Base station synchronization in a wireless network

• US 20040202119A1
• Filed: 04/10/2003
• Published: 10/14/2004
• Est. Priority Date: 04/10/2003
• Status: Abandoned Application

• Current Assignee: Nokia Siemens Networks GmbH & Company KG (Nokia Corporation)
• Original Assignee: Nokia Siemens Networks GmbH & Company KG (Nokia Corporation)
• Inventors: Edge, Stephen William
• Application Number:
  US10/410,843
• Publication Number:
  US 20040202119A1
• Time in Patent Office: Days
• US Class Current: 370/324
• CPC Class Codes: H04B 7/2687 Inter base stations synchronisation

Abstract

A wireless communications network and method of synchronizing network base stations. Wireless mobile units, e.g., cell phones, periodically measure transmission timing differences between pairs of adjacent base stations and each provide the measurements to a server base station. An absolute transmission timing difference (ATD) is determined for each difference measurement. ATDs are collected and combined for each pair of base stations. A timing relationship is developed for all base stations from the combined ATDs. A timing correction is extracted for each base station from the timing relationship. Application of the timing corrections synchronizes the base stations.

Claims

20. A method for synchronizing base station transmission timing in a wireless network, said method comprising the steps of:

a) measuring timing differences between pairs of a plurality of base stations;

b) aggregating measured said timing differences;

c) combining aggregated said timing differences for each pair of said plurality of base stations;

d) reducing errors in combined said timing differences; and

e) adjusting base station timing responsive to said combined timing differences.

21. A method as in claim 20, wherein said plurality of base stations initiate timing difference measurements in step (a), mobile units served by said plurality of base stations measure transmission timing differences between said pairs, each of said mobile units returning said measured transmission timing differences to a server base station.

22. A method as in claim 21, wherein said timing differences are timing differences relative to a sub-unit of transmission for the wireless technology.

23. A method as in claim 20, wherein the step (b) of aggregating said measured timing differences comprises the steps of:

i) extracting an absolute timing difference (ATD) from each measured timing difference; and

ii) averaging extracted ATDs.

24. A method as in claim 23, wherein said averaging step (ii) averages said ATDs with a moving weighted average.

25. A method as in claim 23, wherein said mobile units measuring base station timing differences extract ATDs in extracting step (i).

26. A method as in claim 20, wherein the step (c) of combining said aggregated timing differences includes transferring said aggregated timing differences from said plurality of base stations to a central entity.

27. A method as in claim 20, wherein a network graph being formed in the step (d) of reducing errors, the error reducing step (d) comprising the steps of:

i) representing each of said plurality of base stations being represented as a graph node;

ii) providing links between pairs of graph nodes, each of said link representing the availability of a combined said timing differences between each corresponding represented pair of said plurality of base stations;

iii) identifying paths between nodes, identified said paths defining timing difference relationships between said nodes; and

iv) reducing errors from defined said timing difference relationships.

28. A method as in claim 27, wherein the step (iii) of identifying paths comprises identifying distinct loops in an ordered list, each distinct loop being an identified path and having at least one link not included in any earlier listed said distinct loop.

29. A method as in claim 20, wherein the base station adjustment step (e) comprises the steps of:

i) selecting a reference base station from said plurality of base stations;

ii) calculating a timing synchronization adjustment relative to said reference base station for each remaining one of said plurality of base stations;

iii) providing calculated timing adjustments to corresponding base stations; and

iv) adjusting base station timing responsive to provided said calculated timing adjustments.

30. A method as in claim 29, wherein the reference base station includes a universal clock source.

31. A method as in claim 30, wherein said universal clock source is a global positioning system (GPS) receiver.

32. A method as in claim 30, wherein each said timing synchronization adjustment calculated in calculating step (ii) is calculated relative to a clock from said universal clock source.

33. A method for synchronizing the transmission timing of base stations in a wireless network comprising:

a) measuring a transmission timing difference between at least one base station pair;

b) adjusting said measured timing difference (MTD) to obtain an adjusted transmission timing difference;

c) extracting an absolute transmission timing difference (ATD) from said adjusted transmission timing difference;

d) transferring said ATD to a central network entity;

e) obtaining a transmission timing difference relative to a reference base station for at least one base station of said pair using said ATD;

f) reducing errors in said transmission timing difference;

g) obtaining a timing adjustment relative to said reference base station for said at least one base station of said pair;

h) transferring said timing adjustment to said at least one base station of said pair; and

j) applying said timing adjustment to transmission timing of said at least one base station of said pair.

