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US5220584A
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System for demodulation and synchronizing multiple tone waveforms
Assignee: DATA DESIGN AND DEVELOPMENT Corp
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Filing: 1990-12-21
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Granted: 1993-06-15
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Expires: 2010-12-21
Abstract
Abstract A multiple tone signal is demodulated and synchronization is obtained by precisely locating frame boundaries in order to ensure reliable communication in the presence of anomalies such as Doppler shift, multipath propagation, and additive electronic noise. One such signal, TADIL-A or Link-11, employs multiple tone audio waveforms that are used to modulate RF carriers for the transmission of digital data. Initially, the signal is frequency divided and classified to determine if the incoming frame is a preamble, data or noise. For preamble frames, a digital filter is used to extract the 605 Hz tone in the time domain, and the resulting real-valued signal is passed through a first Hilbert transform to generate the corresponding imaginary complex-valued signal. Unfiltered data frames are passed through a second Hilbert transform and the output is multiplied by the complex conjugate of the stored Doppler reference signal to produce a Link-11 Signal stripped of Doppler. As a result of the demodulation process, the entire waveform now includes frequencies separated by an integer multiple of 110 Hz. Therefore, the composite waveform has a period of 9.09 milliseconds. The composite waveform partially repeats itself at least once in each 13.33 msec frame interval. Using this property, it is possible to determine the location at which the composite signal becomes discontinuous and thereby locate the synchronization frame boundaries.
Inventors
- 👤 Joseph R. Burns
Bibliographic Details
Application Number
—
Jurisdiction
US
Data Source
google_patents
Last Synchronized
2026-11-08 21:03
CPC Classifications
H; H04; H04L; H04L27/00; H04L27/26; H04L27/2601; H04L27/2647; H04L27/2655; H04L27/2662; H04J; H04J3/00; H04J3/02; H04J3/06; H04J3/0602; H04J3/0605; H04J3/0608; H04L27/2656; H04L27/2668; H04L27/2673; H04L27/2675; H04L7/00; H04L7/04; H04L7/041; H04L2007/047; H04L27/2657; H04L7/06
Claims Analysis
Total Claims: 29
Claim 1
Independent Claim
227 words
A system for demodulating a multiple tone signal S including a Doppler reference tone f DR , a Doppler frequency shift f DP and a plurality of data tones at frequencies f n transmitted in frames, said system comprising: a digital filter means for passing f DR and f DP and forming a real signal R 1 =cos (2π [f DR +f DP ] t); first Hilbert transform means connected to said digital filter means for forming from R 1 an imaginary signal in the form of I 1 =sin (2π [f DR +f DP ] t); second Hilbert transform means for transforming the real part R 2 of S=cos (2π [f n +f DP ] t) into an imaginary signal having the form of I 2 =sin (2π [f n +f DP ] t); and, complex multiply means for multiplying R 2 +jI 2 =cos (2π[f n +f DP ] t)+j sin (2π [f n +f DP ] t)=Σe jwnt by the complex conjugate of R 1 +jI 1 =cos (2π [f DR +f DP ] t)+j sin (2π [f DR +f DP ] t)=Σe jwot , thereby producing the signal Σe j (wn-wo)t, wherein the Doppler reference signal f DR and the Doppler frequency shift f DP are stripped from the signal S to produce a periodic signal having the form of Σe 7 (wn-wo)t.
Claim 2
Independent Claim
227 words
A system for demodulating a multiple tone signal S including a Doppler reference tone f DR , a Doppler frequency shift f DP and a plurality of data tones at frequencies f n transmitted in frames, said system comprising: a digital filter means for passing f DR and f DP and forming a real signal R 1 =cos (2π [f DR +f DP ] t); first Hilbert transform means connected to said digital filter means for forming from R 1 an imaginary signal in the form of I 1 =sin (2π [f DR +f DP ] t); second Hilbert transform means for transforming the real part R 2 of S=cos (2π [f n +f DP ] t) into an imaginary signal having the form of I 2 =sin (2π [f n +f DP ] t); and, complex multiply means for multiplying R 2 +jI 2 =cos (2π[f n +f DP ] t)+j sin (2π [f n +f DP ] t)=Σe jwnt by the complex conjugate of R 1 +jI 1 =cos (2π [f DR +f DP ] t)+j sin (2π [f DR +f DP ] t)=Σe jwot , thereby producing the signal Σe j (wn-wo)t, wherein the Doppler reference signal f DR and the Doppler frequency shift f DP are stripped from the signal S to produce a periodic signal having the form of Σe 7 (wn-wo)t.
Claim 3
Dependent Claim
17 words
The system of claim 1 wherein said digital filter means comprises a low pass digital filter means.
