GB2366490A - Method of locating a transceiver - Google Patents

Abstract

A method to determine the location of a first transceiver 4 comprising the steps of providing (at least) two reference stations 1 having known locations, at least one of which is also a second transceiver, determining the time delay and thus the absolute distance between the first and second transceiver and sending synchronised signals from each reference station to the first transceiver. From the time of arrival of each signal t<SB>1-4</SB> from previous steps, computing the distance between each base station and the transceiver, and using all of the said distances to determine the location of the receiver. This is preferably achieved by measuring the propagation delay t <SB>1</SB> on one of the reference stations. Any inherent delays are subtracted from the propagation delay before determining said distance. The signal to determine absolute distance may be initiated from the reference station rather than the first transceiver.

Description

<Desc/Clms Page number 1> METHOD OF LOCATING A TRANSCEIVER This invention relates to a method of location of transceivers, and has particular but not exclusive application to providing location services for mobile communication systems, such as mobile telephones.

Third generation mobile communication systems may require a need for the terminal (e.g. mobile telephone) to locate its position or for the network to locate the terminal's position. At present there are a number of way s of achieving this..

Where, for example in a cellular communications network, the cells are small in size, locating the terminal to the serving cell may provide adequate accuracy. Alternatively the terminals may be equipped with satellite navigation receivers to receive location data from Global Positioning Satellites (GPS) A further known method is that of radio based location where signals transmitted within the bandwidth which supports the communications services are used to perform measurements permitting computation of the terminal locations. Methods based on these include the following:- Observed Time Difference of Arrival (OTDOA) This is illustrated by figure 1. This approach can be supported by one way transmission and is similar to that used by GPS. In an example of this system, the object to be located is a receiver 1 and the reference equipment for

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location are transmitters viz satellites 2. It is important for this that the time of the satellites are synchronised. Where the reference equipment, i.e. satellites and the receiver are in three dimensions, it is required to compute four variables; time and three dimensions of space. The times at which the signals are received at the receiver are indicative of the satellites' relative distance and they feed into four simultaneous equations, which may be solved to obtain the location and time.

In general the required number of signals is equal to the number of dimensions plus one. Where the object to be located and the reference equipment all lie in a plane, then the required number of reference equipment is reduced to a minimum of three.

With this method, the positioning is determined from the relative distance to the satellites and the absolute distances from the satellites are not measured directly.

From a positioning point of view it is immaterial whether the reference equipments are transmitters or receivers. Where the terminal transmits and the base stations listen, noting the time of arrival and reporting the information back to a common point where location can be computed; this is referred to as active location. Where the terminal receives is referred to as passive location. Time of Arrival.

In this technique the terminal to be located and the reference equipments for location are transceivers. This is illustrated by figure 2. A transmission is made in one direction from the terminal 3 to the reference equipment 4

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according to the source's local clock. The reference equipment would then respond to receiving the signal by making its own transmission timed with respect to the reception time of the first signal. Where the terminal receives the second transmission it compares its time of arrival with the time expected according to an assumption of zero propagation delay. The delay will determine the round trip propagation delay. Halving this gives the gives a delay time which can be related to the distance between the object and the reference equipment by the speed of light. Thus the absolute distance between the reference and the object can be determined. This is then repeated for a second and further reference equipments. Where the positions of the reference equipment are known (they may be fixed radio beacons for example) then the location of the object can be determined as the point where the circles of distance from each reference signal intersect.

Where the terminal and reference equipment lie in a common plane then the location of the object is determined by the two points of intersection of circles, representing distances from two reference equipments. A further reference equipment is used sometimes to resolve this ambiguity although: in some circumstances it may not be necessary and the ambiguity may be resolved by using other addition information, e.g. for vehicle known locations of roads. Another possibility is that paths to the third reference equipment may be available spasmodically.

The problem with the above technique is that, although the clocks of the reference equipment s do not have to be synchronised, the two way measurement has to be performed for each reference equipment.

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It is an object of the invention to eliminate the need to do two way measurement on every reference equipment and at the same time to minimise the number of reference equipments.

The inventor has determined that, by providing a innovative hybrid system, the above disadvantages are overcome and this also allows a positioning system to reduce its requirement for the number of reference links.

