Methods and apparatus for use in obtaining frame synchronization in an OFDM communication system

• US 6,807,147 B1
• Filed: 06/13/2000
• Issued: 10/19/2004
• Est. Priority Date: 06/23/1999
• Status: Expired due to Term

• Current Assignee: AT&T Mobility II LLC (AT&T, Inc.)
• Original Assignee: AT&T Wireless Services Incorporated (AT&T, Inc.)
• Inventors: Hirano, Michael R., McMeekin, Steven E., Heinonen, Jari M.
• Primary Examiner(s): HO, DUC CHI
• Application Number:
  US09/592,961
• Time in Patent Office: 1,589 Days
• Field of Search: 370/208, 370/210, 370/252, 370/350, 370/321, 370/333, 370/347, 370/442, 370/503, 375/293, 375/349, 375/362, 375/368, 455/150.1
• US Class Current: 370/208
• CPC Class Codes: H04L 27/2656 Frame synchronisation, e.g. packet synchronisation, time division duplex [TDD] switching point detection or subframe synchronisation
• CPC Class Codes: H04L 27/2675 Pilot or known symbols

Abstract

An orthogonal frequency division multiplexing (OFDM) communication system includes base units which simultaneously transmit pilot tones in a predetermined pilot tone pattern during one or more time slots of a message slot. OFDM signals are received within a time slot; an OFDM signal level is determined at each frequency of a plurality of pilot tone frequencies; the signal levels and the predetermined pilot tone pattern are correlated; and a pattern detection value based on performing the correlation is generated and stored. This process is repeated for each time slot of a plurality of time slots. This process may be repeated for a plurality of message frame durations to produce a plurality of averaged pattern detection values, following which the plurality of averaged pattern detection values and a predetermined message frame pattern may be correlated. Then, a boundary of a message slot may be detected based on the correlation.

Claims

1. A method for use in obtaining frame synchronization in an Orthogonal Frequency Division Multiplexing (OFDM) communication system, the OFDM communication system having a base unit which transmits a plurality of pilot tones in a first predetermined pattern in one or more time slots of a plurality of time slots, the method comprising:

receiving OFDM signals within a time slot;

determining a signal level in the OFDM signals at each frequency of a plurality of pilot tone frequencies;

comparing the signal levels in the OFDM signals with a first predetermined pattern;

detecting whether a match exists between the signal levels in the OFDM signals and the first predetermined pattern;

repeating the receiving, determining, comparing, and detecting for each time slot of the plurality of time slots; and

identifying a boundary of a message slot based on results of the repeated receiving, determining, comparing, and detecting.

2. The method according to claim 1, wherein the message slot comprises a plurality of time slots.

3. The method according to claim 1, wherein multiple time slots define a message slot and multiple message slots define a message frame.

4. The method according to claim 1, further comprising:

wherein detecting comprises storing a detection result, to thereby store a plurality of detection results associated with the plurality of time slots;

comparing the plurality of detection results with a second predetermined pattern; and

wherein identifying the boundary of the message slot further comprises identifying the boundary based on results of comparing the plurality of detection results with the second predetermined pattern.

5. The method according to claim 4, further comprising:

wherein comparing the plurality of detection results with the second predetermined pattern comprises performing a correlation between the plurality of detection results and the second predetermined pattern.

6. The method according to claim 4, wherein the second predetermined pattern comprises a detected/undetected pattern corresponding to 1111111100000000000000000000000000000000, where 1 is indicative of a detected first predetermined pattern and 0 is indicative of an undetected first predetermined pattern.

7. The method according to claim 1, wherein multiple time slots define a message slot and multiple message slots define a message frame, the method further comprising:

wherein detecting comprises storing a detection result, to thereby store a plurality of detection results associated with the plurality of time slots;

repeating the repeated receiving, determining, comparing, and detecting over multiple message frame durations to generate a plurality of averaged detection results; and

wherein the results used to identify the boundary of the message slot comprise the plurality of averaged detection results.

8. The method according to claim 1, wherein multiple time slots define a message slot and multiple message slots define a message frame, the method further comprising:

wherein detecting comprises storing a detection result, to thereby store a plurality of detection results associated with the plurality of time slots;

repeating the repeated receiving, determining, comparing, and detecting over multiple message frame durations to generate a plurality of averaged detection results; and

comparing the plurality of averaged detection results with a second predetermined pattern; and

wherein identifying the boundary of the message slot further comprises identifying the boundary based on results of comparing the plurality of averaged detection results with the second predetermined pattern.

