 1.2 bil-lion people in the world used cellular phones. Without any

modifications in the network structure, a simple positioning ispossible within the GSM network, simply by determining inwhich cell a mobile telephone is registered.

The mobile participant can also access location-related datavia the radio signals from the base stations. A base stationcan broadcast such data via so-called Cell Broadcast Channels(CBCHs), a logical data channel in the GSM data stream. Amobile phone has to listen for specific frames where smallpieces of data, such as locations about the emergency phones,hotels, hospitals, gas station, and so on, can be transferred. Theresolution of the position is too inaccurate for some services.The cell radius varies from less than 1 km in city centers upto 35 km in the countryside. If a mobile user stays in a smallcell, the position is relatively exact. The 35 km as a maximumare, however, far too large for most services.

Ericsson has developed a system called the Mobile Position-ing System (MPS) [8], that allows a more precise positioningamong large cells. MPS cooperates with standard GSM sys-tems and needs only minimal modifications for installationat the communication infrastructure. The mobile terminals(i.e., the cell phones) do not have to be modified, which isparticularly important because customers often reject cost-intensive modifications of the terminals. The precision by MPScan be improved by GPS.

To determine the positions, MPS uses several mechanisms:• Cell of Global Identity (CGI): (Figure 10) Using the

identification of a cell, the position of a mobile participantcan be roughly determined. This inaccurate method isonly used if more precise procedures are not available.

Figure 10. Cell of Global Identity (CGI)

• Segment antennas: (Figure 11). Base stations often haveseveral antennas, which divide the 360 degrees into (usu-ally two, three, or four) segments. Thus, a base station canlimit the location of a mobile user to an angular segmentof 180, 120, or 90 degrees.

Figure 11. Segment Antennas

• Timing Advance (TA): (Figure 12). Base stations and mo-bile terminals use certain time slots for communication.

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Because the timing must be exact, the mechanism takesinto account the signal runtime between terminal andbase station. A mobile terminal sends a data burst earlierwhen the distance to the base station increases. With thismechanism, a burst always arrives at the base stationexactly within a time slot. This information can be usedto determine the position within a cell more precisely.The distance to the base station is measured in steps ofapproximately 555 m. Timing advance can be combinedwith segment antennas to increase precision.

Figure 12. Timing Advance

• Uplink Time of Arrival (UL-TOA): (Figure 13). Thebetter positioning possible in GSM networks occurs whena mobile participant is in the reach of at least fourbase stations. By measuring the signal runtimes from amobile terminal to the base stations, the position can bedetermined with a precision of 50 to about 150 m. Asimilar computation is used as in the case of satellitenavigation.

Figure 13. Uplink Time of Arrival

B. UMTS - Universal Mobile Telecommunications SystemIn GSM, services like voice, fax and data are standard-

ized, ensuring compatibility between different networks andterminals, but it difficults development of new services. Theintroduction of SAT and WAP was the first step towardsan open service environment. However, both concepts arenot sufficient for complex UMTS services because they aredesigned for very limited GSM phones and do not provideaccess to all relevant network elements. To overcome thecurrent limitations, a flexible service environment for UMTShas been standardized. The main objectives are to facilitatequick service development and convenient service access.

VII. CONCLUSION

Until now, LBS applications were difficult to implementmainly because of the technology used. The availability ofnew network technologies including 2.5G and 3G technologiesincreased the use of data services. The ‘always-on’ dataconnection, the higher data transfer rates, and the charging pervolume and per user-value, will enable LBS to benefit fromthese technologies. The ability to push data to users based ontheir location and preferences, in a seamless and inexpensivemanner, is likely to help LBS services to proliferate. Futurereleases of 2.5G and 3G technologies are likely to benefit fromthe fruits of the ongoing effort to standardize different aspectsof LBS.

As for the standardization of LBS [5], a big effort isbeing made, both on the network and application side. Mainforces are the 3G Partnership Program (3GPP), definingmainly the addition of LBS capabilities to future releases of3G networks, and the Location Interoperability Forum (LIF),formed by vendors and interested parties to developing andpromote common and ubiquitous solutions for LBS which arenetwork and location technology independent. The result ofthese efforts will have an significant effect on the success ofLBS, affecting the technology choice operators will make, therequired investment to launch or upgrade existing LBS, as wellas on the actual availability, usability, and cost of services.

LBS must have attractive and accessible services and appli-cations to take off. Some of these future services are likelyto benefit from higher accuracy location technologies [2].The ability to offer such services requires tight cooperationbetween mobile operators, application developers and equip-ment vendors. This requires the understanding of subscriberspreferences and usage habits as well technology expertise.Standardization is likely to facilitate the development andlaunch of services, but the key is still in attracting thesubscribers. Only a joint effort by the different players is likelyto enable that [1].

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[5] P. M. Adams, G. W. B. Ashwell and R. Baxter, “Location-based services— an overview of the standards”, BT Technology Journal, vol. 21, no. 1,Jan. 2003

[6] M. Olynik, M.G. Petovello, M.E. Cannon and G. Lachapelle, “Temporalvariability of GPS error sources and their effect on relative positioningaccuracy”.

[7] Wlodzimierz Lewandowski and Claudine Thomas, “GPS Time Transfer”,Proceedings of the IEEE, vol. 79, no. 7, Jul. 1991.

[8] David Mountain and Jonathan Raper, “Positioning techniques forlocation-based services (LBS): characteristics and limitations of proposedsolutions”, Aslib Proceedings, vol. 53, no. 10, Nov./Dec. 2001.

[9] Paolo Bellavista, Axel Küpper, and Sumi Helal, “Location-based services:back to the future”, Published by the IEEE CS, vol. 7, no. 2, Apr./Jun.2008

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Cristina Estorninho Born in 16/07/1986 at Golegã,has 3rd level of Computer Science/Equipment Main-tenance taken at Escola Profissional Gustave Eiffel(2004), works as a volunteer at Pastoral Socialof parish church of Golegã since 2003 and is astudent of Electrical and Computers Engineering inthe telecommunications area at Escola Superior deTecnologia de Setúbal of the Polytechnic Institute ofSetúbal.

João Neto Born in 1/04/1982 at Lisbon, has 3rd levelof Power Engineering/Electronics taken at EscolaSecundária Sebastião da Gama (2003), is a traineeat Sadofone, Lda - Telecommunications, ITED In-staller and Designer Technician registered under thenumber ITS50216PI and a student of Electrical andComputers Engineering in the telecommunicationsarea at Escola Superior de Tecnologia de Setúbal ofthe Polytechnic Institute of Setúbal.