MXPA98004355A - Method to locate a motion station - Google Patents

Abstract

The present invention relates to a method that allows the detection of geographical location of a mobile station within a mobile network. The mobile station detects signal characteristics for signals generated by neighboring cells, to create a report of signal characteristics corresponding to the location in which the mobile station resides. This report is then compared against the database of signal signatures that identify geographic locations within the cell, where the mobile station is known to be located. The signal signatures are then used to detect, either an exact correspondence or an approximate correspondence to give position information of the mobile station inside the cell.

Description

METHOD FOR LOCATING A MOBILE STATION BACKGROUND DB THE INVENTION The present invention relates to a method for locating a mobile station. More particularly, the present invention relates to detecting the position of a mobile station in a cell area, based on certain received signal characteristics. In emergency situations, it is critical to know the location of the emergency and where the help should be sent. This is easily done with communication systems that are wired. For example, it is well known that when a wired line subscriber or physical line activates an emergency code such as "911", facilities can determine the user's location, so that assistance can be sent to that site. It is equally important to provide emergency assistance to those who can not access a physical line connection such as a person in a vehicle. These people can use mobile communication devices such as mobile cell phones. However, it is more difficult to provide such assistance only because of the nature of the communications instrument, since the instrument can be moved to very different places. In this way, it would be convenient to provide a method by which the location of a mobile station can be determined quickly and simply. A number of prior art systems for mobile system location identification have detected signal strengths to effect triangulation. This is a complex solution that requires real-time calculations related to signal strength measurements. A simpler solution to the problem is convenient. COMPENDIUM OF THE I VENTION The present invention provides a method for locating mobile stations using signal strength information in a new form. In particular, in one embodiment of the present invention, a mobile station is notified of the channel frequencies of the neighboring cells. The station then measures the signal parameters with respect to these frequencies of neighboring cells. The compiled signal measurements are transmitted back to a central processing station via the mobile communication network. A data feed in that central processing station stores signal measurements corresponding to locations within the cell, where the mobile station is located. The central processor searches the database for a signal measurement corresponding to the received signal measurements. If correspondence is found, then the geographical location corresponding to the signal measurement is chosen as the geographical location of the mobile station, however if, the central processor does not detect an exact correspondence by the signal signature, then the processor can choose the closest corresponding signal signature and use the corresponding location as representative of the location of the mobile station. Alternatively, the processor can calculate an approximate location, based on location information corresponding to the nearest N signal measurements. The present invention reduces the location operation to a simple task of searching a database for corresponding information. This technique can also be used to improve the precision of a triangulation technique. BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 illustrates an example of a mobile cellular configuration in which the present invention can be employed. FIGURE 2 illustrates an example of an information table that can be used in an embodiment of the present invention. FIGURE 3 illustrates a flowchart of a method according to a -modality of the present invention. FIGURE 4 illustrates a sample map of a region within a cell for purposes of explaining how to create a database to be used in conjunction with the present invention. FIGURE 5 illustrates an example of a database entry in an embodiment of the present invention. FIGURE 6 illustrates, in block diagram form, an example of a system employing the present invention.

