WO2001095659A1

WO2001095659A1 - Positioning of mobile station by determining synchronisation differences between base stations

Info

Publication number: WO2001095659A1
Application number: PCT/SE2001/001305
Authority: WO
Inventors: Tjaart Van Der Walt
Original assignee: Cellpoint Systems Ab
Priority date: 2000-06-08
Filing date: 2001-06-08
Publication date: 2001-12-13
Also published as: AU2001264508A1

Abstract

The present invention relates to a method of determining a synchronisation difference between said BTSs, in a wireless communications network comprising a number of Base Transceiver Stations, BTSs (12-14) and at least one Mobile Station (10) comprising at least one of the steps of: collecting a set of data on said mobile station and transmitting it back to a server for further processing, said data including at least one of a Timing Advance (TA) or Time Difference Of Arrival (TDOA), or collecting information on said communications network and transmitting it back to a server for further processing, said information including a Real Time Difference (RTDs) between said BTS's during a handover. The method further comprises recording said Real Time Differences (RTDs) between the BTSs in a table, said table comprising at least entries characterising behaviours of the RTDs over time and an expiry time attached to each table entry to invalidate it after a pre-set time, applying the RTDs as corrective factors to BTSs' time bases, and updating RTD values continuously as fresh data becomes available.

Description

Title

Positioning of mobile station by determining synchronisation differences between base stations .

Technical field of the invention

The present invention refers to a method of determining a synchronisation difference between a number of Base Transceiver Stations in a wireless communications network comprising said Base Transceiver Stations and at least one Mobile Station.

More specially, the invention relates to method in a wireless communications network comprising a number of Base Transceiver Stations, BTSs, and at least one Mobile Station a method of determining a synchronisation difference between said BTS's, comprising at least one of the steps of: collecting a set of data on said mobile station and transmitting it back to a server for further processing, said data including at least one of a Timing Advance (TA) or Time

Difference Of Arrival (TDOA), or collecting information on said communications network and transmitting it back to a server for further processing, said information including a Real Time Difference (RTDs) between said BTS's during a handover.

Background of the invention and related art

In wireless communications systems, and in particular the GSM, there is a drive towards being able to position a Mobile Station (MS) with better accuracy. Figure 1 shows a typical cellular radio network with the mobile station or a wireless device 10 inside it. The MS 10 is located in a cellular network 11 with a number of Base Transceiving Stations (BTS's) 12-15. The device may communicate with any of these BTS's. The positions of these BTS's are known, but the position of the MS is unknown.

There are currently a few methods of determining the position of a device within a communications network. These methods usually use radio parameters like received signal strength, Time Difference Of Arrival (TDOA) at the BTS of MS transmitted signals, TDOA of the BTS transmitted signals at the MS, or the timing advance (TA) of the MS's transmitted signal as governed by the network.

The only way to use the TDOA parameters for positioning, is if the start time of transmissions and receive times of all the signals are known. This involves costly hardware installations and software modifications to the network. Some means of eliminating the need for synchronising the BTS's of the network is clearly needed.

According to US 6,002,936, a telecommunications system and method is disclosed for allowing a cellular network to determine the optimum positiomng method, having knowledge of all available network-based and terminal-based positioning methods. This can be accomplished by the Mobile Station (MS) sending to the Mobile Switching Center/Visitor Location Register (MSC/NLR) a list of terminal-based positioning methods that the MS is capable of performing. This list can, in turn, be forwarded to the Mobile Positioning Center (MPC) for determination of the optimum positioning method. For example, in a GSM network, the MS CLASSMARK information, which is sent to the MSC/NLR when the MS registers with the MSC/NLR, can be extended to include the MS's positioning capabilities.

In a cellular system configuration, according to US 5519760, the location of a mobile station is determined from the acquisition of cellular network data pertaining to the mobile station, and the translation of such network data into a corresponding geographical position profile. The cellular system includes a mobile station locator entity for receiving from a mobile switching center the network data such as cell and/or sector ID and trunk group member number. The mobile station locator translates the network data into position information such as geographic coordinates (latitude and longitude), resolution (radius), and angle values for sectorized cells.

