Patent Publication Number: US-2006014548-A1

Title: Determination of mobile terminal position

Description:
TECHNICAL FIELD  
      The present invention relates in general to cellular communications systems and in particular to determination of positions of mobile terminals connected to cellular telecommunications systems.  
     BACKGROUND  
      The possibility to determine the position of a mobile device has enabled application developers and wireless network operators to provide location based, and location aware, services. Examples of those are guiding systems, shopping assistance, friend finder and other information services giving the mobile user information about their surroundings.  
      In addition to the commercial services, the governments in several countries have also put requirements on the network operators to be able to determine the position of an emergency call. For instance, the governmental requirements in USA (FCC E911) requires that it must be possible to determine the position of a certain percentage of all emergency calls. There is no difference between the requirements put on indoor environments compared to outdoor environments.  
      In outdoor environments, the position estimation can be done using external methods for position determination, e.g. GPS (Global Positioning System) based methods like Assisted-GPS (A-GPS). Position estimation can also be performed using the wireless network itself. Methods using the wireless network can be grouped in two main groups. The first group comprises methods that are based on the radio cell, to which a mobile terminal is attached, e.g. by using Cell-ID. The second group uses measuring of radio signals from several base stations (BS) and determining the terminal position using e.g. Time Difference (TD).  
      In order to be able to connect to a mobile network or to perform handover when connected, a mobile terminal typically constantly measures available downlink signals, not only from its own base station, but also from other base stations. These signals are typically control signals intended for measuring radio conditions of transmissions, which control signals contain, among other data, information about how to establish a connection to the transmitting base station. In particular, the control signals comprise data, which by itself or in combination with the frequency of the carrier on which the control signal was transmitted constitute base station identification data. A mobile terminal can thus obtain an identity of the transmitting base station and an estimate of the radio conditions. The mobile terminal typically compiles this information, in GSM (Global System for Mobile communications) in a neighbour list, which is transferred to the network as information.  
      Position estimation can be based on measurements in the neighbouring list. One then uses the relation between the distance from the radio base station and the radio condition in combination with knowledge about the exact position of the base station. The base station positions are known within the communications network. This means that the neighbour list easily can be used for position estimating according to different algorithms. The accuracy of the position estimation is generally proportional to the size of the cell.  
      Triangulations, or Time Difference (TD) methods, use signals associated with two or more different base stations. These signals are used to calculate the position or at what distance from the base station a mobile terminal is located. The calculations are based on the relative or absolute difference in the time it takes the signal to propagate between the different base stations and the terminal. The achievable accuracy of TD-methods depends on system architecture, physical conditions and radio conditions. Typically, the accuracy of a TD method in a mobile telephony system is 50 to 150 meters. TD methods are also relatively time and resource consuming.  
      Fingerprinting methods use the fact that all places have a, more or less, unique characteristic signature of the received radio signals. This is the result of multi-pathing and reflections in the buildings and obstacles. By storing the characteristic radio signature of different locations in a database, it is possible to determine the location of a device by comparing the received signature of a signal with the signatures stored in the database. Fingerprinting methods requires an always-updated database. A good result typically also relies on being able to match signals from several different sources or base stations.  
      A terminal located indoor typically has a connection to a base station covering the surrounding outdoor area that is of lower quality than if the terminal would have been located outdoors. To improve the indoor coverage situation, many larger buildings are equipped with indoor mobile telephony systems. The indoor system most often consists of one base station and a distributed antenna system or a leaking cable antenna. For a building spread over large areas repeaters may also be used. This results in that the entire building appears as one large radio cell and that it is impossible to determine where the terminal is located within the building. Furthermore, due to weak signals from base stations located outdoors, more sophisticated methods using e.g. triangulation is normally impossible to apply to indoor positioning.  
