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| United States Patent |
6,304,216
|
|
Gronemeyer
|
October 16, 2001
|
Signal detector employing correlation analysis of non-uniform and disjoint
sample segments
Abstract
A signal detector is provided in which correlation analyses of multiple
segments of a received signal from separate and distinct periods of time
are combined to improve the detection capability of the signal detector.
In one embodiment, the signal detector is part of a GPS receiver.
| Inventors:
|
Gronemeyer; Steven A. (Cedar Rapids, IA)
|
| Assignee:
|
Conexant Systems, Inc. (Newport Beach, CA)
|
| Appl. No.:
|
281741 |
| Filed:
|
March 30, 1999 |
| Current U.S. Class: |
342/378; 342/357.06; 342/357.12; 375/130; 375/140; 375/147; 375/150 |
| Intern'l Class: |
G01S 003/16 |
| Field of Search: |
342/357.01-357.17,378
375/130-153,224-228
|
References Cited [Referenced By]
U.S. Patent Documents
| 3604911 | Sep., 1971 | Schmitt.
| |
| 3975628 | Aug., 1976 | Graves et al.
| |
| 4426712 | Jan., 1984 | Gorski-Popiel.
| |
| 4445118 | Apr., 1984 | Taylor et al.
| |
| 4463357 | Jul., 1984 | MacDoran.
| |
| 4578678 | Mar., 1986 | Hurd.
| |
| 4667203 | May., 1987 | Counselman, III.
| |
| 4701934 | Oct., 1987 | Jasper.
| |
| 4754465 | Jun., 1988 | Trimble.
| |
| 4785463 | Nov., 1988 | Janc et al.
| |
| 4809005 | Feb., 1989 | Counselman, III.
| |
| 4821294 | Apr., 1989 | Thomas, Jr.
| |
| 4890233 | Dec., 1989 | Ando et al.
| |
| 4894662 | Jan., 1990 | Counselman.
| |
| 4894842 | Jan., 1990 | Broekhoven et al.
| |
| 4992720 | Feb., 1991 | Hata.
| |
| 4998111 | Mar., 1991 | Ma et al.
| |
| 5014066 | May., 1991 | Counselman, III.
| |
| 5018088 | May., 1991 | Higbie.
| |
| 5036329 | Jul., 1991 | Ando.
| |
| 5043736 | Aug., 1991 | Darnell et al.
| |
| 5108334 | Apr., 1992 | Eschenbach et al.
| |
| 5148042 | Sep., 1992 | Nakazoe.
| |
| 5153591 | Oct., 1992 | Clark.
| |
| 5179724 | Jan., 1993 | Lindoff.
| |
| 5202829 | Apr., 1993 | Geier.
| |
| 5225842 | Jul., 1993 | Brown et al.
| |
| 5253268 | Oct., 1993 | Omura et al.
| |
| 5276765 | Jan., 1994 | Fremman et al.
| |
| 5293170 | Mar., 1994 | Lorenz et al.
| |
| 5293398 | Mar., 1994 | Hamao et al.
| |
| 5297097 | Mar., 1994 | Etoh et al.
| |
| 5311195 | May., 1994 | Mathis et al.
| |
| 5323164 | Jun., 1994 | Endo.
| |
| 5343209 | Aug., 1994 | Sennott et al.
| |
| 5345244 | Sep., 1994 | Gildea et al.
| |
| 5347536 | Sep., 1994 | Meehan.
| |
| 5352970 | Oct., 1994 | Armstrong, II.
| |
| 5363030 | Nov., 1994 | Ford et al.
| |
| 5378155 | Jan., 1995 | Eldridge.
| |
| 5379224 | Jan., 1995 | Brown et al.
| |
| 5396515 | Mar., 1995 | Dixon et al.
| |
| 5402346 | Mar., 1995 | Lion et al.
| |
| 5402347 | Mar., 1995 | McBurney et al.
| |
| 5410747 | Apr., 1995 | Ohmagari et al.
| |
| 5416712 | May., 1995 | Geier et al.
| |
| 5418818 | May., 1995 | Marchetto et al.
| |
| 5420593 | May., 1995 | Niles.
| |
| 5440313 | Aug., 1995 | Osterdock et al.
| |
| 5450344 | Sep., 1995 | Woo et al.
| |
| 5498239 | Mar., 1996 | Galel et al.
| |
| 5504684 | Apr., 1996 | Lau et al.
| |
| 5546445 | Aug., 1996 | Dennison et al.
| |
| 5548613 | Aug., 1996 | Kahu et al.
| |
| 5550811 | Aug., 1996 | Kahu et al.
| |
| 5568473 | Oct., 1996 | Hemmati.
| |
| 5577023 | Nov., 1996 | Marum et al.
| |
| 5577025 | Nov., 1996 | Skinner et al.
| |
| 5592173 | Jan., 1997 | Lau et al.
| |
| 5594453 | Jan., 1997 | Rodal et al.
| |
| 5608722 | Mar., 1997 | Miller.
| |
| 5623485 | Apr., 1997 | Bi.
| |
| 5625668 | Apr., 1997 | Loomis et al.
| |
| 5640429 | Jun., 1997 | Michels et al.
| |
| 5640431 | Jun., 1997 | Bruckert et al.
| |
| 5642377 | Jun., 1997 | Chung et al.
| |
| 5644591 | Jul., 1997 | Sutton | 375/142.
|
| 5649000 | Jul., 1997 | Lee et al.
| |
| 5650792 | Jul., 1997 | Moore et al.
| |
| 5654718 | Aug., 1997 | Beason et al.
| |
| 5663734 | Sep., 1997 | Krasner.
| |
| 5663735 | Sep., 1997 | Eschenbach.
| |
| 5689814 | Nov., 1997 | Hagisawa et al.
| |
| 5722061 | Feb., 1998 | Hutchinson, IV et al.
| |
| 5734674 | Mar., 1998 | Fenton et al.
| |
| 5734966 | Mar., 1998 | Farrer et al.
| |
| 5737329 | Apr., 1998 | Horiguchi.
| |
| 5739596 | Apr., 1998 | Takizawa et al.
| |
| 5749067 | May., 1998 | Barrett.
| |
| 5781156 | Jul., 1998 | Krasner.
| |
| 5784695 | Jul., 1998 | Upton et al.
| |
| 5786789 | Jul., 1998 | Janky.
| |
| 5812087 | Sep., 1998 | Krasner.
| |
| 5825327 | Oct., 1998 | Krasner.
| |
| 5828694 | Oct., 1998 | Schipper.
| |
| 5831574 | Nov., 1998 | Krasner.
| |
| 5832021 | Nov., 1998 | Kondo.
| |
| 5841396 | Nov., 1998 | Krasner.
| |
| 5845203 | Dec., 1998 | LaDue.
| |
| 5854605 | Dec., 1998 | Gildea.
| |
| 5862465 | Jan., 1999 | Ou.
| |
| 5867535 | Feb., 1999 | Phillips et al.
| |
| 5867795 | Feb., 1999 | Novis et al.
| |
| 5872540 | Feb., 1999 | Casabona et al.
| |
| 5874914 | Feb., 1999 | Krasner.
| |
| 5877724 | Mar., 1999 | Davis.
| |
| 5877725 | Mar., 1999 | Kalafus.
| |
| 5881371 | Mar., 1999 | Reynolds.
| |
| 5883594 | Mar., 1999 | Lau.
| |
| 5884214 | Mar., 1999 | Krasner.
| |
| 5889474 | Mar., 1999 | LaDue.
| |
| 5903654 | May., 1999 | Milton et al.
| |
| 5907809 | May., 1999 | Molnar et al.
| |
| 5909640 | Jun., 1999 | Farrer et al.
| |
| 5917444 | Jun., 1999 | Loomis et al.
| |
| 5917829 | Jun., 1999 | Hertz et al.
| |
| 5920283 | Jul., 1999 | Shaheen et al.
| |
| 5923703 | Jul., 1999 | Pon et al.
| |
| 5924024 | Jul., 1999 | Ikeda et al.
| |
| 5926131 | Jul., 1999 | Sakumoto et al.
| |
| 5936572 | Aug., 1999 | Loomis et al.
| |
| 5943363 | Aug., 1999 | Hanson et al.
| |
| 5945944 | Aug., 1999 | Krasner.
| |
| 5956328 | Sep., 1999 | Sato.
| |
| 5963582 | Oct., 1999 | Stansell, Jr.
| |
| 5970084 | Oct., 1999 | Honda et al.
| |
| 5977909 | Nov., 1999 | Harrison et al.
| |
| 5982324 | Nov., 1999 | Watters et al.
| |
| 5987016 | Nov., 1999 | He.
| |
| 5991309 | Nov., 1999 | Jensen et al.
| |
| 5991613 | Nov., 1999 | Euscher et al.
| |
| 5995537 | Nov., 1999 | Kondo.
| |
| 5999124 | Dec., 1999 | Sheynblat.
| |
| 6002362 | Dec., 1999 | Gudat.
| |
| 6002363 | Dec., 1999 | Krasner.
| |
| 6002709 | Dec., 1999 | Hendrickson.
| |
| 6009551 | Dec., 1999 | Sheynblat.
| |
| 6016119 | Jan., 2000 | Krasner.
| |
| 6023462 | Feb., 2000 | Nieczyporowicz et al.
| |
| 6041222 | Feb., 2000 | Horton et al.
| |
| 6047016 | Apr., 2000 | Ramberg et al.
| |
| 6047017 | Apr., 2000 | Cahn et al.
| |
| 6049715 | Apr., 2000 | Willhoff et al.
| |
| 6052081 | Apr., 2000 | Krasner.
| |
| 6061018 | May., 2000 | Sheynblat.
| |
| 6064336 | May., 2000 | Krasner.
| |
| 6064688 | May., 2000 | Yanagi.
| |
| 6075809 | Jun., 2000 | Naruse.
| |
| 6104338 | Aug., 2000 | Krasner.
| |
| 6104340 | Aug., 2000 | Krasner.
| |
| 6107960 | Aug., 2000 | Krasner.
| |
| 6108372 | Aug., 2000 | Tidemann, Jr. et al. | 375/225.
|
| 6111540 | Aug., 2000 | Krasner.
| |
| 6131067 | Oct., 2000 | Girerd et al.
| |
| 6133871 | Oct., 2000 | Krasner.
| |
| 6133873 | Oct., 2000 | Krasner.
| |
| 6133874 | Oct., 2000 | Krasner.
| |
| 6150980 | Nov., 2000 | Krasner.
| |
| Foreign Patent Documents |
| 0511741 A1 | Nov., 1992 | EP.
| |
| 0639901 A2 | Feb., 1995 | EP.
| |
| 0511741 B1 | Nov., 1997 | EP.
| |
| 0639901 A3 | Nov., 1998 | EP.
| |
| 08/065205 A | Mar., 1996 | JP.
| |
| WO 92/13392 A1 | Aug., 1992 | WO.
| |
| WO 00/19644 A2 | Apr., 2000 | WO.
| |
Other References
D. J. R. Van Nee and A. J. R. M. Coenen, "New Fast GPS Code-Acquisition
Technique Using FFT," Jan. 17, 1991, Electronics Letters, vol. 27, No. 2,
pp. 158-160.
|
Primary Examiner: Gregory; Bernarr E.
