Communication system and method for acquiring pseudonoise codes or carrier signals under conditions of relatively large chip rate uncertainty

• US 7,221,696 B1
• Filed: 03/03/2003
• Issued: 05/22/2007
• Est. Priority Date: 03/03/2003
• Status: Active Grant

• Current Assignee: Harris Corporation (L3Harris Technologies, Inc.)
• Original Assignee: ITT Manufacturing Enterprises, Inc. (ITT, Inc.)
• Inventors: Weinberg, Aaron, McLeod, Gene Allan
• Primary Examiner(s): Burd; Kevin
• Assistant Examiner(s): Aghdam; Freshteh N
• Application Number:
  US10/376,450
• Time in Patent Office: 1,541 Days
• Field of Search: 375/130, 375/134, 375136-137, 375/140, 375/142, 375/143, 375/326, 375/329, 375/339, 375/149, 342/357
• US Class Current: 375/140
• CPC Class Codes: H04B 1/7077 Multi-step acquisition, e.g. multi-dwell, coarse-fine or validation
• CPC Class Codes: H04L 2027/003 at baseband only
• CPC Class Codes: H04L 2027/0053 Closed loops
• CPC Class Codes: H04L 27/0014 Carrier regulation of chaotic carriers H04L27/001; for multicarrier receivers H04L27/2657

Abstract

A Spread Spectrum system acquires signals with frequency uncertainty. The present invention independently computes the number of frequency bands necessary to cover the carrier frequency uncertainty and the chip rate uncertainty and uses the largest of the two quantities in the acquisition process. A receiver cycles through pseudonoise code phases until a matched filter output exceeds a threshold. The local code is adjusted based on that code phase and the signal is processed for carrier acquisition, where an FFT estimates the carrier frequency. The code phase and nearest neighbors are used to determine the presence of a false alarm. When a false alarm is absent, the detection is examined for an alias or noise spur. If an alias is absent, the signal search is complete. Otherwise, processing continues to search for a pseudonoise code. The process continues by examining all code phases for each band until a signal is detected.

Claims

1. In a communication system receiver utilizing a local carrier signal and a local code to respectively determine a carrier frequency of a received signal and a phase and rate of a received signal code, wherein the received signal carrier frequency and code rate each reside within a respective uncertainty region, a signal acquisition unit to acquire a transmitted signal including a code comprising:

a frequency band module to determine a quantity of frequency bands to employ for said local carrier signal to acquire said transmitted signal, wherein said frequency band module includes a band determination module to identify a quantity of frequency bands sufficient to cover said frequency uncertainty region and a quantity of frequency bands sufficient to cover said code rate uncertainty region, to compare said frequency band quantities for said frequency and code rate uncertainty regions to each other and to select the frequency bands associated with the greatest quantity as said determined quantity of frequency bands to acquire said transmitted signal; and

an acquisition module to employ said determined quantity of frequency bands for said local carrier signal to determine said transmitted signal carrier frequency and said phase and code rate of said transmitted signal code in order to acquire said transmitted signal.

2. The signal acquisition unit of claim 1, wherein said transmitted signal includes a spread spectrum signal.

3. The signal acquisition unit of claim 2, wherein said spread spectrum signal includes a Direct Sequence spread spectrum signal.

4. The signal acquisition unit of claim 3, wherein said spread spectrum signal includes a Binary Phase Shift Keying modulated signal.

5. The signal acquisition unit of claim 1, wherein said acquisition module includes:

a code search module to examine a plurality of code phases of said local code and determine a local code phase substantially in phase with said transmitted signal code;

a frequency estimate module to employ said determined local code phase and measure a frequency offset between said carrier frequency of said transmitted signal and said local carrier signal to estimate said transmitted signal carrier frequency and code rate;

a false alarm module to employ said frequency and code rate estimates for said local carrier and said local code and examine said determined local code phase and at least one neighbor of that code phase to identify a local code phase substantially in phase with said transmitted signal code in order to determine the presence of a false alarm, wherein said false alarm module indicates a false alarm in response to the absence of a local code phase substantially in phase with said transmitted signal code; and

an alias module to employ said identified local code phase to determine the presence of an alias of said transmitted signal and to indicate acquisition of said transmitted signal in response to the absence of said transmitted signal alias.

6. The signal acquisition unit of claim 5, wherein said code search module includes:

a correlator to correlate phases of said local code with said transmitted signal code and produce correlation information including correlation values for each said local code phase indicating a correlation between that code phase and said transmitted signal code;

a threshold module to determine a threshold based on said correlation values from said correlator; and

a threshold crossing module to compare said correlation values for each code phase to said threshold to determine said local code phase substantially in phase with said transmitted signal code.

7. The signal acquisition unit of claim 6, wherein said frequency estimate module includes:

a local code module to supply said determined local code phase to said correlator;

a data stripping module to receive and process said correlation information from said correlator to enable measurement of said carrier frequency offset; and

a transform module to transform said processed correlation information from a first domain to a second domain and identify a magnitude within said transformed information indicating said carrier frequency offset.

8. The signal acquisition unit of claim 7, wherein said transform module includes:

a Fourier module to perform a Fourier transform on said processed correlation information and identify the largest magnitude produced by said Fourier transform indicating said carrier frequency offset.

