METHOD AND SYSTEM FOR EVALUATION OF A POSITION INDICATION FOR A MOBILE UNIT IN A CELLULAR RADIO NET

Abstract: A method and a system for evaluating a positional indication of a mobile unit in a cellular radio net, such as the GSM-system, in the form of a horizontal position line. Signal bursts are transmitted essentially simultaneously via a respective antenna of two base stations. Information relating to the exact positions of the antennas is also sent via said antennas. The net is synchronized, i.e. has a common clock source. The actual time delay between a burst triggering pulse in the clock source up to the actual time point at which said burst leaves the antenna concerned is measured for each antenna. The mobile unit detects the time difference between the bursts and also receives information relating to the time delay of said antennas, via the antenna signals. The mobile unit is now able to calculate itself a position line along which it is located. By repeating the procedure with another pair of antennas in the net for instance, there is obtained a further position line, wherewith the position of the mobile unit lies at the intersection between the position lines. The position of the mobile unit along said position line may alternatively be determined with other information obtainable from the net.

Description: METHOD AND SYSTEM FOR EVALUATION OF A POSITION INDICATION FOR A MOBILE UNIT IN A CELLULAR RADIO NET The present invention relates to a method and to a system for evaluating the position of a mobile unit in a cellular radio net, such as the GSM-system, and of the kind defined in the preamble of the method claim and in the preamble of the system claim respectively. A cellular radio net, for instance the GSM-system, is able to provide many other services than a purely communications service. Many of these alternative services, however, require sound information concerning the position of a given mobile unit (mobile telephone) in the cell. Although systems and methods are available with which the position of a mobile unit can be estimated to some extent, the majority of the systems and methods are not particularly accurate and normally require large quantities of data to be passed through long routes through the net and processed by the operator. It would, of course, be of great benefit if the mobile unit were to be able to evaluate its own position. The object of the present invention is to provide a technique which will enable a mobile unit in a cellular radio net, for instance the GSM-system, to readily calculate by itself a position indication in the form of a position indicating line on the basis of information delivered by an antenna signal from each of two mutually separated base stations. In one further development of the invention, the position of the mobile unit can then be determined with the aid of a further position indication. In this regard, the object of the invention also includes the provision of a technique which requires only relatively small modifications, changes or supplementary additions to the base stations of the radio net and to the mobile units. This object is achieved with the method according to the accompanying independent method claim. The object is also achieved with the system according to the accompanying independent system claim. Further embodiments of the invention will be apparent from the accompanying dependent claims. One requirement is for the mobile station to receive information relating to the exact position of the two antennas concerned. The signals must also have an identity. Positional information can be sent from respective base stations/antennas over a broadcast channel dedicated to this end. Alternatively, the signal identity can be sent together with the positional information. It is also necessary for the net to be synchronized, i.e. have a common clock source (exchange) . The actual time delay between a given clock pulse in the clocking source for which a burst shall be triggered and the actual time at which this burst leaves the antenna concerned is measured for each antenna. As is well known, this time delay will depend on the distance between the clocking source and the antenna concerned, among other things. Each antenna in the radio net thus obtains an own time delay value of this nature. Several broadcast channels are available in the GSM-system. Such a broadcast channel can be utilized to mark those bursts that shall be used by the mobile unit in measuring the time difference. The signal received from an antenna in each of the two mutually separated base stations is detected in a mobile telephone. The mobile telephone thus receives simultaneously information relating to the precise positions of the antennas, the identity and/or the respective time delay values of said antennas. The mobile telephone also detects the time difference between the arrival time of the reference burst from the two antennas, wherewith the transit or propagation time of the signals corresponds to a transit distance. Naturally, there are many different positions along the ground surface where a mobile telephone detects one and the same time difference in respect of the signal bursts from the two