. FIGURE 1 schematically illustrates the distribution of a cellular network in which the present invention can be employed. Three cells are illustrated: cell 1, cell 2 and cell 3. Each cell has its own set of channels, for example a control channel and voice channels, to provide on-the-air communications with mobile stations. In the example shown, cell 1 has a channel having frequency fl, cell 2 has a channel with frequency f2 and cell 3 has a channel with frequency f3. Each cell can have multiple frequencies, but only one frequency is illustrated for ease of description. In the example, a portable device 10 is illustrated located within cell 1. The present invention provides a method for locating mobile station 10 at some geographic position within the geographic service area of the cellular system of cell 1. It is known in existing mobile systems to notify portable device 10, while it is in cell 1, of frequencies used by the cells neighboring cell 1, for example, frequency f2 for cell 2 and frequency f3 for cell 3. Information is useful in at least two situations. First, the mobile station can be registered in the mobile network, but in fact it can not be involved in a communication, that is, it is in a sleep mode. In this mode, the mobile chooses a control channel based on certain signal criteria. Under these circumstances, while the mobile is moving, it is possible that a time will come when the mobile is located, so that it is more optimal to choose the control channel either from cell 2 or cell 3 instead of the channel control of cell 1. This is known as a re-selection process. The mobile makes this determination by periodically measuring the signal intensities of the control channels in the neighboring cells. An algorithm is made using those measurements and the mobile then determines if the frequency of one of the neighboring cells is re-selected. This information can be sent back to the system to assist in channel allocation. This is referred to as a mobile-assisted channel assignment (MACA = Mobile Assisted Channel Allocation). A second situation in which the signal characteristics of the neighbor cell channels are useful, refers to those occasions when a mobile station engages in a communication as it moves through a given cell and reaches the outer perimeter of that cell. cell and it needs to be transferred to a neighborhood cell. This is referred to as a mobile assisted transfer (MAHO = Mobile Assisted Hand Off). In this situation, the mobile station detects signal characteristics of the neighboring cells and transmits that characteristic information back to the cellular system, in such a way that the system can coordinate a transfer of the mobile station from one cell to another as it travels through the cells . These two MACA and MAHO operations are described in detail in the IS-136 protocol specifications (TIA / EIA / IS 136.1-A October 1996). The present invention takes advantage of these known operations which detect the signal strength for signals from the neighboring cells. One embodiment of the present invention provides that the mobile station is notified of channel frequencies of the neighboring cells and is requested to make a signal measurement that detects one or more certain parameters that characterize the signals of the neighbor cells. An example of a table that correlates the signals of neighboring cells, (fl, f2, f3) to a signal parameter such as signal strength, is illustrated in FIGURE 2. The present invention is based on the concept that each location geographic within a given cell site has its own signature signal. By this it is understood that given a sufficiently large number of signals from neighboring cells, the detected set of parameters of those signals at a given site, it will be unique. A database to use in the location operation can be generated with this idea in mind. FIGURE 4 illustrates a sample cell area, where there are six roads or routes shown (41 to 46) that are normally traversed by the mobile stations. Each X marks a geographical location on one of the roads or roads where measurements can be made with respect to the signals coming from the neighboring cells. A network operator can collect this data during a normal course of test drive on these roads in a cell. This would mean placing a call on a telephone connected to a personal computer (PC = Personal Computer) that collects information from mobile-assisted channel assignment or mobile-assisted transfer. The computer will also have a global positioning system (GPS = Global Position System) card to collect latitude and longitude information to geographically fix the information. The collection team can be automated and placed in taxis, delivery vehicles, trucks or other vehicles that commonly go through the cell. To improve the signature that is associated with each geographic location, the data can be collected and averaged in such a way that real-time variations in the signal parameters, such as those due to RF conditions, fading, tree leaves, trucks, buses , etc., would be removed from the data. In addition, it may be beneficial to create an average progress time to take into account changes in the terrain, such as a new building construction, which may affect the signal parameters. When collecting information in this way, a database containing the latitude, longitude, frequency and signal parameters of all the test positions in the cell will be created. Of course, this process can then be repeated for each of the cells through the system. The result is that the database creates, for each cell, a set of signal signatures that specifically - identify geographic locations within the cell. It can be difficult to physically detect a signal signature for any location within a cell. In that circumstance, the signal signature for particular locations can be detected and signatures for other locations can be extrapolated from the information detected. In the currently described embodiment, the signal parameter that is measured by the mobile station is the signal strength of the neighboring cell site signals. However, it is envisaged that other signal parameters that can vary with location or distance from a given transmitter and that can be used to create a substantially unique signal signature for a geographical location, can be used either in conjunction with the signal strength or as alternatives to signal strength. FIGURE 3 illustrates a flow diagram for one embodiment of a method for detecting mobile station location in accordance with the present invention. The method can be executed upon receiving an emergency or distress call. Alternatively, the method may be executed in response to a location request from the mobile station. The method also has applicability to verify or track the location of a fleet of vehicles. First, a neighborhood cell frequency list is sent to the mobile station, step 301. The central processor then receives a report of signal parameters for the cell frequencies from the mobile station, step 302. These parameters may include signal characteristics such as signal strength. Once the central processor has received the report it compares the report with the information in a database for position location, step 303. This database can be generated as described above with reference to FIGURE 4. FIGURE 5 illustrates an example of entries in said position location database. Here a certain position defined by length, latitude (500) is associated with a set of signal parameters, for example signal strengths of neighboring cell signals (510). The search operation determines whether there is a signal signature in the database that is an exact match for the received report, step 304. If this exact match exists, then the method proceeds to send output to global position system coordinates (GPS) corresponding to the corresponding location and which are stored in the database together with the signal signatures, step 305. However, if there is no exact correspondence, then in the method shown in FIGURE 3, the processor retrieves the signal signature for the N closest correspondences (eg, N-1, 2, 3, etc.) to the received report, step 306. The processor then extrapolates a location corresponding to signal parameters in the received report of the sites of the nearest N maps, according to a predetermined algorithm, step 307. The system then outputs an appropriate location based on the extrapolation, step 308. The number The closest correspondence can be selected based on various criteria such as how closely the correspondence closest to fact corresponds to the report and the accuracy of the algorithm to extrapolate the location information. The algorithm can also take into account that the data is collected in routes that were used to create the database. The algorithm can also consider whether mobile stations were used inside houses, offices or other constructions. The model can take into account whether trajectory loss models for the neighborhood, transfer boundaries and the location of cell sites with respect to the collection point. Other factors can also be considered according to the signal characteristics that are considered when representing the signal signature of a geographical location. In one embodiment, the location information can be extrapolated with a simple averaging technique that is based on a number of geographic locations that surround the location that the reported set of measurements has generated. In an alternate structure, the mobile station may be approached to be located at or approximately at the position of the closest correspondence signal signature. Once the position is determined or approximated, that information can be provided to dispatch assistance to the subscriber at the station. FIGURE 6 illustrates a block diagram representation of a system employing the present invention. The base stations 601 are provided for a plurality of neighboring cells. The base stations are connected to a mobile switching center (MSC) 602. A processor 603 associated with the MSC receives the signal measurements made by the mobile stations 610. The processor then searches for a corresponding entry in the base of data 604. The present invention therefore provides a simple database search operation for determining the location of a mobile station within a given area serviced by the mobile network. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention. Having described the invention as above, property is claimed as contained in the following:

