Adaptive frequency correction in a wireless communications system, such as for GSM and IS54

• US 6,522,696 B1
• Filed: 04/11/1997
• Issued: 02/18/2003
• Est. Priority Date: 04/11/1997
• Status: Expired due to Fees

• Current Assignee: Avago Technologies General IP PTE Limited (Broadcom, Inc.)
• Original Assignee: Agere Systems Incorporated (Broadcom, Inc.)
• Inventors: Mobin, Mohammad Shafiul, Mondal, Kalyan
• Primary Examiner(s): Chin, Stephen
• Assistant Examiner(s): GHAYOUR, MOHAMMAD H
• Application Number:
  US08/832,719
• Time in Patent Office: 2,139 Days
• Field of Search: 375/262, 375/326, 375/327, 375/344, 375/346, 375/371, 375/373
• US Class Current: 375/262
• CPC Class Codes: H04L 1/20 using signal quality detector
• CPC Class Codes: H04L 2025/03407 Continuous phase
• CPC Class Codes: H04L 2027/0028 at passband only
• CPC Class Codes: H04L 2027/0065 Frequency error detectors H04L2027/0067 takes precedence
• CPC Class Codes: H04L 2027/0095 in a preamble or similar structure
• CPC Class Codes: H04L 25/03273 with carrier recovery circuitry

Abstract

In accordance with one embodiment of the invention, a method for compensating frequency offset errors in a communication system comprising the steps of receiving a plurality of signal bursts that correspond to a plurality of transmitted signal bursts. A channel estimation is performed based on at least the predetermined information contained in the transmitted signal bursts. For each received signal sample, a Viterbi decoding is performed using the channel estimations. A final updated Viterbi state cost is calculated for each received signal. A good equalizer quality signal is provided when the final updated Viterbi state cost is below a given threshold. The transmitted signal bursts are then reconstructed from the receive signal bursts, by applying the channel estimation to reencode the decoded signal bursts retrieved from the Viterbi decoder. The quality of the channel estimation is determined based on a given threshold of bit error rate in a plurality of received signal bursts. The frequency offset error is updated in accordance with the reconstructed transmitted signals, when the quality of the channel is such that the bit error rate in the received signal bursts is below the given threshold and, when the good equalizer quality signal is provided. Each received signal is adjusted in accordance with the updated frequency offset error.

Claims

1. A method for reducing frequency offset errors in a communications system, said method comprising the steps of:

receiving a plurality of signal bursts, each signal burst containing a plurality of signals;

decoding said plurality of received signal bursts by employing an equalizer;

generating a plurality of burst quality indication signals each burst quality indication signal corresponding to the bit error rate in each of said plurality of received signal bursts;

generating a plurality of residual frequency offset signals, each of said plurality of generated residual frequency offset signals corresponding to one of said plurality of received signal bursts;

accumulating said plurality of residual frequency offset signals having acceptable burst quality indication signals;

acquiring a rotation frequency offset signal based on said plurality of accumulated residual frequency offset signals; and

adjusting the phase of each of said plurality of signals in said plurality of received signal bursts in accordance with said rotation frequency offset signal.

2. A method for reducing frequency offset errors in a communications system, said method comprising the steps of:

receiving a plurality of signal bursts, each signal burst containing a plurality of signals;

performing a channel estimation based on predetermined signal information contained in said plurality of signal bursts;

decoding said plurality of received signal bursts by employing a first Viterbi decoder in accordance with said channel estimation to produce first Viterbi decoded signals; and

generating a burst quality indication signal in accordance with updated cost generated by said first Viterbi decoder for decoding a signal burst;

generating a plurality of residual frequency offset signals, each of said plurality of generated residual frequency offset signals corresponding to one of said plurality of received signal bursts;

accumulating a plurality of residual frequency offset signals having acceptable burst quality indication signals;

acquiring a rotation frequency offset signal based on said plurality of accumulated residual frequency offset signals; and

adjusting the phase of each of said plurality of signals in said plurality of received signal bursts in accordance with said rotation frequency offset signal.

3. The method of claim 2, wherein said step of generating a plurality of residual frequency offset signals further comprises the steps of:

re-encoding said first Viterbi decoded signals in accordance with said channel estimation so as to provided re-encoded signals;

multiplying said plurality of re-encoded signals with signals provided to said equalizer to generate said plurality of residual frequency offset signals.

4. The method of claim 3 further comprising the steps of:

channel decoding a block of said first Viterbi decoded signals;

channel reencoding said block that was channel decoded of said first Viterbi decoded signals; and

comparing said block that was channel reencoded with said first Viterbi decoded signals so as to generate a channel quality indication signal.

5. The method of claim 4 further comprising a training mode and a tracking mode, said training mode further comprises the steps of:

determining a course frequency offset signal by employing a synchronizer, in response to a plurality of predetermined known signals;

accumulating said plurality of residual frequency offset signals over a predetermined number of received synchronization bursts;

said tracking mode further comprising the steps of;

measuring the average value of a plurality of accumulated residual frequency offset signals based on said accumulating step to obtain an average value;

accumulating residual frequency offset signals over a plurality of burst signals, only when said burst quality indication signal is smaller than a first predetermined threshold value, and a plurality of channel quality signals is smaller than a second predetermined threshold value and said average value is less than a third predetermined threshold value; and

updating said rotation frequency offset signal in accordance with said plurality of accumulated residual frequency offset signals.