      One straightforward solution is to use an additional system for positioning, a system that is not based on any mobile telephony system. This can be an indoor GPS system, a WLAN (Wireless Local Area Network) or a Bluetooth based system or some other sensor solution. However, such systems require additional complex equipment and also the terminals have to be equipped with special hardware and/or software, which makes the solution expensive.  
      Another straightforward solution is to increase the number of indoor base stations, thus reducing the size of the cells. However, a base station is an expensive piece of equipment and such a solution would therefore become very costly. A drastic increase at base station level also means that network control and truncating capabilities also have to be extended, which also is associated with large costs.  
      In the published US patent application U.S. 2003/0008664 A1, a method and apparatus for estimating the position of a terminal within a radio system having a repeater is disclosed. A dedicated identity code is transmitted for each repeater. The terminal is provided with hardware and/or software for receiving and interpreting these codes. In a preferred embodiment of a CDMA system, the identification codes can be implemented with pseudo-noise sequences at defined offsets, specifically reserved for repeater identification. The repeater identity can then be used to give improved position estimation. Such a solution has the drawback that it typically needs additional software in the terminals for being able to identify the dedicated identity codes as well as additions in different communications standards, even if some special solutions may be possible within existing frames of standards.  
     SUMMARY  
      In prior art solutions, an improved accuracy in position estimation is associated with large investments in expensive additional equipment. Furthermore, some solutions require that special hardware or software is added to the mobile terminals, which means that all terminals already on the market either will not be possible to position, or that they must be upgraded. Moreover, solutions operable within present or near future standards are to prefer.  
      An object of the present invention is thus to provide for position estimation of mobile terminals with improved accuracy that involves limited investments in additional equipment. A further object of certain embodiments of the present invention is to provide methods and devices that do not require any changes in existing cellular standards and with no need for new or updated mobile terminals. Another object of certain embodiments of the present invention is to provide for improved position estimation suitable to be comprised in systems involving distributed antenna systems, leaking cable antennas and/or systems comprising repeaters. Yet another object of certain embodiments of the present invention is to provide for improved position estimation of indoor systems.  
      The above objects are achieved by devices and methods according to the enclosed patent claims. In general words, a multitude of measurement units is distributed over a cell area. The measurements units are instructed to measure properties of uplink signals of radio resources utilized by mobile terminals, whose position is requested. The measurements are in particular embodiments signal strength measures. The measurements are provided to a position determination node, which by comparing the measurements can estimate a position of the mobile terminal. Several possibilities for implementing ordering and reporting routines as well as implementing the communication between the measurement units and the position determination node are provided. The measurement units comprise two communication interfaces, one arranged for measuring the signal strength and one for communicating with the position determination node. The measurement units prepare measurements reports on the achieved measured properties, which are transmitted to the central position determination node for final evaluation.  
      The present invention has many advantages compared to prior art solutions. Since the present invention make use of already existing uplink signaling, the proposed embodiments are easy and inexpensive to introduce in already existing systems. The complexity of additional functionality or devices is low. Furthermore, mobile terminals already on the market can be used with the proposed system without any modifications at all. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which;  
       FIG. 1  is a schematic illustration of a cellular communications system;  
       FIG. 2  is a schematic illustration of a distributed antenna system according to prior art;  
       FIG. 3  is an illustration of an embodiment of a communications system according to the present invention;  
       FIG. 4  is an illustration of another embodiment of a communications system according to the present invention;  
       FIG. 5  is an illustration of yet another embodiment of a communications system according to the present invention;  
       FIG. 6  is an illustration of an embodiment of a communications system according to the present invention having an antenna system involving a repeater;  
       FIG. 7  is an illustration of an embodiment of a communications system according to the present invention having an additional coarse positioning arrangement;  
      FIGS.  8 A-C are illustrations of embodiments of connection possibilities of measurement units according to the present invention;  
       FIG. 9  is a flow diagram of main steps of an embodiment of a method according to the present invention;  
       FIG. 10  is a block diagram of an embodiment of a measurement unit according to the present invention; and  
       FIG. 11  is a block diagram of an embodiment of a position determination node according to the present invention. 
