Method and system for multiple-shift code acquisition of optical orthogonal codes in optical CDMA systems

• US 20030137956A1
• Filed: 05/08/2002
• Published: 07/24/2003
• Est. Priority Date: 01/15/2002
• Status: Active Grant

• Current Assignee: Abtin Keshavarzian, Jawad Ahmed Salehi
• Original Assignee: Abtin Keshavarzian, Jawad Ahmed Salehi
• Inventors: Keshavarzian, Abtin, Salehi, Jawad Ahmed
• Granted Patent: US 7,120,205 B2
• Time in Patent Office: Days
• US Class Current: 370/335
• CPC Class Codes: H04B 1/7075 with code phase acquisition
• CPC Class Codes: H04B 2201/70715 with application-specific features
• CPC Class Codes: H04J 13/10 Code generation

Abstract

A technique for determining an arbitrary delay in a source code word propagated over a channel that introduces the arbitrary delay in the source code word to produce a received code word. The technique is based upon a multiple-shift algorithm to determine the delay by a coarse synchronization process to locate, synchronization as contained within a composite code pattern, and a fine synchronization process to pinpoint the delay by examining shifts in the source code word used to form the composite code pattern.

Claims

1. A method for determining an arbitrary delay in a source code word propagated over a channel that introduces the arbitrary delay in the source code word to produce a received code word, the code words encompassing a given time interval, the method comprising

partitioning the time interval into cells and grouping a plurality of cells into groups of cells,

examining each of the groups to locate a particular one of the groups containing the arbitrary delay with reference to the code words, and

estimating the arbitrary delay within the particular one of the groups with reference to the code words.

2. The method as recited in claim 1 wherein the examining includes generating a composite code pattern with reference to the source code word to examine each of the groups.

3. The method as recited in claim 2 wherein the examining includes

randomly selecting one of the groups, and

computing an overlap value between the received code word and the composite code pattern for the randomly selected one of the groups.

4. The method as recited in claim 3 wherein the particular one of the groups is determined whenever the overlap value exceeds a given threshold.

5. The method as recited in claim 4 wherein

the generating further includes generating the composite code pattern from a plurality of shifted versions of the source code word, and

the estimating includes

(a) computing another overlap value for a selected one the shifted versions, and

(b) whenever said another overlap value exceeds a predetermined threshold, identifying the arbitrary delay with reference to said selected one of the shifted versions and the particular one of the groups;

otherwise, returning to (a) until all shifted versions are processed.

6. The method as recited in claim 4 wherein, if the overlap value is less than the given threshold, randomly selecting another one of the groups to re-compute the overlap value and continuing the randomly selecting until the re-computed overlap value exceeds the given threshold.

7. The method as recited in claim 2 wherein the generating includes augmenting the composite code pattern to produce a modified composite code pattern.

8. The method as recited in claim 7 wherein the examining includes

randomly selecting one of the groups, and

computing an overlap value between the received code word and the modified composite code pattern for the randomly selected one of the groups.

9. The method as recited in claim 8 wherein the particular one of the groups is determined whenever the overlap value exceeds a given threshold.

10. The method as recited in claim 9 wherein

the generating further includes generating the modified composite code pattern from a plurality of shifted versions an expanded source code word, and

the estimating includes

(a) computing another overlap value for a selected one the shifted versions, and

(b) whenever said another overlap value exceeds a predetermined threshold, identifying the arbitrary delay with reference to said selected one of the shifted versions of the expanded source code word and the particular one of the groups;

otherwise, returning to (a) until all shifted versions are processed.

11. The method as recited in claim 9 wherein, if the overlap value is less than the given threshold, randomly selecting another one of the groups to re-compute the overlap value and continuing the randomly selecting until the re-computed overlap value exceeds the given threshold.

12. The method as recited in claim 9 wherein

the generating further includes generating the modified composite code pattern from a plurality of shifted versions of an expanded source code word, and

the estimating includes

(a) computing another overlap value for a selected one the shifted versions, and

(b) whenever said another overlap value exceeds a predetermined threshold, identifying the arbitrary delay with reference to said selected one of the shifted versions of the source code word only and the particular one of the groups;

otherwise, returning to (a) until all shifted versions are processed.

13. The method as recited in claim 12 wherein, if the overlap value is less than the given threshold, randomly selecting another one of the groups to re-compute the overlap value and continuing the randomly selecting until the re-computed overlap value exceeds the given threshold.

14. A method for determining an arbitrary delay in a source code word propagated over a channel that introduces the arbitrary delay in the source code word to produce a received code word, the code words encompassing a given time interval, the method comprising

(a) partitioning the bit interval into a number of cells,

(b) grouping the cells into a plurality of groups,

(c) generating a composite code pattern by combining shifts in the source code word with reference to a shift parameter and a previously unused one of the groups,

(d) computing an overlap value by evaluating the overlap of the received code word with the composite code pattern,

(e) if the overlap value exceeds a given threshold, continuing with (f);

otherwise returning to (c),

(f) sequentially comparing the received code word for a selected one of the shifted versions of the source code word used to form the composite code pattern to produce a second overlap value, and

(g) whenever the second overlap value exceeds a predetermined threshold, equating the shift in the compared shifted version of the source code word to the arbitrary delay;

otherwise, either returning to (f) until all shifted versions are processed or to (c) until all groups are processed.

15. The method as recited in claim 14 wherein the generating includes augmenting the composite code pattern to produce a modified composite code pattern.

