Frequency discrimination in a spread spectrum signal processing system

• US 6,178,197 B1
• Filed: 07/30/1999
• Issued: 01/23/2001
• Est. Priority Date: 06/23/1997
• Status: Expired due to Term

• Current Assignee: Landis+Gyr Innovations Incorporated (Landis+Gyr Group AG)
• Original Assignee: CellNet Data Systems, Inc. (Landis+Gyr Group AG)
• Inventors: Fulton, Forrest, Froelich, Robert K.
• Primary Examiner(s): Bocure, Tesfaldet
• Application Number:
  US09/365,108
• Time in Patent Office: 543 Days
• Field of Search: 375/326, 375/150, 375/147, 375/343
• US Class Current: 375/150
• CPC Class Codes: H04B 1/707 using direct sequence modulation
• CPC Class Codes: H04B 1/7075 with code phase acquisition
• CPC Class Codes: H04B 1/70752 Partial correlation
• CPC Class Codes: H04B 1/708 Parallel implementation
• CPC Class Codes: H04B 1/7085 using a code tracking loop, e.g. a delay-locked loop
• CPC Class Codes: H04B 1/709 Correlator structure
• CPC Class Codes: H04B 1/7156 Arrangements for sequence synchronisation
• CPC Class Codes: H04B 2001/71563 Acquisition

Abstract

The actual carrier frequency of a spread spectrum signal is found by tuning a pair of correlators to a pair of search frequencies and then correlating the spread spectrum signal against one or more reference signals to produce a pair of correlation magnitude values. The ratio of the difference between the correlation magnitude values to the sum of the correlation magnitude values determines the offset between the actual carrier frequency and an estimated carrier frequency.

Claims

1. A method for use in determining an actual carrier frequency of a spread spectrum signal, the method comprising:

tuning a pair of correlators to a pair of search frequencies;

correlating the spread spectrum signal against a reference signal in each of the correlators to produce a pair of correlation magnitude values;

determining a ratio of a difference between the correlation magnitude values to a sum of the correlation magnitude values to produce a frequency offset; and

determining the actual carrier frequency by using the frequency offset and an estimated carrier frequency.

2. The method of claim 1, wherein one of the search frequencies is greater than the estimated carrier frequency and the other search frequency is less than the estimated carrier frequency.

3. The method of claim 1, wherein both of the search frequencies are greater than the estimated carrier frequency.

4. The method of claim 1, wherein both of the search frequencies are less than the estimated carrier frequency.

5. The method of claim 1, wherein the spread spectrum signal is correlated against the reference signal by each of the correlators over multiple correlation periods to produce multiple correlation magnitude values for each correlator.

6. The method of claim 5, further comprising calculating, for each correlation period, the difference and the sum of correlation magnitude values for the pair of correlators.

7. The method of claim 5, wherein the ratio of the difference to the sum of the correlation magnitude values is calculated for the pair of correlators during each correlation period.

8. The method of claim 7, further comprising summing the ratios of all of the correlation periods.

9. The method of claim 1, further comprising tuning at least one additional correlator to another search frequency and correlating the spread spectrum signal against the reference signal to produce at least one additional correlation magnitude value.

10. The method of claim 9, wherein the search frequencies are evenly spaced.

11. The method of claim 9, wherein the ratio is determined using the difference and the sum of correlation magnitude values for a chosen pair of the correlators tuned to adjacent search frequencies.

12. The method of claim 11, wherein the offset is proportional to the ratio and includes a constant determined by which pair of the correlators is chosen.

13. The method of claim 11, further comprising calculating a cumulative total of correlation magnitude values for each pair of correlators tuned to adjacent search frequencies.

14. The method of claim 13, wherein the chosen pair of correlators includes the correlators that produce the highest cumulative total of correlation magnitude values.

15. The method of claim 9, wherein the estimated carrier frequency is equidistant from the highest and lowest search frequencies.

16. An apparatus for use in determining an actual carrier frequency of a spread spectrum, the apparatus comprising:

a pair of correlators tunable to a pair of search frequencies and adapted to correlate the spread spectrum signal against a reference signal to produce a pair of correlation magnitude values; and

processing circuitry adapted to determine a frequency offset by using a ratio of a difference between the correlation magnitude values to a sum of the correlation magnitude values and determine the actual carrier frequency by using the frequency offset and an estimated carrier frequency.

17. The apparatus of claim 16, wherein one of the search frequencies is greater than the estimated carrier frequency and the other search frequency is less than the estimated carrier frequency.

18. The apparatus of claim 16, wherein both of the search frequencies are greater than the estimated carrier frequency.

19. The apparatus of claim 16, wherein both of the search frequencies are less than the estimated carrier frequency.

20. The apparatus of claim 16, wherein each correlator correlates the spread spectrum signal against the reference signal over multiple correlation periods to produce multiple correlation magnitude values for each correlator.

21. The apparatus of claim 20, wherein the processing circuitry is configured to calculate, for each correlation period, the difference and the sum of correlation magnitude values for the pair of correlators.

22. The apparatus of claim 20, wherein the processing circuitry is configured to calculate the ratio of the difference to the sum of the correlation magnitude values for the pair of correlators during each correlation.

23. The apparatus of claim 22, wherein the processing circuitry is configured to sum the ratios of all of the correlation periods.

24. The apparatus of claim 16, further comprising at least one additional correlator tunable to another search frequency and adapted to correlate the spread spectrum signal against the reference signal to produce at least one additional correlation magnitude value.

25. The apparatus of claim 24, wherein the search frequencies are evenly spaced.

26. The apparatus of claim 24, wherein the processor is configured to determine the ratio using the difference and the sum of correlation values for a chosen pair of the correlators tuned to adjacent ones of the search frequencies.

27. The apparatus of claim 26, wherein the offset is proportional to the ratio and includes a constant having a value that is determined by which pair of the correlators is chosen.

28. The apparatus of claim 26, wherein the processing circuitry is configured to calculate a cumulative total of the correlation values for each pair of correlators tuned to adjacent ones of the search frequencies.

29. The apparatus of claim 28, wherein the chosen pair of correlators includes the correlators that produce the highest cumulative total.

30. The apparatus of claim 28, wherein the estimated carrier frequency is equidistant from the highest and lowest search frequencies.

31. A method for use in determining an actual carrier frequency of a spread spectrum signal, the method comprising:

tuning a pair of correlators to a pair of search frequencies;

correlating the spread spectrum signal against a reference signal in each of the correlators to produce a pair of correlation magnitude values;

determining a frequency offset expressed as

32. An apparatus for use in determining an actual carrier frequency of a spread spectrum, the apparatus comprising:

a pair of correlators tunable to a pair of search frequencies and adapted to correlate the spread spectrum signal against a reference signal to produce a pair of correlation magnitude values; and

a processing circuitry adapted to determine the actual carrier frequency by adding a frequency offset to an estimated carrier frequency, said frequency offset expressed as