antennas. These mobile telephone positions form a straight line along which the mobile telephone may be situated. This position line for the mobile telephone extends between the two base stations. The position line is perpendicular to the direction between the two stations concerned. When the signal bursts are transmitted simultaneously from both of the antennas concerned, and when the time difference measured in the mobile telephone is zero, each point on the position line will lie equidistantly from the two antennas. When the detected time difference between the bursts have a value which deviates from zero, then this (time difference) value corresponds to a given distance. The position line for the mobile telephone then lies parallel with the first position line but at a distance therefrom which corresponds to half the measured time difference multiplied by the propagation rate of the radio signals. Correspondingly, it is necessary to make a correction for the difference in the measured time delay between said signal bursts in order to establish the actual position line on which the mobile telephone/mobile unit may be situated. This actual position line can be readily determined geographically, since the positions of the base stations/antenna are known and are sent to the mobile unit. All calculations can be carried out in the mobile unit. One prerequisite is that the antennas have identity, in other words that the antenna signals include information from which the mobile unit can determine the geographical positions of the antennas. Another assumption is that the mobile unit has or receives information relating to the signal delay of respective antennas in relation to the clock. Subsequent to having cal- culated the true line of possible positions for the mobile unit, the actual position of said unit can be determined with the aid of one of several available techniques. For instance, the direction from a base station to the mobile unit can be calculated with the technique described by Swedish Patent Application No 9801439-2. Alternatively, the distance from the mobile unit to a base station whose position is known can be calculated with the aid of a known method to this end. Another alternative is one in which the aforedescribed procedure is repeated for antenna signals from another pair of base station antennas, so as to obtain another true district line for the mobile station concerned, wherewith the position of the mobile station lies on the point at which the two position lines intersect. The invention will now be described in more detail by way of example and with reference to the accompanying drawing. Figure 1 is a schematic illustration of two base stations in a GSM-system, which have a common clocking source. Figure 2 illustrates schematically the evaluation of the position of a mobile telephone with the aid of three base stations in a GSM-system. Figure 1 shows two mutually separated antennas 6, 7 which belong to two mutually separated base stations. The stations send groups of signals, "bursts", that shall be essentially simultaneous and that reappear at a given periodicity. The positions of the antennas 6, 7 are known. The co-ordinates of the antenna positions can be sent together with the signal transmitted via respective antennas. The base stations are connected to a common clock source (exchange) 5 which controls the burst transmissions. If the clock signal from the clock source 5 should arrive at the base stations β, 7 with no time delay, this will mean that the bursts were transmitted simul- taneously from the antennas 6,7. In reality, however, there is a certain delay from the time at which the clock source reference pulse to was generated in the source 5 and the time t B7 , t B 6 a which the bursts actually respective antennas. It can be assumed that the stations 6, 7 send a signal "burst" at that moment at which the reference pulse t 0 arrives at respective antennas. If it is assumed initially that the delays are of equal magnitude, i.e. that t B , t B 6 are simultaneous, there are a number of positions along the ground in which a mobile telephone will detect these bursts simultaneously. These positions lie along a straight line 1 0 . lo is perpendicular to the direction 1 6 , 1 between the antennas 6 and 7 and lies equidistant, Si, from each station 6, 7. If the time difference between the bursts detect- ed by the mobile telephone deviate from zero, the position of a line li parallel with the line 1 0 can be calculated such that a mobile telephone situated anywhere along the line li would discern the bursts with this time difference. The distance s 2 between 1 0 and IT . is v (t B7 -t B6 ) /2 where v is the propagation rate of the radio wave. Correspondingly, compensation must be made for the different time delays for a pulse from the source 5 to a respective station/antenna, i.e. the time delays Δt 7 and Δt 6 respectively. In this case t 7B = t 0 + Δt 7 and t 6B = t 0 + Δt 6 . The distance s 3 between l x and a line 1 2 parallel therewith is thus v(.