9. The method according to claim 1, wherein multiple time slots define a message slot and multiple message slots define a message frame, the method further comprising:

wherein detecting comprises storing a detection result, to thereby store a plurality of detection results associated with the plurality of time slots;

repeating the repeated receiving, determining, comparing, and detecting over multiple message frame durations to generate a plurality of averaged detection results; and

performing a correlation between the plurality of averaged detection results and a second predetermined pattern; and

wherein identifying the boundary of the message slot further comprises identifying the boundary based on performing the correlation.

10. The method according to claim 1, wherein the first predetermined pattern comprises an active/inactive pilot tone pattern corresponding to 011100101, where 1 is indicative of an active pilot tone position and 0 is indicative of an inactive pilot tone position.

11. A method for use in obtaining frame synchronization in an Orthogonal Frequency Division Multiplexing (OFDM) communication system, the OFDM communication system including a base unit which transmits a plurality of pilot tones having a first predetermined pattern in one or more time slots of a plurality of time slots, thereby defining a second predetermined pattern of one or more time slots having the first predetermined pattern and one or more other time slots not having the first predetermined pattern, the method comprising:

receiving OFDM signals within a time slot;

determining a signal level in the OFDM signals at each frequency of a plurality of pilot tone frequencies;

comparing the signal levels in the OFDM signals with the first predetermined pattern;

detecting whether a match exists between the signal levels in the OFDM signals and the first predetermined pattern;

storing a detection result;

repeating the receiving, determining, comparing, detecting, and storing for each time slot of a plurality of time slots, to thereby produce a plurality of detection results;

comparing the plurality of detection results and the second predetermined pattern; and

identifying a boundary of a message slot based on results of the comparing.

12. The method according to claim 11, further comprising:

wherein the plurality of pilot tones having the first predetermined pattern are simultaneously broadcasted by each base unit of a plurality of base units.

13. The method according to claim 11, wherein multiple time slots define a message slot and multiple message slots define a message frame.

14. The method according to claim 11, further comprising:

repeating the repeated receiving, determining, comparing, detecting, and storing over multiple message frame durations to generate a plurality of averaged detection results; and

wherein comparing the plurality of detection results with the second predetermined pattern comprises comparing the plurality of averaged detection results with the second predetermined pattern.

15. The method according to claim 11, wherein the first predetermined pattern comprises an active/inactive pilot tone pattern corresponding to 011100101, where 1 is indicative of an active pilot tone position and 0 is indicative of an inactive pilot tone position.

16. The method according to claim 11, wherein the second predetermined pattern comprises a detected/undetected pattern corresponding to 1111111100000000000000000000000000000000, where 1 is indicative of a detected first predetermined pattern and 0 is indicative of an undetected first predetermined pattern.

17. A method for use in obtaining frame synchronization in an Orthogonal Frequency Division Multiplexing (OFDM) communication system, the OFDM communication system including a plurality of base units which simultaneously transmit a plurality of pilot tones in a predetermined pilot tone pattern during some of a plurality of time slots of a message slot but do not simultaneously transmit the predetermined pilot tone pattern during some of the other time slots in the message slot, the method comprising:

receiving OFDM signals within a time slot;

determining a signal level in the OFDM signals at each frequency of a plurality of pilot tone frequencies;

performing a correlation between the signal levels and the predetermined pilot tone pattern;

generating and storing a pattern detection value based on performing the correlation;

repeating the receiving, determining, performing, generating and storing for each time slot of the plurality of time slots;

repeating, for a plurality of message frame durations, the above steps of repeated receiving, determining, performing, and generating to produce a plurality of averaged pattern detection values; and

performing a correlation between the plurality of averaged pattern detection values and a predetermined message frame pattern; and

detecting a boundary of a message slot based on performing the correlation.

18. The method according to claim 17, wherein the predetermined pilot tone pattern comprises an active/inactive pilot tone pattern corresponding to 011100101, where 1 is indicative of an active pilot tone position and 0 is indicative of an inactive pilot tone position.

19. The method according to claim 17, wherein the predetermined message frame pattern corresponds to 1111111100000000000000000000000000000000, where 1 is indicative of a detected predetermined pilot tone pattern and 0 is indicative of an undetected predetermined pilot tone pattern.

20. The method according to claim 17, wherein the predetermined pilot tone pattern comprises a first predetermined pilot tone pattern, and wherein each base unit takes a turn, on a message frame by message frame basis, to broadcast a plurality of pilot tones in a second predetermined pilot tone pattern during the other time slots of the plurality of time slots, the method further comprising:

receiving OFDM signals within a message frame;

determining a signal level in the OFDM signals at each frequency of a plurality of pilot tone frequencies;

repeating the above receiving and determining for each message frame of a plurality of message frames to produce a plurality of signal levels;

identifying a maximum signal level from the plurality of signal levels; and

selecting a message frame for decoding messages based on identifying the maximum signal level.