Claim 4
Dependent Claim
31 words
The system of claim 2 further comprising: Fourier Transform means connected to said complex conjugate multiply means for determining the frequencies f n and phase angles φ n present in S.
Claim 5
Dependent Claim
105 words
The system of claim 3 further comprising frame boundary determining means including: function generating means for sampling said signal S a total of m samples per frame and generating a voltage difference function D=S(m+x)-S(m) volts, wherein x is the sample point where the signal S begins to repeat itself within a single frame; threshold detecting means for determining when D is greater than a predetermined threshold voltage T, wherein T is substantially greater than zero volts; and, frame boundary setting means for generating a frame boundary signal when said threshold detecting means detects that the difference voltage D is greater than the threshold voltage T.
Claim 6
Dependent Claim
34 words
The system of claim 4 further comprising: classifying means connected to said digital filter means for receiving said signal S and for determining if a signal containing data is present in said signal S.
Claim 7
Dependent Claim
27 words
The system of claim 5 further comprising: frequency dividing means connected to said classifying means for receiving said signal S and for reducing the frequency of S.
Claim 8
Dependent Claim
21 words
The system of claim 6 further comprising: correction means for correcting the signal from said Fourier Transform means for phase errors.
Claim 9
Dependent Claim
17 words
The system of claim 7 wherein said Fourier Transform means comprises a Fast Fourier Transform (FFT) means.
Claim 10
Independent Claim
216 words
A method for demodulating a multiple tone signal S including a Doppler reference tone f DR , a Doppler frequency shift f DP and a plurality of data tones at frequencies f n transmitted in frames, said method comprising the steps of: digitally filtering said signal S to pass f Dr and f DP and form the real signal R 1 =cos (2π [f DR +f DP ] t); transforming said signal R 1 with a first Hilbert transform into an imaginary signal I 1 =sin (2π [f DR +f DP ] t); transforming the real part R 2 of S=cos (2π [f n +f DP ] t) with a second Hilbert transform into an imaginary signal I 2 =sin (2π [f n +f DP ] t); and, multiplying R 2 +jI 2 =cos (2π [f n +f DP ] t)+j sin (2π [f n +f DP ] t)=Σe jwnt by the complex conjugate of R 1 +jI 1 =cos (2π [f DR +f DP ] t)+j sin (2π [f DR +f DP ] t)=Σe jwot , thereby producing the signal Σe j (wn-wo)t, whereby the Doppler reference signal f DR and the Doppler signal f DP are stripped from the signal S to produce a periodic signal having the form of Σ e j (wn-wo)t.
Claim 11
Independent Claim
216 words
A method for demodulating a multiple tone signal S including a Doppler reference tone f DR , a Doppler frequency shift f DP and a plurality of data tones at frequencies f n transmitted in frames, said method comprising the steps of: digitally filtering said signal S to pass f Dr and f DP and form the real signal R 1 =cos (2π [f DR +f DP ] t); transforming said signal R 1 with a first Hilbert transform into an imaginary signal I 1 =sin (2π [f DR +f DP ] t); transforming the real part R 2 of S=cos (2π [f n +f DP ] t) with a second Hilbert transform into an imaginary signal I 2 =sin (2π [f n +f DP ] t); and, multiplying R 2 +jI 2 =cos (2π [f n +f DP ] t)+j sin (2π [f n +f DP ] t)=Σe jwnt by the complex conjugate of R 1 +jI 1 =cos (2π [f DR +f DP ] t)+j sin (2π [f DR +f DP ] t)=Σe jwot , thereby producing the signal Σe j (wn-wo)t, whereby the Doppler reference signal f DR and the Doppler signal f DP are stripped from the signal S to produce a periodic signal having the form of Σ e j (wn-wo)t.
Claim 12
Dependent Claim
32 words
The method of claim 9 wherein f DP is approximately 605 Hz and said digital filtering step comprises low pass digital filtering to pass frequencies in the range of 605±f DP Hz.
Claim 13
Dependent Claim
24 words
The method of claim 10 further comprising the step of: Fourier transforming said signal S to determine the frequencies f n present in S.
Claim 14
Dependent Claim
88 words
The method of claim 11 further comprising steps for locating boundaries between frames, said steps comprising: sampling said signal S a total of m samples per frame; generating the voltage difference function D=S(m+x)-S(m) volts, wherein x is the sample point where the signal S begins to repeat itself within a single frame; detecting when D is greater than a predetermining threshold voltage T, wherein T is substantially greater than zero; and generating a frame boundary signal when the difference voltage D is greater than the threshold voltage T.
Claim 15
Dependent Claim
30 words
The method of claim 12 further comprising the step of: classifying said signal S prior to said digital filtering step to determine if data is present in said signal S.
Claim 16
Dependent Claim
28 words
The method of claim 13 further comprising the step of: frequency dividing said signal S prior to said classifying step in order to reduce the frequency of S.
