INTRODUCTION. Location related products are the next major class of value added services

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1 INTRODUCTION Location related products are the next major class of value added services that mobile network operators can offer their customers. Not only will operators be able to offer entirely new services to customers, but they will also be able to offer improvements on current services such as location-based prepaid or information services. factors: The deployment of location based services is being spurred by several Competition The need to find new revenue enhancing and differentiating value added services has been increasing and will continue to increase over time. Regulation The Federal Communications Commission (FCC) of the USA adopted a ruling in June 1996 (Docket no ) that requires all mobile network operators to provide location information on all calls to 911, the emergency services. The FCC mandated that by 1 st October 2001, all wireless 911 calls must be pinpointed within125 meters, 67% of the time. On December , the FCC amended its ruling to allow terminal based solutions as well as network based ones (CC Docket No , Waivers for Handset-Based Approaches). 1

2 There are a number of regulations that location based services must comply with, not least of all to protect the privacy of the user. Mobile Streams believes that it is essential to comply with all such regulations fully. However, such regulations are only the starting point for such services- there are possibilities for a high degree of innovation in this new market that should not be overlooked. Technology There have been continuous improvements in handset, network and positioning technologies. For example, in 1999, Benefon, a Finnish GSM and NMT terminal vendor launched the ESC! GSM/ GPS mapping phone. 2

3 NEEDS OF CELLULAR POSITIONING There are a number of reasons why it is useful to be able to pinpoint the position of a mobile telephone, some of which are described below. Location-Sensitive billing Different tariff can be provided depending upon the position of the cell phone. This allows the operator without a copper cable based PSTN to offer competitive rates for calls from home or office. Increased subscriber safety A significant number of emergency calls like US.911 are coming from cell phones, and in most of the cases the caller can not provide the accurate information about their position. As a real life example let us take the following incident. In February 1997 a person became stranded along a highway during a winter blizzard (Associated press 1997).She used her cellular phone to call for help but could not provide her location due to white-out conditions. To identify the callers approximate position authorities asked her to tell them when she could hear the search plane flying above. From the time of her first call forty hours elapsed before a ground rescue team reached her. An automatic positioning system would have allowed rescuers to reach her far sooner. 3

4 Enhanced network performance At the microscopic level, accurately positioning a moving mobile phone enable the communication network operator to take better decisions on when to hand over from one cell to next. Macroscopically, long-term monitoring of mobile telephone positions provides excellent input to the planning of the cellular network Intelligent transport systems Many services envisaged under the ITS initiative will require position information, often in conjunction with a communications channel, to be effective. The ability to position a mobile telephone could enable services such as providing information to travelers, more effective dispatch of vehicles in fleets, and detecting traffic incidents and congestion 4

5 POSITIONING SYSTEM CLASSIFICATION The positioning systems are classified according to where the measurements are made and where the position information is used. Two broad classifications are made: self positioning and remote positioning. The classification is useful when evaluating a given positioning system for a particular application. Self positioning In self positioning the positioning receiver makes the appropriate signal measurements. There will be geographically distributed transmitters. The positioning may be collocated with the receiver. The position information can be transmitted to distant stations from the positioning receiver if needed. An example of self positioning is GPS. Remote positioning In remote positioning there will be receivers at more than one location, the object to be positioned being the transmitter. The signal measurement is done at the receivers and the data collected are transmitted to a central site for processing. Indirect positioning It is possible to send the position measurement from a self positioning receiver to a remote site or vice versa. A self positioning system sending data to a remote 5

6 site is called indirect remote positioning system and the other is called indirect self positioning system 6

