Methods and apparatus for performing timing synchronization with base stations

• US 20060281476A1
• Filed: 07/18/2005
• Published: 12/14/2006
• Est. Priority Date: 06/13/2005
• Status: Active Grant

• Current Assignee: Qualcomm, Inc.
• Original Assignee: Qualcomm, Inc.
• Inventors: Li, Junyi, Larola, Rajiv, Lane, Frank A.
• Granted Patent: US 7,574,224 B2
• Time in Patent Office: Days
• US Class Current: 455/502
• CPC Class Codes: H04B 7/185 Space-based or airborne stations; Stations for satellite systemsH04B7/204 takes precedence
• CPC Class Codes: H04B 7/2125 Synchronisation
• CPC Class Codes: H04L 27/261 Details of reference signals
• CPC Class Codes: H04L 27/2655 Synchronisation arrangements
• CPC Class Codes: H04L 27/2662 Symbol synchronisation
• CPC Class Codes: H04L 5/0007 the frequencies being orthogonal, e.g. OFDM(A), DMT
• CPC Class Codes: H04L 5/0051 of dedicated pilots, i.e. pilots destined for a single user or terminal
• CPC Class Codes: H04W 48/12 using downlink control channel
• CPC Class Codes: H04W 56/0005 synchronizing of arrival of multiple uplinks
• CPC Class Codes: H04W 74/04 Scheduled access hybrid access H04W74/02
• CPC Class Codes: H04W 84/06 Airborne or Satellite Networks space-based or airborne stations H04B7/185
• CPC Class Codes: H04W 88/08 Access point devices

Abstract

A wireless terminal using OFDM signaling supporting both terrestrial and satellite base station connectivity operates using conventional access probe signaling in a first mode of operation to establish a timing synchronized wireless link with a terrestrial base station. In a second mode of operation, used to establish a timing synchronized wireless link with a satellite base station, a slightly modified access protocol is employed. The round trip signaling time and timing ambiguity between a wireless terminal and a satellite base station is substantially greater than with a terrestrial base station. The modified access protocol uses coding of access probe signals to uniquely identify a superslot index within a beaconslot. The modified protocol uses multiple access probes with different timing offsets to further resolve timing ambiguity and allows the satellite base station access monitoring interval to remain small in duration. Terrestrial base station location/connection information is used to estimate initial timing.

Claims

1. A method of operating a communications device for use in a communications system where beacon time slots occur on a periodic basis, a beacon signal being transmitted by a base station during each beacon time slot according to a periodic downlink timing structure, said downlink timing structure including a plurality of superslots within each beacon slot, the individual superslots within a beacon slot being identifiable through the use of a superslot index, each superslot including a plurality of symbol transmission time periods, the method comprising:

receiving at least one beacon signal;

processing the received beacon signal to determine a downlink timing reference point, superslots occurring within a beacon slot having a predetermined relationship to the determined downlink timing reference point;

transmitting a first access probe at a time corresponding to a first timing offset relative to the start of a super slot in a beacon slot;

transmitting a second access probe at a time corresponding to a second timing offset relative to the start of a super slot, said second access probe being transmitted at a point in time, which is less than the longer of i) twice the time required for a transmitted signal to travel from the communications device to the base station and ii) the duration of a superslot, from the point in time at which the first access probe was transmitted;

monitoring to determine if a response to one of the transmitted access probes was received from the base station; and

if it is determined that a response was received from the base station, performing a transmission timing adjustment as a function of information included in said response.

2. The method of claim 1, wherein the second access probe is transmitted before the response to the first access probe can be received from the base station.

3. The method of claim 1, wherein a maximum timing ambiguity is less than the duration of a superslot and where the time between the transmission of the first and second access probes is less than the duration of a superslot.

4. The method of claim 1, further comprising:

wherein the first and second access probes are transmitted at intervals from one another less than or equal to an access interval during which the base station will respond to received access probes.

5. The method of claim 4, further comprising:

coding in at least one of the first and second access probes an access probe identifier.

6. The method of claim 5, wherein the received response includes information identifying the access probe to which the response corresponds, and

wherein performing a transmission timing adjustment as a function of information included in said response includes:

determining an amount of a timing adjustment to be performed from timing correction information received from the base station and information about the transmission time of identified probe relative to the determined downlink timing reference point.

7. The method of claim 1, wherein said first and second access probes are OFDM signals;

and

wherein the method further comprises;

coding in at least one of the first and second access probes a wireless terminal identifier.

8. The method of claim 7,

wherein transmitting the first access probe includes transmitting said first access probe signal into a wireless communications channel;

and

wherein said at least one beacon signal is a single tone signal having a duration which is less then the duration of 4 OFDM symbol transmission time periods.