    
    
     DETAILED DESCRIPTION  
      In order to fully understand the operation of the present invention, first a short review of general prior art position estimations in cellular networks is given.  
      The basic idea with cellular networks  10 , one of which is schematically illustrated in  FIG. 1 , is to structure the network as a grid of cells  4 A-J where each cell  4 A-J is the area covered by one radio base station  2 A-J. The communication takes place via different radio resources. To avoid interference between mobile phones  6  and radio base stations  2 A-J in neighboring cells, the communication between the mobile phone  6  and the base station  2 A-J uses different resources, i.e. slightly different configurations or settings, e.g. of frequencies or codes. The number of those resources or “configurations” is limited. In GSM systems, the resources are formed by a limited number of allowed carrier frequencies, and they are used to separate communication in different cells. In WCDMA (Wideband Code Division Multiple Access) systems, the resources are characterized by a limited number of different codes. The result of the limited number of radio resources means that it is important to plan the network  10  carefully.  
      Mobile Station (MS), Mobile Phone, Mobile Terminal and Handset all refer to the device that is to be positioned. These terms will be used in the present disclosure as equivalent expressions. This device is typically a mobile telephone, hand held computer so-called Personal Digital Assistance (PDA) or other device or apparatus equipped with a radio receiver for cellular or mobile networks.  
      In most cellular networks  10 , the mobile terminal  6  continuously measures the receiving conditions of the downlink radio signals. The reasons are several. One is to be able to modify the transmission power in order to avoid sending at unnecessary high transmission power. In general, but not necessarily, the radio base station with the best radio conditions is the one used for connecting to the cellular network. The base station with the best radio conditions is in most cases also the one that is located closest to the mobile telephone  6 . In  FIG. 1 , the mobile telephone  6  is connected via base station  2 F. The mobile telephone  6  is thus located within the cell  4 F of that particular base station  2 F. The radio cell is defined as the area surrounding a base station, in which the base station is the base station with the best radio connection to a mobile telephone. Since the positions of the transmission points associated with the base stations are known by the cellular network, the identity of the base station with the best radio conditions hence also gives an approximate location estimate of the mobile telephone. The size of a cell is proportional to the density of base stations. In  FIG. 1 , one may therefore conclude that mobile telephone  6  is present within cell  4 F.  
      In order to know which base station to connect to, the mobile telephones constantly measures downlink signals sent also from other base stations. These signals are special control signals intended for measuring the radio conditions between the base stations and the mobile telephone. The signals contain, among other data, information about how to establish a connection to the base station sending the signal. As mentioned above, the communications in neighboring cells are done over links with slightly different configurations in order to avoid interference. The control signals are typically transmitted using those different configurations. As an example, in GSM, the control signal from one base station is sent on a different frequency than the control signal sent from the neighboring base station. However, base stations further away could use the same frequency in a reuse pattern. To be able to separate the base stations associated with different cells, but that are sending control signals on the same frequency, from each other, the control signals also contain other information making is possible to distinguish a control signal from one base station from the other. This information, alone or in combination with the frequency of the control signal, gives a possibility to identify a particular base station. In other words, the control signals comprise base station identification data. In GSM, so-called color codes are used to separate different base stations from each other.  
      In the present disclosure, the expressions “position” and “location” will be used. Position is intended to mean a geographical position given as coordinates or degrees (e.g. the WGS-84 datum). It may also contain orientation and/or heading, speed, acceleration etc. A position may also be given as a relative measure. The location is a more subjective position defined by the type of (or relation to) facility or place. Examples of locations are: “military area/facility”, “hospital”, “office”, “theatre”, “near emergency exit” The expression “location” is assumed to comprise also what is comprised by “position”.  