7 POSITIONING TECHNIQUES There are a variety of ways in which position can be derived from the measurement of signals and these can be applied to any cellular system including GSM. The important measurements are the Time of Arrival (TOA), the Time Difference of Arrival (TDOA), the Angle of Arrival (AOA) and Carrier phase. All these measurement put the object to be positioned on a particular locus. Multiple measurements give multiple loci and the point of their intersection gives the position. If the density of the base stations is such that more measurements can be done than required then a least square approach can be used. If the measurements are too few in number the loci will intersect at more than one point result in ambiguous position estimate. In the following discussion we assume that the mobile station and base station are lying in the same plane. This is approximately true for most networks unless the geography include hilly topology or high rise buildings. Time of Arrival (TOA) In a remote positioning system this involves the measurement of the propagation time of a signal from the mobile phone to a base station. Each measurement fixes the position of the mobile on a circle. With two stations there will be two circle and they can intersect in a maximum of two points. This gives rise to an ambiguity and it is resolved by including a priory information of the trajectory of the mobile phone or making a propagation time measurement to a third base station. The TOA measurement requires exact time synchronization between the base stations and the receiver should 7

8 have an accurate clock, so that the receiver knows the exact time of transmission and an exact TOA measurement have made by the receiver. The measurement of the round-trip time of a signal from a source to destination and echoed back to the source can be made. This does not require exact synchronization and is the common method of measurement of TOA. Figure 1 Circular trilateration 8

9 A system that makes use of TOA for positioning can be called as circularcircular-circular system. Time Difference of Arrival (TDOA) A mobile phone can listen to a series of base station and measure the time difference between each pair of arrival. Each TDOA measurement fixes the position of the mobile phone on a hyperbola. With more than two stations there will be more than two hyperbola and they intersect at a unique point. This is a self positioning system. The inverse approach yield a remote positioning system i.e. each base station listen to the mobile station and measure the TOA and sent each TOA to a central site where the TDOA measurement is made and the position is estimated. A positioning system based on TDOA measurement is called hyperbolic-hyperbolic system. Figure 2 Hyperbolic trilateration 9

10 An important issue in TDOA approach is the requirement of time synchronization irrespective of the positioning method-self or remote. In a self positioning system the base stations are the transmitters. The transmitted signal should leave the transmitters at the same time. Otherwise there will be a bias error in the hyperbolic locus. In remote positioning the base stations are the receivers. There must be a known relationship between the receiver clocks at the base station, or again bias error will result. Angle of Arrival (AOA) In this method the angle of arrival of a signal transmitted form a base station to a mobile station (or mobile station to base station) is measured. The locus is a straight line. With two measurement two straight lines are obtained and the uniquely intersect at a point. This is also called triangulation method. Figure 3 Location by triangulation 10

11 Carrier Phase The phase of the carrier has the potential to provide position information. The receiver can measure the phase of the carrier but it can not measure the integer number of cycles between transmitter and receiver. Another problem is the need of maintaining continuous lock on the carrier signal. Despite of the problems the carrier phase method is successfully used in the GPS. The application of carrier phase positioning to GSM will be challenging due to the problems associated reconstructing the carrier from the Gaussian minimum shift keying modulated signal, with no guarantee that the carrier will be continuous. Cell of Origin (COO) Cell of Origin (COO) requires no modification to the handset or networks and so is able to be used as the Location Finding System for existing subscribers but is less accurate than the other methods employed. Some would argue however that the accuracy of COO in cities is more than adequate for information services owing to the small cell size. The accuracy of COO is though questionable when the Location Finding System is required for assisting with emergency services. COO is the only technology that is widely deployed in wireless networks today. This scheme is used to meet Phase emergency services requirements in the USA, wireless office location specific billing applications and some location-specific information request services. 11

12 In this system, the mobile network base station (BTS) cell area is used as the location of the caller. Positioning accuracy generally depends upon the size of the cell. Although other schemes offer higher degrees of positioning accuracy than cell of origin, its main advantages are that speed of response in getting a location fix is fast (typically around three seconds) and that as no handset or network upgrade is required, it can be used to provide location specific services to existing customer bases Heterogeneous systems Different positioning methods can be mixed. A common example is radar in which TOA and AOA methods are mixed. Mixing of positioning system has advantages e.g. circular angle positioning system can measurement using a single base station. 12