Attorney, Agent or Firm: Thomas, Kayden, Horstemeyer & Risley, L.L.P.
Claims
What is claimed is:
1. A signal detector comprising:
a receiver for receiving a first segment of a signal, and a second segment
of the signal, the first and second segments representing separate and
distinct periods of time, and the signal comprising a signal of interest
perturbed by noise or pseudo-noise;
a correlator coupled to the receiver for deriving first correlation data,
representative of the correlation between the first segment and a
hypothesis, and second correlation data representative of the correlation
between the second segment and the hypothesis; and a combiner coupled to
the correlator for determining a parameter difference between the first
and second correlation data, for adjusting a selected one of the first and
second correlation data responsive to the parameter difference, and
combining the adjusted data with the other of the first and second
correlation data to obtain cumulative correlation data useful for
detecting the signal of interest or a parameter of the signal of interest.
2. The signal detector of claim 1 in which the receiver is an RF receiver.
3. The signal detector is claim 1 in which the signal of interest is a
carrier signal modulated with a repeating PN code.
4. The signal detector of claim 1 in which the correlator is a matched
filter.
5. The signal detector of claim 1 in which the combiner is a processor.
6. The signal detector of claim 1 in which the parameter difference is a
code phase difference, and the combiner is configured to adjust the
selected one of the first and second correlation data responsive to the
code phase difference.
7. The signal detector of claim 1 in which the combiner is configured to
successively combine the correlation data for successive segments until
the signal of interest or a parameter of the signal of interest can be
accurately and reliably detected.
8. The signal detector of claim 7 in which the parameter of the signal of
interest is the code phase of the signal.
9. The signal detector of claim 7 in which the parameter of the signal of
interest is the presence of the signal.
10. A signal detector comprising:
a receiver for receiving a first segment of a signal and a second segment
of the signal, the signal representative of a plurality of signals of
interest, each signal of interest representative of a repeating PN code
modulated onto a carrier signal, and the first and second segments
representing separate and distinct periods of time;
a correlator coupled to the receiver for deriving first correlation data
representative of the correlation between the first segment and a combined
PN code and code phase hypotheses, and second correlation data
representative of the correlation between the second segment and the
combined PN code and code phase hypothesis; and
a combiner coupled to the correlator for determining a code phase
difference between portions of the first and second correlation data
corresponding to the PN code hypothesis, for adjusting a selected one of
the portions of the first and second correlation data responsive to the
code phase difference, and combining the adjusted data with the other
portion to obtain cumulative correlation data useful for detecting a
signal of interest or a parameter of a signal of interest.
11. The signal detector of claim 10 in which the correlator is configured
to derive correlation data representative of the correlation between a
segment and a plurality of code, code phase, and Doppler shift hypotheses.
12. A GPS receiver including either of the signal detectors of claims 1 and
10.
13. A method for detecting a signal comprising:
receiving a first segment of a signal, and a second segment of the signal,
the signal comprising a signal of interest perturbed by noise or
pseudo-noise, and the first and second segments representing separate and
distinct periods of time;
deriving first correlation data representative of the correlation between
the first segment and a hypothesis, and second correlation data
representative of the correlation between the second segment and the
hypothesis;
determining a parameter difference between the first and second correlation
data;
adjusting a selected one of the first and second correlation data
responsive to the parameter difference; and
combining the adjusted data with the other of the first and second
correlation data to obtain cumulative correlation data useful for
detecting the signal of interest or a parameter of the signal of interest.
14. The method of claim 13 wherein the signal of interest is a carrier
signal modulated with a repeating PN code.
15. The method of claim 13 wherein the parameter difference is a code phase
difference.
16. The method of claim 15 wherein the adjusting step comprises shifting
the selected one of the first and second correlation data responsive to
the code phase difference.
17. A method for detecting a signal comprising:
receiving a first segment of a signal and a second segment of the signal,
the signal representative of a plurality of signals of interest, each
signal of interest representative of a repeating PN code modulated onto a
carrier signal, the first and second segments representing separate and
distinct periods of time;
deriving first correlation data representative of the correlation between
the first segment and a combined PN code and code phase hypothesis, and
second correlation data representative of the correlation between the
second segment and the combined PN code and code phase hypothesis;
determining a code phase difference between portions of the first and
second correlation data corresponding to the PN code hypothesis;
adjusting a selected one of the portions of the first and second
correlation data responsive to the code phase difference; and
combining the adjusted data with the other portion to obtain cumulative
correlation data useful for detecting a signal of interest or a parameter
of a signal of interest.
18. The method of claim 17 further comprising successively combining the
cumulative correlation data with correlation data from successive segments
until the presence and code phase of a signal of interest can be
accurately and reliably detected.
19. The method of claim 17 further comprising deriving correlation data
representative of the correlation between a segment and a plurality of
code, code phase, and Doppler shift hypotheses.
20. The method of claim 19 wherein the portions of the first and second
correlation data correspond to a Doppler shift hypothesis, and the code
phase difference is determined from the time period between the beginning
of the first and second segments and the Doppler shift hypothesis
corresponding to the portions.
21. The method of claim 20 wherein the adjusting step comprises circularly
shifting the selected portion by a value derived from the code phase
difference between the two portions.
22. A signal detector, comprising:
means for receiving a signal of interest comprising a first segment and a
second segment of the signal of interest, each segment representing a
distinct period of time;
means for correlating a first correlation data corresponding to a first
correlation between the first segment and a hypothesis, and for deriving a
second correlation data corresponding to a second correlation between the
second segment and the hypothesis;
means for determining a parameter difference between the first and the
second correlation data;
means for adjusting a selected one of the first and the second correlation
data in response to the parameter difference to determine an adjusted
data; and
means for combining the adjusted data with the non-selected correlation
data for generating a cumulative correlation data for detecting the signal
of interest.
23. The signal detector of claim 22, wherein the time period for the first
segment and the time period for the second segment are separate,
nonoverlapping time periods.
24. The signal detector of claim 22, wherein the signal of interest is
representative of a plurality of signals of interest.
25. The signal detector of claim 22, wherein the signal of interest is
representative of a repeating PN code modulated onto a carrier signal.
26. The signal detector of claim 22, wherein the hypothesis is a combined
PN code and code phase hypothesis.
27. The signal detector of claim 22, wherein the means for correlating
further performs the function of deriving the cumulative correlation data
representative of the correlation between the first segment and a
plurality of code, code phase, and Doppler shift hypotheses, and between
the second segment and the plurality of code, code phase, and Doppler
shift hypotheses.
28. The signal detector of claim 27, wherein a first portion of the first
correlation data corresponds to a Doppler shift hypothesis and a second
portion of the second correlation data corresponds to the Doppler shift
hypothesis, and a code phase difference is determined from a time period
between the beginning of the first segment and the Doppler shift
hypothesis corresponding to the first portion, and between the beginning
of the second segment and the Doppler shift hypothesis corresponding to
the second portion.
29. The signal detector of claim 28, wherein the means for adjusting
circularly shifts the selected portion by a value derived from the code
phase difference between the first and the second portion.
30. The signal detector of claim 22, wherein the signal detector detects an
at least one global positioning system (GPS) signal.
31. The signal detector of claim 22, wherein the parameter difference
comprises a code phase difference.
32. The signal detector of claim 22, wherein the means for adjusting shifts
the selected correlation data.
33. The signal detector of claim 22, wherein the signal of interest is
perturbed by noise.
34. The signal detector of claim 22, wherein the signal of interest is
perturbed by pseudo-noise.
35. The signal detector of claim 22, wherein the means for receiving
comprises a radio frequency receiver.
36. The signal detector of claim 22, wherein the signal of interest
comprises a carrier signal modulated with a pseudo-noise code.
37. The signal detector of claim 22, wherein the means for correlating
comprises a matched filter.
38. The signal detector of claim 22, wherein the means for combining
comprises a processor.
39. The signal detector of claim 22, wherein the means for combining
successively combines the correlation data for a plurality of successive
segments to detect the signal of interest.
40. The signal detector of claim 22, wherein the means for combining
successively combines the correlation data for a plurality of successive
segments to detect a parameter of the signal of interest.
41. The signal detector of claim 40, wherein the parameter comprises a code
phase of the signal of interest.
42. The signal detector of claim 40, wherein the parameter comprises the
presence of the signal of interest.
43. A signal detector, comprising:
a receiver configured to detect a first segment of a signal of interest and
a second segment of the signal of interest, and the first segment and the
second segment representing distinct periods of time;
a correlator, coupled to the receiver, configured to derive a first
correlation data representative of the correlation between the first
segment and a hypotheses, and configured to derive a second correlation
data representative of the correlation between the second segment and the
hypothesis;
a parameter determiner, coupled to the correlator, configured to determine
a parameter difference between the first and the second correlation data;
a processor, coupled to the parameter determiner, configured to adjust a
selected one of the first and second correlation data responsive to the
parameter difference to determine an adjusted data; and
a combiner, coupled to the processor, configured to combine the adjusted
data with the non-selected correlation data to generate cumulative
correlation data useful for detecting the signal of interest.
