US4573205A
Expired / Lapsed
US
⏱️ 0.0 years remaining
Technique for secure communications on FM radio channels
Assignee: Nokia Bell Labs
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Filing: 1983-08-22
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Granted: 1986-02-25
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Expires: 2003-08-22
Abstract
Abstract This present invention relates to a system for providing secure communications without bandwidth expansion using an encryption method called masking whereby a masking signal is generated at the transmitter using a secret key which includes a predetermined threshold signal-to-noise ratio level that when added to a frequency modulated (FM) signal produces an unintelligible signal. Any type of masking signal as, for example, a sine wave, an FM signal, or bandlimited Gaussian noise, may be used. At the receiver, the corresponding masking signal used by the transmitter is regenerated and subtracted from the FM signal before demodulation. Because of the FM threshold effect, perfect removal of the masking signal is not required. It is only necessary to subtract enough of the masking signal such that the resulting signal-to-noise ratio (SNR) is above the required threshold for reliable demodulation of the original signal.
Inventors
- 👤 Randy D. Nash
Bibliographic Details
Application Number
—
Jurisdiction
US
Data Source
google_patents
Last Synchronized
2026-10-24 10:39
CPC Classifications
H; H04; H04K; H04K1/00; H04K1/02
Claims Analysis
Total Claims: 22
Claim 1
Independent Claim
105 words
A transmitter for providing secure communications comprising: a modulator capable of converting an input analog signal to a frequency modulated (FM) output signal at a nominal carrier frequency; means for generating an output masking signal which is limited to the band of the FM output signal and includes a predetermined threshold level such that when said masking signal is added to the FM output signal a resultant signal is generated which is unintelligible when received with a conventional FM receiver; and means for directly adding the output signals from both the modulator and the output masking signal generating means for generating a transmitter output signal.
Claim 2
Dependent Claim
105 words
A transmitter for providing secure communications comprising: a modulator capable of converting an input analog signal to a frequency modulated (FM) output signal at a nominal carrier frequency; means for generating an output masking signal which is limited to the band of the FM output signal and includes a predetermined threshold level such that when said masking signal is added to the FM output signal a resultant signal is generated which is unintelligible when received with a conventional FM receiver; and means for directly adding the output signals from both the modulator and the output masking signal generating means for generating a transmitter output signal.
Claim 3
Dependent Claim
22 words
A transmitter according to claim 1 wherein the output signal from the generating means comprises at least a -5 db signal-to-noise ratio.
Claim 4
Dependent Claim
15 words
A transmitter according to claim 1 wherein the output masking signal comprises bandlimited Gaussian noise.
Claim 5
Dependent Claim
16 words
A transmitter according to claim 1 wherein the output masking signal comprises a separate FM signal.
Claim 6
Dependent Claim
15 words
A transmitter according to claim 1 wherein the output masking signal comprises a sine wave.
Claim 7
Dependent Claim
119 words
A receiver for providing secure communications capable of receiving from a remote transmitter an input signal including a masking signal superimposed on a frequency modulated (FM) signal with a predetermined signal-to-noise ratio such that the FM signal is unintelligible, the receiver comprising: means for generating an output masking signal which corresponds to, and is synchronized with, the masking signal forming part of the input signal to the receiver; means for directly subtracting the output masking signal generated by the generating means from the input signal to the receiver and generating a resultant output signal; and means for demodulating the resultant output signal from the subtracting means for recovering an original input analog signal used to generate the FM signal.
Claim 8
Dependent Claim
119 words
A receiver for providing secure communications capable of receiving from a remote transmitter an input signal including a masking signal superimposed on a frequency modulated (FM) signal with a predetermined signal-to-noise ratio such that the FM signal is unintelligible, the receiver comprising: means for generating an output masking signal which corresponds to, and is synchronized with, the masking signal forming part of the input signal to the receiver; means for directly subtracting the output masking signal generated by the generating means from the input signal to the receiver and generating a resultant output signal; and means for demodulating the resultant output signal from the subtracting means for recovering an original input analog signal used to generate the FM signal.
