TDMA satellite communications system operating in wide and narrow aperture modes for receiving bursts of various timing deviations
First Claim
1. A satellite communications system having a central earth station and a plurality of terminal earth stations establishing a satellite channel with the central earth station via a geosynchronous satellite, the central earth station transmitting a time slot assignment signal and a reference timing signal to the terminal stations via the satellite channel to allow the terminal earth stations to send bursts on time slots specified by the assignment signal and establish a timing reference for timing said bursts, the central station including burst detector means for detecting said bursts from the terminal stations, a carrier and clock recovery circuit for recovering a carrier and clock pulses from the output of the detector means, and a unique word detector for detecting a unique word contained in the burst detected by the burst detector means and generating an output signal representative of the presence of a unique word, the central earth station comprising;
- timing means for generating a local timing signal synchronously with said reference timing signal;
delay means for introducing a delay to the local timing signal by an amount corresponding to a minimum round-trip propagation delay of the satellite communications system;
first control means responsive to said delayed local timing signal for generating a first control signal to recover said carrier and clock pulses, and generating a wide aperture gating pulse;
second control means responsive to a reset pulse applied thereto for generating a second control signal to recover said carrier and clock pulses, and generating a narrow aperture gating pulse; and
aperture control means for initially applying said first control signal to said carrier and clock recovery circuit and allowing said second control means to respond to an output signal from the unique word detector as said reset pulse during the wide aperture gating pulse of first occurrence;
applying the second control signal to the carrier and clock recovery circuit and allowing the second control means to respond to a unique word detector output signal during the narrow aperture gating pulse of first occurrence if a unique word is present during the wide aperture gating pulse of the first occurrence and continuously applying the second control signal to the carrier and clock recovery circuit and continuously allowing the second control means to respond to a unique word detector output signal as said reset pulse during the narrow aperture gating pulse of subsequent occurrence if a unique word is present during the narrow aperture gating pulse of the first occurrence; and
applying the first control signal to the carrier and clock recovery circuit and allowing the second control means to respond to a unique word detector output signal as said reset pulse during the wide aperture gating pulse of subsequent occurrence if a unique word is not present during at least one of the narrow aperture gating pulses of successive occurrences.
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Accused Products
Abstract
In a satellite communications system, a time slot assignment signal and a reference timing signal are transmitted from a central station to terminal stations to allow them to send bursts on assigned time slots and to establish the frame timing. In the central station, first and second self-resettable counters increment their respective binary counts at clock intervals and reset their counts to an initial value at time-slot intervals. A local timing signal is generated synchronously with the reference timing signal and delayed by an amount corresponding to a minimum round-trip propagation delay to reset the first counter. In an initial, wide aperture mode, the output of the first counter is used to generate control signals for recovering a carrier and clock and widely opening an aperture gate for detecting unique words of a burst that is affected by satellite drift. If a unique word is detected, the central station switches to a narrow aperture mode in which the output of the second counter is used instead to generate control signals for recovering the carrier and clock and narrowly opening the gate to reduce the possibility of detecting false unique words. In a preferred embodiment, the output of the second counter is stored in a memory in response to the detection of a unique word and an average value of the stored counts is derived to optimize the reset timing of the second counter.
