GB2349532A

GB2349532A - Mobile unit location method

Info

Publication number: GB2349532A
Application number: GB9909774A
Authority: GB
Inventors: Nigel Horspool
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-04-28
Filing date: 1999-04-28
Publication date: 2000-11-01
Also published as: GB9909774D0

Abstract

A method and system for establishing the location of a mobile unit (20) which may be within a communications network such as a GSM network. A reference unit (30) is provided having a known location and communications are sent sequentially from three or more fixed location base stations (10<SB>1</SB>, 10<SB>2</SB>, 10<SB>3</SB>) to the mobile unit (20) and the reference unit (30). The times of receipt of the communications at the mobile and reference units are established and compared by control unit (40) in order to establish the location of the mobile unit. The position of the reference unit (30) may be coincident with one of the fixed locations (10<SB>1</SB>, 10<SB>2</SB>, 10<SB>3</SB>) and the fixed locations may be mobile telephone network base stations.

Description

Position Identification The present invention relates to the identification of a current position of a mobile unit which is arranged to communicate with a plurality of fixed base stations. It has particular application to the location of mobile telephones, in their networks of base stations, for instance, GSM phones in a GSM network.

As is well known, to establish the position of a mobile unit within a network of fixed locations it is necessary to establish either a) the direction and distance from one of the fixed locations to the mobile unit, or b) the distance of the mobile unit from two or more of the fixed locations. In mobile communication networks and other networks which utilize electro-magnetic radiation it requires the presence of extra equipment in the base stations to establish the direction of the radiation, and therefore the present invention is primarily concerned with using arrangements akin to (b), just mentioned.

Figure 1 illustrates in a schematic form, a single mobile unit located among three fixed locations, for instance base stations in a mobile communication network. The establishing of the position of the mobile unit may be based on measuring the distances between the fixed locations and the mobile unit by measuring the time it takes for electro-magnetic radiation to travel between the fixed locations and the mobile unit. In such arrangements a significant problem lies in ensuring that all of the time measurements are made using the same time reference.

In some previous arrangements it has been proposed that a central timing pulse should be transmitted for use by each device as a time reference.

However, in addition to requiring all the devices in the system to have extra equipment, this system has significant limitations on its accuracy and area of coverage.

According to the present invention location of a mobile unit within a network of fixed locations is achieved by sending communications between the fixed locations and the mobile unit to establish the difference in the distances between the mobile unit and each of at least two pairs of fixed locations. The differences are processed to establish the actual location of the mobile unit.

Preferably, for simplicity, one fixed location is common to the pairs of fixed locations. Depending on the locations, the use of two pairs of fixed locations may identify two possible locations for the mobile unit, and a selection must be made between these two. The use of three or more pairs will give a single location.

In the invention the differences are established by sending a sequence of communications to the mobile unit from the different fixed locations and also sending the same communications to a reference unit, the position of which is known. A comparison of the receipt of the communications at the mobile unit and at the reference unit enables the above mentioned differences in distance to be established, while the use of a single reference unit removes the need for trying to co-ordinate the time bases used by the various fixed locations.

When used in a mobile network, such as a GSM network the present invention can be implemented with only minor changes to the already existing infrastructure and phones. In particular the existing infrastructure need not be changed, and a reference unit is simply added. By being easily implemented in a system for which a significant and increasing proportion of the population carry a mobile unit the invention is a convenient way to establish a useful emergency location system for cars or children for instance and also could be used to summon emergency assistance to a specific location.

In the context of a GSM network, the present invention is implemented using a frequency/phase modulated code of the type which is used for the communication in the GSM network. Frequencies used in the current technology enable the timing of the receipt of the communications to an accuracy of 2-3 nanoseconds, which permits an accuracy of position location within 6-9 metres.

The present invention will be better understood from the following description of a preferred embodiment with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of a mobile unit among a plurality of fixed locations; Figure 2 is a schematic illustration of the relevant portions of a mobile unit and reference unit implementing the invention ; Figure 3 is a diagram of exemplary timing signals used to implement the invention; and Figure 4 is a timing diagram illustrating the operation of the preferred embodiment.

Broadly speaking the preferred embodiment of the invention is a method and system for establishing the location of a mobile unit within a communications network such as a GSM network. A reference unit is provided having a known location and communications are sent sequentially from three or more base stations to the mobile unit and the reference unit. The times of receipt of the communications at the mobile and reference units are established and compared in order to establish the locations of the mobile unit.