      The most trivial position estimation is to determine the approximate position as inside the cell of the base station with best radio connection with the mobile terminal. In  FIG. 1 , this means that it is possible to conclude with a certain probability that the mobile telephone  6  is situated within cell  4 F. Using several measured signals from different base stations for different algorithms means that a better accuracy than the cell where the mobile phone is camping can be calculated. In  FIG. 1 , it is assumed that base station  2 G provides the second best downlink signal. It is then very probable that the mobile telephone is situated in a 60° sector facing the cell  4 G, marked with broken lines in  FIG. 1 . Furthermore, if assuming that base station  21  is the next in quality, it is also probable that the mobile terminal  6  is situated in the half of the sector that is closest to cell  41 .  
      The translation or calculation translating the downlink signal measurements to a position and/or location estimate may take place either in the cellular system or in the terminal. If the position estimation takes place in the system, e.g. in a network server, the mobile terminal has to transmit measurements to the radio base station. If the mobile terminal itself performs the estimation, the estimation can in a basic concept e.g. comprise a determination of a closest base station in form of e.g. a cell-ID. Such position information can in certain cases be enough to support many of the services based on position determination. However, if the actual geographic position is to be estimated, the mobile terminal first needs information about the particular surroundings. Such information should contain at least the known positions of the different base stations and could e.g. be deduced from instructions concerning base stations to be measured. Other information that may be specific to the location, building or surroundings may also be useful. Such specific information about e.g. a specific building could comprise map information, from which it is possible to exclude certain areas where a mobile cannot be located from the position determination. It is e.g. obviously most likely that a mobile terminal is not located inside a wall of the building or hovering in the air 10 meters above the floor.  
      Indoor coverage in cellular systems is often of a lower quality than outdoors. Therefore, many larger buildings have their own local cell or cells. A typical prior art system is illustrated in  FIG. 2 . One single base station  8  serves a distributed antenna system comprising a number of antennas  14  distributed over the indoor area. A repeater  12  can be present in order to enhance the signals during distribution. Since all antennas provides the same information, a mobile terminal  6  experiences all antennas  14  together as one transmitting system, being associated with one single cell  4 . Furthermore, since the mobile terminal  6  is unaware of which antenna it is communicating with, refined position estimation as described above is less likely to operate well.  
      In the present invention distributed antenna systems as well as leaking cable systems and subsystems that are fed by a repeater or any other active component are assumed to be well suited for implementing certain embodiments of the present invention. The term “antenna” is normally used both for an antenna in a distributed antenna system, but also for a section of a leaking cable on a leaking cable antenna.  
      The typically bad connections to the base stations for the outdoor coverage also makes it difficult or even impossible to use base stations located outdoors for triangulation purposes. Since only one base station often is used for the indoor coverage, it is impossible to use internal indoor triangulation for position determination. In some buildings that are spread over large areas (e.g. airports), repeaters are used. The cell then becomes even larger resulting in that the area in which the mobile phone is when connected to that cell is very large, i.e. the position estimation accuracy is low.  
      The accuracy of position estimation based on downlink signal measurements is basically proportional to the cell size. Smaller cells will generally give a better position estimation. However, cells are controlled by a base station, and base stations are generally very expensive.  
      The present invention is applicable to most cellular communications networks. The accuracy of the position determination method according to the invention depends on e.g. the premises or environment where the invention is to be implemented and other pre-requisites as well as various customer requirements. However, a position accuracy of 20-50 meters is believed to be realistic. The present invention could advantageously be used for positioning of mobile terminals located in indoor systems, underground railway systems (subways) and sub-systems connected to cellular macro systems, e.g. tunnels connected to a macro radio cell using a repeater.  
      The positioning method disclosed in the examples below is primarily targeting positioning in cellular mobile radio systems. GSM is the mobile radiotelephony standard used in the exemplary embodiments presented in this disclosure. However, the present invention is also applicable to other cellular mobile radio systems and their related standards, such as e.g. other radio standards based on TDMA (Time Division Multiple Access), CDMA (Code Division Multiple Access), Wideband CDMA (WCDMA), PDC (Personal Digital Cellular) and TDD (Time Division Duplex) technology.  
