Method and system for facilitating timing of base stations in an asynchronous CDMA mobile communications system

• US 6,526,039 B1
• Filed: 02/02/1999
• Issued: 02/25/2003
• Est. Priority Date: 02/12/1998
• Status: Expired due to Term

• Current Assignee: Telefonaktiebolaget LM Ericsson
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Cedervall, Mats, Lundqvist, Patrik, Jamal, Karim, Nyström, Johan Anders, Dahlman, Erik Bengt Lennart
• Primary Examiner(s): Trost, William
• Assistant Examiner(s): PEREZ GUTIERREZ, RAFAEL
• Application Number:
  US09/243,095
• Time in Patent Office: 1,484 Days
• Field of Search: 370/342, 370/335, 370/350, 375/145, 375/149, 375/140, 375/142, 375/143, 375/180, 375/152, 455/501, 455/502, 455/500, 455/442, 455/67.6, 455/524, 455/456
• US Class Current: 370/350
• CPC Class Codes: H04B 1/7083 Cell search, e.g. using a three-step approach
• CPC Class Codes: H04W 56/002 Mutual synchronization
• CPC Class Codes: H04W 92/10 between terminal device and access point, i.e. wireless air interface

Abstract

A method and system are disclosed for facilitating the timing (e.g., the known relative timing differences) of base stations (BSs) in asynchronous CDMA mobile communications systems. A plurality of mobile stations (MSs) measure the relative time differences between various pairs of BSs, and these measurements are stored by the BSs. A source BS sends to an MS, in a neighbor list message, estimates of the relative time difference between the source BS and each of the BSs on the neighboring cell list. Each BS on the list can maintain a relative time difference estimate table, which can be updated continuously from the reports received from MSs. Subsequently, the BSs can send entries from this table to the MS in the neighbor list message. Using this novel technique, the BSs have known relative timing differences. Consequently, when the MS initiates a cell-search for a candidate BS, the MS already has an estimate of the timing of that BS as compared to its source BS. As such, the resulting cell-search procedure has a lower level of complexity and thus can be accomplished much quicker than with prior procedures. In addition, the relative time difference estimates can be compared with corresponding time differences that are measured by a second mobile station. Based on this comparison, the propagation delays of signals between the second MS and various BSs can be calculated to determine the position of the second MS.

Claims

1. A method for estimating a relative timing of a plurality of base stations in an asynchronous mobile communications system, comprising the steps of:

receiving at a first mobile station a first downlink signal transmitted by a first one of said base stations and a second downlink signal transmitted by a second one of said base stations;

transmitting an uplink signal from the first mobile station to the first and second base stations; and

calculating an estimated relative time difference between the time base of said first base station and the time base of said second base station using receive times at the first and second base stations of said uplink signal, transmit times of the first downlink signal and the second downlink signal, and a time difference at the first mobile station between a receive time of the second downlink signal and the transmit time of said uplink signal, wherein the receive times of the uplink signal and the transmit times of the first and second downlink signals are in the time base of the base station transmitting or receiving the respective signal, said calculation accounting for propagation delays between said first mobile station and said first and second base stations.

2. The method of claim 1, further comprising the step of utilizing the estimated relative time difference to calculate at least one possible location of a second mobile station relative to at least one of said first and second base stations by determining a distance between the second mobile station and at least one of said first and second base stations.

3. The method of claim 2, further comprising the step of determining a location of the second mobile station by using a plurality of relative time differences between a plurality of different pairs of base stations to calculate a distance between the second mobile station and each of a plurality of said base stations.

4. The method of claim 1, further comprising the step of determining at least one possible location of a second mobile station using the receive times at said first base station and said second base station of an uplink signal transmitted by said second mobile station and using said estimated relative time difference.

5. The method of claim 1, further comprising the step of determining at least one possible location of a second mobile station using receive times at the second mobile station of downlink signals transmitted by each of said first and second stations and using said estimated relative time difference.

