Method and device for automatic control of the frequency of a local oscillator in a DS-CDMA type receiver

1. A method for controlling frequency of a local oscillator in a DS-CDMA type receiver, the method comprising:

transforming a known spread spectrum signal into a sampled digital signal formed of symbols with a despreaded frequency spectrum;

determining a residual frequency error f e for each symbol including a first residual frequency error f e1 ;

filtering the residual frequency errors;

correcting the frequency of the local oscillator with the filtered residual frequency errors;

storing the first determined residual frequency error f e1 ;

determining an average of absolute values of a predetermined number of successive residual frequency errors; and

comparing the average with a threshold, and if the average is greater than or equal to the threshold, the local oscillator frequency is corrected using an error equal to −sgn(f e1 ) (1/T−|f e1 |) , where sgn is the sign function, | | is the absolute value function and T is duration of a symbol before determining the next residual frequency error associated with the next symbol.

2. A method according to claim 1 , wherein the residual frequency errors are digitally filtered with a digital filter, and if the average is greater than the threshold, the digital filter is set to zero before a next residual frequency error is filtered.

3. A method according to claim 1 , wherein the residual frequency errors are filtered with a first order matching filter.

4. A method according to claim 1 , wherein a current correction applied to the local oscillator is equal to (1−b) times a previous correction plus b times a current residual frequency error, and the coefficient b is chosen to be close to 1 if the current residual frequency error is greater than a first predetermined limiting value, and the coefficient b is chosen to be close to zero if the current residual frequency error is less than a second predetermined limiting value.

5. A method according to claim 4 , wherein the first and second predetermined limiting values are equal to the symbol rate divided by 7.

6. A device for controlling frequency of a local oscillator in a DS-CDMA type receiver comprising pre-processing means for transforming a known spread spectrum signal into a sampled digital signal formed of symbols with a despreaded frequency spectrum, the device comprising:

first calculation means for determining a residual frequency error f e for each symbol including a first residual frequency error f e1 ;

filter means for filtering the residual frequency errors;

correction means for correcting the frequency of the local oscillator with the filtered residual frequency errors;

memory means for storing the first residual frequency error f e1 ;

second calculation means for determining an average of absolute values of a predetermined number of successive residual frequency errors; and

comparison means for comparing the average with a threshold, and if the average is greater than or equal to the threshold, the local oscillator frequency is corrected using an error equal to −sgn(f e1 ) (1/T−|f e1 |), where sgn is the sign function, | | is the absolute value function and T is duration of a symbol before determining the next residual frequency error associated with the next symbol.

7. A device according to claim 6 , wherein said filtering means comprises a digital filter; and further comprising control means for resetting said digital filter to zero before filtering a next residual frequency error if the average is greater than the threshold.

8. A device according to claim 6 , wherein said filtering means comprises a first order matching filter.

9. A device according to claim 6 , wherein a current correction applied to the local oscillator is equal to (1−b) times a previous correction plus b times a current residual frequency error, and the coefficient b is chosen to be close to 1 if the current residual frequency error is greater than a first predetermined limiting value, and the coefficient b is chosen to be close to zero if the current residual frequency error is less than a second predetermined limiting value.

10. A device according to claim 9 , wherein the first and second predetermined limiting values are equal to the symbol rate divided by 7.

11. A device according to claim 6 , wherein said first calculation means, said filter means, said correction means, said memory means, said second calculation means and said comparison means are implemented within a processor.

12. A receiver comprising:

pre-processing circuitry for transforming a known spread spectrum signal into a sampled digital signal formed of symbols with a despreaded frequency spectrum;

a local oscillator connected to said pre-processing circuitry; and

a device for controlling frequency of said local oscillator, said device comprising

a first calculation circuit for determining a residual frequency error f e for each symbol including a first residual frequency error f e1 ,

a correction circuit for correcting the frequency of the local oscillator with the residual frequency errors,

a second calculation circuit for determining an average of absolute values of a predetermined number of successive residual frequency errors, and

a comparison circuit for comparing the average with a threshold, and if the average is greater than or equal to the threshold, the local oscillator frequency is corrected using an error equal to −sgn(f e1 ) (1/T−|f e1 |), where sgn is the sign function, | | is the absolute value function and T is duration of a symbol before determining the next residual frequency error associated with the next symbol.

13. A receiver according to claim 12 wherein said device further comprises a memory for storing the first residual frequency error f e1 .

14. A receiver according to claim 12 wherein said device further comprises a filter for filtering the residual frequency errors.

15. A receiver according to claim 14 , wherein said filter comprises a digital filter; and wherein said device further comprises a control circuit for resetting said digital filter to zero before filtering a next residual frequency error if the average is greater than the threshold.

16. A receiver according to claim 14 , wherein said filter comprises a first order matching filter.

17. A receiver according to claim 12 , wherein a current correction applied to said local oscillator is equal to (1−b) times a previous correction plus b times a current residual frequency error, and the coefficient b is chosen to be close to 1 if the current residual frequency error is greater than a first predetermined limiting value, and the coefficient b is chosen to be close to zero if the current residual frequency error is less than a second predetermined limiting value.

18. A receiver according to claim 17 , wherein the first and second predetermined limiting values are equal to the symbol rate divided by 7.

19. A receiver according to claim 12 , wherein the receiver is integrated within a mobile cell phone.

20. A mobile cell phone comprising:

an antenna; and

a receiver connected to said antenna for receiving a known spread spectrum signal, said receiver comprising

pre-processing circuitry for transforming the spread spectrum signal into a sampled digital signal formed of symbols with a despreaded frequency spectrum,

a local oscillator connected to said pre-processing circuitry, and

a processor for controlling frequency of said local oscillator by

determining a residual frequency error f e for each symbol including a first residual frequency error f e1 ,

filtering the residual frequency errors, correcting the frequency of the local oscillator with the filtered residual frequency errors,

storing the first determined residual frequency error f e1 ,

determining an average of absolute values of a predetermined number of successive residual frequency errors, and

comparing the average with a threshold, and if the average is greater than or equal to the threshold, the local oscillator frequency is corrected using an error equal to −sgn(f e1 ) (1/T−|f e1 |), where sgn is the sign function, | | is the absolute value function and T is duration of a symbol before determining the next residual frequency error associated with the next symbol.

21. A mobile cell phone according to claim 20 wherein said receiver comprises a DS-CDMA type receiver.

22. A mobile cell phone according to claim 20 wherein said processor comprises a memory for storing the first residual frequency error f e1 .

23. A mobile cell phone according to claim 20 wherein said processor filters the residual frequency errors before correcting the frequency of the local oscillator.

24. A mobile cell phone according to claim 23 , wherein said processor resets the filtering to zero before filtering a next residual frequency error if the average is greater than the threshold.

25. A mobile cell phone according to claim 23 , wherein the filtering corresponds to a first order matching filter.

26. A mobile cell phone according to claim 20 , wherein a current correction applied to said local oscillator is equal to (1−b) times a previous correction plus b times a current residual frequency error, and the coefficient b is chosen to be close to 1 if the current residual frequency error is greater than a first predetermined limiting value, and the coefficient b is chosen to be close to zero if the current residual frequency error is less than a second predetermined limiting value.

27. A mobile cell phone according to claim 26 , wherein the first and second predetermined limiting values are equal to the symbol rate divided by 7.