13 PERFORMANCE CONSIDERATIONS There are many error sources that degrade the performance of the positioning system. The measure of the RMS deviation from the measured position (x^, y^) about the position (x, y).this is called RMS accuracy in two dimension and is given by the formula S= (E [(x^-x) 2 +(y^-y) 2 ]) 1/2 where E is the statistical expectation operator. The RMS accuracy is a function of the geometry of the base stations and accuracy of raw locus measurement. The locus errors impose a limit on the accuracy that can be achieved, but the relative geometry of the base station to the mobile station will further degrade the accuracy. The amount by which errors are degraded by geometry is called dilution of precision (DOP). When developing a positioning system the designer should aim to minimize DOP. Coverage is the proportion of an area of interest that is provided with an acceptable level of service by the positioning system. An acceptable level of service could be the ability to make positioning measurement to a predefined level of accuracy. 13

14 REVIEW OF GSM GSM networks are very complex communication systems. In the following paragraphs those aspects of GSM signaling pertinent to positioning considerations are discussed. GSM 900 uses two 25 MHz blocks of the radio frequency spectrum, called uplink and downlink. Each block is divided into125 frequency channels of 200 khz. Other systems derived from GSM specifications have similar frequency channel structure. GSM employs Time Division Multiple Access (TDMA) schemes on each frequency channel, dividing it in to time slots of 577 µ s duration. Blocks of eight time slots are grouped in to form a frame. Frames in turn, are grouped in to multi-frames and super-frames. GSM defines a number of logical channels, each of which has a specific role such as carrying user payload (traffic channels), coordinating base station and mobile (associated and dedicated control channels), establishing links (common control channels), and transmitting system parameters (broadcast channels). These logic channels are mapped in to predetermined time slots of particular frames within the multi-frame sequence. The message contained in various logical channels is fitted in to the time slots using a burst structure. GSM defines a number of burst types with differing formats. 14

15 The format for a normal burst, the most commonly used burst structure, is indicated in fig.4. Of particular note in the context of positioning is the 26-bit training sequence which is located in the middle of the burst. This is a pseudo-random sequence chosen for its correlation properties. By cross-correlating a local copy of the training sequence with the sequence in the received burst, a GSM receiver is able to estimate the impulse response of the radio channel as an aid in demodulating the bits in the burst. This training sequence is also useful for time based positioning measurements. A positioning receiver is able to use the correlation peak as a time reference for the burst. Another useful training sequence is the 64 bit training sequence used in the GSM synchronization burst transmitted by all the base stations on the synchronization channel (SCH). Figure 4 the GSM TDMA format 15

16 The area covered by GSM network is divided in to a number of cells, each served by its own base station, called a base transceiver station (BTS). Each cell is distinguished by a unique cell identifier and is allocated one or more uplink/downlink frequency pairs. More than one BTS may be grouped together under the control of a base station controller (BSC). In turn, several BSCs are controlled by a mobile service center (MSC), which handles tasks such as call routing and serves as the interface between the mobile network and the fixed telephone network. In GSM specification a mobile telephone is referred to as mobile station (MS). GSM uses a number of techniques to increase capacity, some which have implications for positioning. One of these is the use of sectored cells. In this case BTSs of more than one cell may be located at a particular site, each BTS serving only a sector of the area around that site. A common configuration is to have three collocated BTSs, each providing a 120 degree coverage pattern. Where such a configuration is in use, the constraint on the BTS coverage pattern offers extra information for use by a positioning system. The training sequences in different burst structures of GSM lend themselves to measurement of time and indeed there are also higher-level features of GSM signaling which measure time and could be used for positioning. GSM is a TDMA system, the successful operation of which requires that all signals arrive at the BTSs at the appropriate time. Since the signals arriving at the base stations originate from different distances, the time at which the signals are sent must be varied. GSM achieves this by having each BTS send each MS connected to it a timing advance (TA), which is 16