44. The signal detector of claim 43, wherein the time period for the first
segment and the time period for the second segment are separate,
nonoverlapping time periods.
45. The signal detector of claim 43, wherein the signal of interest is
representative of a plurality of signals of interest.
46. The signal detector of claim 43, therein the signal of interest is
representative of a repeating PN code modulated onto a carrier signal.
47. The signal detector of claim 43, wherein the hypothesis is a combined
PN code and code phase hypothesis.
48. The signal detector of claim 43, wherein the correlator is configured
to derive correlation data representative of the correlation between the
first segment and a plurality of code, code phase, and Doppler shift
hypotheses, and between the second segment and the plurality of code, code
phase, and Doppler shift hypotheses.
49. The signal detector of claim 48, wherein a first portion of the first
correlation data corresponds to a Doppler shift hypothesis and a second
portion of the second correlation data corresponds to the Doppler shift
hypothesis, and a code phase difference is determined from a time period
between the beginning of the first segment and the Doppler shift
hypothesis corresponding to the first portion, and between the beginning
of the second segment and the Doppler shift hypothesis corresponding to
the second portion.
50. The signal detector of claim 49, wherein the processor circularly
shifts the selected portion by a value derived from the code phase
difference between the first and the second portion.
51. The signal detector of claim 43, wherein the signal detector detects an
at least one global positioning system (GPS) signal.
52. The signal detector of claim 43, wherein the parameter difference
comprises a code phase difference.
53. The signal detector of claim 43, wherein the processor shifts the
selected correlation data.
54. The signal detector of claim 43, wherein the signal of interest is
perturbed by noise.
55. The signal detector of claim 54, wherein the parameter comprises a code
phase difference.
56. The signal detector of claim 54, wherein the parameter comprises the
presence of the signal of interest.
57. The signal detector of claim 43, wherein the signal of interest is
perturbed by pseudo-noise.
58. The signal detector of claim 43, wherein the receiver comprises a radio
frequency receiver.
59. The signal detector of claim 43, wherein the signal of interest
comprises a carrier signal modulated with a pseudo-noise code.
60. The signal detector of claim 43, wherein the correlator is a matched
filter.
61. The signal detector of claim 43, wherein the combiner comprises a
processor.
62. The signal detector of claim 43, wherein the combiner successively
combines the correlation data for a plurality of successive segments to
detect the signal of interest.
63. The signal detector of claim 43, wherein the combiner successively
combines the correlation data for a plurality of successive segments to
detect a parameter of the signal of interest.
64. A method for detecting signals of interest, comprising the steps of:
receiving a first segment of a signal of interest;
receiving a second segment of the signal of interest;
deriving a first correlation data representative of the correlation between
the first segment and a hypothesis;
deriving a second correlation data representative of the correlation
between the second segment and the hypothesis;
determining a parameter difference between the first aid the second
correlation data;
selecting one of the first and the second correlation data;
adjusting the selected correlation data in response to the parameter
difference to determine an adjusted data; and
combining the adjusted data with the non-selected correlation data to
determine a cumulative correlation data useful for detecting the signal of
interest.
65. The method of claim 64, wherein the period of time for the first
segment and the time period for the second segment are separate,
nonoverlapping time periods.
66. The method of claim 64, wherein the signal of interest is
representative of a plurality of signals of interest.
67. The method of claim 64, wherein the signal of interest is a carrier
signal modulated with a repeating PN code.
68. The method of claim 64, wherein the hypothesis is a combined PN code
and code phase hypothesis.
69. The method of claim 64, wherein the correlation data is representative
of the correlation between the first segment and a plurality of code, code
phase, and Doppler shift hypotheses, and between the second segment and
the plurality of code, code phase, and Doppler shift hypotheses.
70. The method of claim 69, wherein a first portion of the first
correlation data corresponds to a Doppler shift hypothesis and a second
portion of the second correlation data corresponds to the Doppler shift
hypothesis, and a code phase difference is determined from a time period
between the beginning of the first segment and the Doppler shift
hypothesis corresponding to the first portion, and between the beginning
of the second segment and the Doppler shift hypothesis corresponding to
the second portion.
71. The method of claim 70, wherein the adjusting step comprises circularly
shifting the selected portion by a value derived from the code phase
difference between the first and the second portion.
72. The method of claim 64, wherein the receiving step receives at least
one global positioning system (GPS) signal.
73. The method of claim 64, wherein the parameter difference is a code
phase difference.
74. The method of claim 64, wherein the adjusting step is implemented by
shifting the selected correlation data.
75. The method of claim 64, wherein the signal of interest is perturbed by
noise.
76. The method of claim 64, wherein the signal of interest is perturbed by
pseudo-noise.
77. The method of claim 64, wherein the receiving step is implemented with
a radio frequency receiver.
78. The method of claim 64, wherein the signal of interest comprises a
carrier signal modulated with a pseudo-noise code.
79. The method of claim 64, wherein the combining step is implemented with
a matched filter.
80. The method of claim 64, wherein the combining step successively
combines the correlation data for a plurality of successive segments to
detect the signal of interest.
81. The method of claim 64, wherein the combining step successively
combines the correlation data for a plurality of successive segments to
detect a parameter of the signal of interest.
82. The method of claim 81, wherein the parameter comprises a code phase of
the signal of interest.
83. The method of claim 81, wherein the parameter comprises the presence of
the signal of interest.
84. A computer readable medium having a program for storing a series of
instructions for detecting signals of interest, the program for performing
at least the following:
receiving a first segment of a signal of interest;
receiving a second segment of the signal of interest, such that the first
segment and the second segment represent distinct periods of time;
deriving a first correlation data representative of the correlation between
the first segment and a hypothesis;
deriving a second correlation data representative of the correlation
between the second segment and the hypothesis;
determining a parameter difference between the first and the second
correlation data;
selecting one of the first and the second correlation data;
adjusting the selected correlation data in response to the parameter
difference to determine an adjusted data; and
combining the adjusted data with the non-selected correlation data to
determine a cumulative correlation data useful for detecting the signal of
interest.
85. The medium as defined in claim 84, further comprising a series of
instructions determining a code phase difference between portions of the
first correlation data and the second correlation data such that each the
code phase corresponds to a combination of a PN code, Doppler shift, and
code phase hypothesis; and, further comprising the series of instructions
adjusting a selected one of the portions of the first correlation data and
the second correlation data responsive to the code phase difference.
86. The medium as defined in claim 85, wherein the series of instructions
determining the code phase difference is determined in accordance with an
equation .DELTA.CP=[(F.sub.PN +D).times..DELTA.T] modulo T.sub.PN, wherein
.DELTA.CP is the code phase difference, F.sub.PN is a PN rate, D is a
Doppler shift hypothesis, .DELTA.T is a time offset between the beginning
of the first segment and the beginning of the second segment, and TPN is a
period of the PN codes.
87. The medium as defined in claim 86, wherein each of the first segment
and the second segment span at least a frame, and begin at a time which is
an offset from a frame mark, and the offsets for the first segment and the
second segment, an OS.sub.1 and OS.sub.2, respectively, are used to
determine the .DELTA.T in accordance with an equation .DELTA.T=(T.sub.n
-T.sub.2 +OS.sub.1 -OS.sub.2).times.S, where T.sub.n is the timing of the
frame mark which occurs at the beginning of the second segment, and from
which the OS.sub.2 is defined, a T.sub.2 is the timing of the frame mark
which occurs at the beginning of the first segment, and from which the OS,
is defined, and a variable S is the error in a time base.
Description
BACKGROUND
I. Field of the Invention
This invention relates to the field of signal detection using correlation
analysis, and more specifically, to correlation analysis in which the
results of analyzing segments of samples separated in time and possibly
having non-uniform lengths are combined to achieve a greater effective
signal to noise ratio (SNR).
II. Background of the Invention
The Global Positioning System (GPS) is a collection of 24 earth-orbiting
satellites. Each of the GPS satellites travels in a precise orbit about
11,000 miles above the earth's surface. A GPS receiver locks onto at least
3 of the satellites, and responsive, thereto, is able to determine its
precise location. Each satellite transmits a signal modulated with a
unique pseudo-noise (PN) code. Each PN code comprises a sequence of 1023
chips which are repeated every millisecond consistent with a chip rate of
1.023 MHz. Each satellite transmits at the same frequency. For civil
applications, the frequency is known as L1 and is 1575.42 MHz. The GPS
receiver receives a signal which is a mixture of the transmissions of the
satellites that are visible to the receiver. The receiver detects the
transmission of a particular satellite by correlating the received signal
with shifted versions of the PN code for that satellite. If the level of
correlation is sufficiently high so that there is a peak in the level of
correlation achieved for a particular shift and PN code, the receiver
detects the transmission of the satellite corresponding to the particular
PN code. The receiver then used the shifted PN code to achieve
synchronization with subsequent transmissions from the satellite.
The receiver determines its distance from the satellite by determining the
code phase of the transmission from the satellite. The code phase (CP) is
the delay, in terms of chips or fractions of chips, that a satellite
transmission experiences as it travels the approximately 11,000 mile
distance from the satellite to the receiver. The receiver determines the
code phase for a particular satellite by correlating shifted versions of
the satellite's PN code with the received signal after correction for
Doppler shift. The code phase for the satellite is determined to be the
shift which maximizes the degree of correlation with the received signal.
The receiver converts the code phase for a satellite to a time delay. It
determines the distance to the satellite by multiplying the time delay by
the velocity of the transmission from the satellite. The receiver also
knows the precise orbits of each of the satellites. The receiver uses this
information to define a sphere around the satellite at which the receiver
must be located, with the radius of the sphere equal to the distance the
receiver has determined from the code phase. The receiver performs this
process for at least three satellites. The receiver derives its precise
location from the points of intersection between the at least three
spheres it has defined.
The Doppler shift (DS) is a frequency shift in the satellite transmission
caused by relative movement between the satellite and the receiver along
the line-of-sight (LOS). It can be shown that the frequency shift is equal
to
##EQU1##
where .nu..sub.LOS is the velocity of the relative movement between the
satellite and receiver along the LOS, and .lambda. is the wavelength of
the transmission. The Doppler shift is positive if the receiver and
satellite are moving towards one another along the LOS, and is negative if
the receiver and satellite are moving away from one another along the LOS.
