This work has been fully supported by Vodafone I.T., Research & Development Department, TURKEY.Secure User-Plane Location (SUPL) ArchitectureFor Assisted GPS (A-GPS)Tolga Göze 1, Özgün Bayrak 1, Metin Barut 1, M. O ğuz Sunay 2,1Vodafone IT. Research & Development Department, Istanbul, TURKEY{tolga.goze, ozgun.bayrak, metin.barut}@2Dept. of Electrical & Computer Eng., Koç University, Istanbul, TURKEYosunay@.trAbstract—The aim of this paper is to analysis the new as-sisted GPS (A-GPS) architecture based on SUPL (Secure User Plane Location). This architecture improves perform-ance of A-GPS based location estimation approaches. We provide comparative field test results of the developed sys-tem with the conventional GPS in terms of both the positionaccuracy and the TTFF (time to first fix) delay times. We have also examined the effectiveness of initial position esti-mation on the TTFF and the accuracy of calculated position.Field tests are conducted on a real GSM network in urbanareas and indoor environments. Executed analysis has pro-vided that the developed SUPL based A-GPS system achieves high-performance on estimation accuracy and fast-er TTFF.Key words: SUPL, AGPS, mobile positioningI. I NTRODUCTIONGPS is a satellite-based radio-positioning system that provides navigation and timing information to militaryand civilian users [1]. The GPS satellites continually broadcast almanac and ephemeris data to aid users esti-mate their position, velocity and time. The almanac is the orbital parameters for all the GPS satellites while theephemeris is the precise time and orbital data for the in-dividual GPS satellites. While the almanac data can bevalid for months, the ephemeris is valid for a few hours.Upon receiving the broadcast signals from at least 4 sa-tellites, a GPS receiver can predict its location with highaccuracy. GPS receivers work best in open areas that of-fer an unobstructed line of sight to the GPS satellites. In places where GPS signals are weak, obstructed, or scat-tered, as is the case inside an office building or in an ur-ban canyon, traditional GPS receivers compute positionsintermittently at best and usually not at all.In GPS receivers, the time required to first calculatethe position (time to first fix - TTFF) depends on theavailability of current information at the receiver fromprevious fixes as well as the strength of the available GPSsignals at the location of interest. Thus, TTTF varies sig-nificantly; in hot start, it takes merely seconds but in cold start, depending on the availability and quality of satellite signals, it can sometimes take several minutes. A long TTFF is unacceptable for location-based services or emergency calls.Assisted GPS (A-GPS) techniques have been devel-oped to improve the sensitivity of the GPS reception and reduce TTFF [2-7]. The main idea behind A-GPS is toprovide assistance data to the GPS receiver through a wireless network. With this data, mobile devicesequipped with A-GPS can compute positions more quickly and in much more difficult environments. There are three basic types of data that the assistance servermay provide to the GPS receiver: precise GPS satellite orbit and clock information; initial position and time es-timate; and for A-GPS-only receivers, satellite selection, range, and range-rate information. The location estimatemay also be computed at the assistance server using the feedback data received from the A-GPS receiver. In thiscase, the GPS receiver is left with the sole job of collect-ing range measurements. Depending on where the loca-tion estimation calculations are conducted, A-GPS meth-ods are classified as either mobile-based or network-based.The communication exchange between the mobile device and the A-GPS server may be conducted in one oftwo ways: 1. Control-plane A-GPS: Here, the assistance messagesare sent using explicit, pre-defined cellular network (GSM/WCDMA) signaling structures. The initial A-GPS systems all used the control-plane for the transmission of assistance data. However, as the number of A-GPS capable users increase in the cellular system, there is significant potential of the control-plane A-GPS exhausing the system capacity. 2. User-plane A-GPS: Here, the assistance messages are sent using an IP data connection and thus use a GPRS/EDGE/HSPA connection without requiring any wireless standard specific messaging. The user-plane A-GPS has recently been proposed as an alternative to the control-plane A-GPS to avoid system capacity exhaustion due to assistance related signaling. Even though initial work on A-GPS has focused on control-plane solutions and subsequent standards have been written on both sides of the Atlantic (TIA/IS 801-1,3GPP TS 25.331 and 3GPP2 C.S0022-0-1), extensions to these standards have also been made to cover user-plane based A-GPS solutions (3GPP TS 44.031 and IS-801-A).Cellular system agnostic solutions for the user-plane A-GPS approaches have been developed and standardized by the