17 the amount by which the MS must advance the timing of its transmission to ensure that it arrives in the correct time slot. A further mechanism in GSM which may optionally be employed to improve the efficiency of handovers is pseudo synchronization. Each MS monitors the time differences between the epochs of the different BTSs in its vicinity. These measurements are called observed time differences (OTDs) and are used to facilitate handover by estimating the amount the timing of the mobile would have to be advanced /retarded if it were handed over to another BTS. 17

18 LOCUS MEASUREMENT WITHIN GSM SPECIFICATION There are three ways that position loci can be derived using the signaling aspects inherent in GSM specification. Propagation time using TAs, TDOA using observed time difference of arrival (OTD) and angle of arrival from the sector information. It is possible to implement a circular-circular-circular system using TA. For this the mobile station should be artificially forced to listen to more than two Base Transceiver Stations (BTS). There are two difficulties with this scheme. First, artificially forced handover to sub optimal BTS will degrade call quality and reduce system capacity. Second, under GSM specification TA is reported in units of bit period, which equates a locus accuracy of 554m. This is an optimistic value for accuracy, since multi-path will degrade the accuracy further. The rms accuracy of such a system is likely to be even worse due to DOP considerations. Since OTD measurements are made by mobile without forcing a handover, they are potentially greater utility for positioning than TAs. Under the current GSM specification, the resolution of an OTD measurement is 554m. Assuming that network is synchronized, it is possible to use the OTDs to implement a TDOA positioning system. The GSM specification does not require that the network to be synchronized. The level of uncertainty in the degree of synchronization will even further degrade the accuracy of a GSM positioning system which simply operates with the current specifications. 18

19 Some networks use sectored cells. A mobile connected to a sectored base station can be crudely located using knowledge of the base station s reduced geographic coverage. While this information is insufficient to provide an accurate position estimate, on occasion it might be sufficient to resolve a twofold ambiguity resulting from an insufficient number of measurements, thus alleviating the need for additional propagation time (TA) or TDOA (OTD) measurements. 19

20 GSM POSITIONING ARCHITECTURES In this section we examine three different position architectures that could be used to position GSM mobile phones: mobile-based, network-based, hybrid positioning. There could be significant differences in system architectures affecting infrastructure costs, coverage, the total number of users that can be supported and number of users that can be simultaneously positioned. The needs of a given positioning application will determine where the position information is required, the position update rate for each object being tracked, the number of objects to be tracked, and the net value of the position information. The various architectures need to be evaluated in light of these requirements to select the most appropriate one. Mobile-based Positioning This is a form of self positioning. The Mobile Station (MS) uses the signals from the BTSs to determine the position. There are a number of techniques that could be used to calculate position, but the basis is likely to be TDOA. For an MS based TDOA systems to work two fundamental changes need to be made to GSM equipment. The first is to modify the MS so that it is able to make accurate TDOA measurements, much more accurate than the current one bit resolution of OTDs. Such measurements include algorithms to reject multi-path. Accordingly within the mobile there will be a locus function which will accurately determine the TDOA. This can be done by processing the burst information to locate the epoch of the training sequence. The simplest logical channel on which to carry out this processing is the broadcast control 20

21 channel (BCCH) because its bursts are not subject to frequency hopping or power control and is repeated more frequently than the SCH. The second modification arises from the need of network synchronization. There are two options. The first is to tightly synchronize the network. This can be achieved by placing GPS time transfer receivers at each base station. The alternative is to provide information to the MS about the synchronicity of the network. This could be provided by special purpose monitoring receivers which measure the timing offset between different BTSs and sent this timing data to MS via a traditional data link such as the GSM short message service. To accurately provide a robust cellular positioning system with high accuracy and good coverage, it will be necessary to integrate many other sources of information. A key part of such implementation will be sophisticated software called fusion function which will fuse the information from a variety of sources. A key source of information is the location of the base stations. Other possible sources of information, which can be used to increase accuracy and to resolve ambiguities, include TAs, signal strength indications, and sector information. On many occasions, the position information calculated in the mobile will be needed at another location. This position information could be sent from the mobile to another location using SMS. This would constitute indirect positioning system. 21