Through, M. A. Spirito and A. guidarelli MattiolipOn the Hyperbolic Positioning of GSM Mobile Stations", ISSSE 98 (International Symposium on Signals, Systems and Electronics), October 1998, Pisa, Italy, a method is described for collecting TA or TDOA data for a mobile station together with RTDs between the base stations registered during a handover. However, according to this method, at least three base stations are needed for in order to locate a mobile station in two-dimensional system, column two, lines 2-4. Moreover, no suitable way is presented to handle the data in an structured way.

Summery of the invention

One object of the present invention is to enable methods previously thought to be impossible or difficult, to be used to position a mobile station inside a cellular radio network.

According to the invention the synchronisation between BTS's, as demanded by the prior art, is no longer required.

Moreover, the invention improves the accuracy of the positioning services by using time-based, received power level, or hybrid positioning methodologies.

Accordingly, the initially mentioned method comprises, recording said Real Time Differences (RTDs) between the BTSs in a table, said table comprising at least entries characterising behaviours of the RTD's over time and an expiry time attached to each table entry to invalidate it after a pre-set time, applying the RTDs as corrective factors to BTSs' time bases, and updating RTD values continuously as fresh data becomes available.

The method further comprises the step of recording said Real Time Differences (RTDs) between the BTS's in a table. Preferably, the RTD's are applied as corrective factors to BTS's time bases and RTD values are updated continuously as fresh data becomes available. Moreover, an expiry time is attached to each table entry to invalidate it after a pre-set time. Also, characteristics can be applied to live values in real time.

Furthermore, said table comprises entries that characterise behaviours of the RTDs over time. The entries include rate of drift as well as the rate of change of the drift with further time derivatives.

Most preferably, the table is an N x N x M sized table, wherein N is the number of BTSs and M is the number of parameters stored per RTD.

hi one embodiment, the MS synchronises with at least a first and a second BTS's and uses different timing advances for the first and the second BTS during a handover from one cell to another. A new Timing Advance for said second BTS is calculated according to a handover type and said handover is misynchronised, and that the MS receives timing advance instructions at the latest after the second BTS has calculated the TA, and it is carried out after that the MS has transmitted for the first time to the second BTS.

The relevant times are: RTD: which is the Real Time Difference between at least two BTSs, OTD: which is an Observed Time Difference between two BTS's, from the MS's point of view, TAo: which is the Timing Advance for the first BTS, TAj : which is the Timing Advance for the second BTS, t₀: is the TDOA for transmissions from the first BTS, ti: is the TDOA for transmissions from the old BTS.

Preferably, a common time is subtracted from any row or column of measurements to obtain a time that each of BTS's is out of synchronism with the common (subtracted) time.

If a MS receives signals from a BTS, a column is used to obtain the real time differences between a first and a second BTS. The time of arrival of a signal from the MS are taken at different BTS's, which times are then offset by the RTD's. Then said times are sent to a central processing unit, where the adjustments are applied to obtain a time difference for transmission and reception. The synchronisation values are applied to the time differences between the TA of signals received by the BTS's from the transmission of the MS. The values are not synchronised. Actual signal time difference is obtained by subtracting the synchronisation values obtained from the RTD table. The TDOA times of the BTS's as measured in the MS, are sent back to a central server along with the BTS Cell identities.

Moreover, in the server, said table is used to correct said values in order to obtain the time transmission/reception time difference for the signals from the BTS's to the MS.

In one embodiment, TDOA times of the BTS's as measured in the MS, are sent back to a central server along with the BTS cell identities. TDOAs are processed in a central server where the table is applied to remove the RTDs. The timing information are converted to distances, and a position for the MS estimated.

It is also possible to sent RTD using piggy-backing a message transmitted between the BTSs.