The Doppler shift alters the perceived code phase of a satellite
transmission from its actual value. Hence, the GPS receiver must correct
the satellite transmissions for Doppler shift before it attempts to
determine the code phase for the satellite through correlation analysis.
The situation is illustrated in FIG. 1, which shows a GPS receiver 10 and
three GPS satellites 12a, 12b, and 12c. Each satellite 12a, 12b, 12c is
transmitting to the GPS receiver 10. Satellite 12a is moving towards the
GPS receiver 10 along the LOS at a velocity .nu..sub.a.sup.+ 14; satellite
12b is moving away from the GPS receiver 10 along the LOS at a velocity
.nu..sub.b.sup.- 16; and satellite 12c is moving away from the GPS
receiver 10 along the LOS at a velocity .nu..sub.c.sup.- 18. Consequently,
assuming a carrier wavelength of .lambda., the transmission from satellite
12a will experience a positive Doppler shift of
##EQU2##
the transmission from satellite 12b will experience a negative Doppler
shift of
##EQU3##
the transmission from satellite 12c will experience a negative Doppler
shift of
##EQU4##
The GPS receiver functions by sampling a finite portion of the received
signal 20 and then processing the samples. Typically, external constraints
limit the size and occurrence of the sampling period. For example, in the
case of a mobile wireless phone integrated with a GPS receiver, the
sampling window should be limited to those periods in which the phone is
not transmitting. The purpose is to avoid interference between the
satellite transmitter and the GPS receiver 10.
The problem is that the signal to noise ratio of the received signal 20
over a finite sampling window may not be sufficient to detect the presence
and range of the satellite transmitters. For example, the signal may be
such that there is no correlation value for a particular set of hypotheses
which is significantly larger than the correlation values resulting from
the other hypotheses tested.
Moreover, it is difficult to combine segments of samples captured over
different periods of time because each is subject to a different code
phase which must be accounted for before the segments can be combined, and
these code phases are unknown. In an effort to increase the signal to
noise ratio of the received signal, prior art receivers are thus required
to either forgo operation during times in which the received signal is
weak, or to extend the sampling period beyond the limits imposed by
external constraints. In certain applications, such as the case of a GPS
receiver integrated with a mobile wireless phone, extension of the
sampling window is not usually feasible since it would subject the
received signal to unacceptable interference from the phone's transmitter.
In such applications, the practical effect is to forego operation of the
GPS receiver when the received signal is weak. Such occurrences are
frequent because of the approximately 11,000 mile distance traveled by the
GPS satellite transmissions, and because of the noise to a particular
satellite represented by the other satellite's transmissions.
Consequently, there is a need for a signal detector which overcomes the
disadvantages of the prior art. Similarly, there is a need for a GPS
receiver 10 which overcomes the disadvantages of the prior art.
RELATED APPLICATIONS
This application is related to U.S. patent application Ser. No. 09/145,055,
filed Sep. 1, 1998, and entitled "DOPPLER CORRECTED SPREAD SPECTRUM
MATCHED FILTER," and now U.S. Pat. No. 6,044,105, U.S. patent application
Ser. No. 09/281,566, filed on even data herewith, and entitled "SIGNAL
DETECTOR EMPLOYING COHERENT INTEGRATION," both of which are owned in
common by the assignee hereof, and both of which are hereby fully
incorporated by reference herein as though set forth in full.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates an example environment for operation of a GPS receiver.
FIG. 2 is a block diagram of an embodiment of a signal detector in
accordance with an embodiment of the subject invention.
FIG. 3 is a flowchart illustrating an embodiment of a method of operation
of the signal detector of FIG. 2.
FIG. 4 illustrates an embodiment of a GPS receiver in accordance with the
subject invention.
FIG. 5 illustrates the data structures output by one embodiment of a
matched filter in accordance with the subject invention.
FIG. 6 illustrates data flows in one embodiment of a GPS receiver in
accordance with the subject invention.
FIG. 7 is a timing diagram for combining multiple correlation arrays in
accordance with one implementation of the subject invention.
FIG. 8 illustrates an embodiment of a matched filter in accordance with the
subject invention.
FIG. 9 illustrates an embodiment of a method of operation of a GPS receiver
in accordance with the subject invention.
FIGS. 10A-10C illustrate an embodiment of a method of operation of a GPS
receiver in accordance with the subject invention.
FIGS. 11A-11C illustrate an embodiment of a method of operation of a
matched filter in accordance with the subject invention.
FIG. 12 illustrates an embodiment of a method of operation of a GPS
receiver in accordance with the subject invention.
FIGS. 13A-13B illustrate an embodiment of a method of operation of a GPS
receiver in accordance with the subject invention.
SUMMARY OF THE INVENTION
In accordance with the purpose of the invention as broadly described
herein, there is provided a signal detector configured to combine the
results of performing correlation analysis on segments of samples of the
received signal that may have non-uniform lengths and that may have been
obtained over different and non-overlapping periods of time. In one
embodiment, the segments are obtained during sampling windows of arbitrary
length and at arbitrary times, and the results of processing the segments
are successively combined until a threshold signal to noise ratio has been
achieved.
In one embodiment, the signal detector is part of a GPS receiver. In this
embodiment, the GPS receiver comprises a radio frequency (RF) receiver,
sampling circuitry, timing circuitry, offset measurement circuitry, a PN
code generator, a matched filter, and a GPS processor. The RF receiver
demodulates the received signal to obtain a baseband signal. The sampling
circuitry provides, responsive to timing signals produced by the timing
circuitry, a segment of samples of the baseband signal taken over a
defined sampling window. The matched filter processes the segment of
samples in accordance with a plurality of PN code, Doppler shift, and code
phase hypotheses.
In one implementation, for each segment of samples, the matched filter
outputs correlation data derived by correlating various combinations of PN
code, Doppler shift and code phase hypotheses with the segment of samples.
According to this implementation, the correlation data can be grouped into
groupings which correspond to various combinations of specific hypotheses
and ranges of hypotheses. In one implementation example, the correlation
data comprises a plurality of arrays, wherein each array corresponds to a
PN code hypothesis, each row of an array corresponds to a Doppler shift
hypothesis, each column of an array corresponds to a code phase
hypothesis, and each entry in the array is a measure of the degree to
which the combined PN code, Doppler shift, and code phase hypothesis
corresponding to the entry correlates to the samples.
The PN code generator generates the PN code hypotheses which are provided
as inputs to the matched filter. In one embodiment, the Doppler shift
hypotheses are generated internally within the matched filter. The GPS
processor sends out data capture commands to the sampling circuitry and
the matched filter directs the sampling circuitry to capture a segment of
samples, and directs the matched filter to process the segment of samples.
The GPS processor also, responsive to timing signals generated locally by
the GPS radio receiver, generates frame marks which are input to the
offset measurement circuitry. The offset measurement circuitry determines
the offset between the time that a data capture command is issued by the
GPS processor and the timing of the next frame mark. This information is
provided to the GPS processor for use in combining the results of
processing different segments of samples.
In one embodiment, cumulative correlation data is maintained. When the
correlation data for a new segment of samples is obtained, the new
correlation data is combined with the cumulative correlation data.
According to this embodiment, the GPS processor accumulates the
correlation data for a particular satellite until a threshold signal to
noise ratio is achieved. In one implementation, the data for a particular
satellite is maintained until the presence and range of the satellite can
be determined.
The correlation data for different segments of samples are combined using
an algorithm that allows for the differing code phases between the
segments to be accounted for even through the actual code phases are
unknown. In one embodiment, in which the correlation data for a given
segment of samples comprises a plurality of arrays, wherein each array in
the plurality corresponds to a particular PN code hypothesis, and each row
of an array corresponds to a particular Doppler shift hypothesis,
cumulative correlation arrays are initialized with the correlation arrays
for the first segment. Then, the correlation arrays for the second segment
are combined with the cumulative correlation arrays one array at a time,
and within an array, one row at a time. The process continues indefinitely
and ceases for a particular satellite (PN code) when the presence and
range of the satellite is accurately detected.
The code phase difference corresponding to a row is derived from the total
time offset between the start of the first segment and the start of the
second segment, and the Doppler shift hypothesis corresponding to the row.
In one implementation example, the code phase difference is determined from
the following equation:
.DELTA.CP=[(F.sub.PN +D).times..DELTA.T] modulo 1 mS
in which .DELTA.CP is the code phase difference, F.sub.PN is the nominal PN
rate of 1.023 MHz, D is the Doppler shift hypothesis corresponding to the
array, and .DELTA.T is the time offset between the beginning of the
sampling period for the first segment, and that for the second segment.
The value of 1 mS is the period of the PN codes. Since the codes are
periodic, the code phase difference of interest is the fractional part of
the code period. This is reflected in the foregoing equation through the
modulo 1 mS operation.
In this implementation example, the first and second segments each span at
least a frame, and begin at a time which is offset from a frame mark. The
offsets for the first and second segment, OS.sub.1 and OS.sub.2,
respectively, are each determined from the offset measurement circuitry.
According to the implementation example, the offsets are used to determine
.DELTA.T in the foregoing equation as follows:
.DELTA.T=(T.sub.n -T.sub.2 +OS.sub.1 -OS.sub.2).times.S
In this latter equation, T.sub.n is the timing of the frame mark which
occurs at the beginning of the second segment, and from which OS.sub.2 is
defined. Similarly, T.sub.2 is the timing of the frame mark which occurs
at the beginning of the first segment, and from which OS.sub.1 is defined.
The variable S is the error in the local time base generated by a local
crystal oscillator in the GPS receiver in relation to the time base
defined at the GPS satellites. The GPS receiver determines this error and
corrects for it in the foregoing equation.
This code phase difference is then used to combine the rows for the
correlation arrays. In the implementation example, the row of correlation
data for the second segment is circularly shifted by the code phase
difference. Then, the shifted row is added to the corresponding row of
cumulative correlation data, one data element at a time. This process is
then repeated for each of the rows of each of the arrays for the second
segment. The result is a plurality of cumulative correlation arrays which
combine the results of the first and second segments.
The correlation arrays for additional segments are combined with the
cumulative correlation arrays in the foregoing manner. For a given
satellite, the process of combining segments continues until the signal to
noise ratio is sufficient to permit the presence and range of the
satellite to be accurately determined.
