GB2375021A - Process for locating mobile terminals of a cellular radiotelephony network - Google Patents

Abstract

The network comprises a number of base stations (1, 2, 3), in predetermined positions, transmitting signals to the terminals (4). In the terminals (4) the power levels or the radio signals from base stations (1, 2, 3) in radio range are measured and, by comparison, the terminals (4) are located in relation to the stations (1, 2, 3). The cellular network (10, 20, 30) is covered by a network for locating the terminals (4) containing radio beacons (11, 12, 21, 22, 31, 32) for emulating stations and, in the terminals (4), the power levels of radio signals originating from the beacons (11, 12, 21, 22, 31, 32) are also measured to determine the positions of the terminals (4).

Description

237502 1

PROCESS FOR LOCATING MOBILE TERMINALS OF A CELLULAR

RADIOTELEPHONY NETWORK:

This invention concerns a process for locating a mobile terminal of a cellular radiotelephony network, for example of the GSM type, comprising a number of base stations, each covering a particular topographical cell.

Such locating processes are used particularly in emergency call networks such as are to be found in particular along motorways. When the user of a mobile telephone sends an emergency call to an emergency centre, the origin of the call should be accurately located.

There are mainly three types of process for locating a mobile, telephony or some other.

The first process uses a system of location by satellites, called "GPS" (Global Positioning System) and is based on measuring, through the mobile, the differences between the moments of arrival of signals sent by several satellites.

The second process, by "Timing Advance" is based on measurement of the time for propagating radio waves between a cellular telephony mobile and base stations. The accuracy of this is, however, limited.

The third process consists in determining the position of a cellular telephone by triangulation, by comparing the power levels of the signals originating from known transmission relative power stations.

Nowadays, in underground transport systems such as the metro, putting an emergency call service at the disposal of users with a cellular telephony terminal is envisaged. One solution would consist in setting up base stations of a cellular telephone network along the routes of the transport system, the cells of the base stations being thus arranged like a string of beads.

Locating a terminal along an underground route, to go back to this example, does however raise difficulties.

In fact, locating by GPS is impossible in this case.

Timing Advance is too imprecise.

Triangulation is also imprecise for the base stations are aligned.

In short, locating a mobile terminal on a route of an underground transport system has so far not been precise enough.

Now, in practice, if one returns to the example of the metro, it is a question for instance of determining the metro station from which the call originates, if such is the case, and even of determining the relevant platform or passage, so that the emergency services can reach it without wasting time. The invention is not, however, limited to such remote locating and also covers determining its position through the carrier of the terminal, i.e. navigation.

This invention offers a solution for improving the accuracy with which such a terminal can be located.

For this purpose, the invention concerns a process for locating mobile terminals of a cellular radiotelephony network comprising a number of base stations in particular positions, issuing signals to the terminals, whereby: - in the terminals the power levels of the radio signals originating from base stations in radio range are measured, and - through comparison of these measurements, the terminals are located in relation to the positions of the stations, - a process characterised by the fact that the cellular network of a terminal locating network containing radio beacons for emulation of the base stations and particular positions is overlapped and thus, in the terminals, the power levels of radio signals originating from the beacons are measured to determine the positions of these terminals.

Therefore, the beacons functionally complete the cellular radiotelephony network to supply the terminals with an increased number of signals to be measured relating to the extra geographical references which the positions of the beacons form. As the function of the beacons

is not to deal with telephone traffic, it can be seen that they do not constitute an extension of the telephone network, which would risk upsetting its operation, since this telephone network is not functionally related to the function of the beacons. The network of beacons is in parallel on the telephone network.

To advantage, the beacons of the locating system are transmitters and the positions of the terminals are determined in these terminals or else the terminals send their power level measurements to a locating centre.

The terminal can thus obtain its position directly or else allow it to be remote-located.

The invention will be more easily understood by means of the following description of a

preferred mode of putting the process covered by the invention into service, with reference to the appended single illustration which represents a cellular radio telephony network and a network for locating terminals of the radio telephony network.

The radio telephony network shown, here the GSM system, has a number of base stations, three of which are represented and marked 1, 2, 3, spaced in particular positions along a route 9 of a transport system, in this case a metro line. The areas of radio coverage or respective cells 10, 20, 30 are therefore arranged like a string of beads with partial overlapping of the adjacent cells.