Claim 17
Dependent Claim
22 words
The method of claim 14 further comprising the step of: correcting said signal S for phase errors after said Fourier transforming step.
Claim 18
Dependent Claim
17 words
The method of claim 15 wherein said Fourier transforming step comprises a Fast Fourier Transform (FFT) step.
Claim 19
Independent Claim
169 words
A method for demodulating a multiple tone signal S including a multiplicity of tones f n at least one of which is a constant 605 Hz Doppler reference tone and said signal S further includes a Doppler frequency shift f DP , the method comprising the steps of: low pass digital filtering said signal S so as to pass f DP and said 605 Hz Doppler reference tone; transforming said signal passed from said low pass digital filtering step with a first Hilbert transform to produce a complex signal including said 605 Hz tone plus said Doppler frequency shift f DP ; transforming said signal S with a second Hilbert transform to produce a complex signal including the Doppler frequency shift f DP ; and, multiplying the complex conjugate of the 605 Hz and the Doppler frequency shift f DP signal by the complex composite signal f n plus Doppler frequency shift f DP to produce an output signal substantially stripped of said Doppler frequency shift f DP .
Claim 20
Independent Claim
169 words
A method for demodulating a multiple tone signal S including a multiplicity of tones f n at least one of which is a constant 605 Hz Doppler reference tone and said signal S further includes a Doppler frequency shift f DP , the method comprising the steps of: low pass digital filtering said signal S so as to pass f DP and said 605 Hz Doppler reference tone; transforming said signal passed from said low pass digital filtering step with a first Hilbert transform to produce a complex signal including said 605 Hz tone plus said Doppler frequency shift f DP ; transforming said signal S with a second Hilbert transform to produce a complex signal including the Doppler frequency shift f DP ; and, multiplying the complex conjugate of the 605 Hz and the Doppler frequency shift f DP signal by the complex composite signal f n plus Doppler frequency shift f DP to produce an output signal substantially stripped of said Doppler frequency shift f DP .
Claim 21
Dependent Claim
30 words
The method of claim 17 further including the step of: classifying said signal S prior to said digitally filtering step to determine if data is present in said signal S.
Claim 22
Independent Claim
144 words
A system for determining the location of frame boundaries in a complex periodic signal S=Σe jw n t , wherein w n =2πf n and f n is a plurality of frequencies which are separated by an integer multiple of a common frequency f c , said system comprising: function generating means for sampling said signal S a total of m samples per frame and generating the difference voltage function D=S(m+x)-S(m), wherein x is the sample point where the periodic signal S begins to repeat itself within a single frame; threshold detecting means for detecting when the difference voltage D is greater than a predetermined threshold voltage T, wherein T is substantially greater than zero; and, frame boundary generating means for generating a frame boundary signal when said threshold detecting means detects that the difference voltage D is greater than the threshold voltage T.
Claim 23
Independent Claim
144 words
A system for determining the location of frame boundaries in a complex periodic signal S=Σe jw n t , wherein w n =2πf n and f n is a plurality of frequencies which are separated by an integer multiple of a common frequency f c , said system comprising: function generating means for sampling said signal S a total of m samples per frame and generating the difference voltage function D=S(m+x)-S(m), wherein x is the sample point where the periodic signal S begins to repeat itself within a single frame; threshold detecting means for detecting when the difference voltage D is greater than a predetermined threshold voltage T, wherein T is substantially greater than zero; and, frame boundary generating means for generating a frame boundary signal when said threshold detecting means detects that the difference voltage D is greater than the threshold voltage T.
Claim 24
Independent Claim
94 words
A method for determining frame boundary locations of a complex periodic signal S=Σe jw n t , wherein w n =2πf n and f n is a multiple of a common frequency f c , said steps comprising: sampling said complex signal S m times per frame and generating a plurality of first signals S(m); generating a plurality of second signals S(m+x), x samples later, wherein m is greater than x; generating the difference voltage function D=S(m+x)-S(m); and, setting a frame boundary whenever the difference voltage function D=X(m+x)-S(m) is substantially greater than zero volts.
Claim 25
Independent Claim
94 words
A method for determining frame boundary locations of a complex periodic signal S=Σe jw n t , wherein w n =2πf n and f n is a multiple of a common frequency f c , said steps comprising: sampling said complex signal S m times per frame and generating a plurality of first signals S(m); generating a plurality of second signals S(m+x), x samples later, wherein m is greater than x; generating the difference voltage function D=S(m+x)-S(m); and, setting a frame boundary whenever the difference voltage function D=X(m+x)-S(m) is substantially greater than zero volts.