9. A communications device for use in a communications system where beacon time slots occur on a periodic basis, a beacon signal being transmitted by a base station during each beacon time slot according to a periodic downlink timing structure, said downlink timing structure including a plurality of superslots within each beacon slot, the individual superslots within a beacon slot being identifiable through the use of a superslot index, each superslot including a plurality of symbol transmission time periods, the method comprising:

a receiver for receiving at least one beacon signal;

a processing module for processing received beacon signals to determine, based on at least one received beacon signal, a downlink timing reference point, superslots occurring within a beacon slot having a predetermined relationship to the determined downlink timing reference point;

a transmitter module for transmitting a first access probe at a time corresponding to a first timing offset relative to the start of a super slot in a beacon slot and for transmitting a second access probe at a time corresponding to a second timing offset relative to the start of a super slot, said second access probe being transmitted at a point in time, which is less than the larger of i) twice the time required for a transmitted signal to travel from the communications device to the base station and ii) the duration of a downlink superslot, from the point in time at which the first access probe was transmitted;

a monitoring module for determining if a response to one of the transmitted access probes was received from the base station; and

a timing adjustment module for performing a transmission timing adjustment as a function of information included in a response to a transmitted access probe.

10. The device of claim 9, wherein a maximum timing ambiguity is less than the duration of a superslot and where the time between the transmission of the first and second access probes is less than the duration of a superslot.

11. The device of claim 9, further comprising:

wherein said transmitter module includes means for transmitting the first and second access probes are transmitted at intervals from one another less than or equal to an access interval during which the base station will respond to received access probes.

12. The device of claim 11, further comprising:

a coding module for coding in at least one of the first and second access probes an access probe identifier.

13. The device of claim 9, wherein said transmitter is an OFDM transmitter and wherein said first access probe is an OFDM signal.

14. The device of claim 13, wherein said receiver is an OFDM receiver.

27. The method of claim 14, wherein performing a transmission timing adjustment includes determining a timing adjustment amount from the location in a beaconslot of the superslot which included the received access probe response and information about the time the access probe signal was transmitted.

28. The method of claim 27, wherein said information indicating the time the access probe signal was transmitted indicates the index of a downlink superslot as determined from said determined reference point in time in which said access probe, to which a response was received, was transmitted.

15. A method of operating a communications device for use in a communications system where beacon time slots occur on a periodic basis, a beacon signal being transmitted by a satellite base station during each beacon time slot according to a periodic downlink timing structure, said downlink timing structure including a plurality of superslots within each beacon slot, the individual superslots within a beacon slot being identifiable through the use of a superslot index, each superslot including a plurality of symbol transmission time periods, the method comprising:

transmitting an access probe signal to said base station;

receiving a response to the access probe signal from the base station, the response including information indicating at least one of;

i) an amount of an indicated main superslot timing offset correction, the amount of the main superslot timing offset correction being an integer multiple of a superslot time period;

ii) a superslot identifier indicating the position of a superslot within a beacon slot during which the base station received the access probe signal to which the received response corresponds;

iii) an identifier that identifiers the communications device which transmitted the access probe signal to which the received response corresponds and iv) an identifier that identifies the access probe to which the response being responding to; and

performing a transmission timing adjustment as a function of the information included in the received response.

17. The method of claim 15, wherein said received response is transmitted to the communications device at a fixed predetermined superslot time offset from the time it was received by the base station.

18. The method of claim 15, further comprising:

prior to performing said transmitting, receiving and processing steps,

receiving at least one beacon signal from the satellite base station;

processing the received beacon signal to determine a downlink timing reference point, superslots occurring within a beacon slot having a predetermined relationship to the determined downlink timing reference point.

19. The method of claim 18, wherein said access probe signal is an OFDM signal.

21. The method of claim 18,

wherein the received response from the base station includes a superslot identifier indicating the position of the superslot within a beacon slot during which the base station received the access probe signal, and

wherein performing a transmission timing adjustment as a function of information included in said response includes:

determining a main superslot timing offset from the superslot identifier included in the received response and information indicating the superslot position, relative to said downlink timing reference point, within a beacon slot at which said access probe was transmitted, the main superslot timing offset being an integer multiple of the duration of a superslot.

22. The method of claim 21,

wherein the received response further includes sub-superslot timing correction information including a sub-superslot time offset;

and

wherein performing a transmission timing adjustment includes adjusting said transmission timing by an amount corresponding to the sum of the determined main superslot timing offset and said sub-superslot time offset.

23. The method of claim 18,

wherein the received response from the base station includes information indicating a main superslot timing offset which is an integer multiple of the duration of a superslot and a sub-superslot time offset which is a time offset that is less than the duration of a superslot.

24. The method of claim 23, wherein said main superslot timing offset and said sub-superslot time offset are communicated as part of a single coded value.

25. The method of claim 23, wherein said main superslot timing offset and said sub-superslot time offset are communicated as two separately coded values.

26. The method of claim 23,

wherein performing a transmission timing adjustment includes adjusting said transmission timing by an amount correspond to the sum of the main superslot timing offset and said sub-superslot time offset.

29. The method of claim 15, wherein performing a transmission timing adjustment includes determining a main timing adjustment amount from information received from said base station and information indicating a superslot index, determined by said communications device to be the index within a beacon slot at which the probe being responded to was transmitted.

