A CELLULAR RADIO SYSTEM COMPRISING AT LEAST TWO MEASURING MEANS FOR LOCATING A MOBILE STATION

Abstract: The invention relates to a cellular radio system comprising, in each cell, at least one base station (100, 102) communicating (104) with terminals (106) located within its area. To advantageously locate a terminal, the system of the invention comprises at least one set of measuring means (112, 114) for listening to the signals sent by the terminals and for collecting measurement data for at least two different terminal location methods.

Description: A CELLULAR RADIO SYSTEM COMPRISING AT LEAST TWO MEASURING MEANS FOR LOCATING A MOBILE STATION FIELD OF THE INVENTION The invention relates to a cellular radio system comprising, in each cell, at least one base station communicating with terminals located within its area. BACKGROUND OF THE INVENTION A mobile phone system typically covers a wide geographical area, comprising a plural number of coverage areas, or cells, each served usually by one base station. The size of the cells typically varies greatly according to the volume of telephone traffic in the area. In areas of high traffic density cell sizes are usually smaller than in areas where fewer calls are placed, irrespective of cell size, situations often arise when it would be useful to know the geographical location or the direction of travel of a mobile phone. Several different methods have been developed for determining the geographical location of terminals. A prior art method is known as the Direction Finding (DF) method where a signal sent by a terminal is received at least by two DF receivers. The network must roughly know the location of the terminal in advance so as to be able to control the DF receivers that are in the coverage area of the signal sent by the terminal to simultaneously receive the signal the terminal sends. In other words, the DF receiver must know the transmission parameters of the signal coming from the terminal, such as the channel, time slot and any frequency hopping used. The signalling messages thereby created load the network. Each DF receiver estimates the direction the signal sent by the terminal arrives from. On the basis of this information and the known location of the DF receiver, a sector can be drawn from each DF receiver, the sector indicating the direction where the terminal is situated. The terminal can be established to locate within an intersection of two or more sectors. The greater the number of sectors (i.e. DF receivers) available, the better the location accuracy. Each DF receiver informs its measurement results to a network unit responsible for computation, such as a Mobile Positioning Centre MPC. In prior art solutions DF receivers are located at base stations or at other suitable places. The solution requires antenna groups to be used for estimating the direction of arrival of a signal. To allow the DF receivers to better measure the signal coming from the terminal, the terminal can be commanded to perform an intra-cell handover at full transmission power. The DF method is not suitable for situations requiring continuous location monitoring because the required signalling loads the network. A situation where a plural number of terminals are to be located also loads the network. The accuracy of the DF method is not high in urban areas where multipath propagation and reflections from buildings make the determining of the signal's direction of arrival inaccurate. Another known location method is based on the times of arrival of the signals (TOA, Time of Arrival; TDOA, Time Difference of Arrival). In this method an additional receiver is placed at the base station. The receiver listens to a terminal's transmission addressed to another base station serving the terminal. To allow the receivers to better measure the signal coming from the terminal, the terminal is commanded to perform an intra-cell handover at full transmission power, the additional receivers at the neighbouring base stations receiving an access message sent by the terminal in connection with the handover. The above method requires that each base station is able to accurately determine the time of the access message by applying one and the same time reference. This requires a common synchronisation of the receivers. In practice, a GPS receiver can be arranged at the receivers in order to provide the synchronisation. A problem arises from whether the neighbouring base stations are capable of hearing the transmission of the terminal. This restricts the use of the method in areas where base stations are scattered, such as rural areas. In addition, the capacity of the method is limited because each additional receiver can reliably measure only one terminal at a time. A third known method is based on the Observed Time Difference (OTD) between the signals. In this method the terminal measures a timing difference between the base stations. The method also requires information about the difference in synchronisation between the base stations (RTD, Real Time Difference), in case the base stations are not synchronised. The location is determined on the basis of this information. The method is described in greater detail in patent application FI954705. To sum up, it can be stated that a terminal location method applicable to different circumstances is not yet available. BRIEF DESCRIPTION OF THE INVENTION An object of the invention is thus to provide a cellular radio system allowing a terminal to be advantageously located with one or more location methods. This is achieved with a system described in the preamble, characterized in that the system comprises at least one set of measuring means for listening to signals sent by the terminals and for collecting measurement data for at least two different terminal location methods. The preferred embodiments of the