Synchronization preamble method for OFDM waveforms in a communications system

• US 6,643,281 B1
• Filed: 03/05/1998
• Issued: 11/04/2003
• Est. Priority Date: 03/05/1998
• Status: Expired due to Fees

• Current Assignee: Clearwire Ip Holdings LLC (Deutsche Telekom AG)
• Original Assignee: AT&T Wireless Services Incorporated (AT&T, Inc.)
• Inventors: Ryan, David James
• Primary Examiner(s): Yao, Kwang Bin
• Assistant Examiner(s): NGUYEN, HANH N
• Application Number:
  US09/035,210
• Time in Patent Office: 2,070 Days
• Field of Search: 370/203, 370/208, 370/503, 370/514, 370/330, 370/207, 370/350, 370/281, 370/337, 370/336, 370/324, 370/317, 370/509, 370/510, 455/13.2, 455/501, 455/502, 455/63, 375/293, 375/354, 375/356, 375/358, 375/363
• US Class Current: 370/350
• CPC Class Codes: H04B 7/2668 Arrangements for Wireless Code-Division Multiple Access [CDMA] System Synchronisation, for code acquisition H04B1/7075, for code tracking H04B1/7085
• CPC Class Codes: H04L 5/026 using code division
• CPC Class Codes: H04W 56/0065 using measurement of signal travel time

Abstract

A highly bandwidth-efficient communications method is disclosed that enables remote stations to synchronize in time and frequency to their serving base station. The invention enables a base station and its remote stations in a cell to synchronize in a noisy environment where signals interfere from other base stations and remote stations in other cells. The base station forms a forward synchronization burst that includes a plurality of tone frequencies arranged in a distinctive orthogonal frequency division multiplexed pattern unique to the base station. The unique pattern enables a remote station to distinguish the base station'"'"'s bursts from other signals present in a crowded area. The distinctive orthogonal frequency division multiplexed pattern can be a Hadamard code pattern, for example. When the a base station has received a signal on a reverse link from a remote station, having significant interference, the base station selectively forms a request signal requesting the remote station to respond with a reverse synchronization burst that includes a plurality of tone frequencies arranged in the same distinctive orthogonal frequency division multiplexed pattern. The base station then transmits the forward synchronization burst and the request signal at a base station reference instant of time to the remote station. The reverse synchronization signals selectively occupy time slots in the transmission frame from the remote station to the base station, that would otherwise be occupied by channel control or traffic signals. Only when the base station requests the remote station to respond with a reverse synchronization burst, does this burst preempt the time slot from its other uses.

Claims

1. A highly bandwidth-efficient communications method, comprising:

forming a synchronization burst at an antenna element of a base station, including a plurality of tone frequencies arranged in a distinctive orthogonal frequency division multiplexed pattern unique to the base station;

transmitting the synchronization burst from the antenna element at a base station reference instant of time;

receiving the synchronization burst at a remote station during a remote station receive time window which begins at a remote station reference instant of time established by a remote station clock;

recognizing the pattern of the plurality of tone frequencies as having the base station as the source of the synchronization burst;

transmitting an error signal back to the base station at an instant referenced with respect to the remote station reference instant of time, in response to the recognizing;

deriving from the error signal a correction value related to a relative time error between the base station reference instant of time and the remote station reference instant of time; and

transmitting the correction value to the remote station to correct the remote station clock.

2. The highly bandwidth-efficient communications method of claim 1, wherein deriving further comprises:

deriving from the error signal a second value related to a relative phase error between the base station and the remote station; and

transmitting the second value to the remote station to correct the remote station.

3. The highly bandwidth-efficient communications method of claim 1, wherein forming the synchronization burst comprises:

selecting the distinctive orthogonal frequency division multiplexed pattern unique to the base station;

computing spreading weights at the base station to spread an outgoing synchronization signal over a plurality of outgoing frequencies, using the pattern; and

spreading the synchronization signal over the plurality of outgoing frequencies using the computed spreading weights, thereby forming the synchronization burst.

4. The highly bandwidth-efficient communications method of claim 1, wherein deriving the error signal at the base station comprises:

receiving at the base station a spread signal comprising an incoming signal that includes the error signal spread over a plurality of incoming frequencies;

adaptively despreading the spread signal received at the base station by using despreading weights, recovering the error signal;

deriving from the error signal the relative time error;

comparing the relative time error with a desired relative time difference value;

calculating the correction value in response to the comparing, to minimize a difference between the relative time error and the desired relative time difference value.

5. The highly bandwidth-efficient communications method of claim 1, wherein the base station is part of a wireless discrete multitone spread spectrum communications system.

6. The highly bandwidth-efficient communications method of claim 1, wherein a time of arrival of the error signal at the base station is used to derive the correction value.