1. A wireless communications network comprising:

a plurality of wireless mobile units;

a plurality of base stations, each of said plurality of wireless mobile units being in wireless communication with one or more of said plurality of base stations, each of said plurality of base stations being a server base station for wirelessly connecting to one or more of said wireless mobile units, each said server base station collecting timing differences from said wireless mobile units each receiving signals from at least two of said plurality of base stations; and

a central network entity, said plurality of base stations providing said collected timing differences to said central network entity, said central network entity determining a timing synchronization adjustment for each of said plurality of base stations from said collected timing differences, said central network entity providing to at least some of said plurality of base stations a corresponding said timing synchronization adjustment, each of said plurality of base stations adjusting base station transmission timing, whereby transmission timing synchronizes and is maintained synchronized among said plurality of base stations.

2. A wireless communications network as in claim 1, wherein said central network entity is a base station controller in a base station subsystem.

3. A wireless communications network as in claim 1, wherein said central network entity is a serving mobile location center for GSM.

4. A wireless communications network as in claim 1, wherein each said server base station extracts an absolute timing difference from said collected timing differences.

5. A wireless communications network as in claim 4, wherein each said server base station averages said extracted absolute timing differences for corresponding pairs of said base stations.

6. A wireless communications network as in claim 5, wherein averages of said absolute timing differences are moving weighted averages.

7. A wireless communications network as in claim 1, further comprising:

one or more fixed measurement units, each of said fixed measurement units being located at a known location.

8. A wireless communications network as in claim 7, wherein each of said fixed measurement units provides timing differences to one of said plurality of base stations.

9. A wireless communications network as in claim 1, wherein at least one base station includes a universal clock source, said at least one base station being synchronized to said local clock source.

10. A wireless communications network as in claim 9, wherein said universal clock source is a global positioning system (GPS) receiver.

11. A wireless communications network as in claim 9, wherein said central network entity bases timing synchronization adjustments on timing differences from said at least one base station, other ones of said plurality of base stations being synchronized to said at least one base station.

12. A wireless communications network as in claim 1, wherein said timing differences are relative timing differences.

13. A wireless communications network as in claim 1, wherein said central network entity forms a network graph representative of said communications network, each of said plurality of base stations being represented as a graph node, measured delay differences between pairs of said plurality of base stations being represented as links between pairs of corresponding representative nodes.

14. A wireless communications network as in claim 13, wherein paths between each pair of nodes define timing difference relationships between represented said base stations, said timing difference relationships relating errors in said timing differences between pairs of said represented base stations, said central network entity extracting related said errors.

15. A wireless communications network as in claim 13, wherein said central entity identifies an ordered list of distinct loops in said graphs, each distinct loop having at least one link not included in any earlier listed said distinct loop.

16. A wireless communications network as in claim 15, wherein said distinct loops define timing difference relationships between base stations represented by connected nodes on said distinct loops, said timing difference relationships relating errors in said timing differences between pairs of said base stations, said central network entity extracting related said errors.

18. A wireless communications network as in claim 1, wherein at least one of said plurality of mobile units is a cellular telephone.

19. A wireless communications network as in claim 1, wherein at least one of said plurality of mobile units is a computing device with a wireless communications interface.

34. The method of claim 33 wherein said adjusted transmission timing difference obtained in adjusting step (b) is transferred to a serving base station for extracting the ATD in extracting step (c).

35. The method of claim 34 wherein a mobile unit measures the MTD in the measuring step (a) and adjusts the MTD in the adjusting step (b) and provides said adjusted transmission timing difference to said serving base station.

36. The method of claim 35 wherein the mobile unit subtracts a propagation delay difference (P2−

P1) for each said at least one base station pair from said MTD in the adjusting step (b), the difference (MTD−

(P2−

P1)) being provided to said serving base station, said difference being said ATD extracted in step (c) by said serving base station.

37. The method of claim 35 wherein for one base station in each pair of said at least one base station pair, in the adjusting step (b) the mobile unit subtracts a propagation delay (P2) from said MTD, the difference (MTD−

P2) being provided to said serving base station, said ATD being extracted in step (c) by adding another propagation delay (P1) for the other base station in said each pair.

38. The method of claim 35 wherein for one base station of each pair of said at least one base station pair, in the adjusting step (b) the mobile unit adds to said MTD a propagation delay (P1), the sum (MTD+P1) being provided to said serving base station, said ATD being extracted in step (c) by subtracting another propagation delay (P2) for the other base station in said each pair.

39. The method of claim 34 wherein a fixed measurement unit measures the MTD in the measuring step (a) and adjusts the MTD in the adjusting step (b) and provides said adjusted transmission timing difference to said serving base station.

40. The method of claim 39 wherein the fixed measurement unit subtracts a propagation delay difference (P2−

P1) from said MTD in the adjusting step (b), the difference (MTD−

(P2−

P1)) being provided to said serving base station, said difference being said ATD extracted in step (c) by said serving base station.

41. The method of claim 33 wherein a mobile unit measuring the MTD in the measuring step (a) and adjusting the MTD in the adjusting step (b) provides said adjusted transmission timing difference to a central entity for extracting the ATD in extracting step (c).