invention are disclosed in the dependent claims. The invention is based on different location methods comprising characteristics and functions that can be implemented by using one and the same equipment. For instance, in digital time division multiple access systems, such as the GSM system, OTD and TOA location methods require similar measurements and equipment, therefore it is advantageous to provide measuring means that support both methods at the same time. The solution of the invention provides various advantages. When a terminal can be located by applying different methods with one and the same equipment, the location method, or a combination of methods, that best suits a particular situation or environment can be easily selected. The solution of the invention is also advantageous as regards equipment configuration, because overlapping implementations are avoided and, furthermore, a uniform equipment for locating can be provided at different points of the network, irrespective of the circumstances or the environment. This allows significant savings to be made in maintenance costs. The system operator can use the solution of the invention to build a location service by gradually increasing the number of measuring means at a desired pace. The number of the measuring devices determines the measurement accuracy, i.e. one of the service parameters. To be able to measure the right signal in the TOA and DF methods, for example, the measuring means must know the channel (frequency and time slot of a digital time division multiple access cellular system, for example) the terminal is transmitting on. The network element controlling the measuring means, such as a mobile positioning centre or a base station controller, can inform the time slot to the measuring means, but the measuring means must still find out the timing of the terminal. The measuring means can then measure the transmissions of the base station serving the terminal and thereby obtain information about the timing of the base station and the terminal. In the TOA and TDOA methods, after the measuring means have measured the signal sent by the terminal, they must time stamp the measured result. One alternative is to use a GSP signal as a time reference. Another option is that the measuring means produce the time stamp by using the timing of a suitable base station (the base station serving the terminal, for example). The measuring means finds out the timing when receiving transmissions from the base station. In addition, the measuring means can measure signals of other base stations, too, to find out timing differences between base stations. On the basis of the base station measurements made by the measuring means, a network element - such as the mobile positioning centre - can determine the timing differences between the base stations so as to allow the time stamps the different measuring means report for the signals of the terminals to be made to correspond to each other (in other words, all measuring means participating in the locating of a particular terminal need not necessarily be able to measure the base station serving the terminal concerned and to use the timing of the base station in question). In the OTD method the base stations must be synchronised or, alternatively, the timing differences between them must be known. To find out the timing differences, the measuring means can measure the signals sent by the base stations and determine the differences in the times of arrival of the signals. This procedure is similar to the one required in the TOA method (without synchronised transmissions from the base stations), i.e. the requirements the TOA and the TDOA methods set to the measuring means are similar to those set by the OTD method, if the measuring means in the TOA/TDOA method comprise a GPS receiver for time stamping the TOA/TDOA measurements, the GPS can be used for synchronising the transmissions of different base stations, provided that the measuring means is located at the base station. Another possibility in this case is that the timing of the base station is compared with the clock available in the GPS and the information obtained is forwarded to the mobile position centre, for example, to find out the timing differences between the base stations. BRIEF DESCRIPTION OF THE DRAWINGS In the following the invention will be described in greater detail in connection with preferred embodiments and with reference to the accompanying drawings, of which Figure 1 illustrates an example of a cellular radio system of the invention; Figures 2a and 2b illustrate examples of a structure of a measuring equipment of the invention; Figure 3 illustrates an example of signalling in a system of the invention in connection with a locating operation; and Figure 4 illustrates another example of signalling in a system of the invention in connection with a locating operation. DETAILED DESCRIPTION OF THE INVENTION With reference to Figure 1 , a structure of a cellular radio system according to the invention will be examined. In the following, a radio system of the GSM type is used as an example, although a person skilled in the art will find it obvious that a solution of the invention can be similarly applied also to other systems. The system comprises a plural number of base stations 100, 102 communicating with terminals located in their area. Figure 1 shows a situation in which a first base station 100 communicates 104 with a terminal 106. The system further comprises base station controllers 107, 108 controlling the base stations. In the system shown in the Figure, the first base station 100 is controlled by a first base station controller 107 and a second base station 102 by a second base station controller 108. One base station controller can control one or (usually) more base stations. For the sake of clarity, the