Reference 4 designates one of the terminals of the GSM network, therefore receiving transmissions of signals from those of the base stations 1, 2, 3, which are in radio range and can send signals to them.

Further, cells 10, 20, 30, are covered, at least in the working areas concerning the metro line 9, by a tenninal locating system like the one marked 4. The locating system has radio beacons 11, 12, 21, 22, 31, 32 for emulating base stations like stations 1, 2, 3, and particular positions. The process for locating terminals 4 will now be explained.

As already set out, it is a question of a process by triangulation whereby, in the terminals 4, the power levels of the radio signals coming from the base stations 1, 2, 3, in radio range are measured, in order, by comparing these measurements, to locate the terminals 4 in relation to the base stations 1, 2, 3.

All the radio signals of the base stations 1, 2, 3, such as periodic beacon route signals, to manage the mobility of the cell changing terminals 4, or again the communication signals of the various active terminals 4, are detected and used here.

However, the number of base stations 1, 2, 3, transmissions from which are simultaneously received by a terminal 4, is limited due to the in- line or string of beads arrangement of the base stations 1, 2, 3. The signals from a pair (1, 2) of these would theoretically allow a hyperbola of possible positions to be roughly estimated, centered on the line 9, but the interference radio fading would falsify the positioning of this hyperbola which, in addition, would not allow for determining the lateral position of the terminal 4 in relation to the route 9. A different pair (2, 3) of base stations would clearly give another hyperbola, but it would cut the other one at an angle of practically nil, giving a very uncertain lateral position due to its high sensitivity to the position errors of the two hyperbolas.

To locate terminal 4 accurately, the signals sent by the emulation beacons 11, 12, 21, 22, 31, 32, are recognised by it as being extra base station signals. In the terminal 4 in question, the power levels of the radio signals originating from the beacons 11, 12, 21, 22, 31, 32, are therefore also measured to determine the position of terminal 4.

It will be remembered that determining the position by non-directive detection triangulation consists in determining fading relating to radio propagation of the signals received from at least three transmitters, as mentioned above. Each pair of transmitters allows determining a variable, difference in propagation fading and therefore in distances, and it is therefore possible to define two, if necessary three, partly different pairs of transmitters allowing

at least the figures for two variables to be found, and that is sufficient to define the geographical co-ordinates of the intersection point or of the two intersection points of the two hyperbolas thus determined, the third one removing any possible ambiguity on position.

If these transmitters transmit at the same level, known or not, of the receiving terminal 4, or at levels where the difference is known, the differences in the levels received allow the relative distances between the transmitters and terminal 4 to be estimated, and therefore its position to be determined in relation to the positions of the transmitters.

A reminder is given that, as propagation in free space, the amplitude of a transmitted radio signal decreases linearly with the distance of propagation, i.e. its power decreases conversely to the square of the distance.

In practice, in the GSM network, the terminals measure the six strongest levels received in order to obtain redundant information which allows for eliminating the effect of radio paths containing obstacles which weaken the signals and therefore increase the estimate of the distances. The network of beacons like 11, 12, which might otherwise, on its own, allow the terminals 4 to be located, improves the grid of locating transmitters, i.e. the base stations 1, 2, 3, and the beacons 1 1, 12, form a reduced mesh matrix overall.

Each cell 10, 20, 30, could have a single beacon 11 but, preferably as here, each cell 10, 20, 30, has a number of such beacons 11, 12.

The linear network 1, 2, 3, only allows establishing locating triangles by triangulation which are flattened along the direction x of the route 9 and provide sections of hyperbolas, representing each variable above, practically parallel in the area of the route 9. The beacons 11, 12, enable the network 1, 2, 3, to be transformed into a real network or 2dimension (or 3-

dimension) matrix if there is, in each metro station, at least one beacon 11 remote from the route 9 and preferably at least two beacons 11, 12, at different distances from the route 9 and for

example in a direction "y" more or less perpendicular to it. That provides a section of hyperbola more or less parallel to the route 9 and therefore more or less perpendicular to the sections of hyperbolas of the pairs of base stations 1, 2, 3. Movement of the terminal 4 perpendicular to the route 9 thus corresponds to an appreciable change in its distance to the beacon 11 and can therefore be accurately measured.