Open Mobile Alliance (OMA) [8-9-11]. It is envisioned that, due to its inherent advantage in cellularcapacity usage, user-plane based A-GPS solutions will dominate the market. However, currently the user-plane solutions in North America are limited by the fact that simultaneous voice and data feature is non-existent in 2G systems there. This paper is concerned with system development and field test performance evaluation of a user-plane A-GPS system that makes use the Secure User-Plane Location (SUPL) Architecture, a recently standardized system by Open Mobile Alliance (OMA) [9-11]. To the authors’ knowledge, this paper is the first of its kind in presenting an actual, deployed user-plane A-GPS system and associated field test results. To this end, we present in this paper the developed SUPL server, and the SUPL MIDlet for the mobile station for the A-GPS operation. We also present extensive field test measurements to asses the system performance of the SUPL based A-GPS system.The remainder of this paper is organized as follows. In Section II we provide a brief overview of the SUPL architecture. In Section III we describe how we have implemented the A-GPS system using SUPL, describe the deployed SUPL server. In Section IV we analize the effect of initial position accuracy. In Section V we present the SUPL MIDlet we have developed to conduct the system operation at the SET, describe the conducted field tests using the developed system and provide performance results. Finally, we draw conclusions in Section VI.II. O VERVIEW OF THE S UPL A RCHITECTURE The SUPL architecture, as described in [9-11], is composed of two basic elements: a SUPL Enabled Terminal (SET) and a SUPL Location Platform (SLP). The SET is a mobile device, such as a phone or a PDA, capable of supporting SUPL transactions. The SLP is a server or a network equipment stack that handles the tasks associated with user authentication, location re-quests, location-based application downloads, charging, and roaming [9]. The SUPL architecture is illustrated inFigure 1.Figure 1. Overall SUPL Enabled A-GPS ArchitectureThe SUPL architecture utilizes existing protocols, IP connections, and data-bearing channels for GPS assis-tance. SUPL supports two protocols, namely, MLP (Mo-bile Location Protocol) and ULP (User Plane Location Protocol). MLP is used in the exchange of location based service data between SLP and a GMLC, or between twoSLPs. ULP, on the other hand, is used in the exchange of location based service data between an SLP and a SET. The SUPL architecture is flexible in terms of the assis-tance data to be exchanged; depending on the capability of the mobile station as well as the specific request from it, the contents of the assistance data may vary [9-10]. III. I MPLEMENTATION OF THE S UPL A RCHITECTURE At Vodafone Turkey, we have developed a mobile lo-cation center solution composed of a Gateway Mobile Location Center (GMLC) and a Serving Mobile Location Center (SMLC). The SUPL Location Platform (SLP) is part of the GMLC in this design and is responsible of handling secure user plane location technologies. It main-ly utilizes user plane data bearers for transferring location information and carrying positioning technology related protocols between a SET (SUPL Enabled Terminal) and the network. It manages the SET initiation, security, posi-tion calculation, assistance data retrieval from Worldwide Reference Network and delivery to SET for user plane location operations.In this design, when the SET initiates a location esti-mation request, the sequences of events that take place are illustrated in Figure 2. A brief summary of each stepof operations is given below.Figure 2. Overview of the SET-Initiated A-GPS process1. The World Wide Reference Network (WWRN) con-tinually tracks the entire global GPS constellation using a number of Hubs at geographically different locations and stores the raw-GPS data. Currently, a number of com-mercial WWRNs are present and our developed SUPL Location Platform (SLP) utilizes one of these existing networks. The developed SLP periodically requests the complete GPS satellite raw-data from the WWRN. In our design, the connection between the WWRN and MLC is established using a VPN or an SSH connection over the public Internet.2. The raw GPS satellite data includes the almanac, ephemeris, reference time, acquisition assistance data, UTC model parameters, real-time integrity and iono-sphere models. The MLC, upon receiving this data, for-mats it into a form as required by the SET and stores it in the SLC cache.3. For the SET initiated scenario, if the SET is not already attached to Packet Data Network services it will attach itself as GPRS, EDGE or others. The SET may reuse the secure IP connection of an already ongoing SUPL session between the SET and the SLP. Thus the SUPL Agent on the SET can start a request for position.4. The SUPL Agent uses the 'Positioning Server Address' provisioned by the