The invention also concerns an arrangement in a wireless communications network comprising a number of Base Transceiver Stations, BTSs and at least one Mobile Station an arrangement for determining a synchronisation difference between said BTSs. The arrangement comprises means for collecting a set of data on said mobile station and means for transmitting said set of data back to a server for further processing, wherein said data comprises at least one of a Timing Advance (TA) or Time Difference Of Arrival (TDOA), or means for collecting information on said communications network and means for transmitting said information back to a server for further processing, wherein said information comprises a Real Time Difference (RTDs) between said BTS's during a handover. It further comprises recording means for recording said Real Time Differences (RTDs) between the BTS's in a table and a device for updating RTD values continuously as fresh data becomes available. The table comprises entries that characterise behaviours of the RTD's over time. Preferably, the table is a N x N x M sized table, wherein N is the number of BTS's and M is the number of parameters stored per RTD. During a handover from one cell to another, the MS is arranged to synchronise with at least a first and a second BTS's and to use different timing advances for the first and the second BTS. A new Timing Advance (TAi) for said second BTS is calculated according to a handover type. Said handover is unsynchronised, and that the MS is arranged to receive timing advance instructions at the latest after the second BTS has calculated the TA.

Said relevant times are: RTD: which is the Real Time Difference between at least two BTS's, OTD: which is an Observed Time Difference between two BTS's, from the MS's point of view, TAo: which is the Timing Advance for the first BTS, TAi : which is the Timing Advance for the second BTS, to: is the TDOA for transmissions from the first BTS, and ti: is the TDOA for transmissions from the old BTS.

The arrangement comprises means for subtracting common time from any row or column of measurements to obtain a time that each of BTS's is out of synchronism with the common (subtracted) time.

Brief description of the Drawings

In the following, the invention will be described in more detail with reference to attached drawings, in which:

Figure 1 illustrates a mobile station inside a cellular radio network according to known technique, Figure 2 is a possible embodiment of a table with dynamic phase corrections (DPC), according to the present invention, Figure 3 shows the time parameters involved in handovers,

Figure 4 illustrates estimation of the ranges between a mobile station and BTS's inside a cellular radio network, and Figure 5 illustrates estimation of the position of a mobile station inside a cellular radio network.

Description of the preferred embodiments

According to the invention, a solution is proposed whereby synchronisation between BTS's is not needed. This is achieved by recording the Real Time Differences (RTDs) between BTS's in a table. The RTD's may be applied as corrective factors to the BTS's time bases. The RTD values are updated continuously as fresh data becomes available, as they will drift over time. An expiry time (Expiry) is attached to each table entry to invalidate it after a pre-set time. There may also be entries that characterise the behaviour of the RTD's over time, such as rate of drift (dRTD/dt) as well as the rate of change of the drift with further time derivatives (dRTD/dt²). These characteristics may be applied to the live values in real time. This results a N x N x M sized table with N = number of BTS's and M = number of parameters stored per RTD. An example of such a table is shown in Figure 2.

The RTD's can be obtained in two ways: In the MS (during handovers). In the network.

During MS handover in the MS

Referring now to fig. 3, during a handover from one cell to another, the MS 10 has to synchronise with both BTS's 12 and 13, and use different timing advances for the old and the new BTS. The new timing advance (TAi) for the new BTS is calculated according to the handover type. With unsynchronised handovers, the MS will receive timing advance instructions at the latest after the new BTS could calculate the TA (after the MS transmitted for the first time to the new BTS). During this process, six different times are of relevance: a) RTD: is the real time difference between two BTS's. b) OTD: is the observed time difference between two BTS's, from the MS's point of view. c) TAo: is the timing advance for the old BTS. d) TAi : is the timing advance for the new BTS. e) to: is the TDOA for transmissions from the old BTS. f) t_\ : is the TDOA for transmissions from the new BTS.

During MS handover in the network

During a handover from one cell to another, some or all the above information may be extracted from the network.