Related methods of operation and computer readable media are also provided.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
I. Signal Detector Overview
A block diagram of one embodiment of a signal detector in accordance with
the subject invention is illustrated in FIG. 2. As illustrated, the signal
detector comprises a receiver 30 configured to receive segments of a
signal. The segments may be of nonuniform-length and may be disjoint and
separated by arbitrary periods of time. The signal may comprise a signal
of interest perturbed by noise. Alternatively, in accordance with a spread
spectrum environment, the signal may comprise the combination of multiple
signals of interest each encoded using one of a plurality of pseudo-noise
(PN) codes. In this case, other signals appear as noise to a particular
signal of interest.
A hypothesis generator 32 generates a plurality of hypotheses about the
signal of interest. A correlator 34 receives the plurality of hypotheses
from hypothesis generator 32, and the segment received by receiver 30, and
responsive thereto, generates correlation data representative of the
correlation between the received segment and the plurality of generated
hypotheses.
The correlation data is provided to combiner 36 which combines the
correlation data with cumulative correlation data accumulated for previous
segments received by receiver 30. In one embodiment, the combiner 36 first
performs an adjustment on the correlation data so that it is combinable
with the cumulative correlation data. Once the data has been combined,
combiner 36 determines whether the cumulative correlation data is
sufficient to permit a parameter of the signal of interest to be detected.
If so, an output is provided on signal line 38 signaling detection of the
parameter of the signal of interest. If not, a signal is provided to the
receiver on signal line 40 indicating the need for additional segments.
Optionally, the foregoing process iterates until the parameter of the
signal of interest is detected (or a time-out condition is detected).
A flowchart of one embodiment of a method of operation of a signal detector
in accordance with the subject invention is illustrated in FIG. 3. As
illustrated, the process begins at step 50, in which a segment of the type
described earlier is received. Then, in step 52, a plurality of hypotheses
are generated for testing. In step 54, correlation data measuring the
level of correlation between the received sample and the generated
hypotheses is derived.
In step 56, the correlation data from step 54 is combined with cumulative
correlation data which may be present from correlation analysis performed
on previous segments. In one embodiment, the correlation data from step 54
is first adjusted or altered so that it is combinable with the cumulative
correlation data. If no such data is present, the cumulative correlation
data is initialized with the correlation data derived in step 54.
In step 58, a determination is made whether the cumulative correlation data
permits accurate and reliable detection of a desired parameter of the
signal of interest. If so, the process ends. If not, a jump is made to
step 50 and the process repeats itself from this point with a new segment.
Optionally, the process iterates until the desired parameter of the signal
of interest is detected (or a time-out condition occurs).
It is contemplated that the foregoing signal detector can be beneficially
employed in a variety of applications, such as in a GPS receiver 70. One
embodiment of a GPS receiver 70 in accordance with the subject invention
is illustrated in FIG. 4. As shown, the receiver 70 comprises a radio
frequency (RF) receiver 72, sampling circuitry 74, timing circuitry 76,
offset measurement circuitry 78, a PN code generator 80, a matched filter
82, and a GPS processor 84. The RF receiver section 72 demodulates the
received signal to obtain a baseband signal which is provided to the
sampling circuitry 74 over signal line 86. The sampling circuitry 74
provides, responsive to timing signals produced by the timing circuitry
76, a segment of samples of the baseband signal taken over a defined
sampling window. The segment of samples is provided to the matched filter
82 over signal line 88. The matched filter 82 processes the segment of
samples in accordance with a plurality of PN code, Doppler shift, and code
phase hypotheses.
In one implementation, as illustrated in FIG. 5, for each segment of
samples, the matched filter outputs a plurality of correlation arrays 110,
112, 114 of data derived from the segment of samples. According to this
implementation, each array 110, 112, 114 corresponds to a PN code
hypothesis, PN1, PN2, . . . PNr, each row of an array 110, 112, 114
corresponds to a Doppler shift hypothesis, DS1, DS2, . . . DSm, each
column of an array 110, 112, 114 corresponds to a code phase hypothesis,
CP1, CP2, . . . CPn, and each entry in an array 110, 112, 114 is a measure
of the degree to which the combined PN code, Doppler shift, and code phase
hypothesis corresponding to the entry correlates to the samples. Thus, in
FIG. 5, correlation array 110 corresponds to PN code hypothesis PN1;
correlation array 112 corresponds to PN code hypothesis PN2; and
correlation array 114 corresponds to PN code hypothesis PNr.
Returning now to FIG. 4, the PN code generator 80 generates the PN code
hypotheses which are provided as inputs to the matched filter 82 over
signal line 90. In one embodiment, the Doppler shift hypotheses are
generated internally within the matched filter. The GPS processor 84
issues data capture commands on signal line 92 to the sampling circuitry
74 and the matched filter 82 directs the sampling circuitry 74 to capture
a segment of samples, and directs the matched filter 82 to process the
segment of samples. The GPS processor 84 also, responsive to timing
signals generated locally by the GPS radio receiver 72 and received over
signal line 94, generates frame marks which are input to the offset
measurement circuitry 78 over signal line 96. In one implementation, the
timing signals generated by the GPS radio receiver 72 are generated by a
local oscillator within the RF receiver. In one implementation, the timing
signals define a local time base which is related to the time base
maintained by the GPS satellites.
II. Correlation Process
The offset measurement circuitry 78 determines the offset between the time
that a data capture command is issued by the GPS processor and the time of
the next frame mark. This information is provided to the GPS processor 84
for use in combining, in a manner to be described subsequently, the
results of processing different segments of samples.
In one embodiment, the correlation arrays for a segment are grouped by PN
code hypothesis, and by Doppler shift hypothesis for a given PN code
hypothesis. The result is that each grouping corresponds to a particular
combination of PN code hypothesis and Doppler shift hypothesis. In this
embodiment, the correlation arrays are combined one grouping at a time.
According to this implementation, the GPS processor 84 receives these
groupings over signal line 98, and cumulatively combines these correlation
arrays as additional segments are captured. For a particular satellite,
the combination process continues until a threshold signal to noise ratio
is obtained for the satellite. In one implementation example, the
correlation arrays corresponding to a satellite are combined until the
presence and range of the satellite can be accurately determined.
Typically, this occurs when the correlation data for a particular set of
hypotheses is significantly greater than the correlation data for
alternative hypotheses.
The correlation arrays derived from different segments of samples are
combined using an algorithm that accounts for the differing code phases
between the segments even though the actual code phases are unknown. In
one embodiment, a plurality of cumulative correlation arrays are
maintained, which are initially set equal to the correlation arrays
derived from a first segment of samples. The correlation arrays for a
second segment of samples are then combined with the cumulative
correlation arrays one row at a time. According to this embodiment, the
code phase difference corresponding to a row is derived from the total
time offset between the start of the first segment and the start of the
second segment, and the Doppler shift hypothesis corresponding to the row.
A circular shift of the row for the second segment is then performed, with
the amount of the shift being derived from the code phase difference which
has been determined. The entries in the shifted row are then added to the
corresponding entries of the corresponding row in the cumulative arrays.
The foregoing combining process is then repeated for additional data in the
correlation arrays for the second segment. In one implementation, the
process is repeated for each of the rows in each of the arrays remaining
in the correlation data for the second segment. Moreover, once the data
for the second segment has been exhausted, the data for additional
segments is combined with the cumulative arrays in the foregoing manner
until, for a given satellite, the signal to noise ratio is sufficient to
permit the presence and range of the satellite to be accurately
determined.
The situation is illustrated in FIG. 6. Numeral 120 identifies one of the
arrays in the plurality of cumulative correlation arrays which are
maintained. The particular array which is identified is the one
corresponding to PN code hypothesis PNi. These arrays are assumed to have
been initially set equal to the correlation arrays derived from a first
segment of samples. Numeral 122 identifies the corresponding one of the
arrays in the plurality of correlation arrays which are derived from a
second set of samples. Again, this array corresponds to the PN code
hypothesis PNi. In one implementation, the first segment of samples is
obtained during a first time period, and the second set of samples is
obtained during a subsequent non-consecutive time period.
The row 124 from array 122 is combined with the row 126 from array 120 in
the manner shown. Both rows are assumed to correspond to the same Doppler
shift hypothesis, DPi. Row 126 is obtained as indicated by identifying
numeral 128. In addition, row 124 is obtained as indicated by identifying
numeral 130. As indicated by identifying numeral 132, the code phase
difference, .DELTA.CP, defining the code phase difference between the
start of the first and second segments is determined. The row 124 obtained
from array 122 is then circularly shifted by an amount derived from
.DELTA.CP. Such is indicated by identifying numeral 134. The entries in
the shifted row are then added to corresponding entries in row 126. Such
is indicated by identifying numeral 136. The result is then stored in
array 120 in lieu of row 126. Such is indicated by identifying numeral
138.
III. Code Phase Difference Determination
In one implementation example, the code phase difference is determined from
the following equation:
.DELTA.CP=[(F.sub.PN +D).times..DELTA.T] modulo 1 mS (1)
in which .DELTA.CP is the code phase difference, F.sub.PN is the nominal PN
rate of 1.023 MHz, D is the Doppler shift hypothesis corresponding to the
row, and .DELTA.T is the time offset between the beginning of the sampling
period for the first segment, and that for the second segment. The value
of 1 mS is the period of the PN codes. Since the codes are periodic, the
code phase difference of interest is the fractional part of the code
period. This is reflected in the foregoing equation through the modulo 1
mS operation.
In this implementation example, the first and second segments each span at
least a frame, and begin at a time which is offset from a frame mark. The
offsets for the first and second segments, OS.sub.1 and OS.sub.2,
respectively, are each determined from the offset measurement circuitry 78
(FIG. 4). According to the implementation example, the offsets are used to
determine .DELTA.T in the foregoing equation as follows:
.DELTA.T=(T.sub.n -T.sub.2 +OS.sub.1 -OS.sub.2).times.S (2)
In this latter equation, T.sub.n is the timing of the frame mark which
occurs at the beginning of the second segment, and from which OS.sub.2 is
defined. Similarly, T.sub.2 is the timing of the frame mark which occurs
at the beginning of the first segment, and from which OS.sub.1 is defined.