7. The highly bandwidth-efficient communications method of claim 1, wherein a phase of the error signal when it arrives at the base station is used to derive the correction value.

8. The highly bandwidth-efficient communications method of claim 1, wherein a numerical value calculated at the remote station is used to derive the correction value.

9. The highly bandwidth-efficient communications method of claim 8, wherein the numerical value is derived from a measured difference between the remote station reference instant of time and a time of arrival of the synchronization burst at the remote station.

10. The highly bandwidth-efficient communications method of claim 1, wherein the relative time error is the difference between the base station reference instant of time and the remote station reference instant of time less a propagation duration of time of the synchronization burst from the base station to the remote station;

and

wherein the desired relative time difference value is a difference between the base station reference instant of time and a desired remote station reference instant of time less the propagation duration of time of the synchronization burst from the base station to the remote station.

11. A highly bandwidth-efficient communications system, comprising:

means for forming a synchronization burst at an antenna element of a base station, including a plurality of tone frequencies arranged in a distinctive orthogonal frequency division multiplexed pattern unique to the base station;

means for transmitting the synchronization burst from the antenna element at a base station reference instant of time;

means for receiving the synchronization burst at a remote station during a remote station receive time window which begins at a remote station reference instant of time established by a remote station clock;

means for recognizing the pattern of the plurality of tone frequencies as having the base station as the source of the synchronization burst;

means for transmitting an error signal back to the base station at an instant referenced with respect to the remote station reference instant of time, in response to the recognizing means;

means for deriving from the error signal a correction value related to a relative time error between the base station reference instant of time and the remote station reference instant of time; and

means for transmitting the correction value to the remote station to correct the remote station clock.

12. The highly bandwidth-efficient communications system of claim 11, wherein the deriving means further comprises:

means for deriving from the error signal a second value related to a relative phase error between the base station and the remote station; and

means for transmitting the second value to the remote station to correct the remote station.

13. The highly bandwidth-efficient communications system of claim 11, wherein the means for forming the synchronization burst comprises:

means for selecting the distinctive orthogonal frequency division multiplexed pattern unique to the base station;

means for computing spreading weights at the base station to spread an outgoing synchronization signal over a plurality of outgoing frequencies, using the pattern; and

means for spreading the synchronization signal over the plurality of outgoing frequencies using the computed spreading weights, thereby forming the synchronization burst.

14. The highly bandwidth-efficient communications system of claim 11, wherein the means for deriving the error signal at the base station comprises:

means for receiving at the base station a spread signal comprising an incoming signal that includes the error signal spread over a plurality of incoming frequencies;

means for adaptively despreading the spread signal received at the base station by using despreading weights, recovering the error signal;

means for deriving from the error signal the relative time error;

means for comparing the relative time error with a desired relative time difference value;

means for calculating the correction value in response to the comparing means, to minimize a difference between the relative time error and the desired relative time difference value.

15. The highly bandwidth-efficient communications system of claim 11, wherein the base station is part of a wireless discrete multitone spread spectrum communications system.

16. The highly bandwidth-efficient communications system of claim 11, wherein a time of arrival of the error signal at the base station is used to derive the correction value.

17. The highly bandwidth-efficient communications system of claim 11, wherein a phase of the error signal when it arrives at the base station is used to derive the correction value.

18. The highly bandwidth-efficient communications system of claim 11, wherein a numerical value calculated at the remote station is used to derive the correction value.

19. The highly bandwidth-efficient communications system of claim 18, wherein the numerical value is derived from a measured difference between the remote station reference instant of time and a time of arrival of the synchronization burst at the remote station.

20. The highly bandwidth-efficient communications system of claim 11, wherein the relative time error is the difference between the base station reference instant of time and the remote station reference instant of time less a propagation duration of time of the synchronization burst from the base station to the remote station;

and

wherein the desired relative time difference value is a difference between the base station reference instant of time and a desired remote station reference instant of time less the propagation duration of time of the synchronization burst from the base station to the remote station.

21. A highly bandwidth-efficient communications method, comprising:

forming a synchronization burst at a terrestrial base station, wherein the synchronization burst includes a plurality of tone frequencies arranged in a distinctive orthogonal frequency division multiplexed pattern unique to the base station;

transmitting the synchronization burst at a base station reference instant of time, wherein the synchronization burst uniquely identifies the base station to a remote station;

receiving an error signal back from the remote station at an instant referenced with respect to a remote station reference instant of time;

deriving a correction value from the error signal, related to a relative time error between the base station reference instant of time and the remote station reference instant of time; and

transmitting the correction value to the remote station to correct timing at the remote station.

22. The highly bandwidth-efficient communications method of claim 21, wherein deriving further comprises:

deriving from the error signal a second value related to a relative phase error between the base station and the remote station; and

transmitting the second value to the remote station to correct the remote station.