Home Network to establish a secure IP connection to the SLP and sends a SUPL START mes-sage to start a positioning session with the SLP. The SUPL START message contains session-id, SET capa-bilities, location identifier (lid) and the optional parame-ter is the desired quality of position (QoP).The SET capabilities include the supported positioning technology (e.g., SET-Assisted A-GPS, SET-Based AGPS, autonomous GPS), preferred method (agpsSETas-sistedPreferred, agpsSETBasedPreferred, noPreference) and associated positioning protocols (e.g., RRLP, RRC, TIA-801). In this implementation SET-Based A-GPS positioning method and RRLP protocol is used. Gsm Cell Information parameter is the location identifier and it contains MCC, MNC, LAC, cell-id and optionally pro-vides NMR and TA information. According to maxLo-cAge parameter in QoP if the last calculated position is up-to-date and reasonably accurate position estimate at the SLP then SLP sends the position to the SET in the SUPL END message.5. The parameter Cell-id which is the parameter of the location identifier is queried from the database in the GMLC side to provide a latitude and longitude as an ini-tial position for the position calculation.6. The SLP sends a SUPL RESPONSE message to the SET, which determines the posmethod consistent with the SUPL START message including posmethod(s) and con-tains a unique compound session-id that is formed from the IP address allocated by the SET and SLP address al-located by the SLP. This compound session-id will be used in all of the remaining messages between the SET and the SLP.7. After receiving the SUPL RESPONSE message, the SET sends a SUPL POS INIT message to the SLP with the mandatory parameters as SET capabilities and loca-tion identifier. Different from the SUPL START mes-sage, it contains the Requested Assistance Data element. The assistance data element optionally provides the pa-rameters almanac, utc model, ionospheric model, dgps corrections, reference location, reference time, acquisi-tion assistance, real-time integrity and navigation model. The SET may provides its position, if this is supported. 8. The SLP sends a SUPL POS message to the SET to start the positioning procedure based on the initial posi-tion which is sent by the GMLC. If the location of cell-id can not found as an initial position in the database then the position is taken from the parameter of the SET. The SUPL POS message contains the PosPayLoad element which is the rrlpPayload as an octet string. RRLP mes-sage contains the Measure Position Request with Assis-tance Data. The performance of the SET depends on the number of satellites included in the assistance data it re-ceives. On the other hand, it is not necessary to send all satellites ephemeris data every time. It causes the slowly location calculation. Thus the initial position is used to select the suitable satellites in view for the SET. Besides the initial position from the SET, cell-id and mobile country codes (MCC, MCC) can be used to select best satellites.Thus the SET calculates the position estimate based on assistance data obtained from the SLP and sends the re-sponse with SUPL POS message. This time rrlpPayload message contains the RRLP Measure Position Response to return the location information [8].9. Once the position calculation is complete the SLP sends the SUPL END message to the SET informing it that no further positioning procedure will be started and that the location session is completed. Both the SET and SLP release all resources related to this session including the secure IP connection [9].IV.I NITIAL P OSITIONThe effect of initial position on AGPS receiver per-formance is investigated in this section. The receiver can use the initial position and uncertainties to determine the satellites in view, reduce its search space and initialize the navigation solution.There are four different approaches to calculation of the SET initial position:1. Getting initial position from the SET:If the last calculated position is up-to-date which is checked with maxLocAge parameter in QoP, then the reference position from the SET as used the initial posi-tion.2. Cell ID + Signal Strength positioning method:In this method, GSM cell information parameter is used. According to our network statistics, the SET is generally close to the BTS, thus an initial position for the SET can be approximated by the BTS location. Statistic results for TA averages shown in Figure 3.Figure 3. Averages of TA Values in Local Network3. Using only Cell ID location:If the NMR information isn’t present in the optional pa-rameter, then the only cell location is used as the initial position.4. Using country code (MCC-MNC) without Cell ID: Finally, if the Cell ID cannot be found in the database, then the country code (MCC-MNC) is used to locate the initial position as the middle point of the country loca-tion.The Cell ID + signal strength positioning method for GSM requires information