42. The method of claim 41 wherein the mobile unit subtracts a propagation delay difference (P2−

P1) from said MTD in the adjusting step (b), the difference (MTD−

(P2−

P1)) being provided to said central entity, said difference being said ATD extracted in step (c) by said central entity.

43. The method of claim 33 wherein the step (d) of extracting ADTs comprises averaging ADTs for base station pairs.

44. The method of claim 33 wherein the step (d) of extracting ADTs comprises a moving weighted average according to:

ATD₁=ATD₁(n=1) and, ATD_n+1=(1−

w)ATD_n+wATD_n+1(n≧

1);

where,

ATD_n=n^th absolute time difference (obtained after ATD<sub>n−

1</sub> and before ATD_n+1)

ATD_n=n^th value for moving weighted average

w=weight (0<

w<

1).

45. The method of claim 33 wherein the step (e) of obtaining said transmission timing difference comprises the steps of:

i) obtaining a network graph wherein each node represents a base station and each link between a pair of nodes represents an available absolute transmission time difference between the pair of corresponding base stations;

ii) selecting an arbitrary reference node;

iii) finding a continuous path of nodes and links from said reference node to any other node; and

iv) summing the absolute transmission timing differences traversing along all links on said continuous path from said reference node to said other node.

46. The method of claim 45 wherein any said transmission timing differences summed in the step (iv) of summing are each differences of a first traversed node transmission time less a second subsequently traversed adjacent node transmission time.

47. The method of claim 33 wherein the step (f) of reducing errors includes averaging said ATDs for each pair of base stations.

48. The method of claim 33 wherein the step (f) of reducing errors comprises:

i) obtaining a network graph wherein each node represents a base station and each link between a pair of nodes represents an available ATD between a corresponding base station pair;

ii) identifying closed loops in said network graph;

iii) listing said closed loops in an ordered list in ascending order, according to the number of said nodes;

iv) traversing said ordered list in ascending order and removing all closed loops from said ordered list having links in earlier traversed closed loops, each remaining listed loop being a distinct closed loop including at least one link not in any other of said distinct closed loops appearing earlier in said ordered list;

v) producing an equation for each closed loop of nodes still remaining in said ordered list, each said equation summing the ATDs plus error component variables along all the links in said each closed loop, each said sum being equal to zero;

vi) producing additional equations, relating said error component variables, equal in number to the number of distinct links in all closed loops less the number of closed loops;

vii) solving said equations for said error component variables in said ATDs;

viii) obtaining an adjusted value for each ATD, said adjusted value including the corresponding error component obtained from said equations;

ix) selecting a reference node; and

x) summing said adjusted values along a sequence of links on a path from said reference node to each other node, each sum being the transmission timing difference from said reference node to said each other node.

49. The method of claim 33 wherein the step (f) of reducing errors comprises:

i) obtaining a network graph wherein each node represents a base station and each link between a pair of nodes represents an available ATD between a corresponding base station pair;

ii) selecting a reference node;

iii) for each remaining node, obtaining a set of minimum paths from said reference node to said remaining node, no link being in more than one path;

iv) for each path, identifying alternative sub-paths, each identified alternative sub-path connecting two nodes in said each path and including links that are not in any said path or in any other said identified alternative sub-path;

v) for portions of sub-paths, identifying alternative sub-paths, each identified alterative sub-path connecting two nodes in said sub-path portion and including links that are not in any said path, any of said sub-paths or other said sub-path portions;

vi) repeating step (v) until all said alternative sub-paths are found; and

vii) summing ATDs along all said paths, sub-paths and sub-path portions, ATD sums for each said path, sub-path and sub-path portion being summed with a weighted average, said weighted average being a transmission timing difference between said reference node and said remaining node, weights being assigned to each of said ATD sums and to each of said combined ATD sums inversely proportional to its variance;

whereby, variance is calculated for independent delays between pairs of nodes with zero error expectation and a common error variance, and said weighted average minimizes the variance of said transmission timing difference sum between path nodes.

50. The method of claim 33 wherein the step (g) of obtaining said timing adjustment includes dividing said transmission timing difference by a transmission time unit, said timing adjustment being obtained from the remainder.