The invention comprises a method of determining the location of a first transceiver comprising the steps of: a) providing at least two. reference stations having known locations, at least one of which is also a second transceiver, b) transmitting a signal from an originator to a recipient, where the originator is the first transceiver and the recipient is the second transceiver or vice versa; c) sending back a signal from recipient to the originator, d) determining from the time of arrival of said signal, the propagation delay and thus the distance between the originator and recipient; e) sending synchronised signals from each reference station to the first transceiver; f) from the time of arrival of each signal from step e) and the result from step d) computing the distance between each base station and the transceiver, and g) using all of said distances to determine the location of the receiver.

The invention will now be described by way of example arid with reference to figures 3 to 4.

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Figure 3 shows a transceiver 4 and a number of base stations BSI BS2 BS3. If a synchronous signal is sent at (the same time) from each of the reference stations it will arrive at time t1, t2, t3, t4 at the receiver by transceiver 4. The transceiver 4 transmits a signal to one of the reference stations BS 1 This base station then responds by transmitting the signals back. The propagation delay for this link, T1, is determined by the time taken from the signal to be sent out and received at transceiver 4.

This enables the absolute distance between the transceiver and the base station to be determined. When the base stations are synchronised, the absolute distance between each of the remaining ones can be determined with reference to the absolute distance determined above, as follows:- If the transceiver send out a signal, and it is received at times ti, t2, t3, etc by the base stations BSI BS2 and BS3 respectively, the time delay for between transceiver and base station N is:Tn = (tn-ti) + 'r1, as long as the reference stations are synchronised.

From the time delays, the absolute distances for each reference station can be determined and by intersection techniques allows the location of transceiver to be found (assuming the locations of the base stations are known) In this way, in comparison with the OTDOA techniques, the number of base stations can be reduced by one, but the measurement of the round trip time only has to be performed once.

As can be seen only one of the reference station has to be a transceiver the other can be pure transmitters. This is not the case for the TOA technique above.

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In some cases there may be internal delays in the system such that the total propagation delay is made up from the delay due to propagation i.e. through space as well as inherent delay between receiving and re-transmitting the signal. This is shown in figure 4. Such inherent delays are usually easily determined and are known from the system design, or may be ascertained by disconnecting or other various test techniques.

Such delays should be subtracted from r1 before computation of distance. It is not necessary for the transceiver whose position is to be determined to be the originator of the signal to determine the propagation delay. In an alternative embodiment of the invention, the transceiver base station may perform the calculation of propagation delay and distance and then transmit this to the terminal. Such methodology is shown in figure S. This may be more appropriate when the terminal is a mobile telephone and thus the reference stations may have more computational power. In the operation of cellular telephone systems, for example TDMA systems (time divisional multiple access) whereby transmission data is neatly placed into slots which normally OK. However due to delays drift occurs and the system i.e. the transmitter compensates by this by determining a timing advance. In an advantageous embodiment of the system, where there is already use systems a timing advance, it is used to calculate the distance i.e. the timing advance parameter is tapped of in the calculation. In this manner the up/down link transmission do not need to be performed.

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Claims (5)

1. CLAIMS 1. A method of determining the location of a first transceiver comprising the steps of: a) providing at least two reference stations having known locations, at least one of which is also a second transceiver, b) transmitting a signal from an originator to a recipient, where the originator is the first transceiver and the recipient is the second transceiver or vice versa; c) sending back a signal from recipient to the originator, d) determining from the time of arrival of said signal, the propagation delay and thus the distance between the originator and recipient; e) sending synchronised signals from each reference station to the first transceiver; f) from the time of arrival of each signal from step e) and the result from step d) computing the distance between each base station and the transceiver, and g) using a11 of said distances to determine the location of the receiver.
2. 2. A method as claimed in claim 1 wherein any inherent delays are subtracted from the propagation delay before determining said distance in step d).
3. 3. A method as claimed in any preceding claim wherein said steps d) and e) are performed by the second transceiver and including the additional step in of transmitting the location data to the first transceiver.

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4. 4. A method as claimed in any previous claim wherein in step c) the synchronised signals are sent from the first transceiver to the reference stations.
5. 5. A method as claimed in any preceding claim, wherein, if the system uses a timing advance parameter in its protocol, using this to determine the propagation delay in step b).