       FIG. 3  illustrates schematically one particular embodiment of a communications system according to the present invention. A base station  2  is associated with a cell  4 . A mobile terminal  6  is situated within the cell  4 . A multitude of measuring units  7 A-G are distributed over the cell area. The measuring units  7 A-G are connected, in this particular embodiment by wires  3 , to a position determination node  13 . The communication via the wires  3  can be performed by any wired based communication technology, e.g. regular telephone lines, ISDN, Ethernet etc. When the mobile terminal  6  wants to have its position determined by an accuracy better than the cell division, the position determination node  13  demands the measuring unit  7 A-G to measure a property of a signal  11  of an uplink radio resource allocated to the mobile terminal  6  in question. The signal property is in one embodiment associated with signal strength and can e.g. be an absolute signal strength, or a signal-to-noise ratio measure. The measurements are typically compiled in the measuring units, typically involving repeated measurements over a short period of time to insure relatively accurate values. These compiled measurement values are then communicated to the position determination node  13 , where a comparison is made.  
      In one embodiment, the position determination node  13  is arranged just to compare the different values, and the mobile terminal  6  is determined to be present within an area  5 A-SF closest to the measuring unit  7 A-G having the highest measured signal property value. In the situation illustrated in  FIG. 3 , the measuring unit  7 A is probably measuring the highest signal strength, and a first position determination of the mobile terminal  6  is that the mobile terminal  6  is present within area  5 A.  
      In another embodiment, the position determination can be further refined. Again referring to  FIG. 3 , it can be concluded that the signal strength measurements of measuring units  7 B,  7 F and  7 G probably are in the same order of magnitude, which means that the mobile terminal probably is present at about the same distance from measuring units  7 B,  7 F and  7 G, but still within area  5 A. However, since signal strength properties can be influenced in many ways, the accuracy of such refined position determinations is generally not very impressive. Instead, to improve the position accuracy, the density of measuring units can be increased, which reduces the size of the associated areas  5 A-G.  
      Signal strength can be measured easily by many procedures as such known in prior art. A probe measuring signal power is typically easy to implement and is typically associated with low costs. Furthermore, by having a multitude of measuring units relatively close to the mobile terminal, the sensitivity of the signal strength of e.g. obstructing elements is generally relatively low. The reason for this is to be found in the exponential power distribution of the radio signal. In the vicinity of an emitting antenna, the gradient of the power distribution is much steeper than further away from the emitting antenna. Therefore, a decreased signal by e.g. 10 dB, caused by an obstructing element, will result in a large spatial error when the receiver is located a large distance from the emitter. A corresponding spatial error will be considerably smaller if the receiver is located closer to the emitter. A signal-to-noise ratio measure is believed to present an even lower sensitivity for obstructing objects.  
      It should be noted that the measurement units  7 A-G are measuring on uplink signals  11  used by the mobile terminal  6 . There is thus no need for any additional hardware or software in the mobile terminal  6 . The only requirement is that the position determination node  13  should have access to the actual radio resource that is used for the uplink signals of the mobile terminal  6  to be positioned. Furthermore, since already available uplink signals are used for the positioning measurements, the interference situation is unaltered. Solutions based on providing additional signals in the licensed spectrum require a license. Regulations for licensed spectra differ from one country to another and general solutions may be difficult to find. This typically means that only operators having licenses can provide such solutions to the users. By instead using only measurements on already existing signals, no licensing is necessary and external operators may therefore easily be involved.  