21. The method according to claim 20, further comprising:

decoding a message within the selected message frame.

22. The method according to claim 20, further comprising:

decoding a message within the selected message frame;

comparing a base unit identifier in the message with a prestored base unit identifier; and

confirming whether the selected message frame is accurate based on comparing the base unit identifier with the prestored base unit identifier.

24. The method according to claim 21, further comprising:

performing fine time synchronization after selecting the message frame for decoding messages.

25. The method according to claim 21, further comprising:

performing at least a coarse time synchronization process and at least a coarse frequency synchronization process prior to the receiving and the determining.

26. The method according to claim 21, further comprising:

generating a rank of the plurality of base units in accordance with the signal levels.

27. The method according to claim 21, further comprising:

generating a rank of the plurality of base units in accordance with the signal levels; and

wherein identifying the frame boundary for decoding messages comprises selecting a first one in the rank.

23. A method for use in obtaining frame synchronization in an Orthogonal Frequency Division Multiplexing (OFDM) communication system having a plurality of base units, each base unit taking a turn in transmitting a plurality of pilot tone signals over a plurality of message frames, the method comprising:

receiving OFDM signals within a message frame;

determining a signal level of a plurality of pilot tones in the OFDM signals;

repeating the receiving and determining for each message frame of a plurality of message frames to produce a plurality of signal levels associated with the plurality of base units;

identifying a maximum signal level from the plurality of signal levels; and

selecting a message frame for decoding messages based on the identified maximum signal level.

28. A method for use in obtaining frame synchronization in an Orthogonal Frequency Division Multiplexing (OFDM) communication system including a plurality of base units, each base unit taking a turn in transmitting a plurality of pilot tone signals over a plurality of message frames, the method comprising:

receiving OFDM signals within a message frame;

determining a signal level of a plurality of pilot tones in the OFDM signals;

repeating the receiving and determining for each message frame of a plurality of message frames to produce a plurality of signal levels;

generating a rank of the plurality of base units in accordance with the signal levels;

repeating the following steps if necessary until a serving base unit is confirmed;

selecting a candidate base unit from the rank;

decoding a message within the message frame associated with the selected candidate base unit;

comparing a base unit identifier in the message and a prestored base unit identifier; and

confirming that the candidate base unit is the serving base unit based on a match between the base unit identifier and the prestored base unit identifier.

29. The method according to claim 28, further comprising:

performing at least a coarse time synchronization process and at least a coarse frequency synchronization process prior to the receiving and the detecting.

30. A method for use in obtaining frame synchronization in an Orthogonal Frequency Division Multiplexing (OFDM) communication system, the OFDM communication system having a plurality of base units which simultaneously broadcast a plurality of pilot tones in a predetermined pilot tone pattern during one or more time slots of a plurality of time slots, the plurality of time slots defining boundaries of a message slot, a plurality of message slots defining boundaries of a message frame, each base unit taking a turn on a message frame by message frame basis to transmit all of the plurality of pilot tones during the other time slots of the plurality of time slots, the method comprising:

receiving OFDM signals within a time slot;

determining a signal level in the OFDM signals at each frequency of a plurality of pilot tone frequencies;

performing a correlation between the signal levels and the predetermined pilot tone pattern;

generating and storing a pattern detection value based on performing the correlation;

repeating the receiving, determining, performing, and generating and storing for each time slot of a plurality of time slots over a message slot duration;

repeating, for a plurality of message frame durations, the above steps of repeated receiving, determining, performing, and generating to produce a plurality of averaged pattern detection values;

performing a correlation between the plurality of averaged pattern detection values and a predetermined message frame pattern;

detecting a boundary of a message slot based on performing the correlation;

receiving OFDM signals within one of the other time slots;

determining a signal level of a plurality of pilot tones in the OFDM signals;

repeating the receiving and determining for each of the other time slots within the message frame to produce a plurality of signal levels;

generating a rank of the plurality of base units in accordance with the plurality of signal levels;

repeating the following steps if necessary until a serving base unit is confirmed;

selecting a candidate base unit from the rank based on its associated signal level;

decoding a message within a message frame associated with the selected candidate base unit;

comparing a base unit identifier in the message and a prestored base unit identifier; and

confirming that the candidate base unit is the serving base unit based on a match between the base unit identifier and the prestored base unit identifier.