30. The method of claim 29, wherein said main timing adjustment is a timing adjustment corresponding to an integer multiple of a superslot time period, determining the main timing adjustment further being performed as a function of a fixed superslot delay between the time the base station receives an access request and transmits a response to the received access request.

31. The method of claim 29, wherein the main timing adjustment is determined according to the following equation:

main timing adjustment=2×

(index of superslot in which the response to the access probe was received−

index of superslot determined by the communications device in which the access probe was transmitted)−

a fixed superslot delay) times the period of a superslot.

32. The method of claim 31 wherein performing a transmission timing adjustment further includes:

adjusting said timing by a sub-superslot offset specified by said base station in said response to the transmitted access request.

16. The method of 15, wherein said identifier of the received access probe includes information that identifies the downlink superslot in which the communication device determined the probe was transmitted.

20. The method of claiml9, wherein said beacon signal is a single tone signal.

33. A communications device for use in a communications system where beacon time slots occur on a periodic basis, a beacon signal being transmitted by a satellite base station during each beacon time slot according to a periodic downlink timing structure, said downlink timing structure including a plurality of superslots within each beacon slot, the individual superslots within a beacon slot being identifiable through the use of a superslot index, each superslot including a plurality of symbol transmission time periods, the method comprising:

a transmitter module for transmitting an access probe signal to said base station;

a receiver module for receiving a response to the access probe signal from the base station, the response including information indicating at least one of;

i) an amount of an indicated main superslot timing offset correction, the amount of the main superslot timing offset correction being an integer multiple of a superslot time period;

ii) a superslot identifier indicating the position of a superslot within a beacon slot during which the base station received the access probe signal to which the received response corresponds, iii) an identifier that identifiers the communications device which transmitted the access probe signal to which the received response corresponds and iv) an identifier that identifies the access probe to which the response being responding to; and

a transmission timing adjustment module for performing a transmission timing adjustment as a function of the information included in the received response.

34. A method of operating a communications terminal in an OFDM system including terrestrial and satellite base stations which use OFDM signals for both uplink and downlink signaling, the method comprising:

determining if a base station to which said wireless terminal is seeking to send OFDM uplink signals is a satellite base station or a terrestrial base station;

performing a first uplink timing synchronization process if it is determined that said base station is a satellite base station, said first timing synchronization processes; and

performing a second uplink timing synchronization process if it is determined that said base station is a terrestrial base station, said second timing synchronization process being different from said first timing synchronization process.

35. The method of claim 34, where both said terrestrial and satellite base stations have a downlink timing structure that includes a plurality of superslots which recur in a periodic manner, each superslot including a plurality of OFDM symbol transmission time periods.

36. The method of claim 35, wherein said first uplink timing synchronization process supports the communication of an uplink timing correction signal to the communications terminal which indicates an uplink transmission timing synchronization correction exceeding the duration of a downlink superslot.

37. The method of claim 35, wherein performing the first uplink timing synchronization process includes:

transmitting an access probe signal to said satellite base station;

receiving a response to the access probe signal from the base station, the response including information indicating at least one of;

i) an amount of an indicated main superslot timing offset correction, the amount of the main superslot timing offset correction being an integer multiple of a superslot time period; and

ii) a superslot identifier indicating the position of a superslot within a beacon slot during which the base station received the access probe signal to which the received response corresponds; and

performing an uplink transmission timing adjustment as a function of the information included in the received response.

38. The method of claim 37, wherein performing the second uplink timing synchronization process includes:

transmitting an access probe signal to said terrestrial base station;

receiving a response to the access probe signal from the terrestrial base station, the response including information indicating a timing correction which is less than the duration of a superslot; and

performing a transmission timing adjustment as a function of the information included in the response received from the terrestrial base station, the timing adjustment involving changing transmitter timing by an amount less then the duration of a superslot.

39. A wireless terminal for use in communicating with base stations in a in a communications system including a plurality of base stations, the method comprising:

storing base station location information in memory;

receiving a signal from a first base station;

determining a time at which to transmit an access signal to a second base station as a function of stored location information indicating the location of said first base station from which said wireless terminal received said signal; and

transmitting said access signal at the determined time.

40. The method of claim 39, wherein determining a time at which to transmit an access signal to a second base station, further includes:

receiving a signal from the second base station; and

determining from the received signal a timing reference point.

41. The method of claim 40, wherein determining a time at which to transmit an access signal includes calculating an uplink timing correction from said first base station position information and stored information indicating the position of said second base station.

42. The method of claim 40, wherein determining a time at which to transmit an access signal to a second base station, further includes:

said time at which to transmit an access signal is a time determined relative to the determined reference point using said uplink timing correction information, said time at which to transmit an access signal being determined so that the transmitted access signal will be received at the second base station during an access interval in which received access signals are processed by said second base station.

43. The method of claim 42, wherein said first base station is a terrestrial base station and said second base station is a satellite base station.

44. The method of claim 43, wherein said access signal is an OFDM signal and wherein said signal received from said second base station is a beacon signal.

45. The method of claim 43, wherein said first and second base stations each use a periodic downlink timing structure, said downlink timing structure including beacon slots and superslots, a plurality of superslots occurring within each beacon slot.