22 Figure 5 mobile based positioning architecture Network-based Positioning Using transmissions of a mobile to work out its position is referred to as network-based positioning. The simplest implementation is based on TDOA. A number of geographically distributed positioning receivers are required to monitor transmissions from mobiles in the area and to be able to make accurate TOA measurements of the signals from the MSs. A locus function is to be needed to reside in each positioning receiver. The locus function will process the bursts emanating from the uplink from mobiles engaged in a call. A location service center (LSC) will generate TDOA measurement from the different TOAs and produce position estimate. The fusion function will reside in LSC. The LSC will also contain the BTS database. It will receive requests for position 22

23 measurement from various application computers; schedule the appropriate positioning receivers to make the required locus measurement of the nominated MS, collect the locus measurement from the positioning receivers, fuse all information into a position measurement, and then return the position measurement to the requested application. The LSC is likely to sit at the same level of the GSM hierarchy as the MSC. Positioning receivers could be collocated with BTSs, with many resultant advantages. Many of the functional elements of a positioning receiver have equivalent elements in a BTS. For certain BTS configurations, the positioning receiver could tap into the down converted video stream generated within the BTS and thus not have to duplicate subsystems such as antenna system, power supply, amplifiers, down converter, and digitizer. If the LSC is collocated with the MSC, signaling between LSC and the positioning receiver could be achieved using the existing communication link between the MSC and BTSs. As with mobile based positioning, synchronization is an issue. If the network is synchronized, there is likely to be a timing source available at each BTS to unambiguously mark the TOA of a given TCH burst on a given frequency channel. In an unsynchronized network a mechanism is required to synchronize the clocks used at each positioning receiver to mark the epoch at which a nominated burst arrives. Alternatively the positioning receiver collocated with a BTS could measure the TOA relative to their own transmission cycle, and separate monitoring sites would provide the LSC with timing offset of each BTS, thus allowing TDOAs and subsequently position to be calculated. 23

24 Figure 6 network based positioning architecture Network based solutions are important because it allows positioning without modification of the mobile phones. The other advantage is the leverage that can be gained from a single positioning measurement. The network operator could use the information for position based tariffs and provide the mobile user with a variety of position-based services (e.g., route guidance to the nearest service station). Hybrid Positioning Hybrid-positioning combines different aspects of remote-positioning and self-positioning. Possible hybrid architecture has the locus function residing in the mobile but the fusion function situated at the LSC. When requested by the LSC, a given mobile will measure TOA of bursts from various BTSs. These are then sent to the LSC, which generates TDOA measurements and compute position estimate for that mobile. 24

25 As with mobile based positioning, some form of synchronization is required. Either of synchronization methods identified for mobile-based positioning is suitable for use in hybrid architecture. A variant of hybrid architecture is Digital Cursor system. Instead of transmitting the locus information, this system transmits a replica of the signal, with the TDOA calculated using cross-correlation of the different replicas. 25

26 SUMMARY Many people will agree that mobile location services are an important new category of value-added service for the 21 st century. The applications and services explained in the introduction are of interest to customers and will help to differentiate network operators initially before becoming an expected part of service within 10 years. Although the opportunity for operators generated by these new offerings is both broad and deep, the deployment of location-based services will not begin with the most complex, technically demanding and feature-rich offerings. Instead, network operators will use today's technology to differentiate their services, as well as gain market leadership and critical technical skills. Becoming involved in the development of location specific products prepares them to be able to efficiently augment their offerings for when more sophisticated positioning technology and wireless devices enter the market.. 26

27 REFERENCES Christopher Drane, Malcolm Macnaughtan, and Craig Scott, Positioning GSM telephones IEEE Communication Magazine April Jeffery H. Reed, Kevin J. Krizman, Brain D.Woerner, and Theodore S. Rappaport, An overview of challenges and progress in meeting the E-911 requirement for location service IEEE Communication Magazine April Richard Walter Klukas, A Super resolution Based Cellular Positioning System Using GPS Time Synchronization UCGE Reports Number