Figure shows a situation where each base station controller only controls one base station. The base station controllers, in turn, communicate with a mobile switching centre 110 which monitors and co-ordinates their operations and forwards calls to other parts of the network and to external networks. The system of the invention comprises at least one set of measuring means 112, 114 (GEMU, Generic Measurement Unit) which are arranged to collect measurement data for at least two different terminal location methods. Figure 1 shows first and second measuring means 112 and 114. The measuring means can be located at a base station, as for example the second means 114 shown in Figure 1; they can be integrated into a base station; or they can be arranged as separate means. Alternatively, the measuring means can be separate from other network equipment, as for example the first measuring means 112 in Figure 1. Location methods supported by the measuring means comprise, for example, the above mentioned DF, OTD and TOA/TDOA methods. The system further comprises a Mobile Positioning Centre (MPC) 116, which is arranged to calculate, by applying one or more location methods, the location of a desired terminal using the information collected by the measuring means 112, 114, and other information possibly available. Location can also be calculated somewhere else, at a mobile station, for example, provided that additional information, such as timing differences between the base stations and base station co-ordinates, are sent to the mobile station. The mobile positioning centre communicates with the measuring means and the mobile switching centre 110. The measuring means 112, 114 can be logically controlled by one or more network elements. The measuring means can be, for example, equipment controlled by a mobile position centre alone, or they can be controlled both by a mobile positioning centre and a base station controller. Figures 2a and 2b are block diagrams illustrating examples of the structure of the measuring means 112. Let us first study Figure 2a. The measuring means comprise, firstly, an antenna 200 for receiving signals that are to be monitored. The measuring means further comprise a receiver block 202 which is capable of receiving and measuring a desired signal received through the antenna. The receiver block 202 can be implemented in a prior art manner, basically it is similar to a base station receiver block and/or a terminal receiver block. The measuring means further comprise a control unit 204, which can be implemented by means of a processor and the necessary detached circuits. The control unit controls all the operations of the receiver block 202 and the measuring means. The measuring means further comprise a transceiver block 206 which the measuring means use for communicating with the rest of the network. The measuring means 112 are thus located, as shown in Figure 1 , separate from other network equipment. The measuring means have a connection to the mobile positioning centre 116. The connection 118 can be implemented in various different ways, the method of implementation applied determining the details of the structure of the transceiver block 206. For example, the connection 118 between the measuring means 112 and the mobile positioning centre 116 can be implemented as a modem connection, as a PCM connection with fixed cabling, or as a wireless connection. The connection 118 can also be implemented through a public or non-public data network, such as Intranet and Internet networks implemented by means of an X.25 or IP protocol. A wireless connection, in turn, can be implemented as a dedicated call in the cellular radio system, as a packet data connection in the cellular radio system, or by means of a special implementation that allows the signalling load to be avoided. In such cases, the transceiver block 206 conforms to the implementation required by the connection. When a wireless connection is used, for example, the block can be implemented as a transceiver of the cellular radio system or as a modem controlled by the control unit 204 and transferring the measurement data needed. Let us then study Figure 2b. In addition to the parts described above, the measuring means can also comprise a GPS receiver 208 which receives a GPS signal through its antenna 210. The GPS receiver 208 allows the measuring means to synchronise its clock and the measuring times. The GPS receiver is not, however, essential to the operation of the equipment because synchronisation can also be achieved by means of a frame rate of the base stations. If the measuring means are located at a base station, as the measuring means 114 shown in Figure 1 , the structure of the transceiver block 206 naturally corresponds to this implementation. In this alternative, the measuring means 114 are physically connected 120 to the base station 102. Logically they can be controlled by the mobile positioning centre 116 and/or the base station controller 108, depending on the implementation. This provides a logical connection 122 through the base station controller 108 and, possibly, the mobile switching centre 110, to the mobile positioning centre 116. Communication between the measuring means and the mobile positioning centre 116 implemented according to the embodiment concerned will be studied further below, with reference to Figure 3. Let us continue to study Figure 1 and Figures 2a and 2b. As stated, the measuring means 112, 114 are arranged to collect measurement data for at least two different terminal location methods, the measuring means supporting, for example, the above mentioned DF, OTD and TOA/TDOA methods. The measuring means receive the signal 124, 126 from the terminal 106, and measure its time of arrival (for the TOA method). The measurement is carried out by using the receiver