51. The method of claim 33 wherein the step (g) of obtaining said timing adjustment includes the steps of:

i) receiving a current complete transmission timing reference from each of said base stations;

ii) adjusting each received said complete transmission timing reference for a propagation delay to said central network entity from said each of said base stations;

iii) approximating a complete transmission timing difference between said reference base station and each said base stations from the difference between said current complete transmission timing references for each of said base stations;

iv) combining each approximated said complete transmission timing difference with a transmission timing difference relative to a sub-unit of transmission for the wireless technology, whereby said combination yields a

$\begin{array}{c}\mathrm{precise}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{complete}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{time}<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{difference}=\left(n<br><br>\text{\hspace{1em}<br><br>}<br><br>S+t^{*}\right)<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{if}<br><br>\text{\hspace{1em}<br><br>}<br><br>t^{*}\ge <br><br>t,\\ \text{\hspace{1em}<br><br>}<br><br>=\left(\left(n+1\right)<br><br>S+t^{*}\right)<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{if}<br><br>\text{\hspace{1em}<br><br>}<br><br>t^{*}<<br><br>t,<br><br>\text{\hspace{1em}<br><br>}\end{array}$

where

S=said sub-unit of transmission

t*=said timing difference relative to S (0≦

t*<

S)

n S+t=lower bound of said approximate complete time difference

n S+t+E=upper bound of said approximate complete time difference

E=uncertainty range where E<

S is required

0≦

t<

S

n is an integer with n≧

0; and

v) dividing said precise complete transmission timing difference by a second sub-unit of transmission time, said timing adjustment being obtained as the remainder.

52. The method of claim 33 wherein the step (g) of obtaining transmission timing adjustments comprises the steps of:

i) assigning a time adjustment value of zero to the reference base station;

ii) obtaining a forward time adjustment and a backward time adjustment relative to said reference base station for every other base station;

iii) assigning a positive value to each said backward time adjustment and a negative value to each said forward time adjustment;

iv) finding a minimum range of values including a timing adjustment offset for every said base station;

v) choosing a new reference time based responsive to said minimum value range; and

vi) obtaining a new timing adjustment for each said base station relative to said new reference time and offset by a corresponding said timing adjustment offset within said minimum range.

53. The method of claim 52 wherein said timing adjustment offset in step (iv) is a forward adjustment.

54. The method of claim 52 wherein said timing adjustment offset in step (iv) is a backward adjustment.

55. The method of claim 52 wherein said new reference time is chosen in step (v) by selecting a new reference base station.

56. The method of claim 52 wherein said new reference base station is one said base station having a timing adjustment in the middle of said minimum range of values.

57. The method of claim 52 wherein said new reference time is between a pair of base station adjustments in the middle of said minimum range of values.

58. The method of claim 33 wherein the step (i) of applying said timing adjustments comprises adjusting base station transmission timing in each of said base stations over a selected period of time, applied said timing adjustments being spread over said selected period of time.

59. The method of claim 33 wherein the step (i) of applying timing adjustments adjusts timing in one of said base stations, immediately adjusting transmission timing in said one comprising the steps of:

i) handing over one or more served mobile units from said one to other base stations;

ii) allowing remaining served mobile units to complete current service requirements and refusing undertaking additional service requirements;

iii) applying the whole said transmission timing adjustment to base station transmission timing for said one; and

iv) resuming service for said one to mobile units.

60. The method of claim 33 further comprising the steps of:

j) maintaining a common time reference in said central entity for all said base stations;

k) transferring said common time reference to at least one unsynchronized base station; and

l) adjusting the transmission time of said unsynchronized base station to said common transmission time allowing for the additional propagation time from said central entity to said unsynchronized base station.

61. The method of claim 60 wherein said common time reference is provided to said central entity by one or more particular base stations.

62. The method of claim 33 further comprising the steps of:

j) synchronizing transmission timing in at least one base station to a universal time source;

k) assigning said at least one base station as said reference base station; and

l) calculating a timing adjustment for other said base stations relative to said reference base station.

63. The method of claim 62 wherein said universal time source is a GPS time source.

64. The method of claim 33, wherein at least one of said base stations includes a universal time source, said method further comprising the steps of:

j) periodically transferring the transmission timing of said at least one base station and a universal time count to said central entity;

k) synchronizing all of said base stations to eliminate transmission timing differences;

l) synchronizing each said at least one base station to universal time by applying a universal time adjustment;

m) assigning each synchronized said at least one as one said reference base station;

n) calculating a universal time timing adjustment for every other base station based on a difference between each said every other base station with a corresponding said reference base station; and

o) adjusting transmission timing in said every other base station according to said universal time adjustment.

65. The method of claim 64 wherein said universal time source is a GPS time source.

66. The method of claim 33 wherein mobile units in wireless communications with said base stations are GSM mobile units, said central network entity is a GSM SMLC and said measured timing differences are measurements for E-OTD positioning.

67. The method of claim 33 wherein mobile units in wireless communications with said base stations are GSM mobile units and at least one GSM mobile unit provides said transmission time differences to a new base station as part of handover from an old base station to said new base station.

68. The method of claim 33 wherein said transmission timing differences are event initiated, said event selected from a group comprising:

a command from said central entity;

a command from said server base station; and

a time periodic event.

46-1. The method of claim 45 wherein any said transmission timing differences summed in the step (iv) of summing are each differences of a second traversed node transmission time less a first previously traversed adjacent node transmission time.