Beacons 11, 12, are only transmitters here, i.e. they do not detect any transmissions from the terminals 4 and have no telephone function. The associated GSM system 1, 2, 3, is therefore not disturbed by the beacons 11, 12. Each beacon, like the beacons 11, 12, of the cell 10, transmits at a particular frequency, different from the frequencies of the base stations 1 and 2 near these beacons 1 1, 12.

The signals received by the terminal 4, originating from the various transmitters which make up the stations 1, 2, 3, and the beacons like 11, 12, have signals for identifying their transmitter so that the measurements can be used according to the position of their transmitter.

A table of the geographical positions of the base stations 1, 2, 3, and of the beacons 11, 12, is then addressed by the identification signals in order to associate each measurement of level received at the position of the transmitter in question.

The calculations for determining the position of a terminal 4 can be done in a central unit of the terminal and the information on position thus determined is forwarded to the base station 1, 2, 3, which then manages the terminal 4, and it retransmits this information on position to a locating centre, possibly incorporated in the station in question 1, 2 3.

As an alternative, the terminal 4 only takes the measurements of level and forwards these, associated with the signals for identification of the respective transmitters 1, 11, 12, to a location calculation centre.

In both cases, it can be planned for the terminal 4 also to supply its user with its position, which it has calculated or received back from the location calculation centre.

Identification of the beacons 11, 12, uses the principle of identification of the base stations 1, 2, 3, i.e. is inserted into the scheme of addressing or identifying them. The beacons 1 1, 12, are thus virtual base stations for the terminals 4. In particular a distributed code word of 8 bits is used, called BSIC (Base Site Identity Code) containing a BCC (Base Colour Code) field

of 3 bits, identifying a cell, and an NCC (Network Colour Code) field of 3 bits for the network

operator's identity.

Identification of the transmitters 1, 11, 12, can also use MCC (Mobile Country Code), MNC (Mobile Network Code) and LAC (Local Area Code) fields or a group of cells. In

addition, each base station 1, 2, 3, can be identified in the group by a CI (Cell Identity) field.

Base stations 1, 2, 3, are grouped together (LAC) into motifs of assigned frequencies which are repeated in the territory covered and in which each base station 1, 2, 3, has channels at frequencies clearly separated from those of the adjacent stations in the group and any groups in the vicinity. This principle extends to the beacons 11, 12, the radio requirements of which can be limited to one or more channels, which does not appreciably disrupt the frequency assignment plan of the GSM network.

Claims (8)

1. A process for locating mobile terminals (4) of a cellular radio telephony network comprising a number of base stations (1, 2, 3), with predetermined positions, transmitting signals to the terminals (4), a process whereby: - in the terminals (4), the power levels of the radio signals originating from base stations (1, 2, 3) in the radio range are measured, and - by comparing these measurements, the terminals (4) are located in relation to the positions ofthe stations (1, 2, 3), - a process characterised by the fact that the cellular network (10, 20, 30) is covered by a network for locating the terminals (4) which contains radio beacons (11, 12, 21, 22, 31, 32) for emulating the base stations (1, 2, 3) and particular positions, and in the terminals (4) the \ power levels of radio signals originating from beacons (11, 12, 21, 22, 31, 32) are also measured to determine the positions of these terminals (4).

2. A process according to Claim 1 whereby the locating network covers each cell (10,

20,30) of the radio network through a number of beacons (11, 12, 21, 22, 31, 32).

3. A process according to one of the Claims 1 and 2 whereby the beacons (11, 12) of the locating network are transmitters.

4. A process according to one of the Claims 1 to 3 whereby the signals transmitted by the locating beacons (11, 12) contain identification signals.

5. A process according to one of the Claims 1 to 4 whereby the positions of the terminals (4) are determined in these terminals.

6. A process according to one of the Claims I to 4 whereby the terminals (4) send out their power level measurements to a locating centre.

7. A process according to Claim 6 whereby the locating centre sends back the calculated position to the terminals (4).

8. A process substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.