40 Citations
9 Claims
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1. A satellite communications system having a central earth station and a plurality of terminal earth stations establishing a satellite channel with the central earth station via a geosynchronous satellite, the central earth station transmitting a time slot assignment signal and a reference timing signal to the terminal stations via the satellite channel to allow the terminal earth stations to send bursts on time slots specified by the assignment signal and establish a timing reference for timing said bursts, the central station including burst detector means for detecting said bursts from the terminal stations, a carrier and clock recovery circuit for recovering a carrier and clock pulses from the output of the detector means, and a unique word detector for detecting a unique word contained in the burst detected by the burst detector means and generating an output signal representative of the presence of a unique word, the central earth station comprising;
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timing means for generating a local timing signal synchronously with said reference timing signal; delay means for introducing a delay to the local timing signal by an amount corresponding to a minimum round-trip propagation delay of the satellite communications system; first control means responsive to said delayed local timing signal for generating a first control signal to recover said carrier and clock pulses, and generating a wide aperture gating pulse; second control means responsive to a reset pulse applied thereto for generating a second control signal to recover said carrier and clock pulses, and generating a narrow aperture gating pulse; and aperture control means for initially applying said first control signal to said carrier and clock recovery circuit and allowing said second control means to respond to an output signal from the unique word detector as said reset pulse during the wide aperture gating pulse of first occurrence;
applying the second control signal to the carrier and clock recovery circuit and allowing the second control means to respond to a unique word detector output signal during the narrow aperture gating pulse of first occurrence if a unique word is present during the wide aperture gating pulse of the first occurrence and continuously applying the second control signal to the carrier and clock recovery circuit and continuously allowing the second control means to respond to a unique word detector output signal as said reset pulse during the narrow aperture gating pulse of subsequent occurrence if a unique word is present during the narrow aperture gating pulse of the first occurrence; and
applying the first control signal to the carrier and clock recovery circuit and allowing the second control means to respond to a unique word detector output signal as said reset pulse during the wide aperture gating pulse of subsequent occurrence if a unique word is not present during at least one of the narrow aperture gating pulses of successive occurrences. - View Dependent Claims (2, 3, 4, 5, 6)
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7. In a satellite communications system having a central earth station and a plurality of terminal earth stations establishing a satellite channel with the central earth station via a geosynchronous satellite, the central earth station transmitting a time slot assignment signal and a reference timing signal to the terminal stations via the satellite channel to allow the terminal earth stations to send bursts on time slots specified by the assignment signal and establish a timing reference for timing said bursts, the central station including burst detector means for detecting said bursts from the terminal stations, a carrier and clock recovery circuit for recovering a carrier and clock pulses from the output of the detector means, and a unique word detector for detecting a unique word contained in the burst detected by the burst detector means and generating an output signal representative of the presence of a unique word, a method comprising the steps of:
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a) generating a local timing signal in synchronism with said reference timing signal; b) introducing a delay to the local timing signal by an amount corresponding to a minimum round-trip propagation delay of the satellite communications system; c) applying a control signal to the carrier and clock recovery circuit in response to the delayed timing signal to recover said carrier and clock pulses and detecting a unique word detector output signal through a wide-open window; d) applying said control signal to the carrier and clock recovery circuit in response to the detection of a unique word by the step (c) to recover said carrier and clock pulses and searching for a unique word detector output signal through a narrow window; and e) repeating the step (c) if a unique word is not detected by the step (d) and repeating the step (d) if a unique word is detected by the step (d).
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8. In a satellite communications system having a central earth station and a plurality of terminal earth stations establishing a satellite channel with the central earth station via a geosynchronous satellite, the central earth station transmitting a time slot assignment signal and a reference timing signal to the terminal stations via the satellite channel to allow the terminal earth stations to send bursts on time slots specified by the assignment signal and establish a timing reference for timing said bursts, the central station including burst detector means for detecting said bursts from the terminal stations, a carrier and clock recovery circuit for recovering a carrier and clock pulses from the output of the detector means, and a unique word detector for detecting a unique word contained in the burst detected by the burst detector means and generating an output signal representative of the presence of a unique word, a method comprising the steps of:
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a) generating a local timing signal in synchronism with said reference timing signal; b) introducing a delay to the local timing signal by an amount corresponding to a minimum round-trip propagation delay of the satellite communications system; c) applying a control signal to the carrier and clock recovery circuit in response to the delayed timing signal to recover said carrier and clock pulses and detecting a unique word detector output signal through a wide-open window; d) incrementing a binary count at clock intervals; e) storing said binary count into a memory in response to the detection of a unique word detector output signal; f) counting the detected unique word detector output signals; g) if the count given by the step (f) reaches a predetermined value, deriving an average value of the binary counts stored in the memory; h) comparing the binary count of the step (d) with the average value; i) if the binary count of the step (d) is equal to the average value, applying said control signal to the carrier and clock recovery circuit to recover said carrier and clock pulses and searching for a unique word detector output signal through a narrow window; and j) repeating the steps (c) to (i) if a unique word is not detected by the step (i) and repeating the steps (f) to (i). - View Dependent Claims (9)
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Specification