6. The method of claim 1, further comprising the step of transmitting the value of the estimated relative time difference to a second mobile station.

7. The method of claim 6, further comprising the steps of:

estimating a propagation delay of signals transmitted between the second mobile station and the first base station and of signals transmitted between the second mobile station and the second base station based on an approximate location of the second mobile station;

adjusting the estimated relative time difference value by factoring in the estimated propagation delays to determine a local estimated relative time difference value at the mobile station;

said second mobile station correlating said local estimated relative time difference value with a matched filter output signal; and

initiating a cell search based on a result of the correlating step.

8. The method of claim 6, further comprising the steps of:

said second mobile station correlating the estimated relative time difference value with a matched filter output signal; and

initiating a cell search based on a result of the correlating step.

9. The method of claim 1, wherein said estimated relative time differences are used to synchronize said first and second base stations.

10. The method of claim 1, further comprising the step of storing the computed difference in a relative time difference table.

11. The method of claim 1, wherein said first mobile station is in a state of handover.

12. A method for facilitating timing between mobile stations and base stations in an asynchronous mobile telecommunications network, comprising the steps of:

receiving relative timing difference data from each of a plurality of mobile stations, the relative timing difference data from each mobile station including a measured difference between the time bases of at least two base stations as measured by said mobile stations;

determining a relative timing difference estimate based on the received relative timing difference data, the relative timing difference estimate representing an estimate of a difference between the time bases of at least two base stations;

accounting for propagation delays between a measuring mobile station and the at least two base stations of which the time base difference is measured in said relative timing difference estimate;

storing the relative timing difference estimate in a relative timing difference table;

transmitting the relative timing difference estimate to a receiving mobile station.

13. The method of claim 12, wherein the relative timing difference estimate is determined by calculating a relative timing difference estimate value from a plurality of measured differences received from the plurality of mobile stations.

14. The method of claim 12 further comprising the steps of:

estimating an error range for the relative timing difference estimate; and

transmitting the error range to the receiving mobile station.

15. The method of claim 12, wherein the relative timing difference estimate transmitted to the receiving mobile station is used to estimate a position of the receiving mobile station.

16. The method of claim 12, wherein the relative timing difference estimate comprises the measured difference received from one of the plurality of mobile stations.

17. The method of claim 12, wherein the relative timing difference estimate transmitted to the receiving mobile station is used to assist in synchronizing the receiving mobile station with a cell.

18. The method of claim 12, wherein data from a neighbor cell list is transmitted along with the relative timing difference estimate.

19. An asynchronous mobile telecommunications system, comprising:

a plurality of base stations for transmitting data to and receiving data from a plurality of mobile stations, the plurality of base stations individually receiving relative timing difference data, the relative timing difference data from each mobile station comprising a measured difference between the time bases of two of the plurality of base stations as measured by the mobile station;

a register storing a relative timing difference table, each of a plurality of entries in said table comprising a relative timing difference estimate calculated from the relative timing difference data and said register storing error data for each relative timing difference estimate; and

wherein a first one of the plurality of base stations transmits a relative timing difference estimate to a receiving mobile station to facilitate timing of communications between the mobile station and a second one of the plurality of base stations.

20. The system of claim 19 wherein the register further stores a neighbor cell list.

21. A method for facilitating the timing of a plurality of base stations in an asynchronous mobile communications system, comprising the steps of:

at least one of said plurality of base stations sending at least one estimated relative time difference value to a mobile station, said at least one estimated relative time difference value comprising an estimated timing difference between said at least one of said plurality of base stations and a neighbor base station;

said mobile station receiving said at least one estimated relative time difference value;

said mobile station correlating said at least one estimated relative time difference value with a matched filter output signal; and

initiating a cell search based on a result of the correlating step.

22. The method of claim 21, wherein the correlating step comprises:

comparing said at least one estimated relative time difference value to said matched filter output signal; and

determining if said at least one estimated relative time difference value is likely to correspond to a matched filter output signal peak.