of the serving sector direction and sectorization scheme of the BS in addition to coordi-nates of the serving Base Station. This method basically estimates the handset's coordinates by combining Cell Identity (CI), Timing Advance (TA, used to estimate the distance between MS and serving BTS), and strongest neighbor cells measurements of the strength of signals received by the Mobile Station (MS). This system deter-mines the mobile phone position based on the intersection of the distance circles. The Cell ID + signal strength posi-tioning method for GSM requires information of the serv-ing sector direction and sectorization scheme of the BS in addition to coordinates of the serving BS. One BTS usu-ally covers a single pointing azimuths 120 degree sector of an area. Usually a tower with 3 BTSs accommodate all 360 degrees around the tower. The TA value is normally between 0 and 63, and changes for each 550m. TA value allows for rough position estimation. Position error can be 550m for TA0, and more than 35km for TA64. Initial position estimates by TA values and TA ranges for sector cell shown in Figure 4.Figure 4. Initial position estimation by TA rangesIn order to investigate the effect of range between user and reference point, distance was set to 0.5 km, 2 km, 5 km, 10 km, 25 km and 35 km, respectively. The average TTFF for each user to reference point distance tested is shown in Figure 5. Obviously, as the user to reference distance increases, the TTFF also increases. Table I shows the accuracy errors. When the user to reference distance increased more than 10 km, a significant in-crease in the TTFF was noticed. Initial position with the accuracy at the level of 500m-2km significantly improves the AGPS TTFF delay and the position estimation accu-racy.Figure 5. The effect of initial position accuracy on the TTFFTABLE IACCURACY ERRORS FOR DIFFERENT REFERENCE POSITIONDISTANCE IN INDOOR AREAS.Mobile Terminal to InitialPosition Distance(km)Accuracy Error (m)0.5 82 105 1215 1425 1735 23V.F IELD T ESTS AND P ERFORMANCE R ESULTSWe have conducted extensive field tests to assess the performance of the developed SUPL A-GPS architecture. The system performance metrics were chosen as TTFF and position accuracy. Data were collected in various geographical areas within Istanbul covering different sce-narios, namely, shopping malls, indoor buildings and urban canyon in-vehicle environments.All of the tests presented herein were performed using a various commercial state of the art mobile phones which using and having a built-in GPS receiver. In order to calculate and record the TTFF period, we have devel-oped a MIDlet program which was subsequently installed in all of the mobile phones used in the data collection.Figure 6. The results of all tests were collected with theMIDlet program.The developed MIDlet collects and logs information about the user's coordinates, serving base station Cell Identity (CI), total number of satellites that is used to calculate the coordinates. The TTFF was measured rela-tive to the time when the assistance data was received at the mobile terminal. The user interface of the MIDlet program is depicted in Figure 6. [12-13]During the data collection, all positions were calcu-lated independently. In other words, it has no a priori knowledge about GPS time, signal propagation and its current position. The GPS receivers were cold-started during all tests. Before cold tests were performed, the mobile terminal was turned off for a period of between 5-6 hours that resulting a cold start. As stated before, in the cold start mode, the receiver has no acquisition aiding information available; it has no information about the current time, the orbits of the satellites or its current posi-tion. Thus, a complete search is necessary.During the in-vehicle tests, the mobile phones were powered up inside the vehicle and they were placed be-tween the front seats.All of the tests performed for this research were done in static conditions. All tests were performed with no orbital, atmospheric or any other errors.In order to SUPL enabled terminal to get assistance da-ta from server, ‘Assisted GPS’ should be selected as a ‘Positioning Method’, and ‘Positioning Server Address’ must be entered in settings of mobile terminal.SUPL enabled A-GPS allows the mobile terminal to request up to date ephemeris (satellite position) data from the server over a packet data network.Figure 7. The total GPRS packet data and duration time forall assistance dataThe transferred average assistance data is 1.28kB up-load and 3.11kB download, 4.39kB at total as shown in Figure 7. Content of transferred information has ex-plained in section 2.When A-GPS info is received successfully, there should be a list of satellites with very short strength bars as shown in Figure 8a. As satellites are found a hollow black bars will appear with the satellites number beside it. Once the GPS chip "locks" onto the satellites signal the