The variable S is the error in the local time base generated by a local
crystal oscillator in the GPS radio receiver 72 in relation to the time
base defined at the GPS satellites. The GPS receiver determines this error
and corrects for it in the foregoing equation.
The situation is illustrated in FIG. 7. Local time base 150 is divided into
frames by frame marks T.sub.1, T.sub.2, . . . T.sub.n. In one
implementation example, the duration of a frame is 20 mS. Upon detecting
the occurrence of frame mark T.sub.1., identified in the figure with
numeral 152, GPS processor 84 (FIG. 4) issues a command, identified with
numeral 154, to capture a first segment of samples. The capture of the
first segment of samples begins at time 156, and the segment itself is
identified in the figure with numeral 158. As 20 indicated, the first set
of samples is assumed to span at least one frame. The offset measurement
circuitry 78 (FIG. 4) detects the next occurrence of a frame mark,
T.sub.2, identified in the figure with numeral 160, and, responsive
thereto, determines the offset, OS.sub.1, between the time 156 that
capture of the segment began, and the time 160 of the frame mark T.sub.2.
The segment of samples 158 is typically representative of the combination
of transmissions from multiple ones of GPS satellites, each of which has a
different code phase as measured at the receiver. In relation to a
particular satellite of interest, the segment 158 will have a specific
code phase, CP.sub.1, which is identified in the figure with numeral 162.
After the occurrence of a number of additional frame marks, T.sub.3, . . .
T.sub.n-1, the GPS processor 84 (FIG. 4) issues another command to capture
a segment of samples. This second command is identified in FIG. 7 with
numeral 164, and is assumed to occur after the GPS processor 84 has
detected the occurrence of frame mark T.sub.n-1. The capture of the second
segment, which is identified with numeral 166, begins at time 168. Again,
the second segment is assumed to span at least one frame. The next frame
mark, T.sub.n, occurs at time 170. The offset measurement circuitry 78
measures the offset, OS.sub.2, between the beginning 168 of the second
segment, and the occurrence 170 of the next frame mark T.sub.n. This
second frame is assumed to have a particular code phase, CP.sub.2, which
is defined in relation to a particular satellite. The time .DELTA.T in the
foregoing equations is the difference in time between the beginning 168 of
the second segment, and the beginning 156 of the first segment, and the
value .DELTA.CP in the foregoing equations is the difference in code phase
between the code phase for the first segment, CP.sub.1, and that for the
second segment, CP.sub.2. Significantly, this value is determined from the
foregoing equations even though the underlying code phases CP.sub.1 and
CP.sub.2 are unknown.
IV. Embodiment of a Matched Filter
FIG. 8 illustrates one embodiment of matched filter 82 in FIG. 4. Compared
to FIG. 4, like elements in FIG. 8 are referenced with like identifying
numerals. As illustrated, this embodiment of the matched filter comprises
random access memory (RAM) 180 which is configured to receive samples from
sampling circuitry 74 (FIG. 4) over signal line 88. In one implementation
example, a frame is 20 mS in duration, and the RAM 180 is configured to
receive one 20 mS frame of samples at a time. According to this
implementation example, each 20 mS frame of samples comprises 40920
samples, obtained by sampling the baseband signal at a nominal sampling
rate of 20.46 MHz, and then performing decimation filtering.
Also provided is Doppler shift correction circuitry 182, and Doppler shift
generator 184. The RAM 180 is configured to provide to Doppler shift
correction circuitry 182 over signal line 186 at least a portion of a
frame of samples stored therein at a time. In one implementation, the
frame of samples stored in RAM 180 is divided into subframes, and is
provided to Doppler shift correction circuitry 182 one subframe at a time.
In one implementation example, the duration of a subframe is equal to the
period of the PN codes, which is currently set to 1 mS. In this
implementation example, each subframe of samples comprises 2046 samples,
with each sample represented as a complex number having an in-phase (I)
and a quadrature (Q) component which are typically expressed in
combination as I+jQ. According to this implementation example, each
component, whether I or Q, is represented by 2 bits, and can take on any
one of the discrete values -1, 0, or +1.
Doppler shift generator 184 generates a plurality of Doppler shift
hypotheses which are provided to Doppler shift correction circuitry 182
over signal line 188 one hypothesis at a time. In one implementation
example, the Doppler shift generator 184 generates Doppler shift
hypotheses in the range of .+-.62,000 Hz, to allow for additional
inaccuracy in the local time base that is not corrected by the input
sampling process.
Sample register 190 is also provided. Doppler shift correction circuitry
182 receives a subframe of samples from RAM 180 over signal line, and a
Doppler shift hypothesis from Doppler shift generator 184, and, responsive
thereto, generates a corrected subframe of samples which are stored in
sample register 190. Additional detail about this procedure is available
in U.S. patent application Ser. No. 09/145,055, filed Sep. 1, 1998,
entitled "DOPPLER CORRECTED SPREAD SPECTRUM MATCHED FILTER," now U.S. Pat.
No. 6,044,105, previously incorporated by reference herein as though set
forth in full. In one implementation example, each corrected subframe of
samples continues to comprise 2046 complex samples, each having I and Q
components. Each of the samples in this implementation example can take on
any one of the discrete values -2, -1, 0, +1, and +2, which are
represented by 3 bits for each of I and Q.
PN code register 192 is provided to store the current PN code hypothesis
provided by PN code generator 80 (FIG. 4) over signal line 90. In one
implementation, each PN code hypothesis represents one period of a PN
code. In one implementation example, the PN code period is 1 mS, and each
PN code hypothesis represents 1023 chips which repeats every 1 mS,
representing a chip rate of 1.023 MHz. In this implementation example, the
PN code register is configured to store 1023 chips at a time.
As indicated by signal line 193, the PN code register is capable of being
circularly shifted by an amount which corresponds to a code phase delay
hypothesis. In the implementation example which has been discussed, in
which the period of a PN code is 1023 chips, the value of the code phase
delay can range from 0 to 2045 half chip increments. The PN code register
is configured in this implementation example to be circularly shifted by
any number or fraction of chips which correspond to a code phase delay
hypothesis under consideration.
Sum of products circuitry 194 is also provided. This circuitry is
configured to form the integration of the product between the subframe of
corrected samples stored in sample register 190 and the PN code hypothesis
stored in the PN code register 192.
In the implementation example discussed earlier in which the subframe of
samples stored in sample register 190 comprises 2046 samples, each having
I and Q components, and the PN code hypothesis stored in PN code register
192 comprises 1023 chips, a correspondence is present between two of the
samples in the sample register 190, and one of the chips in PN code
register 192. The I and the Q components of each of the two samples is
multiplied by the corresponding PN chip. Then, the sum of the I component
products is determined, and the sum of the Q component products is
separately determined. The sum of the I component products is output on
signal line 195, and the sum of the Q component products is output on
signal line 196.
In equation form, the function of the sum of products circuitry 194 in this
implementation example can be represented as follows:
##EQU5##
where CHIP.sub.i is the ith chip in the PN code hypothesis, I.sub.i.sup.1
is the I component of the first of the two samples corresponding to
CHIP.sub.i, I.sub.i.sup.2 is the I component of the second of the two
samples corresponding to CHIP.sub.i, Q.sub.i.sup.1 is the Q component of
the first of the two samples corresponding to CHIP.sub.i, and
Q.sub.i.sup.2 is the Q component of the second of the two samples
corresponding to CHIP.sub.1.
Sqrt. of sum of squares circuitry 198, adder 202, and RAM 200 are also
provided. The sqrt. of sum of squares circuit 198 is configured to receive
the sum of the I component products (SI) and the sum of the Q component
products (SQ) on signal lines 195 and 196 respectively, and to determine
the square root of the sum of the squares of these two values. In equation
form, the circuit computes the value:
SS=(SI+L ).sup.2 +L +(SQ+L ).sup.2 +L . (5)
If the subframe under consideration and stored in sample register 190 is
the first subframe for the frame of interest, the foregoing value is
stored in an array entry in RAM 200 corresponding to the combination of
the PN code, Doppler shift, and code phase hypotheses under consideration.
The arrays are of the same format as those depicted in FIG. 5 and will
eventually become the correlation arrays for the current PN hypothesis.
If the subframe under consideration and stored in sample register 190 is
not the first subframe analyzed for the frame of interest, there may
already be a value derived from a previous subframe stored in RAM 200 in
the entry corresponding to the combination of the PN code, Doppler shift,
and code phase hypotheses under consideration. In this case, the SS value
determined above is added by adder 202 with the previously stored value
which is provided to the adder 202 over signal line 204. The result is
then stored in lieu of the previously stored value in the array entry
corresponding to the combined PN code, Doppler shift, and code phase
hypotheses.
The next code phase hypothesis is then selected, and the PN code register
192 circularly shifted in accordance with the selected code phase
hypothesis. The foregoing process is then repeated. This process continues
for each of the code phase hypotheses which are desired to be tested. In
one implementation, 2046 code phases are tested for each 1 mS subframe,
corresponding to the repetition period of the PN codes. In this
implementation, the code phase hypotheses which are tested range from 0 to
2045 half-chip increments, and the next code phase hypothesis is selected
simply by circularly shifting the PN code register 192 by one-half chip.
The matched filter of FIG. 8 is configured to perform the foregoing tasks
for each of the subframes of the frame stored in the RAM 180. When this
process has been completed, correlation arrays of the form shown in FIG. 5
are present in RAM 200. These correlation arrays are provided to the GPS
processor 84 (FIG. 4) over signal line 98. GPS processor 84 combines these
correlation arrays with correlation arrays derived from a previous segment
in the manner described previously.
V. Method of Operation for a First Embodiment
A first embodiment of an overall method of operation of the subject
invention is illustrated in FIG. 9. As illustrated, the process 220 is
iterative, and each cycle of the process 220 begins at step 222. There, a
segment of samples is received. In one embodiment, the segment size is
variable from cycle to cycle. In another embodiment, the segment size is
fixed. In one implementation example, the segment size is 20 mS.