23. The highly bandwidth-efficient communications system of claim 21, wherein the base station is part of a wireless discrete multitone spread spectrum communications system.

24. The highly bandwidth-efficient communications system of claim 21, wherein a time of arrival of the error signal at the base station is used to derive the correction value.

25. The highly bandwidth-efficient communications system of claim 21, wherein a phase of the error signal when it arrives at the base station is used to derive the correction value.

26. A highly bandwidth-efficient communications system, comprising:

means for forming a synchronization burst at a terrestrial base station, wherein the synchronization burst includes a plurality of tone frequencies arranged in a distinctive orthogonal frequency division multiplexed pattern unique to the base station;

means for transmitting the synchronization burst at a base station reference instant of time, wherein the synchronization burst uniquely identifies the base station to a remote station;

means for receiving an error signal back from the remote station at an instant referenced with respect to a remote station reference instant of time;

means for deriving a correction value from the error signal, related to a relative time error between the base station reference instant of time and the remote station reference instant of time; and

means for transmitting the correction value to the remote station to correct timing at the remote station.

27. The highly bandwidth-efficient communications system of claim 26, wherein the deriving means further comprises:

means for deriving from the error signal a second value related to a relative phase error between the base station and the remote station; and

means for transmitting the second value to the remote station to correct the remote station.

28. The highly bandwidth-efficient communications system of claim 26, wherein the base station is part of a wireless discrete multitone spread spectrum communications system.

29. The highly bandwidth-efficient communications system of claim 26, wherein a time of arrival of the error signal at the base station is used to derive the correction value.

30. The highly bandwidth-efficient communications system of claim 26, wherein a phase of the error signal when it arrives at the base station is used to derive the correction value.

31. A highly bandwidth-efficient communications method, comprising:

receiving at a base station a signal on a reverse link from a remote station, having significant interference;

forming a forward synchronization burst at the base station, including a plurality of tone frequencies arranged in a distinctive orthogonal frequency division multiplexed pattern unique to the base station;

selectively forming at the base station a request signal requesting the remote station to respond with a reverse synchronization burst, including a plurality of tone frequencies arranged in the distinctive orthogonal frequency division multiplexed pattern;

transmitting the forward synchronization burst and the request signal at a base station reference instant of time to the remote station;

receiving at the base station a signal on the reverse link from the remote station the reverse synchronization burst and an error signal at an instant referenced with respect to a remote station reference instant of time;

recognizing at the base station the reverse synchronization burst and deriving a correction value from the error signal, related to a relative time error between the base station reference instant of time and the remote station reference instant of time; and

transmitting the correction value to the remote station to correct the remote station.

32. The highly bandwidth-efficient communications method of claim 31, wherein the deriving further comprises:

deriving from the error signal a second value related to a relative phase error between the base station and the remote station; and

transmitting the second value to the remote station to correct the remote station.

33. The highly bandwidth-efficient communications system of claim 31, wherein the base station is part of a wireless discrete multitone spread spectrum communications system.

34. The highly bandwidth-efficient communications system of claim 31, wherein a time of arrival of the error signal at the base station is used to derive the correction value.

35. The highly bandwidth-efficient communications system of claim 31, wherein a phase of the error signal when it arrives at the base station is used to derive the correction value.

36. A highly bandwidth-efficient communications system, comprising:

means for receiving at a base station a signal on a reverse link from a remote station, having significant interference;

means for forming a forward synchronization burst at the base station, including a plurality of tone frequencies arranged in a distinctive orthogonal frequency division multiplexed pattern unique to the base station;

means for selectively forming at the base station a request signal requesting the remote station to respond with a reverse synchronization burst, including a plurality of tone frequencies arranged in the distinctive orthogonal frequency division multiplexed pattern;

means for transmitting the forward synchronization burst and the request signal at a base station reference instant of time to the remote station;

means for receiving at the base station a signal on the reverse link from the remote station the reverse synchronization burst and an error signal at an instant referenced with respect to a remote station reference instant of time;

means for recognizing at the base station the reverse synchronization burst and deriving a correction value from the error signal, related to a relative time error between the base station reference instant of time and the remote station reference instant of time; and

means for transmitting the correction value to the remote station to correct the remote station.

37. The highly bandwidth-efficient communications system of claim 36, wherein the deriving means further comprises:

means for deriving from the error signal a second value related to a relative phase error between the base station and the remote station; and

means for transmitting the second value to the remote station to correct the remote station.

38. The highly bandwidth-efficient communications system of claim 36, wherein the base station is part of a wireless discrete multitone spread spectrum communications system.

39. The highly bandwidth-efficient communications system of claim 36, wherein a time of arrival of the error signal at the base station is used to derive the correction value.

