Digital signal timing synchronization process

• US 6,717,996 B1
• Filed: 03/23/2000
• Issued: 04/06/2004
• Est. Priority Date: 03/26/1999
• Status: Expired due to Fees

• Current Assignee: Orange S.A.
• Original Assignee: Orange S.A.
• Inventors: Du Reau, Philippe, Yuan-Wu, Julie, Duponteil, Daniel
• Primary Examiner(s): Tran, Khai
• Assistant Examiner(s): TRAN, KHANH C
• Application Number:
  US09/533,924
• Time in Patent Office: 1,475 Days
• Field of Search: 375/142, 375/143, 375/145, 375/148-150, 375/152, 375/260, 375/285, 375/326, 375/327, 375/340, 375/343, 375/349, 375/350, 375/355, 375/365, 455/10, 455/501, 455/502, 455/504, 455/506, 455/63, 455/65, 455/67.3, 455/135, 455/134, 455/143, 455/226.2, 455/226.3, 455/277.2, 455/296, 370/320, 370/332, 370/335, 370/342, 370/441, 370/503, 370/509, 370/516
• US Class Current: 375/343
• CPC Class Codes: H04B 1/70754 Setting of search window, i.e. range of code offsets to be searched H04B1/70758 takes precedence
• CPC Class Codes: H04B 1/70775 Multi-dwell schemes, i.e. multiple accumulation times
• CPC Class Codes: H04B 1/709 Correlator structure
• CPC Class Codes: H04L 7/007 detection of error based on maximum signal power, e.g. peak value, maximizing autocorrelation

Abstract

Definition takes place of a sliding window of width Ne times the sampling period (Te); for each sliding window calculation takes place of the sum of the elementary powers of the correlation samples located in said window; determination takes place of the window for which the sum of the powers is at a maximum, the synchronization then being defined by the position of the synchronized window on the window having the maximum power sum and by the rank of each correlation sample within said window.

Claims

1. Process for the timing synchronization of a digital signal, in which sampling takes place with a certain sampling period of an analog signal resulting from the transmission of a signal modulated with the aid of a shaping function, there is a matched filtering of the samples, said filtering being matched to the shaping function used for the modulation and leading to correlation samples, said process comprising:

the elementary power of each correlation sample is calculated,

a sliding window NeTe of width Ne times the sampling period Te is defined and commencing with a certain rank, said sliding window scans a given range of sampling times,

for each sliding window, a calculation takes place of the sum of the elementary powers of the correlation samples located in said sliding window for a symbol or a given number of symbols,

the window for which the sum of the powers is at a maximum is determined,

the synchronization is then defined by the position of the sliding window with the maximum power sum and by the rank of each correlation sample within said sliding window,

wherein two operation types are performed and working takes place in two modes;

a) in the first type of operations, known as scanning operations, there is a successive examination of all possible positions of the sliding window and for each position a calculation takes place of the global power of the correlation samples contained in the sliding window, the sliding window for which a global power is highest from the start of the cycle up to the present position is identified and the value of said highest power is stored,

b) in the second type of operations, called tracking operations, account is only taken of the correlation sample, whose rank falls within a tracking window and calculation takes place on the one hand of the global power of said samples and on the other of a signal making it possible to lock the center of said sliding window on a mean position of the elementary powers which it contains,

c) in a first operating mode, known as an acquisition mode,

on the one hand whenever appearance takes place in a sliding window of a global power higher than the last power stored since the start of the scanning cycle and up to the present position, to the tracking window is allocated the present position of the sliding window and a verification procedure is initiated,

on the other hand when the scanning cycle is ended, there is a passage into a tracking mode,

d) in a second operating mode, known as a tracking mode,

on the one hand the transfer mechanism of the position of the sliding window to the tracking window is inhibited,

on the other when the permanent verification fails there is a return to the acquisition mode,

wherein a choice is made of a relative power threshold λ

between 0 and 1, the product of said threshold by the highest stored power is calculated and it is verified that the global power obtained in the tracking window is statistically higher than the value of the product.

2. Process according to claim 1, wherein a first or initial verification takes place consisting of examining whether, among Na successive values of the global power, at least Nb among them exceed the product of the threshold by the highest power, the numbers Na and Nb being chosen of the order of a few units and, if this is the case, there is a passage to a second or permanent verification consisting of comparing the global power with a quantity μ

Po, where μ

is a threshold slightly below 1 and where Po designates the mean power expected in the synchronized state, the comparison relating to a large number of successive values of the global power.

3. Process according to claim 2, wherein, in the acquisition mode, the appearance in said sliding window of a global power higher than the last stored power relaunches a new initial verification.

4. Process according to claim 1, wherein the global powers calculated in the sliding window and in the tracking window result from the sum of the powers obtained for several symbols.

5. Process according to claim 1, wherein, a significance is associated with each sample rank within the tracking window, said significance being a function of the position of said rank with respect to the center of the window, a determination takes place of the location of the mean position of the elementary powers of the weighted samples with respect to the center of the sliding window and there is a correction of the sampling times of the analog signal in order to lock the center of the window on said mean position.