Figure 1 is a schematic illustration of a GSM network implemented as a plurality of base stations 101, 102, 103 by way of which mobile unit 20 can communicate in the usual way. There is also provided reference unit 30 the position of which is known, in particular the distance between each of base stations 101, 102, 103 and reference unit 30 are known. It is possible for reference unit 30 to be incorporated into one of the base stations but for clarity it will be assumed to be separate for the purposes of this description. Within a complete GSM system a number of reference units 30 may be necessary to enable the invention to operate throughout the network. As will be apparent from the description below it is necessary for pairs of base stations to be able to communicate with common reference units.

There is also provided a control unit 40. This controls the operation of the system to implement the invention and is in communication via the GSM network, and therefore the base system, with the reference units 30 and mobile units 20. The physical position of control unit 40 is not significant and only one or a few control units 40 may be required to enable the present invention to be implemented within a complete GSM network.

As is shown in Figure 2, a mobile unit 20 comprises an aerial means 21 and modulation/demodulation means 22 arranged to transmit and receive signals to and from the base stations within the GSM networks in the usual way.

Mobile unit 20 also comprises control means 24 for carrying out the steps necessary at the mobile unit for implementing the invention which has associated with it a counter 26 arranged to increment at a predetermined rate. Control means 24 may be integrated with the means implementing the usual function of the mobile unit 20 within the GSM network or may be separately provided.

Also as shown in Figure 2, reference unit 30 correspondingly comprises aerial means 31, modulation/demodulation means 32, control means 34 and counter 36.

In the following description of the operation of the invention it is assumed that the position location procedure is initiated by the mobile unit 20 indicating that it desires to be located. However, the procedure could equally be indicated by control unit 40 having been notifie that mobile unit 20 should be located.

To initiate the procedure then in this example, mobile unit 20 initiates a call to control unit 40 via one of the network base stations 101, and sends various information to control unit 40 including the carrier frequency it is using to communicate with base station 10,. While still connected to mobile unit 20, control unit 40 also communicates to reference unit 30 indicating the carrier frequency which is being used for communication between base station 10, and mobile unit 20. Then, as part of the call between mobile unit 20 and control unit 40, control unit 40 causes a specific signal to be sent from base station 10, to mobile unit 20 which is also received by reference unit 30.

The significant part here is that base station 10, should be caused to send a signal which is received by both mobile unit 20 and reference unit 30 and the above described method of achieving this as part of a call between mobile unit 20 and control unit 40 is one possible way of achieving this. In any event, on receipt of the communication at mobile unit 20 and reference unit 30, the control means 24,34 reset respective counters 26,36.

The code which is sent by base station 10, to reset the counters is preferably of a form which facilitates accurate timing by control means 24,34 and one suitable form is illustrated in Figure 3. As shown, the timing code preferably starts with a plurality of training codes, for instance a regular signal of a predetermined frequency, onto which the receiver can lock. This is then followed by the measuring symbol which is of a predetermined format, for instance being of a predetermined length, and on the basis of which the counters are reset. For best accuracy, the control means 24,34 are responsive to an edge of the measuring symbol which may be, as shown, the trailing edge. This simple binary code is encoded onto the carrier frequency, in the same way that voice data is encoded.

After the counters have been reset as described above, the mobile unit 20 initiates a second call to control unit 40 using a second base station 102.

In a similar manner to what is described above, control unit 40 causes base station 102 to transmit a communication which is received by mobile unit 20 and by reference unit 30. Upon receipt of this second communication, control means 24, 34 record the current values in respective counters 24,36 and the counters continue to increment.

Subsequently, the mobile unit 20 initiates a further call to control unit 40 using a third base station 103, via which a further communication is caused to be sent to both mobile unit 20 and reference unit 30. Upon receipt of this third communication the current values of counters 26,36 are again recorded. The recorded values are then communicated to control unit 40 which calculates the position of mobile unit 20 as described in the following with reference to Figure 4.

In Figure 4, time to is the time at which the first communication is transmitted by base station 10,. This is received by mobile unit 20 at a time t, later and counter 26 is reset. It is also received by reference unit 30 at a time t2 later when counter 36 is reset.

Time t3 is the time at which the second communication is transmitted by base station 102. This is received by mobile unit 20 at a time t later and the first reading is taken from counter 26. This reading, combined with the rate at which counter 26 is incremented, gives time interval tMl. The second communication is received by reference unit 30 at a time t5 after t3 and the first reading is taken from counter 36. This reading, combined with the rate at which counter 36 is incremented, gives time interval tRl.

