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

• US 6,700,866 B1
• Filed: 06/14/2000
• Issued: 03/02/2004
• Est. Priority Date: 06/23/1999
• Status: Expired due to Fees

• 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., Warner, William H., Savola, Reijo
• Primary Examiner(s): Chin, Wellington
• Assistant Examiner(s): Mais, Mark A
• Application Number:
  US09/594,890
• Time in Patent Office: 1,357 Days
• Field of Search: 370/203, 370/208, 370/491, 370/500, 370/516, 375/344, 375/362, 375/371, 455/21, 455/136, 455/138, 455/139, 455/150.1, 455/179.1, 455/182.2, 455/182.3, 455/192.1, 455/192.2, 455/192.3, 455/219, 455/234.1, 455/237.1, 455/239.1, 455/240.1, 455/245.1, 455/247.1, 455/251.1
• US Class Current: 370/208
• CPC Class Codes: H04L 27/2659 Coarse or integer frequency offset determination and synchronisation
• CPC Class Codes: H04L 27/266 Fine or fractional frequency offset determination and synchronisation
• CPC Class Codes: H04L 27/2675 Pilot or known symbols

Abstract

An iterative method involves performing a coarse frequency correction process which adjusts receiver frequency so that a pilot tone signal is within a predetermined frequency range and, after performing the coarse frequency correction process, performing a fine frequency correction process which adjusts receiver frequency so that the pilot tone signal is substantially aligned with a pilot tone reference within the predetermined frequency range. By performing these processes, the receiver frequency may be adjusted so that the alias pilot tone signal is substantially aligned with the pilot tone reference and the pilot tone signal is undesirably shifted outside the predetermined frequency range. To eliminate this condition, the method further involves performing the coarse frequency correction process again and, after performing the coarse frequency correction process again, performing the fine frequency correction process again. By performing the coarse frequency correction process again, receiver frequency is adjusted so that the pilot tone signal is within both the predetermined frequency range and the Nyquist sampling frequency range. By performing the fine frequency correction process again, receiver frequency is adjusted so that the pilot tone signal is substantially aligned with the pilot tone reference.

Claims

1. A method for use in obtaining frequency synchronization in an Orthogonal Frequency Division Multiplexed (OFDM) communication system, the method comprising:

receiving OFDM communication signals;

adjusting receiver frequency so that an alias pilot tone signal is substantially aligned with a pilot tone reference and a pilot tone signal is shifted outside a predetermined frequency range;

performing a coarse frequency correction process which is operative to adjust the receiver frequency so that the pilot tone signal is within the predetermined frequency range; and

performing a fine frequency correction process which is operative to adjust the receiver frequency so that the pilot tone signal is substantially aligned with the pilot tone reference within the predetermined frequency range.

2. The method according to claim 1, wherein adjusting the receiver frequency further comprises:

performing the coarse frequency correction process, such that the pilot tone signal is within the predetermined frequency range but outside a Nyquist sampling frequency range to thereby cause the alias pilot tone signal to be within the Nyquist sampling frequency range; and

performing the fine frequency correction process, such that the alias pilot tone signal is substantially aligned with the pilot tone reference and the pilot tone signal is shifted outside the predetermined frequency range.

3. The method according to claim 1, wherein the coarse frequency correction process comprises:

generating a plurality of tone values for a plurality of tone bins, the plurality of tone bins including a first set of tone bins assigned to a first frequency range and a second set of tone bins assigned to a second frequency range, the first and second frequency ranges corresponding to lower and upper edge portions of a frequency band of interest;

performing complex conjugate multiplication between the tone values of the first and the second sets of tone bins;

identifying a maximum value from results of the complex conjugate multiplication; and

shifting receiver frequency based on a location of the maximum value relative to a predetermined pilot tone location.

4. The method according to claim 3, further comprising:

taking absolute values of results from the complex conjugate multiplication prior to identifying the maximum value.

5. The method according to claim 3, further comprising:

generating a first set of tone values based on tones received in a current time slot;

retrieving a second set of tone values generated based on tones received in a previous time slot; and

performing complex conjugate multiplication between the first and the second sets of tone values generated from the previous and current time slots, thereby suppressing tones that vary in phase over time.