Claim 26
Independent Claim
238 words
A system for demodulating a multiple tome signal S including a Doppler reference tone f DR , a Doppler frequency shift f DP and a plurality of data tones at frequencies f n transmitted in frames, said system comprising: a digital filter means for passing f DR and f DP and forming a real signal R 1 =cos (2π [f DR +f DP ] t); first Hilbert transform means connected to said digital filter means for forming from R 1 an imaginary signal in the form of I 1 =sin (2π [f DR +f DP ] t); storing means for storing R 1 and I 1 ; second Hilbert transform means for transforming the real part R 2 of S=cos (2π [f n +f DP ] t) into an imaginary signal having the form of I 2 =sin (2π [f n +f DP ] t); and, complex multiply means for multiplying R 2 +jI 2 =cos (2π [f n +f DP ] t)+j sin (2π [f n +f DP ] t)=Σe jwnt by the complex conjugate of R 1 +jI 1 =cos (2π [f DR +f DP ] t)+j sin (2π [f DR +f DP ] t)=Σe wjot , thereby producing the signal Σe j (wn-wo)t, wherein the Doppler reference signal f DR and the Doppler frequency shift f DP are stripped from the signal S to produce a periodic signal having the form of Σe j (wn-wo)t.
Claim 27
Independent Claim
238 words
A system for demodulating a multiple tome signal S including a Doppler reference tone f DR , a Doppler frequency shift f DP and a plurality of data tones at frequencies f n transmitted in frames, said system comprising: a digital filter means for passing f DR and f DP and forming a real signal R 1 =cos (2π [f DR +f DP ] t); first Hilbert transform means connected to said digital filter means for forming from R 1 an imaginary signal in the form of I 1 =sin (2π [f DR +f DP ] t); storing means for storing R 1 and I 1 ; second Hilbert transform means for transforming the real part R 2 of S=cos (2π [f n +f DP ] t) into an imaginary signal having the form of I 2 =sin (2π [f n +f DP ] t); and, complex multiply means for multiplying R 2 +jI 2 =cos (2π [f n +f DP ] t)+j sin (2π [f n +f DP ] t)=Σe jwnt by the complex conjugate of R 1 +jI 1 =cos (2π [f DR +f DP ] t)+j sin (2π [f DR +f DP ] t)=Σe wjot , thereby producing the signal Σe j (wn-wo)t, wherein the Doppler reference signal f DR and the Doppler frequency shift f DP are stripped from the signal S to produce a periodic signal having the form of Σe j (wn-wo)t.
Claim 28
Independent Claim
225 words
A method for demodulating a multiple tone signal S including a Doppler reference signal f DR , a Doppler frequency shift f DP and a plurality of data tones at frequencies f n transmitted in frames, said method comprising the steps of: digitally filtering said signal S to pass f DR and f DP and form the real signal R 1 =cos (2π [f DR +f DP ] t); transforming said signal R 1 with a first Hilbert transform into an imaginary signal I 1 =sin (2π [f DR +f DP ] t); storing said signals R 1 and I 1 ; transforming the real part R 2 of S=cos (2π [f n +f DP ] t) with a second Hilbert transform into an imaginary signal I 2 =sin (2π [f n +f DP ] t); and, multiplying R 2 +jI 2 =cos (2π [f n +f DP ] t)+j sin (2π [f n +f DP ] t)=Σe jwnt by the complex conjugate of R 1 +jI 1 =cos (2π [f DR +f DP ] t)+j sin (2π [f DR +f DP ] t)=Σe jwot , thereby producing the signal Σe j (wn-wo)t, whereby the Doppler reference signal f DR and the Doppler signal f DP are stripped from the signal S to produce a periodic signal having the form of Σ e j (wn-wo)t.
Claim 29
Independent Claim
225 words
A method for demodulating a multiple tone signal S including a Doppler reference signal f DR , a Doppler frequency shift f DP and a plurality of data tones at frequencies f n transmitted in frames, said method comprising the steps of: digitally filtering said signal S to pass f DR and f DP and form the real signal R 1 =cos (2π [f DR +f DP ] t); transforming said signal R 1 with a first Hilbert transform into an imaginary signal I 1 =sin (2π [f DR +f DP ] t); storing said signals R 1 and I 1 ; transforming the real part R 2 of S=cos (2π [f n +f DP ] t) with a second Hilbert transform into an imaginary signal I 2 =sin (2π [f n +f DP ] t); and, multiplying R 2 +jI 2 =cos (2π [f n +f DP ] t)+j sin (2π [f n +f DP ] t)=Σe jwnt by the complex conjugate of R 1 +jI 1 =cos (2π [f DR +f DP ] t)+j sin (2π [f DR +f DP ] t)=Σe jwot , thereby producing the signal Σe j (wn-wo)t, whereby the Doppler reference signal f DR and the Doppler signal f DP are stripped from the signal S to produce a periodic signal having the form of Σ e j (wn-wo)t.
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