Claim 9
Dependent Claim
15 words
A receiver according to claim 6 wherein the output masking signal comprises bandlimited Gaussian noise.
Claim 10
Dependent Claim
16 words
A receiver according to claim 6 wherein the output masking signal comprises a separate FM signal.
Claim 11
Dependent Claim
15 words
A receiver according to claim 6 wherein the output masking signal comprises a sine wave.
Claim 12
Dependent Claim
26 words
A receiver according to claim 6 wherein the predetermined signal-to-noise ratio of the masking signal forming part of the input signal is at least -5 db.
Claim 13
Independent Claim
106 words
A method of providing secure communications comprising the steps of: at a transmitter, (a) converting an input analog signal to a frequency modulated (FM) output signal at nominal carrier frequency; (b) generating an output masking signal which is limited to the band of the FM output signal and includes a predetermined threshold level such that when said masking signal is added to the FM output signal a resultant signal is generated which is unintelligible when received with a conventional FM receiver; and (c) directly adding the FM output signal from step (a) and the output masking signal from step (b) for generating a transmitter output signal.
Claim 14
Independent Claim
106 words
A method of providing secure communications comprising the steps of: at a transmitter, (a) converting an input analog signal to a frequency modulated (FM) output signal at nominal carrier frequency; (b) generating an output masking signal which is limited to the band of the FM output signal and includes a predetermined threshold level such that when said masking signal is added to the FM output signal a resultant signal is generated which is unintelligible when received with a conventional FM receiver; and (c) directly adding the FM output signal from step (a) and the output masking signal from step (b) for generating a transmitter output signal.
Claim 15
Dependent Claim
83 words
A method of secure communications according to claim 11 comprising the further steps of: at a receiver, (d) generating an output masking signal which corresponds to, and is synchronized with, the masking signal forming part of the transmitter output signal; (e) subtracting the output masking signal from step (d) from the transmitter output signal received at the receiver for generating a resultant output signal; (f) demodulating the resultant output signal from step (e) to generate the original input analog signal to the transmitter.
Claim 16
Dependent Claim
27 words
A method of providing secure communications according to claim 11 wherein in performing step (b) the output masking signal comprises at least a -5 db signal-to-noise ratio.
Claim 17
Dependent Claim
23 words
A method of providing secure communications according to claim 11 wherein in performing step (b) the output masking signal comprises bandlimited Gaussian noise.
Claim 18
Dependent Claim
24 words
A method of providing secure communications according to claim 11 wherein in performing step (b) the output masking signal comprises a separate FM signal.
Claim 19
Dependent Claim
23 words
A method of providing secure communications according to claim 11 wherein in performing step (b) the output masking signal comprises a sine wave.
Claim 20
Dependent Claim
23 words
A method of providing secure communications according to claim 12 wherein in performing step (d) the output masking signal comprises bandlimited Gaussian noise.
Claim 21
Dependent Claim
24 words
A method of providing secure communications according to claim 12 wherein in performing step (d) the output masking signal comprises a separate FM signal.
Claim 22
Dependent Claim
23 words
A method of providing secure communications according to claim 12 wherein in performing step (d) the output masking signal comprises a sine wave.
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USPTO Annuity & Maintenance Windows
| Annuity Stage | Window Opens | Payment Deadline | Surcharge Period Until | Estimated Fee | Status |
|---|---|---|---|---|---|
| 3.5 Years (Year 4) | 1989-02-24 | 1989-08-26 | 1990-02-25 | $2000.00 | Window Closed / Lapsed |
| 7.5 Years (Year 8) | 1993-02-24 | 1993-08-26 | 1994-02-25 | $3760.00 | Window Closed / Lapsed |
| 11.5 Years (Year 12) | 1997-02-24 | 1997-08-26 | 1998-02-25 | $7700.00 | Window Closed / Lapsed |