23. The method of claim 22, wherein the initiating step comprises selecting a scrambling code based on a result of the determining step.

24. The method of claim 21, further comprising the step of estimating a propagation delay between said mobile station and at least one of said base stations, said estimated propagation delay used to reduce uncertainty in said correlating step.

25. The method of claim 21, wherein said mobile station transmits said at least one estimated relative time difference value along with a neighboring-cell quality report to said at least one of said plurality of base stations.

26. The method of claim 25, wherein a base station controller associated with said at least one of said plurality of base stations stores said at least one estimated relative time difference in a database.

27. The method of claim 21, wherein the sending step comprises broadcasting or transmitting said at least one estimated relative time difference value in a neighbor list message.

28. The method of claim 27, wherein said neighbor list message includes at least-one scrambling code associated with said neighbor base station.

29. The method of claim 21, wherein the sending step further comprises sending an uncertainty value associated with said at least one estimated relative time difference value.

30. The method of claim 21, wherein the mobile communications system comprises an asynchronous DS-CDMA system.

31. A method for facilitating the timing of a plurality of base stations in an asynchronous mobile communications system, comprising the steps of:

at least one of said plurality of base stations sending at least one estimated relative time difference value to a mobile station, said at least one estimated relative time difference value comprising an estimated timing difference between said at least one of said plurality of base stations and a neighbor base station;

said mobile station receiving said at least one estimated relative time difference value; and

determining an approximate position of the mobile station using said at least one estimated relative time difference value.

32. The method of claim 31 wherein said step of determining an approximate position comprises the steps of:

calculating a time difference at the mobile station between receive times of a first downlink signal transmitted by said at least one of said plurality of base stations and a second downlink signal transmitted by said neighbor base station; and

comparing said at least one relative time difference value with said calculated time difference to determine at least one possible location of said mobile station relative to said at least one of said plurality of base stations and said neighbor base station.

33. A system for synchronizing a plurality of base stations in a mobile communications system, comprising:

a first base station of said plurality of base stations, said first base station operable to broadcast or transmit at least one estimated relative time difference value, said at least one estimated relative time difference value comprising an estimated timing difference between said first base station and a neighbor base station;

a mobile station for receiving said at least one estimated relative time difference value; and

a processor for determining an approximate position of the mobile station using said at least one estimated relative time difference value.

34. The system of claim 33, wherein said processor compares said at least one estimated relative time difference value with a time difference measured by said mobile station to determine at least one possible location of said mobile station relative to said first base station and said neighbor base station.

35. A system for synchronizing a plurality of base stations in a mobile communications system, comprising:

a first base station of said plurality of base stations, said first base station operable to broadcast or transmit at least one estimated relative time difference value and at least one uncertainty value associated with said at least one estimated relative time difference value, said at least one estimated relative time difference value comprising an estimated timing difference between said first base station and a neighbor base station; and

a mobile station for receiving said at least one estimated relative time difference value and said at least one uncertainty value.

36. The system of claim 35, wherein said mobile station is operable to correlate said at least one estimated relative time difference value with a matched filter output signal and initiate a cell search.

37. The system of claim 36, wherein said mobile station is operable to compare said at least one estimated relative time difference value to said matched filter output signal, and determine if said at least one estimated relative time difference value is likely to correspond to a matched filter output signal peak.

38. The system of claim 36, wherein said mobile station is operable to select a scrambling code based on a result of correlating said at least one estimated relative time difference value with a matched filter output signal peak.

39. The system of claim 35, wherein said mobile station transmits said at least one estimated relative time difference value along with a neighboring-cell quality report to said first base station.

40. The system of claim 35, wherein a base station controller associated with said first base station stores said at least one estimated relative time difference in a database.

41. The system of claim 35, wherein said first base station is operable to broadcast or transmit said at least one estimated relative time difference value in a neighbor list message.

42. The system of claim 35, wherein the mobile communications system comprises an asynchronous DS-CDMA system.