6. The method of claim 5 wherein said method further comprises the step of determining an initial frequency offset signal, said determining step further comprising the steps of:

derotating a plurality of predetermined received signals to produce derotated signals;

determining plurality of phase differences between consecutive derotated signals; and

calculating the average value of the phase differences over the plurality to obtain said initial frequency offset signal.

7. The method according to claim 6, wherein said plurality of predetermined received signals comprise “

zero”

bits.

8. A method for reducing frequency offset errors in a communications system, said method comprising the steps of:

receiving a plurality of signal bursts corresponding to a plurality of transmitted signal bursts;

decoding a plurality of received signal bursts by employing an equalizer so as to generate decoded signal bursts, said equalizer further generating a burst quality indication signal corresponding to the bit error rate in each received signal burst;

channel decoding said decoded signal bursts by employing a first Viterbi decoder, and generating a channel quality indication signal corresponding to the bit error rate in a plurality of said signal bursts;

generating a plurality of residual frequency offset signals;

accumulating residual frequency offset signals corresponding to signal bursts having acceptable burst quality indication signals, and acceptable channel quality indication signals;

acquiring a rotation frequency offset signal based on a plurality of accumulated residual frequency offset signals; and

adjusting the phase of said plurality of received signal bursts in accordance with said rotation frequency offset signal.

9. The method according to claim 8, wherein said decoding step further comprises the steps of:

performing a channel estimation based on predetermined signal information contained in said plurality of transmitted signal bursts;

Viterbi decoding said plurality of received signal bursts by employing a second Viterbi decoder in accordance with said channel estimation to produce decoded signal bursts; and

generating a burst quality signal in accordance with updated cost generated by said second Viterbi decoder for decoding a signal burst.

10. The method according to claim 9, further comprising a training mode and a tracking mode, said training mode further comprises the steps of:

determining an initial frequency offset signal by employing a synchronizer, in response to a plurality of predetermined known signals;

updating said rotation frequency offset signal in accordance with said initial frequency offset signal;

accumulating said residual frequency offset signals over a predetermined number of received signals, when said channel quality indication signal is within an acceptable predetermined value;

said tracking mode further comprising the steps of;

measuring an average value of a plurality of accumulated residual frequency offset signals;

accumulating residual frequency offset signals over a plurality of burst signals, when a burst quality signal is smaller than a first predetermined threshold value, and said channel quality indication signal is smaller than a second predetermined threshold value and said average value is less than a third predetermined threshold value; and

updating said rotation frequency offset signal in accordance with a plurality of accumulated residual frequency offset signals.

11. The method of claim 10 wherein said step of determining an initial frequency offset signal further comprises the steps of:

derotating a plurality of predetermined received signals to produce derotated signals;

determining plurality of phase differences between consecutive derotated signals; and

calculating the average value of the phase differences to obtain said initial frequency offset signal.

12. The method according to claim 11, wherein said plurality of predetermined received signals comprise “

zero”

bits.

13. The method according to claim 10 further comprising a warm start cycle comprising the steps of accumulating said residual frequency offset signals when said channel quality signal is within an acceptable value, and the number of received signal bursts is within a predetermined value.

14. An integrated circuit at the receiving end of a communications system for receiving signal bursts, comprising:

a frequency derotator for adjusting the phase of signal bursts received by said communications system;

an equalizer for decoding said signal bursts received by said communications system, said equalizer generating a decoded version of said signal bursts, a burst quality signal indication and a residual frequency offset signal;

a channel decoder for further decoding said decoded version of said signal bursts so as to generate signal bursts having substantially eliminated bit errors;

a channel reencoder for encoding said signal bursts generated by said channel decoder;

a comparator for comparing a plurality of the output signals generated by said channel reencoder with signals generated by said equalizer so as to generate a channel quality indication signal; and

a frequency correction unit configured so as to receive said residual frequency offset signals, said burst quality signal indication, and said channel quality signal indication so as to generate a rotation frequency offset signal, to control said frequency derotator.

15. The system in accordance with claim 14 further comprising a synchronizer having an output port coupled to said frequency correction unit, for generating an initial frequency offset signal in response to a predetermined plurality of transmitted signals.

16. The system in accordance with claim 14 wherein said initial frequency offset signal is coupled to said frequency derotator.

17. The system in accordance with claim 15, wherein said frequency correction unit further comprises measuring means to derive an average value of said residual frequency offset signals.

18. The system according to claim 16, wherein said average value is employed to determine said rotation frequency offset signal.

19. The system according to claim 17, wherein said frequency correction unit accumulates said residual frequency offset signals when said average value signal is below a given threshold, and said burst quality signal indication is within a predetermined acceptable range, said frequency correction unit further accumulates residual frequency offset signals over four burst signal bursts.