In step 224, the PN code, Doppler shift, and code phase hypotheses to be
tested are determined. In one embodiment, these hypotheses are variable
from cycle to cycle. In another embodiment, these hypotheses are fixed. In
one implementation, the PN code hypotheses to be tested are determined
responsive to the satellites which have already been successfully
detected. If a satellite has previously been successfully detected, then
its PN code is removed from the set of PN code hypotheses to be tested.
Similarly, the set of Doppler shift hypotheses to be tested may be reduced
based on the Doppler shift values at which previous satellites have been
detected.
In step 226, correlation analyses are performed on the segment of samples
in accordance with the set of hypotheses to be tested as determined in
step 224. The result is correlation data which measures the level of
correlation between various combinations of the tested hypotheses and the
segment of samples. In one implementation example, the correlation data is
computed using equations (3), (4), and (5) above.
In step 228, this data is selectively combined with correlation data which
has been accumulated from previous segments. In one embodiment, this step
involves adjusting for the code phase difference between the current
segment and the previous segments. In one implementation, this step
involves combining correlation arrays row by row after shifting at least
one of the rows by the code phase difference which has been determined. In
one implementation example, the code phase differences are computed in
accordance with equations (1) and (2) above.
In step 230, a determination is made whether the data which has been
accumulated permits detection of the presence and range of a satellite. If
so, then step 232, in which the range of the satellite is determined, is
performed. In one implementation, this step involves determining the range
from the code phase hypothesis which has the highest correlation with the
samples which have been accumulated. If not, a jump is made back to step
222, whereupon the process 220 repeats itself with a new segment of
samples.
VI. Method of Operation for a Second Embodiment
A second embodiment of an overall method of operation of the subject
invention is illustrated in FIGS. 10A-10C. Again, the process 220' is
iterative, and each cycle of the process begins with step 240. According
to this process, it is assumed that a time base exists which is divided up
into successive periods known as epochs.
In step 240, the process loops until the beginning of an epoch is detected.
In one implementation, an epoch is a frame period. In one implementation
example, an epoch is a 20 mS frame period. In the implementation which is
the subject of the timing diagram of FIG. 7, epochs are the time periods
marked by T.sub.0, T.sub.1, T.sub.2, . . . , T.sub.n, T.sub.n+1, etc. In
the scenario depicted in FIG. 7, numeral 152 identifies the beginning of a
new epoch which is represented by T.sub.1.
Once the beginning of an epoch is detected, step 242 is performed. In step
242, the storage of receiver samples is initiated before the commencement
of the next epoch. In the scenario depicted in FIG. 7, the initiation of
the new epoch at T.sub.1 generates an interrupt which causes the GPS
processor 84 (FIG. 4) to issue a command to begin data capture. Such a
command is identified in FIG. 7 with numeral 154. The command is such that
data capture begins before the beginning of the next epoch. With reference
to FIG. 7, the next epoch is identified with numeral 160, and is
represented by T.sub.2. In accordance with the method of FIGS. 10A-10B,
the capture of data samples from the receiver is commenced at the time
identified with numeral 156, which is before the commencement of the next
epoch.
When the next epoch is detected, a counter begins counting. This is
represented in FIG. 10A by step 244 which is performed after step 242. In
one implementation example, the counter is part of the offset measurement
circuitry 78 in FIG. 4.
Step 246 is then performed. In step 246, the system loops until the next
epoch is detected. In the scenario depicted in FIG. 7, the next epoch
isT.sub.2, and is identified with numeral 160. Once the next epoch has
been detected, step 248 is performed.
In step 248, the counter is stopped, and the contents of the counter used
to derive the time offset OS.sub.I between the initiation of data capture
and the beginning of the next epoch. In the scenario depicted in FIG. 7,
this value is represented as CP.sub.1, and is the time offset between the
initiation of the capture of data segment 158 at the time identified with
numeral 156, and the time identified with numeral 160 (FIG. 7), which is
the beginning of epoch T.sub.2.
Step 250 is then performed. In step 250, the system loops until data
capture has been completed. In one implementation, the completion of data
capture is detected by GPS processor 84 (FIG. 4) either by an interrupt
which is generated or through a polling method. In the scenario depicted
in FIG. 7, the completion of the capture of data samples 158 (FIG. 7) is
identified with numeral 172 (FIG. 7).
Upon the completion of data capture, step 252 (FIG. 10B) is performed. In
step 252, the contents of the counter, which defines OS.sub.I, is saved
and associated with the captured data samples. In one implementation
example, the GPS processor 84 (FIG. 4) reads the contents of the counter
maintained in the offset measurement circuitry 78, and defines this value
as CP.sub.1. The GPS processor 84 then saves this value and associates it
with the data samples 158 (FIG. 7).
The system then generates correlation data which describes the degree of
correlation between the PN code, Doppler shift, and code phase hypotheses
which are desired to be tested. In step 254, the PN code hypothesis to be
tested is selected, and in step 256, the Doppler shift hypothesis to be
tested is selected. In one implementation, the selected PN code hypothesis
is generated by PN code generator 80 (FIG. 4), and then stored in PN code
register 192 (FIG. 8). In addition, the Doppler shift hypothesis is
generated by Doppler shift generator 184 (FIG. 8).
Step 258 is then performed. In step 258, the receiver samples which have
been captured are corrected for the selected Doppler shift hypothesis. In
one implementation, the receiver samples are stored in RAM 180 (FIG. 8),
and Doppler shift correction circuitry 182 corrects these samples for the
selected Doppler shift hypothesis one subframe at a time. Additional
detail regarding this implementation is available in U.S. Ser. No.
09/145,055, previously incorporated herein by reference.
Step 260 is then performed. In step 260, a cross-correlation analysis is
then performed between the adjusted data from step 258, and the PN code
hypothesis which has been selected. According to this step, the integral
of the cross-product between the adjusted receiver samples and the PN
code, shifted in accordance with a selected code phase hypothesis, is
determined for each of a plurality of code phase hypotheses. In one
implementation, 2046 code phase hypotheses are tested. These code phase
hypotheses are generated by circularly shifting the selected PN hypothesis
by a shift value ranging from 0 to 2045 half-chip increments. In one
implementation example, this step is performed one subframe at a time. In
this implementation, the integration values for the various subframes for
a given PN code, Doppler shift and code phase hypothesis are simply added
together to obtain the integration value for the frame. In one
implementation, this step is performed by the matched filter 82 (FIG. 4
and FIG. 8). In one implementation example, this step is performed in
accordance with equations (3), (4), and (5) above.
When step 260 has been completed, step 262 is performed. In step 262, the
correlation data obtained in step 260 is combined with any previous
correlation data for the same PN code/Doppler shift hypotheses in a manner
to be described subsequently. Of course, if this is the first set of
correlation data obtained for the given PN code/Doppler shift hypotheses,
these values are simply stored. In one implementation example, the
completion of the cross-correlation analysis of step 260 is detected by
GPS processor 84 (FIG. 4) through either an interrupt or a polling method.
In one implementation example, the correlation data for a segment is
combined with any previous correlation data using equations (1) and (2)
above.
In step 264, a determination is made whether there are any more Doppler
shift hypotheses to be tested. If so, a jump is made back to step 256, and
the process is repeated beginning at this point. If not, step 266 is
performed.
In step 266, a determination is made whether there are any more PN code
hypotheses to be tested. If so, a jump is made back to step 254, and the
process is repeated beginning at this point. If not, step 268 (FIG. 10C)
is performed.
In step 268, a determination is made whether there are any more data
segments to be captured for combination with the results which have been
accumulated. If so, a jump is made back to step 240 (FIG. 10A), and the
process is repeated beginning at this point. If not, the process
terminates.
Typically, as the process iterates, there will be fewer PN code and Doppler
shift hypotheses to test on each pass. This reduction occurs because some
PN codes (satellites) will have been detected and because the range of
Doppler shift uncertainty that needs to be tested may be reduced based on
observing the Doppler shift values at which previous satellites have been
detected.
Also, it should be appreciated that the parameters which are collected for
each set of data samples change. For example, with reference to FIG. 7,
the first pass is associated with the parameters OS.sub.1, T.sub.2, and
CP.sub.1. The second pass, however, is associated with the parameters
T.sub.n, OS.sub.2, and CP.sub.2.
It should also be appreciated that the data capture process is not limited
to a fixed 20 mS epoch or segment length. In general, it can be adjusted
on each pass to a variable length. In one implementation example, it can
be adjusted on each pass to a variable length of from 1 to 20 mS, in 1 mS
increments. In other implementation examples, longer periods are possible
simply by increasing the amount of RAM 400.
Upon the completion of the process 220' of FIGS. 10A-10C, in one
implementation, the GPS processor 84 (FIG. 4) has stored within its memory
a plurality of correlation arrays, with each array corresponding to a
particular PN code hypothesis, and with each row of an array corresponding
to a particular Doppler shift hypothesis. Each array represents the
combined results derived from multiple sets of samples.
VII. Operation of the Matched Filter
A method 300 of operation of a matched filter in accordance with the
subject invention is illustrated in FIGS. 11A-11C. In step 302, a frame of
samples is stored. In the matched filter of FIG. 8, the frame of samples
is stored in RAM 180.
In step 304, a PN code hypothesis is selected for testing and stored in a
circular shift register. In the matched filter of FIG. 8, the PN code
hypothesis is stored in PN code register 192.
In step 306, a subframe of the frame of samples stored in step 302 is
selected.
In step 308, a Doppler shift hypothesis is selected for testing. In the
matched filter of FIG. 8, this step is implicitly performed by Doppler
shift generator 184 which successively generates Doppler shift hypothesis
responsive to constraints or bounds imposed by a user. In one
implementation example, the Doppler shift hypotheses range from .+-.62,000
Hz.
In step 310, the subframe selected in step 306 is corrected for the Doppler
shift hypothesis selected in step 308. In the matched filter of FIG. 8,
this step is performed by Doppler shift correction circuitry 182. In one
implementation example, this step is performed as described in U.S. Ser.
No. 09/145,055, filed Sep. 1, 1998, previously incorporated herein by
reference, now U.S. Pat. No. 6,044,105.
In step 312, the corrected data from step 310 is stored, In the matched
filter of FIG. 8, the corrected data is stored in sample register 190.
