Mechanism for deriving accurate frequency reference for satellite communications burst demodulator
First Claim
1. For use with a communication system having a first station and a second station remote with respect to said first station, said first and second stations communicating with one another over a communication link that subjects the carrier frequencies of signals transmitted thereover to an offset of their respective frequencies, said first station modulating information signals for delivery to said second station onto a first carrier frequency that is transmitted by said first station over said link on a continuous basis, said second station modulating information signals for delivery to said first station onto a second carrier frequency that is transmitted over said link on a burst mode basis, a method of enabling information signals, modulated onto said second carrier frequency which is transmitted on a burst mode basis from said second station over said link to said first station and thereby subjected to said frequency offset, to be recovered at said first station comprising the steps of:
- at said second station,(a) generating said second carrier frequency in accordance with a reference signal derivable from said first carrier frequency received thereby; and
at said first station,(b) receiving each of said first and second carrier frequencies that have been transmitted over said communication link; and
(c) deriving, from said first and second carrier frequencies, a filtered burst demodulation reference frequency for demodulating information signals that have been modulated onto said second, burst mode carrier frequency by said second station.
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Accused Products
Abstract
Accurate tuning of a satellite system'"'"'s burst demodulator to a signal which is subject to frequency offset in the course of its transmission over the satellite link is achieved without the use of high precision oscillators at remote sites, or the transmission of a dedicated pilot tone. Instead, a dedicated high precision clock is used for the purpose of establishing both the outlink carrier and the return channel carrier. The modulation of the data on the outlink carrier is also derived from the precision clock source. At each remote station, the outlink channel is monitored to recover the high precision clock. This recovered clock is then used as a reference for generating the return channel carrier. The burst demodulator equipment at the master station monitors both the outlink channel continuous carrier and burst mode transmissions from the remote stations. Each of the continuous and burst mode carriers, having been transmitted through the satellite, will undergo the same frequency offset or modification, so that the frequency difference between the outlink channel carrier and the return link channel carrier will always be constant. The outlink channel carrier, which is available on a continuous basis, is used to derive a local oscillator frequency reference for establishing the burst demodulator'"'"'s local oscillator reference.
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Citations
23 Claims
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1. For use with a communication system having a first station and a second station remote with respect to said first station, said first and second stations communicating with one another over a communication link that subjects the carrier frequencies of signals transmitted thereover to an offset of their respective frequencies, said first station modulating information signals for delivery to said second station onto a first carrier frequency that is transmitted by said first station over said link on a continuous basis, said second station modulating information signals for delivery to said first station onto a second carrier frequency that is transmitted over said link on a burst mode basis, a method of enabling information signals, modulated onto said second carrier frequency which is transmitted on a burst mode basis from said second station over said link to said first station and thereby subjected to said frequency offset, to be recovered at said first station comprising the steps of:
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at said second station, (a) generating said second carrier frequency in accordance with a reference signal derivable from said first carrier frequency received thereby; and at said first station, (b) receiving each of said first and second carrier frequencies that have been transmitted over said communication link; and (c) deriving, from said first and second carrier frequencies, a filtered burst demodulation reference frequency for demodulating information signals that have been modulated onto said second, burst mode carrier frequency by said second station. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8)
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9. An apparatus for generating a burst demodulation reference frequency, for enabling a first station, within a communication system having first and second stations geographically separated from one another and communicating over a communication link that subjects the carrier frequencies of signals transmitted thereover to an offset of their respective frequencies, said first station modulating information signals for delivery to said second station onto a first carrier frequency that is transmitted by said first station over said link on a continuous basis, said second station modulating information signals for delivery to said first station onto a second carrier frequency that is transmitted over said link on a burst mode basis, to recover information signals that have modulated onto said second burst mode carrier frequency and transmitted from said second station over said link to said first station and thereby subjected to said frequency offset, comprising:
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at said second station, first means for generating said second carrier frequency in accordance with a reference signal derivable from said first carrier frequency received thereby; and at said first station, second means for receiving each of said first and second carrier frequencies that have been transmitted over said communication link; and third means, coupled to said second means, for deriving, from said first and second carrier frequencies received thereby, a burst demodulation reference frequency for demodulating information signals that have been modulated onto said second, burst mode carrier frequency by said second station. said second, burst mode carrier frequency by said second station. - View Dependent Claims (10, 11, 12, 13, 14, 15, 16)
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17. An apparatus for generating a burst demodulation reference frequency, for enabling a first station, within a communication system having first and second stations geographically separated from one another and communicating over a communication link that subjects the carrier frequencies of signals transmitted thereover to an offset of their respective frequencies, said first station modulating information signals for delivery to said second station onto a first carrier frequency that is transmitted by said first station over said link on a continuous basis, said second station modulating information signals for delivery to said first station onto a second carrier frequency that is transmitted over said link on a burst mode basis, to recover information signals that have modulated onto said second burst mode carrier frequency and transmitted from said second station over said link to said first station and thereby subjected to said characteristic modification, comprising:
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at said second station, first means for generating said second carrier frequency in accordance with a reference signal derivable from said first carrier frequency received thereby and, at said first station, second means for receiving each of said first and second carrier frequencies that have been transmitted over said communication link, a plurality of third means, each of which is coupled to said second means, for selectively deriving, from said first and second carrier frequencies received thereby, a burst demodulation reference frequency for demodulating information signals that have been modulated onto said second, burst mode carrier frequency by said second station, and fourth means, coupled to each of said third means, for selectively defining frequency control characteristics of each of said third means with respect to first and second carrier frequencies and enabling a selected third means to derive said burst demodulation reference frequency in accordance with said selectively defined frequency control characteristics. - View Dependent Claims (18, 19, 20)
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- 21. An apparatus according to claim 21, wherein said third means includes down-conversion means for down-converting respective continuous mode and burst mode signals transmitted on said first and second carrier frequencies through said satellite link and received by said first station to a first, lower frequency continuous mode signals and a second, lower frequency burst mode signal, means for deriving, from said first, lower frequency continuous mode signal, a continuous reference frequency, and means for combining said continuous reference frequency with said second, lower frequency burst signal to produce said burst demodulation reference frequency.
Specification