Referring back to figure 2 in conjunction with figure 3, a common time is subtracted from any row or column of measurements to obtain the time that each of the BTS's is out of synchronism with the^'common (subtracted) time. If a unit receives signals from, for example BTSl, 12, BTS2, 13, and BTS3, 14, the first column can be used to obtain the real time differences between BTSl and BTS2, and BTSl and BTS3. In this instance, the synchronisation adjustments for the positional solution will look as follows:

For BTSl : 0

For BTS2 : BS_2θffset-BSι₀ffset - RTDBSI-BS2 For BTS3 : BS_3θffset-BSι₀ffset = RTD_BSI-BS3

This data may be applied in two ways:

In the MS

The time differences between the TDOA of signals received by the MS from the BTS's are recorded. These times are sent to a central server (not shown), where the adjustments are applied to obtain the 'fly time' of the signals. In the network

The synchronisation values are applied to the time differences between the TDOA of signals received by the BTS's from the transmission of the MS. Say the TDOA's recorded by a set of BTS's for one MS, are : BTSl : txooAi

BTS2 : tτD0A2 BTS 3 : tχooA3

These values are not synclironised, but it is possible to obtain the actual signal 'fly time', by subtracting the synchronisation values obtained from the RTD table :

BTSl : tfly timel ⁼ tTDOAl - 0

BTS2 : tfly _tirne2 ⁼ tτD0A3 " RTD_BS2-BS1

BTS3 : tfly time3 - tτD0A3 " RTD_BS3-BS1

These calculated times may be multiplied by the speed of light to obtam the distances of each of

There are MS centric, or network centric approaches to implement TDOA values to estimate a position of the MS, as illustrated in fig. 4.

In the MS centric approach, the TDOA times of the BTS's (with which the MS is synclironised) as measured in the MS, are sent back to a central server along with the BTS Cell ID's and possibly some other parameters. In the server, the DPC table is used to correct these values in order to obtain the 'fly times' for the signals from the BTS's to the MS. These are converted to distances, and a position for the MS estimated.

In the network centric approach, the time of arrival of a signal from the MS at different BTS's are taken. These times will again be offset by the RTD's. These TDOA's are processed in a central server where the DPC table is applied to remove the RTD's. The processed TDOA's can be converted to distances, and the MS position estimated. The MS centric, and the network centric approaches may be combined with other techniques and even with each other, to enable a hybrid approach to positioning.

If the TDOA's of more neighbours become available, it should be possible to use them to increase the accuracy of the position of the MS.

The RTDs can be sent using piggy-backing of other messages.

While we have illustrated and described preferred embodiments of the invention, it is appreciated that several variations and modifications within the scope of the attached the claims can occur.

Claims

1. In a wireless communications network comprising a number of Base Transceiver Stations, BTSs, (12-14) and at least one Mobile Station (10) a method of determining a synchronisation difference between said BTS's, comprising at least one of the steps of:

• collecting a set of data on said mobile station and transmitting it back to a server for further processing, said data including at least one of a Timing Advance (TA) or Time Difference Of Arrival (TDOA), or • collecting information on said communications network and transmitting it back to a server for further processing, said information including a Real Time Difference (RTDs) between said BTS's during a handover characterised by recording said Real Time Differences (RTDs) between the BTSs in a table, said table comprising at least entries characterising behaviours of the RTD's over time and an expiry time attached to each table entry to invalidate it after a pre-set time, applying the RTDs as corrective factors to BTSs' time bases, and updating RTD values continuously as fresh data becomes available.

2. The method of claim 1, characterised in that an expiry time is attached to each table entry to invalidate it after a pre-set time.

3. The method of claim 1 , characterised in that said entries include rate of drift as well as the rate of change of the drift with further time derivatives.

4. The method of claim 1, characterised in that said characteristics are applied to live values in real time.

5. The method of claim 1, characterised in that said table is aN xN x M sized table, wherein N is the number of BTS's and M is the number of parameters stored per RTD.

6. The method of claim 1, characterised in that during a handover from one cell to another, the MS (10) synchronises with two BTSs (12, 13), a first BTS and a second BTS, and uses different timing advances for the first and the second BTS.

7. The method of claim 6, characterised in that a new Timing Advance (TA for said second BTS is calculated according to a handover type.

8. The method of claim 6, characterised in that said handover is unsynchronised, and that the MS receives timing advance instructions at the latest after the second BTS has calculated the TA.

9. The method of claim 8, characterised in that it is carried out after that the MS has transmitted for the first time to the second BTS.