Claims (13)

1. CLAIMS 1. - A method for locating a mobile station within a given cell, the method is characterized in that it comprises the steps of: transmitting to the mobile station a list of neighbor cell site frequencies; receiving from the mobile station a set of measurements associated with the list of neighborhood cell site frequencies; compare the set of measurements with a location location database, which contain information about the given cell; and identifying an approximate geographical location of the mobile station, based on the results of the comparison operation. 2. - The method according to claim 1, characterized in that the location database contains a set of measurements associated with the neighboring cell site frequency list for each of the plurality of geographic location coordinates within a determined cell. 3. - The method according to claim 2, wherein the step of comparing comprises the step of searching the location database for a set of measurements that corresponds more closely to the set of measurements received from the mobile station. 4. - The method according to claim 3, characterized in that the step of identifying comprises the step of reading from the database, the geographical location coordinates that correspond to the set of measurements that most closely correspond to the set of measurements received from the mobile station. 5. The method according to claim 3, characterized in that the step of identifying comprises the step of extrapolating geographical location coordinates from the geographical location coordinates that correspond to the set of measurements that most closely correspond to the set of measurements that they are received from the mobile station. 6. - The method according to claim 2, wherein the step of comparing comprises the step of searching the location database for a predetermined number of sets of measurements that more closely correspond to the set of measurements received from the mobile station . The method according to claim 6, characterized in that the step of identifying comprises the step of extrapolating coordinates of geographical location apartird »-:« geographical location coordinates corresponding to a predetermined number of sets of measurements that more closely correspond to the set of measurements that are received from the mobile station. 8. - Method for generating a directory of geographical locations for a given cell site, characterized in that it comprises the steps of determining a list of frequencies for cell sites neighboring the sites of given cells; measuring signal characteristics of the frequencies from a list in a number of the plurality of geographic coordinates; and compiling the signal characteristics measured in a database, whereby a set of measurements of the signal characteristics of the frequencies for the neighboring cell sites correlates with a location defined by the geographical coordinates from the plurality of Geographical coordinates. 9. The method according to claim 8, characterized in that it comprises the additional steps of: extrapolating signal characteristics for a geographical location from compiled measured signal characteristics; and store the extrapolated signal characteristics for the geographic location in the database. 10. Method for locating a mobile station within a given region, the method is characterized in that it comprises the steps of: receiving a report from the mobile station describing measured signal characteristics for a predetermined set of signals; compare the information of the received report with an index that correlates the measured signal characteristics and geographical positions within the given region; and estimating as a location of the mobile station, a geographical position that corresponds more closely to the information of the received report. 11. The location method according to claim 10, characterized in that the measured signal characteristics include signal strength of the predetermined set of signals. 12. - A database for use in a system for locating a geographical location of a mobile station, the database stores for each of the plurality of geographical locations, a signal signature. 13. A system for locating a mobile station within a cell of a cellular communication system, the system for locating is characterized in that it comprises: a plurality of base stations, each associated with a cell and transmitting at least one signal that has a frequency associated with that cell; a processor that receives a message to determine a location of a mobile station within the given cell, and in response to the message, interrogates the mobile station for a measurement of the signals of neighboring cells; and a database, coupled with the processor, which stores for a plurality of cells, data that correlates to a position within a cell and a signal signature for that position, where the signal signature is related to signal characteristics of neighboring cells in that position. INVENTION The present invention relates to a method that allows the detection of geographical location of a mobile station within a mobile network. The mobile station detects signal characteristics for signals generated by neighboring cells, to create a report of signal characteristics corresponding to the location in which the mobile station resides. This report is then compared against the database of signal signatures that identify geographic locations within the cell, where the mobile station is known to be located. The signal signatures are then used to detect, either an exact correspondence or an approximate correspondence to give position information of the mobile station inside the cell.