16. A method for detecting an arbitrary delay in a received test code word corresponding to a source test code word propagated from a transmitter to a receiver over a channel that introduces the arbitrary delay in the source code word, each of the code words covering a bit interval subdivided into a plurality of chip intervals, the method comprising

(a) partitioning the bit interval into F cells, each of the cells having a duration substantially equal to the chip interval,

(b) determining a group number Q from the relation

$Q=\lceil$

FL⌉

,

where L is a shift parameter, 1<

L<

F,

(c) generating a composite code pattern by combining L shifts in the source code word with reference to a randomly selected number from the set {1, 2, . . . , Q}, excluding previously-selected members of the set,

(c) computing an overlap value by evaluating the overlap of the received code word with the composite code pattern,

(d) if the overlap value exceeds a given threshold, continuing with (e);

otherwise returning to (c),

(e) comparing the received code word with a previously unprocessed one of the L shifted versions of the source code word used to form the composite code pattern to produce a second overlap value, and

(f) whenever the second overlap value exceeds a predetermined threshold, equating the shift in the compared shifted version of the source code word to the arbitrary delay;

otherwise, if shifts remain to be processed, returning to (f);

if not, returning to (c).

17. The method as recited in claim 16 wherein a stream of information-bearing received code words is propagated from the transmitter upon determination of the arbitrary delay and the method further comprises synchronizing the stream in the receiver with reference to the arbitrary delay.

18. The method as recited in claim 16 wherein the generating includes augmenting the composite code pattern to produce a modified composite code pattern.

19. The method as recited in claim 18 wherein a stream of information-bearing received code words is propagated from the transmitter upon determination of the arbitrary delay and the method further comprises synchronizing the stream in the receiver with reference to the arbitrary delay.

20. The method as recited in claim 16 wherein the comparing includes augmenting the source code word when forming the L shifted versions of the source code word.

21. A method for synchronizing a received code word with a source code word, the received code word corresponding to the source code word transmitted from a transmitter to a receiver over a channel that introduces an arbitrary delay in the source code word to produce the received code word, the method comprising

generating in the receiver a composite code pattern composed of shifted versions of the source code word,

computing a set of overlap values by evaluating the overlap of each corresponding one of the shifted versions of the composite code pattern with the received code word,

selecting from the set of overlap values the values exceeding a given threshold as determining corresponding ones of the shifted versions of the composite code pattern containing the received code word,

determining for each of the values exceeding the given threshold another set of overlap values by comparing each of the shifted versions of the source code word used to form the composite pattern with the received code word,

selecting the largest of the overlap values in the second set exceeding a predetermined threshold to determine the corresponding one of the shifted versions of the source code word containing the received code word, and

synchronizing the received code word with the shift in said corresponding one of the shifted versions of the source code word containing the received code word by utilizing the shift as an estimate of the arbitrary delay.

22. The method as recited in claim 21 wherein the generating includes augmenting in the receiver a modified composite code pattern based upon shifted versions of the source code word.

23. A receiver for determining an arbitrary delay in a source code word propagated over a channel that introduces the arbitrary delay in the source code word to produce a received code word, the code words encompassing a given time interval, the method comprising

means for partitioning the time interval into cells and for grouping a plurality of cells into groups of cells,

means for examining each of the groups to locate a particular one of the groups containing the arbitrary delay with reference to the code words, and

means for estimating the arbitrary delay within the particular one of the groups with reference to the code words.

24. An opto-electrical receiver for synchronizing a received optical code word with a source optical code word, the received code word corresponding to the source code word propagated from a transmitter to a receiver over a channel that introduces an arbitrary delay in the source code word to produce the received code word, the code words occupying a bit interval partitioned into F cells, the receiver comprising

code acquisition circuitry for synchronizing the receiver to detect the received code word,

decoding circuitry to decode the received code word, and

an optical switch for switching between the code acquisition circuitry and the decoding circuitry,

the code acquisition circuitry including

a controller for storing a group number Q obtained from the relation

$Q=\lceil$

FL⌉

,

where L is a shift parameter, 1<

L<

F,

a pattern generator for generating L shifted versions of the source code word, under control of the controller, to produce a composite optical code pattern with reference to a randomly selected number from the set {1, 2, . . . , Q }, excluding previously-selected members of the set as registered by the controller,

opto-electrical circuitry for comparing the received optical code word with the composite code pattern to produce an electrical overlap value,

a threshold detector, coupled to the controller, for comparing the electrical overlap value to a given threshold to produce a threshold detector output,

a verifier, responsive to the threshold detector, for signaling the controller, whenever the threshold detector output exceeds the given threshold, to re-process the received code word by instructing the pattern generator to successively generate, as needed, each of the shifted versions of the source code word used to form the composite code pattern and by resetting the integrator after each of the shifted versions is processed, and wherein the output of the threshold detector produces a second threshold value such that, whenever the second threshold value exceeds a predetermined threshold, equating the shift in the shifted version of the source code word to the arbitrary delay, the verifier further controlling the optical switch to couple the received code word to the decoding circuitry upon determination of the arbitrary delay, and

a code tracker, coupled to the verifier, to provide a signal indicative of the arbitrary delay to the decoding circuitry.

25. The receiver as recited in claim 24 wherein the opto-electrical circuitry includes

an optical multiplier for multiplying the received optical code word with the composite code pattern to produce a plurality of multiplier output values,

a photodetector for converting the multiplier output values to corresponding electrical values, and

an integrator for integrating the electrical values to produce the electrical overlap value.

26. The receiver as recited in claim 24 wherein the pattern generator replaces, under control of the controller, the composite code pattern with a modified composite code pattern formed by augmenting the composite code pattern.