Δt 7 - Δt 6 ) /2 The mobile telephone 8 is thus actually situated somewhere along 1 2 and the position of said telephone can easily be calculated since the positions of respective stations 6, 7 are known and delivered to the mobile telephone together with the aforesaid signals, and since the direction of the line 1 3 will be perpendicular to the lines 1 6 , 7 and the distances Si, s 2 , s 3 can easily be calculated in accordance with the above. The technique in which the actual calculation is made may, of course, be simpler than that described. The mobile telephone includes a clock that has an adapted frequency for measuring time precisely so as to enable the distances s x -s 3 to be calculated accurately. A broadcast channel can be dedicated to mark the pulses from the clock 5 at which bursts shall be transmitted via the antennas. Figure 2 illustrates schematically how the aforedescribed procedure is repeated for the antennas 6, 8 of a further pair of mutually separated stations, wherewith a further line 1' 2 is calculated for the position line on which the mobile telephone is situated. The point at which the lines 1' 2 and 1 2 intersect shows the position of the mobile telephone 9 and can be calculated by the telephone itself. The mobile telephone can send information relating to its thus calculated position to a register, via the net. The mobile telephone 9 need not, of course, be an actual telephone, but may be a general purpose "mobile unit" having the significant advantage of being able to calculate accurately its own position on the basis of a position line and some further supplementary information, for instance the distance to a system base station whose position is known, and also the direction from a system base station whose position is known. One advantage afforded by the invention is that position determining information is sent via the existing antennas of the cellular radio net. This enables existing net software to be used.

Claims: C l a i s 1. A method for evaluating the position of a mobile unit in a cellular radio net which includes base stations, in which signal bursts are transmitted periodically via at least two mutually separated antennas that each belong to a respective base station, characterized by transmitting via said antennas antenna position information that can be read by the mobile unit; synchronising the base stations from a common clock source for triggering said bursts at respective antennas with the same clock signal pulse from said source (5) ; measuring the actual time delay (Δt) of the clock-source clock pulse (to) from the clock source (5) to respective base station antennas (6, 7); making the time delay (Δt) of respective antennas available to the mobile unit; detecting the time difference between the arrivals of respective bursts from said two antennas with the mobile unit; and causing the mobile unit (9) to calculate as a position indication an essentially straight, generally horizontal position line that indicates the position of the mobile unit, on the basis of the antenna position information, the measured time delays and the detected time difference. 2. A method according to Claim 1, characterized by generating a further indication of the position of the mobile unit in the net for determining the position of said mobile unit. 3. A method according to Claim 1, characterized by determining a further position line for the mobile unit in the cellular radio net on the basis of a further pair of mutually separate antennas (6, 8) . 4. A system for evaluating a positional indication of a mobile unit in a cellular radio net that includes mutually separate base stations which each has at least one antenna (6, 7) via which signal bursts are transmitted, characterized in that the base stations are synchronized from a common clock source for - transmitting said bursts when the clock source signal pulse arrives at the base station; in that the respective antenna signals contain antenna position information; in that the respective antenna signals contain measured information relating to the time delay from the clock source signal pulse to the transmission of the burst from the antenna; and in that the mobile unit is adapted to calculate as a positional indication a position line for its own location on the basis of the detected time difference between the bursts from said antennas, information relating to the time delays, and information relating to the positions of said antennas. 5. A system according to Claim 4, characterized in that the system includes means for generating a further positional indication with which the mobile unit can determine its own position along its position line. 6. A system according to Claim 4, characterized in that the mobile unit is designed to calculate a further position line with respect to its own position on the basis of a further pair of base station antennas; and in that the mobile unit is also designed to determine its position on the basis of the intersection between its position lines. 7. A system according to any one of Claims 4-6, characterized in that the signals sent via the antennas contain information relating to the identity, time delay and position of said antennas.

Inventors: SICKING MAGNUS, SICKING, MAGNUS
Applicants: SICKING MAGNUS, UNWIRE AB, SICKING, MAGNUS
Application year: 1999
Main classification: H04Q
IPC codes: G01S5