block 202 controlled by the control unit 204. Further, the measuring means can measure the signal's direction of arrival (for the DF method). To determine the direction of arrival, the antenna 200 of the measuring means can be implemented by using an antenna group which can determine the direction of arrival of a received signal by phasing the signal. The antenna group can be implemented and controlled by applying prior art methods. For the OTD method, the measuring means receive the signal 128, 130 from the base stations and carry out the RTD measurement. The measurement is performed by using the receiver block 202 controlled by the control unit 204. Also for the OTD method, the terminal 106 measures the timing of the signals 104, 134 coming from the base stations. The measuring means forward the measurement results to the mobile positioning centre 116, which is arranged to calculate, by applying one or more location methods, the location of a desired terminal on the basis of the information collected by the measuring means. In addition to the information thus obtained, the DF and TOA methods, for example, also need other information, such as co-ordinates of the measuring devices, which can be obtained, however, by using prior art methods. The mobile positioning centre also controls the measuring means, possibly together with a base station controller and/or a mobile switching centre. It can also have other functions associated with locating. The mobile positioning centre can check whether a terminal is entitled to use a service dependent on location, whether a terminal can be located at all, or whether an application using the mobile positioning centre is entitled to locate a terminal. Furthermore, pricing of calls and their invoicing can be carried out as a location function controlled by the mobile positioning centre. The mobile positioning centre 116 can be a separate equipment in the network or it can be integrated into another network component (to a base station controller or mobile switching centre, for example), or its functions can be decentralized. To be able to carry out measurements, the measuring means need information about the traffic channel or signalling channel used by the terminal. The mobile positioning centre can obtain this information from the base station controller or the mobile switching centre either by inquiring it or automatically, and it can transfer the information to the measuring means. The mobile positioning centre can, for example, inquire channel information from the mobile switching centre by using an MAP (Mobile Application Part) protocol. If the measuring means concerned is logically controlled also by the base station controller, the base station controller can relay the information and any commands that may be needed directly to the measuring means. If the measuring means are located as a separate unit apart from other network equipment, transmission and signalling between the measuring means and the mobile positioning centre can be freely designed, i.e. it is not necessary to adhere to the signalling of a cellular radio system. For example, messages can be transmitted over a conventional telephone line. However, the measuring means are preferably located at base stations, whereby it is advantageous to utilize the existing transmission and possibly also the existing signalling channels. Figure 3 illustrates a possible model of a signalling diagram, the system concerned being assumed to be a GSM system. The signalling is controlled from the mobile positioning centre. The signalling example in question assumes that there are three base stations BTS1 , BTS2 and BTS3 in the area, the base stations being controlled by one and the same base station controller BSC and each base station being provided with measuring means GEMU1 , GEMU2 and GEMU3. Let us further assume that a terminal MS is located within the area of the base station BTS1 and communicates with it. In step 301 the mobile positioning centre requests radio channel information from the mobile switching centre by using a new MAP message. In step 302 the mobile switching centre requests radio channel information from the base station controller by using a new A interface message. In step 303 the base station controller replies and sends information about the used channel, time slot and frequency hopping sequence to the mobile switching centre by using a new A interface message. In step 304 the mobile switching centre forwards the radio channel information to the mobile positioning centre by using a new MAP message. In step 305 the mobile positioning centre determines the measuring means that are located close to the base station serving the terminal, and sends a start measurement message through the mobile switching centre to the base station controller controlling the base stations where the measuring means are located. The message is sent by using a new MAP message. In step 306 the mobile switching centre forwards the start measurement message to the base station controller by using a new A interface message. In step 307 the base station controller forwards the start measurement message to the base station by using a new operations and maintenance (O&M) message. In step 308 the base station removes operation and maintenance headers from the message and forwards the message to the measuring means. In step 309 the measuring means carry out the necessary measurements and forward the results to the base station. In step 310 the base station packs the measurement results into the operation and maintenance message and forwards it to the base station controller. In step 311 the base station controller transmits the measurement results to the mobile switching centre by using a new A interface message. In step 312 the mobile switching centre transmits the measurement results to