In step 314, a code phase hypothesis is selected for testing. In the
matched filter of FIG. 8, this step is implicit in the operation of PN
code register 192 which successively and circularly shifts through each of
the possible code phase hypotheses in a PN code repetition period, which,
in one implementation, comprises 2046 half-chip increments.
In step 318 (FIG. 11B), the PN code hypothesis selected and stored in step
304 is circularly shifted by an amount derived from the code phase
hypothesis selected in step 314. In the matched filter of FIG. 8, the
selected code phase hypothesis ranges from 0 to 2045 half-chip increments,
and step 318 is implemented by circularly shifting the PN code hypothesis
by the number of half-chip increments comprising the selected code phase
hypothesis.
In step 320, the product of the shifted PN code from step 318, and the
corrected subframe of samples from step 310 is obtained. In one
implementation, this step comprises forming the I and Q component sum of
products, SI and SQ. In one implementation example, SI and SQ are derived
in accordance with equations (3) and (4), presented earlier. In the
matched filter of FIG. 8, this step is performed by sum of products
circuitry 194.
In step 322, the square root of the sum of the squares of SI and SQ from
step 320 is determined in accordance with equation (5), presented earlier.
In the matched filter of FIG. 8, this step is performed by sqrt. of sum of
products circuitry 198.
In step 324, the value determined in step 322 is added to any similar
values derived from the same hypotheses for previous subframes of the
frame which is the subject of step 302, and the combined result is then
stored. In the matched filter of FIG. 8, this step is performed by the
adder 202 in combination with RAM 200, with the combined values being
maintained in RAM 200.
In step 326, a determination is made whether there are any further code
phase hypotheses which are to be tested for the selected PN code and
Doppler shift hypotheses. If so, a jump is made to step 314 and the
process beginning at this point repeated for the new code phase
hypothesis. If not, step 328 (FIG. 11C) is performed. In the matched
filter of FIG. 8, this step is implicit in the operation of PN code
register 192, which successively shifts through the 2046 code phase
hypotheses to be tested for a given PN code and Doppler shift hypothesis.
In step 328 (FIG. 11C), a determination is made whether there are any
further Doppler shift hypotheses which are to be tested for the selected
PN code hypothesis. If so, a jump is made to step 308, and the process
beginning at this point repeated for the new Doppler shift hypothesis. If
not, step 330 is performed. In the matched filter of FIG. 8, this step is
implicit in the operation of Doppler shift generator 184, which cycles
through a plurality of Doppler shift hypotheses for a given PN code
hypothesis. In one implementation example, the Doppler shift hypotheses
tested for a given PN code hypothesis range from .+-.62,000 Hz.
In step 330, a determination is made whether there are any further
subframes to be analyzed of the frame which is the subject of step 302. If
so, a jump is made to step 306 and the process beginning at this point
repeats itself using the new subframe. If not, the process terminates. In
one implementation, the method illustrated in FIGS. 11A-11C is then
repeated for each PN code hypothesis which is to be tested. In one
implementation example, the coordination of this task is performed by GPS
processor 84 (FIG. 4).
VIII. First Embodiment of a Method to Update Cumulative Correlation Data
Next, the discussion turns to a suitable method for combining the data. An
embodiment of method of updating the cumulative correlation data with the
correlation data derived from a new segment in accordance with the subject
invention is illustrated in FIG. 12. In step 350, a portion of the
cumulative correlation data which is to be updated is obtained. In one
implementation, this portion is the cumulative correlation data which
corresponds to a particular combination of PN code and Doppler shift
hypotheses.
In step 352, the corresponding portion of the incremental correlation data,
that is, the correlation data derived from a new segment, is obtained. In
one implementation, this portion is the incremental correlation data which
corresponds to a particular combination of PN code and Doppler shift
hypotheses.
In step 354, the code phase difference, ACP, between the two portions is
determined. In one implementation, this value is derived from the Doppler
shift hypothesis corresponding to the two portions, and the time
difference between the start of a first segment of samples and a second
segment of samples. In one implementation example, ACP is determined in
accordance with equations (1) and (2) above.
In step 356, the portion of the incremental correlation data obtained in
step 352 is adjusted responsive to the code phase difference between the
two portions determined in step 354. In one implementation, this step
comprises circularly shifting a row of the incremental data by a value
derived from the code phase difference determined in step 354. In one
implementation example, the value ACP is computed using equations (1) and
(2) above, and the shift value, SHIFT, is determined from the following
equation:
SHIFT=-.DELTA.CP modulo 2046 (6)
In step 358, the portion of the cumulative correlation data obtained in
step 350 is updated with the adjusted data determined in step 358. In one
implementation, this step comprises adding a row of circularly shifted
incremental data element by element to a corresponding row of cumulative
data.
In step 360, a determination is made whether there are any additional
portions of cumulative data to update. If so, the process repeats
beginning with step 350 for another portion of the cumulative data which
is to be updated. If not, the process ends as indicated by identifying
numeral 362.
IX. Second Embodiment of a Method to Update Cumulative Correlation Data
A second embodiment of a method of updating cumulative correlation data
with incremental correlation data is illustrated in FIGS. 13A and 13B. In
this embodiment, it is assumed that each of the cumulative data and the
incremental data are of the form shown in FIG. 5.
Again, the process is iterative, and each cycle of the process begins at
step 370, in which a PN code hypothesis is selected. Then, in step 372,
the cumulative correlation array for the selected PN code hypothesis is
obtained, and in step 374, the incremental correlation array for the
selected PN code hypothesis is obtained.
In step 376, a Doppler shift hypothesis is selected, and in step 378 (FIG.
13A), the code phase difference, ACP, for the selected Doppler shift
hypothesis is determined. In one implementation, this value is determined
in accordance with equations (1) and (2) above.
In step 380, the corresponding row of the incremental array obtained in
step 374 is circularly shifted by an amount derived from the code phase
difference determined in step 378. In one implementation, this value is
determined in accordance with equation (6) above.
Then, in step 382, the shifted row from step 380 is added element by
element to the corresponding row in the cumulative array.
In step 384 (FIG. 13B), a determination is made whether there are
additional Doppler shift hypotheses to be updated. If so, a jump is made
to step 376 (FIG. 13A) whereupon the process beginning at this step is
repeated for one of the additional Doppler shift hypotheses to be updated.
If not, step 386 is performed. In step 386, a determination is made
whether there are additional PN code hypotheses to be updated. If so, a
jump is made to step 370 (FIG. 13A) whereupon the process beginning at
this step is repeated for one of the additional PN code hypotheses to be
updated, the process terminates, as indicated by identifying numeral 388.
X. Implementation Example
A matched filter chip code-named "Magna" which combines the functionality
of the sampling circuitry 74, the timing circuitry 76, offset measurement
circuit 78, and the matched filter 82 of FIG. 4 has been developed by the
assignee of the subject application (Conexant Systems, Inc. of Newport
Beach, Calif.). A processor chip which embodies the functionality of the
GPS processor 84 of FIG. 4 code-named "Scorpio", Part No. 11577-11, is
available from the assignee of the subject application. In one
implementation, the processor has additional GPS-specific circuits, such
as tracking channels for continuously tracking a number of GPS satellite
signals 20 (FIG. 1). Typically, the processor includes at least an
embedded microprocessor with an external bus. In one configuration, the
processor views the matched filter chip as a memory mapped peripheral. It
issues commands to the matched filter chip, and retrieves results after it
has completed processing for a given set of commands. An RF receiver chip
which embodies the functionality of the GPS radio receiver 72 of FIG. 4
code-named "Gemini/Pices Monopack", Part No. R6732-13, is available from
the assignee of the subject application. Additional details regarding this
implementation example are available in U.S. Ser. No. 09/145,055, filed
Sep. 1, 1998 previously incorporated herein by reference, now U.S. Pat.
No. 6,044,105.
The foregoing embodiments, implementations, and implementation examples
improve the signal to noise ratio (SNR) of a correlation array through
repeated application of a matched filtering operation. The approach
described has a number of advantages, which are as follows:
1. The embodiments, implementations, and examples are adaptable to any time
base. This means that they can be applied to GPS chip sets, cellular and
PCS chip sets, and standard microprocessors.
2. The embodiments, implementations, and implementation examples allow for
the combination of non-uniform receiver sample capture lengths. This is
critical for integrated applications such as cellular and PCS, in which it
is desirable to receive GPS when the phone is not transmitting. Since the
available idle slots will have different durations in the various phone
standards, adaptability of time intervals is important.
3. The embodiments, implementations, and implementation examples allow
arbitrary offsets in the start of the data capture times. This again is
most important for phone applications, but it is also important for the
basic GPS application. In the GPS application, a flexible start time
capability allows the same capture to be used with any satellite in the
received samples, regardless of their relative code phases.
4. Multiple data capture combining allows SNR to be built up incrementally.
After each increment is added, the arrays can be tested for signal
detection, and processing for successful acquisitions curtailed.
5. Processor RAM and throughput is minimized. This is important for phone
applications, in which the baseband device's digital signal processor
(DSP) and protocol stack processors may be busy and RAM limited. For high
SNR cases, the entire required RAM for a given PN code is located on or
within the matched filter. When data combining is required to improve SNR,
only those satellite that have not been detected need be processed.
Further, it is often possible in phone systems to reduce the size of the
array to less than 2046 elements per PN code and Doppler shift hypothesis,
and to a few Doppler shift hypotheses, using information that is inherent
in mobile phone network operation.
6. While the discussion here has focused on GPS, the invention can be
applied to any system using correlation methods for detecting signals.
This would include most spread spectrum systems, and general signal
detectors.
7. Although embodiments and implementations have been described which
utilize a GPS processor to perform various functions, it should be
understood that embodiments are possible in which a generic processor is
used to perform these functions. For purposes of this disclosure, a
generic processor is defined to mean any device, including a computer,
DSP, baseband processor, microprocessor, or microcomputer, which is
capable of executing a discrete series of instructions stored in a memory
accessible by the processor. It should also be understood that embodiments
are possible in which analog circuitry is used to perform these functions.
While various embodiments of the application have been described, it will
be apparent to those of ordinary skill in the art that many more
embodiments and implementations are possible that are within the scope of
the subject invention. Accordingly, the invention is not to be restricted
except in light of the appended claims and their equivalents.
* * * * *
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