Automatic frequency correction method and receiver for time division duplex modes of 3G wireless communications

• US 7,103,339 B2
• Filed: 06/12/2003
• Issued: 09/05/2006
• Est. Priority Date: 09/28/2001
• Status: Expired due to Fees

• Current Assignee: Interdigital Technology Corporation (InterDigital, Inc.)
• Original Assignee: Interdigital Technology Corporation (InterDigital, Inc.)
• Inventors: Sternberg, Gregory S.
• Primary Examiner(s): Feild, Joseph
• Assistant Examiner(s): Nguyen, Huy D.
• Application Number:
  US10/460,099
• Publication Number:
  US 20030224744A1
• Time in Patent Office: 1,181 Days
• Field of Search: 455/257, 455/255, 455/259, 455/260, 455/264, 455/422.1, 375/344
• US Class Current: 455/264
• CPC Class Codes: H04B 1/707 using direct sequence modulation
• CPC Class Codes: H04B 1/7087 Carrier synchronisation aspects
• CPC Class Codes: H04B 1/709 Correlator structure
• CPC Class Codes: H04L 2027/0065 Frequency error detectors H04L2027/0067 takes precedence

Abstract

A receiver for use in a wireless communication system includes a voltage controlled oscillator and estimating means for obtaining a frequency estimate to adjust the voltage controlled oscillator, thereby correcting an oscillator frequency error. The estimating means performs a number of block correlations on a received signal with a known midamble reference. The output of the block correlators are conjugately multiplied and summed to produce a low-variance estimate of the phase change between correlators. A number of the largest summed values are located, and the located values that exceed a detection threshold are summed to provide a single complex number whose angle is an estimate of the phase change between the correlators.

Claims

1. A receiver for use in a wireless communication system, said receiver including a voltage controlled oscillator and estimating means for obtaining a frequency estimate to adjust said voltage controlled oscillator, said estimating means comprising:

receiving means for receiving a communication signal including time slots containing data symbols and a midamble;

correlating means for performing a given number N of block correlations of the received signal samples with a known midamble reference;

forming means for forming a conjugate product of the N block correlations to form N−

1 conjugate products;

summing means for forming a sum of the N−

1 conjugate products;

accumulating means for accumulating a given number of sums of said N−

1 conjugate products obtained from said summing means;

determining means for determining the magnitude of each accumulated summed value;

searching means for searching for a given number of largest summed values;

detection means for performing a threshold detection of the given number of largest summed values other than the largest summed value, employing a threshold which is a function of the largest summed value;

combining means for combining the largest summed value with the given number of the largest summed values other than the largest summed value and which are greater than the threshold;

computing means for computing the magnitude of the sum obtained by said combining means;

normalizing means for normalizing the complex value obtained by said combining means employing the magnitude obtained by said computing means; and

generating means for generating an argument of the normalized value obtained by said normalizing means for a frequency estimate.

2. A receiver according to claim 1, further comprising applying means for applying the frequency estimate generated by said generating means to a loop filter for adjusting an operating frequency of said voltage controlled oscillator.

3. A receiver according to claim 2, wherein said generating means produces a digital formatted output and includes converting means for converting the digital formatted output into an analog signal applied to said voltage controlled oscillator.

4. A receiver according to claim 1, wherein said correlating means includes means to perform four (4) block correlations.

5. A receiver according to claim 1, wherein said accumulating means includes means for adjusting the number of sums accumulated based on the absolute frequency error, wherein the smaller the frequency error the smaller the number of sums accumulated.

6. A receiver according to claim 1, wherein said detection means uses a threshold value between 0.56 and 0.707 times the largest summed value.

7. A receiver according to claim 1, wherein the received signal has a midamble length of 512 chips, all of which are correlated by said correlating means.

8. A receiver according to claim 1, wherein said detection means includes

second determining means to determine if only one path meets the threshold;

and

second combining means responsive to said second determining means for combining the two paths with equal gain.

9. A receiver according to claim 1, wherein convergence to the required accuracy in a given number of frames with a probability of 0.99 is determined by comparing means for comparing the estimated frequency error with a threshold, wherein convergence is obtained when the estimated frequency error lies below a given detection threshold.

10. A receiver according to claim 1, wherein convergence to the required accuracy in a given number of frames with a probability of 0.99 is determined by means for identifying convergence when two frequency estimates lie below a given detection threshold.

11. A receiver according to claim 1, wherein convergence to the required accuracy in a given number of frames with a probability of 0.99 is determined by means for identifying convergence when two successive frequency estimates lie below a given detection threshold.

12. A receiver according to claim 1, wherein convergence to the required accuracy in a given number of frames with a probability of 0.99 is determined by means for identifying convergence when a two-point moving average of frequency estimates is below said detection threshold.

13. A receiver according to claim 1, further comprising:

measuring means for measuring a frequency offset; and

reading means for reading a broadcast channel when the frequency offset is less than 400 Hz.

14. A receiver according to claim 1, wherein said searching means searches for the three largest values.

15. A receiver according to claim 1, wherein said correlating means includes means for adjusting block correlation size based on frequency offsets wherein the larger the offset the smaller the block correlation size.

16. A method for obtaining a frequency estimate to adjust a voltage controlled oscillator, comprising the steps of:

(a) receiving a signal containing data frames and a midamble;

(b) converting the signal to baseband by the voltage controlled oscillator;

(c) for each of N frames, iteratively performing the following steps;

(1) block correlating the received signal with the midamble at X block correlators;

(2) calculating the conjugate product of the output of each block correlator, to form X-1 conjugate products;

(3) adding the X-1 conjugate products into a summed value;

(4) storing the summed value;

(d) searching for the largest summed value and Y of the next largest summed values;

(e) calculating the magnitude of each of the summed values;

(f) applying a threshold to the Y summed values, wherein the Y summed values will be used only if they exceed the threshold;

(g) adding the largest summed value and the summed values from step (f) to obtain a second sum;

(h) normalizing the second sum; and

(i) outputting the second sum as the frequency estimate.

17. The method according to claim 16, wherein in step (c), the value of N varies as a function of the absolute value of the frequency error.

18. The method according to claim 17, wherein N is between 1 and 30.

19. The method according to claim 16, wherein step (c)(1) includes using four block correlators, whereby step (c)(2) forms three conjugate products.

20. The method according to claim 16, wherein step (d) includes calculating the magnitude of the second and third largest summed values, whereby Y=2.

21. The method according to claim 16, wherein the threshold in step (f) is between 0.56 and 0.707 times the largest summed value.

22. The method according to claim 16, wherein step (h) includes

scaling the second sum to unit magnitude;

and

approximating the complex absolute value function.

23. The method according to claim 16, further comprising the step of:

(j) applying the frequency estimate to a loop filter to obtain a voltage value for adjusting the operating frequency of the voltage controlled oscillator.

24. The method according to claim 23, wherein step (j) includes the step of determining convergence of the loop filter.

25. The method according to claim 24, wherein the determining step includes comparing the estimated frequency error with a second threshold, whereby convergence is obtained when the estimated frequency error is below the second threshold.

26. The method according to claim 24, wherein the determining step includes comparing the estimated frequency error with a second threshold, whereby convergence is obtained when two frequency estimates are below the second threshold.

27. The method according to claim 24, wherein the determining step includes comparing the estimated frequency error with a second threshold, whereby convergence is obtained when two successive frequency estimates are below the second threshold.

28. The method according to claim 24, wherein the determining step includes comparing the estimated frequency error with a second threshold, whereby convergence is obtained when a two-point moving average of the frequency estimate is below the second threshold.