Time t6 is the time at which the third communication is transmitted by base station 103 and this is received by mobile unit 20 and reference unit 30 at time intervals t, and t8 later, respectively. At these time counters 26,36 are read as described above and time intervals tM2 and tR2 are determined accordingly.

As discussed above, the location of reference unit 30 in relation to base stations 101, 102, 103 is known and therefore time intervals t2, 8, ; are also known, being the transmission times between the base stations and the reference unit 30. The recorded count values are converted to time intervals tel, tM2, tR,, t and transmitted to control unit 40 and this information is used to calculate the location of mobile unit 20 as follows.

As can be seen from Figure 4, the time interval between the pulses can be calculated, by using the reference unit measurements: t3-tO t2 + tRI-t5... (1) Similarly for the mobile unit: t3-to = tl + tMI t4 (2) Combining equations (1) and (2) gives: t4-tl = tMI-tRi + t5-t2... (3) This time difference is the difference in the times taken for the communication to travel between base stations 102 and mobile unit 20 and between base stations 10l and mobile unit 20. Combined with the known speed of travel of the communications in air this time difference translates into a difference in the distances between mobile unit 20 and base stations 101, 102.

In a graphical consideration, this difference can be used to calculate a line or curve of possible locations for mobile unit 20 as between base stations 10,, 102 where the difference in distance is as established from equation (3) above.

Similarly, for base stations 101, 103 : t7-tl =tI"-t+t8-t2... (4) Again the difference can be used to calculate a second line or curve of possible locations for mobile unit 20 as between base stations 101, 103 and the coincidence of the two lines gives the locations of mobile unit 20.

It will be understood that the above is described by way of example only. It is not necessary to consider a graphical representation and a mathematical calculation may be done on the basis of the known relationship of the base stations and equations (3) and (4). Also the two pairs of the base stations for which differences are established may differ from what is described above. For instance the counters may in fact be reset on receipt of the second communication so that the pairs of base stations for the calculation may be 10"102 and 102, 103. Further the pairs may be quite independent from each other, using a fourth base station in total.

However it will be appreciated from the above that the present invention utilizes differences in the times taken for communications to travel to the mobile unit and a common reference unit. This removes the need for complex establishment of a reference time frame (it being noted that times to, t3, t6) do not appear in equations (3) and (4) above) as reference unit 30 is common to the whole calculation.

The reference units can also"snoop"for the signals from the mobile phone. The time between the measuring symbol sent by the control unit and the start of the transmission from the mobile unit is fixed. The distance between the reference unit and the mobile unit can be calculated by measuring the time between when the reference unit expects the transmission from the mobile unit, and the time it actually receives the transmission.

As described above, the frequencies used at present in mobile communications enable the above system to locate a mobile unit within 6-9 metres. Final positioning can be achieved by ringing the mobile phone.

Claims

CLAIMS : 1. A method for establishing the location of a mobile unit with reference to at least three fixed locations comprising sending and timing communications between the fixed locations and the mobile unit to establish the difference in distances between the mobile unit and each of at least two different pairs of fixed locations, and calculating on the basis of said differences one or two possible locations of said mobile unit.
2. The method according to claim 1 comprising sending time-separated communications from each fixed location in each said pair to both the mobile unit and a reference location, the position of which with reference to said fixed locations is known, timing, at the mobile unit and the reference unit, the time interval between receiving the first and second communications respectively from the two fixed locations in each said pair, and establishing said difference in distance for each said pair on the basis of the position of said reference unit and said time intervals.
3. The method according to claim 2 in which counters are provided at said mobile unit and said reference location and said step of timing comprises resetting the counters on receipt of the first communication, incrementing the counters at known rates, and reading the values of said counters on receipt of the second communication.
4. The method according to claim 2 or 3 in which the position of said reference unit is coincident with one of said fixed locations.
5. The method according to claim 1,2,3 or 4 in which one of said fixed locations is common to at least two of said pairs.
6. The method according to any preceding claim in which said mobile unit is a mobile telephone and said fixed locations are corresponding network base stations.
7. The method according to claim 6 in which said telephone is a GSM telephone.
8. The method according to claim 6 or 7 further comprising finally establishing an accurate position for said mobile telephone by causing it to ring.