       FIG. 4  illustrates schematically another particular embodiment of a communications system according to the present invention. The base station  2  is connected by a connection  15  to a base station controller or radio network controller  16 . (This was of course also the case in  FIG. 3 , however not explicitly illustrated.) Also here, measurement units  7 A-G are distributed over the cell area  4 . In this particular embodiment, the position determination node  13  is connected to the base station controller  16 . In another embodiment, the position determination node  13  can even be comprised in the base station controller  16 . The communication between the position determination node  13  and the measurement units  7 A-G utilizes in this embodiment the resources of the communications system itself. That is, the measurement units  7 A-G communicates on radio resources  9  with the base station  2 , which forwards the information to the base station controller  16  and the position determination node  13 . The communication between the measurement units  7 A-G and the position determination node  13  can be provided by control signaling or by including reports and orders in ordinary data packets transmitted over the user plane, or a combination thereof.  
      In this embodiment, the measurement unit  7 A-G will occupy some radio resources of the communications system  10  that otherwise could be used for ordinary traffic. However, if the number of measurement units that are active at the same time is relatively limited, and furthermore if the measurement units could share some radio resources, the impact on the amount of available radio resources could be kept small. The large portion of the communication will consist of the actual measurement results and are sent in an uplink direction, where there typically are more available radio resources. The downlink communication consists of measurement orders, which should be possible to send as a multicast or broadcast message to the measurement units. The advantage of this particular embodiment is that no additional wiring or separate communications system for the measurement units  7 A-G is needed, which reduces the installation costs significantly. Furthermore, the measurement units  7 A-G could be based on the same hardware and software as ordinary mobile phones, which opens up for very low-cost solutions.  
       FIG. 5  illustrates yet another particular embodiment of a communications system according to the present invention. The measurement units  7 A-G are in this embodiment equipped for allowing a wireless communication  17  with the position determination node  13 . This wireless communication is separate from the main communications system and can be based on any wireless communication technology, e.g. Bluetooth, wireless LAN (such as e.g. according to the standards 802.11b and 802.11g), GPRS, UMTS etc. In this embodiment, no resources of the main communications system are occupied, however, the installation of the additional wireless communications system increase the costs for such a solution.  
       FIG. 6  illustrates a particular embodiment of the present invention applied to a cell using a repeater arrangement. A base station  8  has an antenna  19  for the outdoor part of the cell  4 . A repeater based implementation is here suitable, since it makes it possible to determine if a mobile phone using the base station is located inside a building  20  or not. A receiver  18  receives the signal and a repeater  12  feeds an indoor system of e.g. distributed antennas  14 . A measurement unit  7  provided within the building and another measurement unit  7  is provided outside the building. From the measurements, it can easily be determined if the mobile terminal  6  is situated within the building  20  or not.  
      If a very accurate positioning is requested, the number of measurement units has to be high. If the number of measurement units within one single cell becomes too large, the efforts for handling measurements from all measurement units may become large and occupy resources of communication as well as Of processing. A particular embodiment of the present invention comprises an initial coarse positioning, on which the orders for the measurement units are based. In  FIG. 7 , a base station  8  serves a distributed antenna system having a number of distributed antennas  14 , together being associated with a cell  4 . In connection with the different antennas  14 , beacons or any other type of transmitters  22  are provided. Signals emitted from the transmitters  22  can be detected by a mobile terminal  6 , and if the signals from the different transmitters  22  are distinguishable, the mobile terminal  6  or any node connected thereto can determine a coarse position of the mobile terminal  6 . Each transmitter  22  therefore has an associated area  21 . Such downlink signal positioning methods are known as such in prior art and are therefore not further discussed in detail. However, the coarse positioning provides a better position estimation that the simple cell identity.  
      Once the mobile terminal  6  knows in which associated area  21  it is present, the position determination node  13  can be informed, and a refined position determination can be performed along the earlier described ideas, but with a restricted number of measurement units  7  involved. In  FIG. 7 , the mobile terminal  6  finds that it is situated in the leftmost associated area  21 . The position determination node  13  subsequently restricts the measurements and reporting on which the refined position determination is going to be based to only involve the four measurement units  7  at the left side of  FIG. 7 . The remaining measurement units  7  in the other parts of the cell  4  can remain inactive, and the amount of signaling is therefore significantly reduced.  