9. The signal acquisition unit of claim 7, wherein said false alarm module includes:

a code module to determine at least one neighbor of said determined local code phase and supply said determined local code phase and said at least one neighbor to said correlator to correlate those local code phases with said transmitted signal code and produce correlation information including correlation values for each of said determined local code phase and said at least one neighbor indicating a correlation between that code phase and said transmitted signal code; and

a threshold crossing detection module to apply said correlation values for each code phase to said threshold crossing module to identify said local code phase substantially in phase with said transmitted signal code.

10. The signal acquisition unit of claim 9, wherein said alias module includes:

an alias detection module to apply said identified local code phase to said correlator and examine information processed by said data stripping and transform modules to identify the proximity of a magnitude within said transformed information to a particular transform component in order to determine the presence of said transmitted signal alias.

11. The signal acquisition unit of claim 10, wherein said transform module includes:

a Fourier transform module to perform a Fourier transform on said processed correlation information;

and said alias detection module includes;

a transform search module to identify the proximity of a largest magnitude produced by said Fourier transform to a DC component to determine the presence of said transmitted signal alias.

12. A method of acquiring a transmitted signal including a code in a communication system receiver utilizing a local carrier signal and a local code to respectively determine a carrier frequency of said transmitted signal and a phase and rate of said transmitted signal code, wherein the transmitted signal carrier frequency and code rate each reside within a respective uncertainty region, said method comprising:

(a) determining a quantity of frequency bands to employ for said local carrier signal to acquire said transmitted signal, wherein step (a) further includes;

(a.1) identifying a quantity of frequency bands sufficient to cover said frequency uncertainty region and a quantity of frequency bands sufficient to cover said code rate uncertainty region; and

(a.2) comparing said frequency band quantities for said frequency and code rate uncertainty regions to each other and selecting the frequency bands associated with the greatest quantity as said determined quantity of frequency bands to acquire said transmitted signal; and

(b) employing said determined quantity of frequency bands for said local carrier signal to determine said transmitted signal carrier frequency and said phase and code rate of said transmitted signal code in order to acquire said transmitted signal.

13. The method of claim 12, wherein said transmitted signal includes a spread spectrum signal.

14. The method of claim 13, wherein said spread spectrum signal includes a Direct Sequence spread spectrum signal.

15. The method of claim 14, wherein said spread spectrum signal includes a Binary Phase Shift Keying modulated signal.

16. The method of claim 12, wherein step (b) further includes:

(b.1) examining a plurality of code phases of said local code and determining a local code phase substantially in phase with said transmitted signal code;

(b.2) employing said determined local code phase and measuring a frequency offset between said carrier frequency of said transmitted signal and said local carrier signal to estimate said transmitted signal carrier frequency and code rate;

(b.3) employing said frequency and code rate estimates for said local carrier and said local code and examining said determined local code phase and at least one neighbor of that code phase to identify a local code phase substantially in phase with said transmitted signal code in order to determine the presence of a false alarm, wherein said false alarm is indicated in response to the absence of a local code phase substantially in phase with said transmitted signal code; and

(b.4) employing said identified local code phase to determine the presence of an alias of said transmitted signal and indicating acquisition of said transmitted signal in response to the absence of said transmitted signal alias.

17. The method of claim 16, wherein step (b.1) further includes:

(b.1.1) correlating phases of said local code with said transmitted signal code and producing correlation information including correlation values for each said local code phase indicating a correlation between that code phase and said transmitted signal code;

(b.1.2) determining a threshold based on said correlation values; and

(b.1.3) comparing said correlation values for each code phase to said threshold to determine said local code phase substantially in phase with said transmitted signal code.

18. The method of claim 17, wherein step (b.2) further includes:

(b.2.1) correlating said determined local code phase with said transmitted signal code to produce said correlation information;

(b.2.2) processing said correlation information to enable measurement of said carrier frequency offset; and

(b.2.3) transforming said processed correlation information from a first domain to a second domain and identifying a magnitude within said transformed information indicating said carrier frequency offset.

19. The method of claim 18, wherein step (b.2.3) further includes:

(b.2.3.1) performing a Fourier transform on said processed correlation information and identifying the largest magnitude produced by said Fourier transform indicating said carrier frequency offset.

20. The method of claim 18, wherein step (b.3) further includes:

(b.3.1) determining at least one neighbor of said determined local code phase and correlating said determined local code phase and said at least one neighbor with said transmitted signal code to produce correlation information including correlation values for each of said determined local code phase and said at least one neighbor indicating a correlation between that code phase and said transmitted signal code; and

(b.3.2) determining said threshold based on said correlation values and comparing said correlation values for each code phase to said threshold to identify said local code phase substantially in phase with said transmitted signal code.

21. The method of claim 20, wherein step (b.4) further includes:

(b.4.1) correlating said identified local code phase with said transmitted signal code to produce said correlation information; and

(b.4.2) processing said correlation information and transforming said processed correlation information from a first domain to a second domain to identify the proximity of a magnitude within said transformed information to a particular transform component in order to determine the presence of said transmitted signal alias.

22. The method of claim 21, wherein step (b.4.2) further includes:

(b.4.2.1) performing a Fourier transform on said processed correlation information and identifying the proximity of a largest magnitude produced by said Fourier transform to a DC component to determine the presence of said transmitted signal alias.