40. The highly bandwidth-efficient communications system of claim 36, wherein a phase of the error signal when it arrives at the base station is used to derive the correction value.

41. A highly bandwidth-efficient communications method, comprising:

forming a synchronization burst at an antenna element of a base station, including a plurality of tone frequencies arranged in a distinctive orthogonal frequency division multiplexed code pattern unique to the base station;

transmitting the synchronization burst from the antenna element at a base station reference instant of time;

receiving the synchronization burst at a remote station;

recognizing the code pattern of the plurality of tone frequencies as having the base station as the source of the synchronization burst;

transmitting an error signal back to the base station;

deriving from the error signal a correction value; and

transmitting the correction value to the remote station to synchronize the remote station.

42. A highly bandwidth-efficient communications method, comprising:

receiving at a base station a signal on a reverse link from a remote station, having significant interference;

forming a forward synchronization burst at the base station, including a plurality of tone frequencies arranged in a distinctive orthogonal frequency division multiplexed pattern unique to the base station;

selectively forming at the base station a request signal requesting the remote station to respond with a reverse synchronization burst, including a plurality of tone frequencies arranged in the distinctive orthogonal frequency division multiplexed pattern;

transmitting the forward synchronization burst and the request signal to the remote station;

receiving at the base station a signal on the reverse link from the remote station the reverse synchronization burst; and

recognizing at the base station the reverse synchronization burst.

43. A wireless communications method to enable a base station and its remote station to synchronize in a noisy environment, comprising:

receiving at a base station a signal on a reverse link from a remote station, accompanied by significant interference;

forming at the base station a request signal in response to the significant interference, the request signal requesting the remote station to respond with a reverse synchronization burst;

transmitting from the base station a forward synchronization burst and the request signal to the remote station;

preparing a reverse synchronization signal at the remote station in response to the request; and

preemptively transmitting the reverse synchronization signal from the remote station during a time slot in a transmission frame from the remote station to the base station, that would otherwise be occupied by control or traffic signals.

44. The wireless communications method of claim 43, wherein the reverse synchronization burst includes a plurality of tone frequencies arranged in a distinctive orthogonal frequency division multiplexed pattern also used by the base station in the forward synchronization burst.

45. A wireless communications method to enable a base station and its remote station to synchronize in a noisy environment, comprising:

receiving at a base station a signal on a reverse link from a remote, accompanied by significant interference,

forming at the base station a request signal in response to the significant interference, the request signal requesting the remote station to respond with a reverse synchronization burst;

transmitting from the base station a forward synchronization burst and the request signal to the remote station;

preparing a reverse synchronization signal at the remote station in response to the request;

preemptively transmitting the reverse synchronization signal from the remote station during a time slot in a transmission frame from the remote station to the base station, that would otherwise be occupied by control or traffic signals, the reverse synchronization signal having an error;

receiving at the base station the reverse synchronization signal having the error;

deriving a correction value from the error at the base station; and

transmitting the correction value from the base station to the remote station to correct the remote station.

46. The wireless communications method of claim 45, wherein a time of arrival of the reverse synchronization signal at the base station is used to derive the correction value.

47. The wireless communications method of claim 45, wherein a phase of the reverse synchronization signal when it arrives at the base station is used to derive the correction value.

48. The wireless communications method of claim 45, wherein the reverse synchronization burst includes a plurality of tone frequencies arranged in a distinctive orthogonal frequency division multiplexed pattern used by the base station in the forward synchronization burst.

49. A wireless communications method to enable a base station and its remote station to synchronize in a noisy environment, comprising:

receiving at a base station a signal on a reverse link from a remote, accompanied by significant interference,

forming at the base station a request signal in response to the significant interference, the request signal requesting the remote station to respond with a reverse synchronization burst;

transmitting from the base station a forward synchronization burst and the request signal to the remote station, the synchronization burst including a plurality of tone frequencies arranged in a distinctive orthogonal frequency division multiplexed pattern unique to the base station;

preparing a reverse synchronization signal at the remote station in response to the request;

preemptively transmitting the reverse synchronization signal from the remote station during a time slot in a transmission frame from the remote station to the base station, that would otherwise be occupied by control or traffic signals, the reverse synchronization signal having an error;

the reverse synchronization burst includes a plurality of tone frequencies arranged in the distinctive orthogonal frequency division multiplexed pattern used by the base station in the forward synchronization burst;

receiving at the base station the reverse synchronization signal having the error;

deriving a correction value from the error at the base station; and

transmitting the correction value from the base station to the remote station to correct the remote station.

50. The wireless communications method of claim 49, wherein a time of arrival of the reverse synchronization signal at the base station is used to derive the correction value.

51. The wireless communications method of claim 49, wherein a phase of the reverse synchronization signal when it arrives at the base station is used to derive the correction value.