10. The method of claim 7, characterised in that relevant times are:

• RTD: which is the Real Time Difference between at least two BTS's, • OTD: which is an Observed Time Difference between two BTS's, from the MS's point of view,

• TAo: which is the Timing Advance for the first BTS,

• TAi : which is the Timing Advance for the second BTS,

• to: is the TDOA for transmissions from the first BTS, • ti : is the TDOA for transmissions from the old BTS.

11. The method of claim 1 , characterised in that a common time is subtracted from any row or column of measurements to obtain a time that each of BTS's is out of synchronism with the common (subtracted) time.

12. The method of claim 1- characterised in that if a MS receives signals from a BTS, a column is used to obtain the real time differences between a first and a second BTS.

13. The method of claim 12, characterised in that the time of arrival of a signal from the MS are taken at different BTSs, which times are then offset by the RTDs.

14. The method of claim 13, characterised in that said times are sent to a central processing unit, where the adjustments are applied to obtain a time difference for transmission and reception.

15. The method of claim 14, characterised in that synchronisation values are applied to the time differences between the TA of signals received by the BTSs from the transmission of the MS.

16. The method of claim 15, characterised in that said values are not synchronised.

17. The method of claim 1- characterised in that actual signal time difference is obtained by subtracting the synchronisation values obtained from the RTD table.

18. The method of claim 1, characterised in that TDOA times of the BTSs as measured in the MS are sent back to the central server along with the BTS Cell identities.

19. The method of claims 1 and 18, characterised in that in said server, said table is used to correct said values in order to obtain the time transmission and reception time difference for the signals from the BTSs to the MS.

20. The method of claim 1, characterised in that TDOA times of the BTS's as measured in the MS, are sent back to a central server along with the BTS Cell identities.

21. The method of claim 1 and 20, characterised in that TDOAs are processed in the central server where the table is applied to remove the RTDs.

22. The method of any of claims 17-20, characterised in that the timing information are converted to distances and a position for the MS is estimated.

23. The method according to any of preceding claims, characterised in that said RTD is sent by piggy-backing a message transmitted between the BTSs.

24. In a wireless communications network comprising a number of Base Transceiver Stations, BTSs, (12-14) and at least one Mobile Station (10) an arrangement for determining a synchronisation difference between said BTSs, said arrangement comprises means for collecting a set of data on said mobile station and means for transmitting said set of data back to a server for further processing, wherein said data comprises at least one of a Timing Advance (TA) or Time Difference Of Arrival (TDOA), or means for collecting information on said communications network and means for transmitting said information back to a server for further processing, wherein said information comprises a Real Time Difference (RTDs) between said BTS's during a handover, characterised in that said arrangement comprises a table for recording said Real Time Differences (RTDs) between the BTSs in a table, said table comprising at least entries characterising behaviours of the RTD's over time and an expiry time attached to each table entry to invalidate it after a pre-set time, means for applying the RTDs as corrective factors to BTSs' time bases, and means for updating RTD values continuously as fresh data becomes available.

25. The arrangement of claim 24, characterised in that said table is a N x N x M sized table, wherein N is the number of BTS's and M is the number of parameters stored per RTD.

26. The arrangement of claim 25, characterised in that during a handover from one cell to another, the MS (10) is arranged to synchronise with at least a first and a second BTS's (12, 13) and to use different timing advances for the first and the second BTS.

27. The arrangement of claim 25, characterised in that a new Timing Advance (TAi) for said second BTS is calculated according to a handover type.

28. The arrangement of claim 26, characterised in that said handover is unsynchronised, and that the MS is arranged to receive timing advance instructions at the latest after the second BTS has calculated the TA.

29. The arrangement of claim 26, characterised in that relevant times are:

• RTD: which is the Real Time Difference between at least two BTS's,

• OTD: which is an Observed Time Difference between two BTS's, from the MS's point of view,

• TA₀: which is the Timing Advance for the first BTS, • TAi : which is the Timing Advance for the second BTS,

• t₀: is the TDOA for transmissions from the first BTS,

• ti : is the TDOA for transmissions from the old BTS.

30. The arrangement of claim 25, characterised in that it comprises means for subtracting common time from any row or column of measurements to obtain a time that each of BTS's is out of synchronism with the common (subtracted) time.