6. The method according to claim 1, wherein the fine frequency correction process comprises:

receiving, in a first time slot, a first set of tones associated with a frequency range;

computing a first set of tone values based on the first set of tones;

receiving, in a second time slot, a second set of tones associated with the frequency range, the first and the second time slots being separated by a difference in time;

computing a second set of tone values based on the second set of tones;

performing complex conjugate multiplication between the first and the second set of tone values;

performing an arctangent function on results from the complex conjugate multiplication to compute a difference in phase between the first and the second set of tones; and

computing a difference in frequency based on a quotient of the difference in phase over the difference in time.

7. The method according to claim 6, further comprising:

adjusting receiver frequency in accordance with a frequency adjustment signal that varies in accordance with the computed difference in frequency.

8. The method according to claim 6, further comprising:

wherein performing the complex conjugate multiplication results in a plurality of conjugated values; and

summing the plurality of conjugated values to thereby provide the results from the complex conjugate multiplication used in performing the arctangent function.

9. The method according to claim 2, further comprising:

wherein the coarse frequency correction process comprises;

generating a plurality of tone values for a plurality of tone bins, the plurality of tone bins including a first set of tone bins assigned to a first frequency range and a second set of tone bins assigned to a second frequency range, the first and second frequency ranges corresponding to lower and upper edge portions of a frequency band of interest;

performing complex conjugate multiplication between the tone values of the first and the second sets of tone bins;

identifying a maximum value from results of the complex conjugate multiplication;

shifting receiver frequency based on a location of the maximum value relative to a predetermined pilot tone location;

wherein the fine frequency correction process comprises;

receiving, in a first time slot, a first set of tones associated with a frequency range;

computing a first set of tone values based on the first set of tones;

receiving, in a second time slot, a second set of tones associated with the frequency range, the first and the second time slots being separated by a difference in time;

computing a second set of tone values based on the second set of tones;

performing complex conjugate multiplication between the first and the second set of tone values;

performing an arctangent function on results from the complex conjugate multiplication to compute a difference in phase between the first and the second set of tones;

computing a difference in frequency based on a quotient of the difference in phase over the difference in time; and

shifting the receiver frequency in accordance with the computed difference in frequency.

10. A digital signal processing apparatus for use in obtaining frequency synchronization in an Orthogonal Frequency Division Multiplexed (OFDM) communication system, the digital signal processing apparatus comprising:

memory;

processor instructions embedded in said memory and executable for;

performing a coarse frequency correction process which is operative to adjust receiver frequency so that a pilot tone signal is within a predetermined frequency range, wherein receiver frequency is adjusted so that the pilot tone signal is within the predetermined frequency range but outside a Nyquist sampling frequency range to thereby cause an alias pilot tone signal to be within the Nyquist sampling frequency range;

after performing the coarse frequency correction process, performing a fine frequency correction process which is operative to adjust receiver frequency so that the pilot tone signal is substantially aligned with a pilot tone reference within the predetermined frequency range, wherein receiver frequency is adjusted so that the alias pilot tone signal is substantially aligned with the pilot tone reference and the pilot tone signal is shifted outside the predetermined frequency range;

after performing the coarse and the fine frequency correction processes, performing the coarse frequency correction process again, wherein receiver frequency is adjusted so that the pilot tone signal is within both the predetermined frequency range and the Nyquist sampling frequency range; and

after performing the coarse frequency correction process again, performing the fine frequency correction process again, and wherein receiver frequency is adjusted so that the pilot tone signal is substantially aligned with the pilot tone reference.

11. The digital signal processing apparatus according to claim 10 wherein, for performing the coarse frequency correction process, said processor instructions are further executable for:

generating a plurality of tone values for a plurality of tone bins, the plurality of tone bins including a first set of tone bins assigned to a first frequency range and a second set of tone bins assigned to a second frequency range, the first and the second frequency ranges corresponding to lower and upper edge portions of a frequency band of interest;

performing complex conjugate multiplication between the tone values of the first and the second sets of tone bins;

identifying a maximum value from results of the complex conjugate multiplication; and

shifting receiver frequency based on a location of the maximum value relative to a predetermined pilot tone location.

12. The digital signal processing apparatus according to claim 11, wherein for shifting the receiver frequency, said processor instructions are further executable for:

adjusting an input voltage of a voltage-controlled oscillator (VCO) based on a difference between the location of the maximum value and the predetermined pilot tone location.

13. The digital signal processing apparatus according to claim 11, wherein said processor instructions are further executable for:

taking absolute values of results from the complex conjugate multiplication prior to identifying the maximum value.