      As anyone skilled in the art understands, the positioning method according to the present invention can be combined with any other positioning method giving a coarse position. The communications system may e.g. be equipped with a positioning system measuring time-of-flight to different neighboring base stations. If such a system has access to too few base station signals, an exact position can not be determined. Instead an area can be determined in which the mobile terminal is situated. A refined position determination can then be performed by the ideas of the present invention, having the number of used measurement unit reduced by only including measurement units within or in the vicinity of the determined area.  
      There are several embodiments of measurement units that could be used in the present invention.  FIG. 8A  illustrates a block diagram revealing the communication possibilities with the surrounding of a first particular embodiment of a measurement unit  7  according to the present invention. Here, the measurement unit  7  has a measurement terminal  26  connected to a receiving antenna  25 . Uplink signals from the mobile terminal to be position determined are recorded by the antenna and provided to the measurement unit  7  via the measurement terminal  26 . The measurement unit  7  of this embodiment has a second reporting terminal  27  being connected to a second antenna  29 . This second antenna  29  is utilized for communicating measurement reports and receiving measurement orders. This embodiment is well suite to be used in a system similar to the one illustrated in  FIG. 5 .  
       FIG. 8B  illustrates an embodiment of a measurement unit  7 , which is suitable for use in a system similar to the one illustrated in  FIG. 3 . Here the reporting terminal  27  is connected to a communication wire system  3 .  
       FIG. 8C  illustrates an embodiment of a measurement unit  7 , which is suitable for use in a system similar to the one illustrated in  FIG. 4 . Here the reporting terminal  27  is also connected to the antenna  25  by a connection  28 . The antenna can then alternately be used for measuring purposes and for communication with the position determination node. In an alternative embodiment, the connection  28  can be an internal connection. In all embodiments, however, two interfaces are present, one for measuring uplink signal properties, and one for communication with the central position determination node.  
      The main steps of an embodiment of a method according to the present invention are illustrated in  FIG. 9 . The procedure starts in step  200 . In step  210 , properties of an uplink signal used of a mobile terminal are measured at a multitude of positions within one single cell. The measurements are reported to a position determination node in step  212 . This can be performed in any of the earlier described manners, using wired or wireless connections. In step  214 , the position of the mobile terminal is determined by comparing the measurements originating at the multitude of measurement positions. In a simple particular embodiment, the position is determined to be within an area associated with the position where the signal with the highest strength was measured. The procedure is ended in step  299 .  
      In a particular embodiment, a further step of compiling measurements into a report is performed at the site of the measurements. Such compiling could e.g. comprise averaging over a certain time, adjusting for rapidly changing signals etc. The measurements or data related thereto can also be stored at the measurement site for later reporting or for comparisons with later measurements.  
      In another particular embodiment, the results of the measurement are instead reported in a more or less unprocessed condition. Compiling and processing of the measurements could then be included in the position determination step.  
      The initiation of the measurements can also be performed in different ways. In one particular embodiment, a measurement order is sent to the sites where measurements are to be performed. The order comprises typically an identification of the particular radio resource that should be targeted, e.g. a time slot, a code, a frequency or a combination thereof. Upon reception of such an order, measurements will be performed at that particular radio resource and reported back. If the equipment at the measurement site so admits, several uplink signals could be monitored simultaneously or alternately, to provide more than one simultaneous measurement. In another particular embodiment, the measurements can be performed more or less continuously, also here either simultaneously or alternately. The measurements are stored. When an order of a measurement is received, the latest stored measured value is reported back immediately without any delay for measurements.  