the mobile positioning centre by using a new MAP message. Figure 4 illustrates another example of a possible signalling diagram. The system concerned is assumed to be a GSM system. In this example signalling is controlled both at the mobile positioning centre and at the base station controller. In the signalling example concerned, it is further assumed that there are three base stations BTS1 , BTS2 and BTS3 in the area, the base stations being controlled by one and the same base station controller BSC, each base station being provided with measuring means GEMU1 , GEMU2 and GEMU3. Let us further assume that the terminal MS is located within the area of the base station BTS1 and communicates with it. In step 401 the mobile positioning centre requests the mobile switching centre to initiate the measurements needed in the locating operation. In step 402 the mobile switching centre requests the base station controller to initiate the measurements needed in the locating operation. In step 403 the base station controller determines the measuring means that are located close to the base station serving the terminal and forwards a start measurement message to the base station by using a new operation and maintenance (O&M) message. In step 404 the base station removes operation and maintenance headers from the message and forwards the message to the base station controller. In step 405 the measuring means carry out the necessary measurements and forward the results to the base station. In step 406 the base station packs the measurement results into the operation and maintenance message and forwards it to the base station controller. In step 407 the base station controller transmits some of the measurement results, or all of them, to the mobile switching centre by using a new A interface message. In step 408 the mobile switching centre transmits the measurement results to the mobile positioning centre by using a new MAP message. It should be noted that although in both examples described above all the base stations are controlled by one and the same base station controller, this need not necessarily be the case in a real situation. Instead, base stations can also be controlled by different base station controllers. Various alternatives are then available for signalling. Signalling can be made to circulate from the base station controller serving the terminal to the other base station controllers through the mobile switching centre. Another option is that the mobile switching centre controls all base station controllers. Although the invention is described above with reference to an example shown in the attached drawings, it is apparent that the invention is not restricted to it, but can vary in many ways within the inventive idea disclosed in the attached claims.

Claims: CLAIMS 1. A cellular radio system comprising, in each cell, at least one base station (100, 102) communicating (104) with terminals (106) located within its area, characterized in that the system comprises at least one set of measuring means (112, 114) for listening to signals sent by the terminals and for collecting measurement data for at least two different terminal location methods. 2. A system according to claim ^ characterized in that the measuring means (112, 114) are arranged to listen to signals sent by base stations and terminals. 3. A system according to claim 2, characterized in that the measuring means are arranged to obtain timing information from the signals sent by the terminals. 4. A system according to claim ^characterized in that the measuring means (114) are located at a base station. 5. A system according to claim 1, characterized in that the system comprises a base station controller (107, 108) controlling one or more base stations, and a mobile switching centre (110). 6. A system according to claim 2, characterized in that the measuring means (112, 114) are arranged to collect measurement data for a locating operation based on an observed time difference (OTD) between the signals. 7. A system according to claim ^ characterized in that the measuring means (112, 114) are arranged to collect measurement data for a locating operation based on direction finding (DF) of the signals. 8. A system according to claim 1, characterized in that the measuring means (112, 114) are arranged to collect measurement data for a locating operation based on times of arrival (TOA, TDOA) of the signals. 9. A system according to claim 1, characterized in that the system comprises means (116) to apply one or more location methods for calculating the location of a terminal by using the data collected by the measuring means. 10. A system according to claim 5, characterized in that the system comprises means (116) to control the measuring means (112, 114) to perform the measurements required by one or more location methods for locating a particular terminal. 11. A system according to claim 10, characterized in that to control the measuring means (112, 114), the means (116) co-operate with another network element (107, 108, 110). 12. A system according to claim 10, characterized in that the base stations (102), the base station controller (106, 108) and the mobile switching centre (110) are arranged to transmit messages between the measuring means and the control means (116) of the measuring means.

Inventors: ALI-VEHMAS TIMO, KASSINEN JUHA P, KURRONEN KARI, PELTOLA JUKKA, RANTALAINEN TIMO M, RUUTU VILLE, ALI-VEHMAS, TIMO, KASSINEN, JUHA, P., KURRONEN, KARI, PELTOLA, JUKKA, RANTALAINEN, TIMO, M., RUUTU, VILLE
Applicants: ALI VEHMAS TIMO, KASSINEN JUHA P, KURRONEN KARI, NOKIA TELECOMMUNICATIONS OY, PELTOLA JUKKA, RANTALAINEN TIMO M, RUUTU VILLE, ALI-VEHMAS, TIMO, KASSINEN, JUHA, P., KURRONEN, KARI, PELTOLA, JUKKA, RANTALAINEN, TIMO, M., RUUTU, VILLE
Application year: 1999
Main classification: H04Q7
IPC codes: H04W64