bar turns solid black as shown in Figure 8b.Results for measurements in both SET Based and Inte-grated GPS TTFF and position accuracy for measurement results are shown in Table II for three different environ-ments, which include 200 measurements in total. Meas-ured median position errors and Average TTFF for SET based A-GPS are 34m and 50.5 seconds in urban indoor, 32m and 50.3 seconds in shopping mall 25m and 25.2 seconds in urban inside vehicle. And median position errors and Average TTFF for conventional integrated GPS are 42m and 81 seconds in urban indoor, 45m and 70.3 seconds in shopping mall, 32m and 41.5 seconds in urban inside.TABLE II.AVERAGES OF POSITION ACCURACIES AND TTFF VALUES FOR SET BASED AND INTEGRATED GPS MEASUREMENTS.SET BasedAveragesIntegrated GPSAveragesType ofEnvironment TTFF(secs)PositionError(m)TTFF(secs)PositionError(m) Urban Indoor 50.5 34 81 42Shopping Mall 50.3 32 70.3 45Urban In-Vehicle25.2 25 41.5 32The comparison results of TTFF values for SET and Conventional GPS are shown in Figures [9-11] for three environments.Figure 9. Comparison of TTFF values for SET and Conventional GPSin Urban In-Vehicle environment.Figure 8.a. After SUPL assis-tance, GPS knows which satellitesit should search for.Figure 8.b. Locked satellitesFigure 10. Comparison of TTFF values for SET Based and Conven-tional GPS in Shopping Mall environment.Figure 11. Comparison of TTFF values for SET and Conventional GPSin Urban Indoor environment.Inside the indoor areas, integrated GPS could not com-pute position under 40 seconds. Because GPS interrupted from a tall building and receiving signal is being reflected due to reception block or multi-paths by high-rise apart-ment complex. If the data isn't received in full, the Ephe-meris data collection has to start again at the next cycle. No lock was seen in the indoor areas unless mobile de-vice was placed very near to windows.The field tests show that SUPL enabled A-GPS ap-proach promise much better position accuracy and faster TTFF period than conventional GPS, because SUPL en-abled A-GPS architecture increases the capability of a conventional GPS receiver to acquire and track more sat-ellites, thereby improving observation geometry, and in-crease sensitivity over a conventional GPS architecture. These enhanced capabilities come from knowledge of the satellite position and velocity, the initial receiver position, and time supplied by the assistance server. A-GPS bene-fits include TTFF improvement, higher sensitivity and maximum availability due to the receiver not having to download and decode navigation data from GPS satel-lites. Hence, the receiver can spend more time and proc-essing power tracking the GPS signal.VI.C ONCLUSIONSIn the study, a SUPL enabled A-GPS architecture was developed. The advantages of the developed system were compared with the conventional GPS in terms of the TTFF delay time and location accuracy in actual urban environment field tests. The study environment is se-lected as indoor environment, because according to the results of the statistics, a normal user stays most of the day in indoor environments and needs GPS help in such environments. The impact of initial position estimation on the both TTFF and accuracy of calculated position also investigated.The performance of SUPL has been evaluated with da-ta collected from field measurements in a GSM networkin different geographic environments. The study wasdone with real mobile phones and real field measure-ments in live GSM network.Previous related work in the literature used either a si-mulator environment or used sophisticated GPS devicesin field tests. This paper presents the first study where aSUPL based A-GSP system was developed and tested inthe field using commercial mobile phones with limitedGPS capabilities.Our experimental results indicate that the SUPL isvery effective on TTFF delay and position accuracy formobile positioning in different environments. When theuser to reference point distance decreased less than 2 km,a significant decrease in the TTFF and the accuracy was noticed.Moreover, the developed system has minimal impacton the cellular infrastructure and does not necessitate any modifications to the mobile handsets or base stations.Currently, we are working on techniques to improvethe SUPL enabled A-GPS accuracy by exploiting thenetwork attributes more.A CKNOWLEDGMENTThis work has been fully supported by Vodafone I.T., Research & Development Department, TURKEY.R EFERENCES[1]J.A. Farrel and M. Barth, The Global Positioning System and Iner-tial Navigation. New York:Mc Graw Hill, 1999.[2]J. Burkowski, J. Niemela, J. Lempiainen, “Cellular Location Tech-niques Supporting AGPS Positioning,” Proceedings of IEEE VTC-Fall, vol. 1, pp. 429-433, Sept. 25-28, 2005.[3]S. Singh, “Comparison of Assisted GPS (AGPS) Performanceusing Simulator and Field Tests,” Proceedings of ION GNSS, Sep-tember 26-29, 2006.[4]G. 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