6. Process according to claim 5, wherein the significances c(i) are of form:

7. Process according to claim 5, wherein the significances (c(i)) are taken equal to +1 in the upper half of the window and equal to −

1 in the lower half of the window.

8. Process according to claim 1, wherein the sampling period of the analog signal is a fraction (Ts/M) of the symbol period.

9. Process according to claim 1, wherein the analog signal corresponds to a signal of the code distribution multiple access or CDMA type corresponding to spread sequences formed from numbers known as chips, to which is applied a shaping, wherein the matched filtering carried out on the analog signal comprises a first filtering matched to the shape of the chips and delivering first samples and a second filtering matched to the spread sequences and applied to the first samples, said second matched filtering delivering second samples on the basis of which are calculated the elementary and global powers respectively located in the sliding and tracking windows.

10. Process according to claim 9, wherein the sampling of the analog signal takes place at a frequency equal to k times the frequency of the chips forming the spread sequences, the second matched filtering then using, at a given time, one sample on k of the first samples.

11. Process according to claim 10, wherein k is equal to 2, the sampling of the analog signal consequently being carried out at a frequency double the frequency of the chips forming the spread sequences, the second matched filtering using, at a given time, one of every two samples.

12. Process according to claim 1, wherein the analog signal is a complex signal with a real component and an imaginary component and wherein each sample is also complex with a real component and an imaginary component, the matched filtering taking place on the real and imaginary components, the elementary and global powers being calculated by forming the sum of the powers of the real and imaginary components of the samples.

13. A process for synchronizing a digital signal, comprising:

modulating a signal with a shaping function, thereby producing an analog signal;

sampling the analog signal at a predetermined sampling period, thereby obtaining a plurality of samples;

filtering the plurality of samples based on the shaping function, thereby obtaining a plurality of correlated samples, wherein the filtering comprises,

calculating at least one elementary power for each of the plurality of correlated samples,

defining a sliding window configured to scan a given range of sampling periods,

calculating a sum of the at least one elementary power for the plurality of correlated samples located within the sliding window for a symbol,

identifying a maximum sliding window having the largest sum of the at least one elementary power,

defining a synchronization based on a position of the maximum sliding window and on a rank of each correlated sample present in the sliding window;

performing at least one of a scanning and a first tracking,

the scanning comprising,

iteratively examining possible positions of the sliding window,

calculating a global power of the correlated samples present in the sliding window at each possible position,

identifying a position having the largest global power, and

storing a value of the largest global power,

the first tracking comprising,

determining which of the plurality of correlated samples falls within a tracking window, thereby locating a tracked sample,

calculating a global power of the tracked sample, and

locating a signal enabling positioning of a center of the tracking window on a mean position of elementary powers contained in the tracking window,

performing at least one of an acquiring and a second tracking,

the acquiring comprising,

allocating the tracking window to a position of the sliding window when the sliding window has a global power higher than the largest global power stored in the scanning, and

verifying the tracking window, the second tracking starting when the scanning ends, and comprising,

 

inhibiting transfer of the sliding window to the tracking window, and

 

returning to the acquiring when the verifying fails.

14. The process according to claim 13, wherein the verifying comprises:

examining Na successive values of the global power;

determining if at least Nb of the Na successive values exceeds the product of a threshold and a highest power; and

successively comparing the global power with a quantity μ

Po, where μ

represents a threshold slightly less than 1 and where Po designates a mean power expected in a synchronized state.

15. The process according to claim 14, wherein the verifying is repeated when a global power higher than the largest global power is located.

16. The process according to claim 13, wherein the global powers calculated in the sliding window and the tracking window represent a sum of powers obtained for a plurality of symbols.

17. The process according to claim 13, further comprising associating a significance with each sample rank within the tracking window, wherein the significance is a function of a position of the rank relative to the center of the tracking window;

determining a location of a mean position of the powers of the plurality of correlated samples relative to the center of the tracking window; and

correcting sampling times of the analog signal to lock the center of the tracking window on the mean position.

18. The process according to claim 17, wherein the significance is represented by:

19. The process according to claim 18, wherein c(i)=(1) in an upper half of the tracking window and c(i)=(−

1) in a lower half of the tracking window.

20. The process according to claim 16, wherein the predetermined sampling period is a fraction of the plurality of symbols.

21. The process according to claim 13, wherein the analog signal includes a signal for code distribution multiple access or CDMA type corresponding to at least one spread sequence formed from at least one chip.

22. The process according to claim 21, wherein the analog signal is sampled at a frequency equal to k times a frequency of the at least one chip forming the at least one spread sequence.

23. The process according to claim 22, wherein k=2.

24. The process according to claim 13, wherein the analog signal includes a complex signal with a real component and an imaginary component,

wherein each of the plurality of samples includes a real component and an imaginary component,

wherein each real component and imaginary component of each of the plurality of samples undergoes matched filtering, and

wherein the at least one elementary power and the global power are calculated by summing powers of the real and imaginary components.