      In another possible embodiment, the measurement unit makes the measurements, but has not the processor capabilities for evaluating them. Instead, the complete raw measurement result is forwarded to the position determination node, where an evaluation is made. The advantage with such a solution is that the measurement units can be made even simpler. However, the communication links to the position determination node has to handle large amounts of data.  
      A measurement order in e.g. a GSM or GPRS system comprises information about which time slot measurements should be performed on for a given base station. The base station is defined by a frequency, BSIC or Cell ID. However, a time slot is defined relative a serving base station clock. The base station clocks are normally not synchronized against any other universal time, such as GPS or UTC. The measurement unit therefore in practice has to know the base station clock relative its internal clock. If the measurement unit communicates with the communications network, e.g. by GSM or GPRS, the measurement unit is already synchronized to at least one base station, and the relative synchronization can thereby easily be arranged. However, if the measurement unit communicates with the position determination node, e.g. by cables or fibers, the measurement unit has to achieve the relative synchronization in another way. One way to solve this problem is to incorporate a GSM downlink receiver, which measures the base station clocks of interest.  
      If signal-to-noise ratio is to be measured, the measurement order has to additionally comprise a training sequence or any other data making it possible to identify which training sequence that is going to be used. In GSM, such a training sequence is uniquely defined by the training sequence code.  
      The measurement orders could be restricted to be valid for only a subset of the measurement positions. Such restrictions could e.g. be based on other coarse positioning methods, thereby reducing the number of measurements needed.  
       FIG. 10  illustrates a block scheme of a particular embodiment of a measurement unit according to the present invention. The measurement unit  7  comprises a processor  30  connected to a first communication interface  35  and a second communication interface  34 . The second communication interface  34  is connected to the measurement terminal  26  and received the actual measured uplink signals. The properties, typically the signal strength, of the uplink signal is evaluated and a measurement report is prepared by an evaluation and report section  31  of the processor. The results are in this particular embodiment also stored in a storage  33 . A transmission section  32  of the processor prepares the report for to be sent to the position determination node and provides the report to the first communication interface  35 , which is arranged for communication with the position determination node.  
      A measurement unit  7  according to  FIG. 10  can easily be provided on a common mobile telephone platform.  
       FIG. 11  illustrates a block scheme of a particular embodiment of a position determination node  13  according to the present invention The position determination node  13  communicates by connection  44  with the measuring units using a communication interface  43 . A processor  40  comprises an ordering section  41 , which is arranged to issue orders of making measurements on particular radio resources. The orders may be restricted to a particular set of measurement units, as decided based on additional coarse position information. Such information is in this particular embodiment achieved from a coarse positioning unit  45 . When measurements are received, a positioning section  42  of the processor  40  evaluates the measurements. The evaluation is typically performed as a comparison between measurements from different sites.  
      The position determination node  13  is in a typical case arranged in or in connection to a base station controller or a radio network controller. However, the position determination node  13  can also be placed anywhere else in the communications system. The position determination node  13  has to have knowledge about which radio resources that are used for which mobile terminal.  
      In a particular embodiment, the position determination node  13  may also be separate from the communications system. In such a case, the information about the uplink radio resource used by a particular mobile terminal has to be provided in some way. One possibility is that there is an agreement between the communications system operator and the position determination node operator that such data is provided by the communications system operator.  
      However, another possibility is that such information is provided by the mobile terminal itself. The knowledge about allocated uplink resources for a specific mobile terminal is available for the communications system, but at allocation also for the mobile terminal in question. It is therefore possible for the mobile terminal to extract such information and communicate it to a position determination node not incorporated in the communications system. Such uplink radio resource data can e.g. be comprised in a data message communicated either to the position determination node or directly to the measuring units. The data message could be transferred e.g. as a SMS message or an electronic mail message.  
      The embodiments described above are to be understood as a few illustrative examples of the present invention. It will be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the scope of the present invention. In particular, different part solutions in the different embodiments can be combined in other configurations, where technically possible. The scope of the present invention is, however, defined by the appended claims.