14. The digital signal processing apparatus according to claim 11, wherein for generating the plurality of tone values for the plurality of tone bins, said processor instructions are further executable for

generating a first set of tone values based on tones received in a current time slot;

retrieving a second set of tone values generated based on tones received in a previous time slot; and

performing complex conjugate multiplication between the first and the second sets of tone values generated from the previous and current time slots, thereby suppressing tones that vary in phase over time.

15. The digital signal processing apparatus according to claim 10 wherein, for performing the fine frequency correction process, said processor instructions are further executable for:

receiving, in a first time slot, a first set of tones associated with a frequency range;

computing a first set of tone values based on the first set of tones;

receiving, in a second time slot, a second set of tones associated with the frequency range, the first and the second time slots being separated by a difference in time;

computing a second set of tone values based on the second set of tones;

performing complex conjugate multiplication between the first and the second set of tone values;

performing an arctangent function on results from the complex conjugate multiplication to compute a difference in phase between the first and the second set of tones; and

computing a difference in frequency based on a quotient of the difference in phase over the difference in time.

16. The digital signal processing apparatus according to claim 15, wherein said processor instructions are further executable for:

adjusting receiver frequency in accordance with the computed difference in frequency.

17. The digital signal processing apparatus according to claim 15, wherein performing the complex conjugate multiplication results in a plurality of conjugated values, and wherein for performing the fine frequency correction process, said processor instructions are further executable for:

summing the plurality of conjugated values to thereby provide the results from the complex conjugate multiplication used in performing the arctangent function.

18. The digital signal processing apparatus according to claim 10, further comprising:

wherein, for performing the coarse frequency correction process, said processor instructions are further executable for;

generating a plurality of tone values for a plurality of tone bins, the plurality of tone bins including a first set of tone bins assigned to a first frequency range and a second set of tone bins assigned to a second frequency range, the first and the second frequency ranges corresponding to lower and upper edge portions of a frequency band of interest;

performing complex conjugate multiplication between the tone values of the first and the second sets of tone bins;

identifying a maximum value from results of the complex conjugate multiplication;

shifting receiver frequency based on a difference between the location of the maximum value and the predetermined pilot tone location;

wherein, for performing the coarse frequency correction process, said processor instructions are further executable for;

receiving, in a first time slot, a first set of tones associated with a frequency range;

computing a first set of tone values based on the first set of tones;

receiving, in a second time slot, a second set of tones associated with the frequency range, the first and the second time slots being separated by a difference in time;

computing a second set of tone values based on the second set of tones;

performing complex conjugate multiplication between the first and the second set of tone values;

performing an arctangent function on results from the complex conjugate multiplication to compute a difference in phase between the first and the second set of tones; and

computing a difference in frequency based on a quotient of the difference in phase over the difference in time.

19. A method for use in obtaining frequency synchronization in an Orthogonal Frequency Division Multiplexed (OFDM) communication system, where receiver frequency is adjusted such that an alias pilot tone signal is substantially aligned with a pilot tone reference and a pilot tone signal is shifted outside a predetermined frequency range, the method comprising:

performing a coarse frequency correction process which is operative to adjust the receiver frequency so that the pilot tone signal is within the predetermined frequency range;

performing a fine frequency correction process which is operative to adjust the receiver frequency so that the pilot tone signal is substantially aligned with the pilot tone reference within the predetermined frequency range;

wherein the fine frequency correction process further comprises the steps of;

receiving, in a first time slot, a first set of tones associated with a frequency range;

computing a first set of tone values based on the first set of tones;

receiving, in a second time slot, a second set of tones associated with the frequency range, the first and the second time slots being separated by a difference in time;

computing a second set of tone values based on the second set of tones;

performing complex conjugate multiplication between the first and the second set of tone values;

performing an arctangent function on results from the complex conjugate multiplication to compute a difference in phase between the first and the second set of tones;

computing a difference in frequency based on a quotient of the difference in phase over the difference in time; and

adjusting the receiver frequency in accordance with the computed difference in frequency.

20. The method according to claim 19, wherein the coarse frequency correction process is operative to adjust the receiver frequency by:

generating a plurality of tone values for a plurality of tone bins, the plurality of tone bins including a first set of tone bins assigned to a first frequency range and a second set of tone bins assigned to a second frequency range, the first and the second frequency ranges corresponding to lower and upper edge portions of a frequency band of interest;

performing complex conjugate multiplication between the tone values of the first and the second sets of tone bins;

identifying a maximum value from results of the complex conjugate multiplication; and

shifting receiver frequency based on a difference between the location of the maximum value and the predetermined pilot tone location.