GB2239758A

GB2239758A - Navigation facility for cellular telephones

Info

Publication number: GB2239758A
Application number: GB9000149A
Authority: GB
Inventors: Andrew Chi Chung Wong; Colin Dougal Mcewen; Ming Kay Wong
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1990-01-04
Filing date: 1990-01-04
Publication date: 1991-07-10
Anticipated expiration: 2010-01-04
Also published as: GB9000149D0; GB2239758B

Abstract

A portable personal telecommunication equipment including means for determining from signals received from two or more fixed base stations the position of the portable equipment in relation to the fixed base stations. The adaptive frame alignment facility may be used to determine the distance to each base station. Other techniques are disclosed. The mobile equipment may relay its position to a central point. <IMAGE>

Description

Navigation Facility for Cellular Telephones This invention relates to the provision of navigation facilities for cellular telephones.

Figure 1 shows the geometry of a cellular radio system. Each cell contains a base station capable of communicating on a number of different radio channels, and the channels are arranged in groups as shown in the diagram. The base stations are linked together and to the telephone network by landlines.

Personal mobile stations are able to communicate on any of the channels allocated to the complete system. When the personal station is in an "A" cell it communicates using one of the "A" group of channels; when in a "B" cell it uses one of the "B" group; and when in a "C" cell it uses one of the "C" group. The system allocates free channels to personal stations on demand, and also deals with the "handover" of personal stations to the next base station as the mobile station moves from one cell to another. The cells vary in size from about 1 km diameter to about 40 km diameter according to geographic and traffic needs.

In the proposed pan-European digital mobile cellular telephone system (GSM) the base stations will use a signal format which includes unique identification of the transmitter in each cell. The system also includes a process called adaptive frame alignment, whereby each base station sends to each mobile station a "timing advance" parameter according to the perceived round trip propagation delay Ei.e. base-mobile-base].

Knowledge of this parameter enables the personal station to determine its range from the base station. Generally a personal mobile station positioned within a cell will in fact be within signal reception range of the transmitter in adjoining cells.

According to the present invention there is provided a portable personal telecommunication equipment including means for determining from signals received from two or more fixed base stations the position of the portable equipment in relation to the fixed base stations.

The term "portable personal telecommunications equipment" as used herein includes any portable telecommunication equipment capable of receiving personal telecommunications messages broadcast from one or more fixed base stations in a cellular radio telephone system. Portable personal telecommunications equipments include cellular telephones and cordless telephones.

The above and other features of the invention will become more apparent from the following descriptvon of embodiments of the invention with reference to the accompanying drawings, in which: Fig. 1 illustrates the concept of a cellular radio telephone system, Fig. 2 illustrates the intersection of three cells of the system of Fig. 1, Fig. 3 illustrates the basis of range finding using hyperbolic techniques, and Fig. 4 illustrates the principle of position fixing using hyperbolic navigation.

Figure 2 shows the intersection of three cells "A", "B", "C" in a cellular radio system. If a mobile personal station is located in the shaded area it will be able to receive signals from each of the three base stations, and can measure the difference in time of arrival of those signals. Knowledge of the base station positions enables the position of the mobile station to be deduced by standard hyperbolic radionavigation techniques.

If two transmitters A and B, in Figure 3, send out respective trains of pulses in synchronism, but on different radio frequencies, then a receiver situated anywhere along the line PQ (which bisects the baseline AB at right angles) will receive the pulses from A and B simultaneously.

If the receiver is at the point X it is apparent that it will receive the pulses from B slightly before the pulses from A. In fact there will be a whole series of points along the line X1XX2 where the pulses from B will be received in advance of those from A by the same amount. The line X1XX2 is the locus of all points that satisfy the equation AX - BX = constant which defines a hyperbola. If a series of values is assigned to the constant in the equation a family of hyperbolae is defined.

In practice, by measurement of the difference in time of arrival of the signals from A and B at the mobile receiver, together with knowledge of the velocity of light it is possible to establish on which hyperbola the receiver is located.

A third transmitter is required, shown at position C in Figure 4, transmitting pulses in synchronism with A and B in order to obtain a position fix. The receiver is located at the intersection of the hyperbola defined by the time difference observed between the A and B signals with the hyperbola defined by the time difference observed between the B and C signals.

The time difference can be measured unambiguously if the repetition rate of the pulses is sufficiently slow that the maximum possible time difference is less than the pulse repetition interval.

However, some systems use continuous signals and phase comparison, rather than pulse modulation, and this can lead to ambiguities if the maximum possible time difference corresponds to more than 2tradians.

The usual technique is to make phase comparisons at a very low frequency, such that the maximum phase difference is less than 2lit, and then to make successively finer comparisons at higher and higher frequencies in order to get more resolution.

As previously explained, the GSM system includes means enabling a mobile equipment to determine its ranges from a single fixed base station using the adaptive frame alignment facility. Repeating the process with other nearby stations enables the mobile station to establish its position. The name "rho-rho" navigation is applied to techniques where absolute range is measured between the mobile and two or more base statIons.

The ambiguity which might appear due to re-use of the frequencies in subsequent cells is resolcved by decoding that part of the signal format which includes the identity of specific base stations.

This resolution of possible ambiguity also leads to a "stripped-down" version of the system, where the only processing is identification of the current cell. Note that densely occupied areas will have very small cells, and hence the system baseline(s) become very short (giving high accuracy) in built-up areas.

There are two main operational modes seen for this type of navigation system - "stand-alone", and "network". In the stand-alone mode, the mobile station deduces its position as described, and the only read-out is at the mobile station. The technique can be offered as part of a de-luxe telephone. In the network mode, the system can keep track of groups of subscribers e.g. fleet members - wherein the mobile stations relay information to a central poInt. This creates a value-added service, paid for by the fleet operator.

Claims

1. A portable personal telecommunication equipment including means for determining from signals received from two or more fixed base stations the position of the portable equipment in relation to the fixed base stations.

2. A portable equipment according to claim 1 wherein the position is fixed by hyperbolic navigation by measuring differential time-of-arrival of signals from three fixed base stations.

3. A portable equipment according to claim 1 wherein the position is fixed by rho-rho navigation by measuring absolute range between the equipment and two or more fixed base stations.

4. A portable equipment according to claim 3 wherein the equipment includes means for decoding timing advance parameter information received from each base station.

5. A portable personal telecommunication equipment substantially as hereinbefore described.