Preamble code structure and detection method and apparatus
First Claim
1. A code detector for detecting a repeated codeword, comprising:
- a correlation filter;
a plurality of delay blocks connected to an output of said correlation filter;
a plurality of test signal locations, one for each subcode in the repeated codeword, wherein each test signal location is separated from the next by one of said delay blocks;
an alert summer connected to said test signal locations;
an alert comparator connected to an output of said summer and a first threshold signal, said alert comparator having as an output an alert signal;
a plurality of confirm comparators;
one for each test signal location;
a second threshold signal connected to each of said confirm comparators; and
a decision block connected to each of said confirm comparators, having as an output a confirm signal.
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Accused Products
Abstract
In a first embodiment of the invention, a concatenated preamble code is formed by a kronecker product between two subcodes of the same or different lengths. The subcodes have favorable correlation properties and may be Barker codes, minimum peak sidelobe codes, Gold codes, Kasami codes, Boztas codes, or other codes. At the receiver a two-stage processor is used to detect the concatenated preamble code. The two-stage processor comprises a series of two filters, one of which is preferably a mismatched filter. In another aspect of the invention, a repeated codeword preamble is formed from a series of the same repeated short subcode. The short subcode may be an augmented or truncated odd-length code. At the receiver a single matched filter is preferably used to generate a series of spikes separated by the period of the subcode. An alert/confirm detector non-coherently adds together individual correlation spikes and reject false alarms. The alert function measures total correlation energy and adjusts detection thresholds based thereon, while the confirm function ensures that the proper number of correlation spikes exceed a detection threshold at the correct times. The preamble codes and their means for detection are preferably employed in a TDMA spread spectrum communication system, wherein a plurality of user stations communicate with a base station, one in each time slot (or virtual time slot).
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Citations
17 Claims
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1. A code detector for detecting a repeated codeword, comprising:
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a correlation filter;
a plurality of delay blocks connected to an output of said correlation filter;
a plurality of test signal locations, one for each subcode in the repeated codeword, wherein each test signal location is separated from the next by one of said delay blocks;
an alert summer connected to said test signal locations;
an alert comparator connected to an output of said summer and a first threshold signal, said alert comparator having as an output an alert signal;
a plurality of confirm comparators;
one for each test signal location;
a second threshold signal connected to each of said confirm comparators; and
a decision block connected to each of said confirm comparators, having as an output a confirm signal. - View Dependent Claims (2, 3, 4, 5)
a second summer connected to a plurality of taps from at least one of said delay blocks;
a first multiple connected to an output of said second summer and a first threshold coefficient, said first multiplier outputting said first threshold signal; and
a second multiplier connected to said output of said second summer and a second threshold coefficient, said second multiplier outputting said second threshold signal.
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6. A method of detecting a repeated codeword, comprising the steps of:
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correlating a received codeword and generating a correlation signal thereby, said codeword comprising a plurality of repeated subcodes;
detecting an envelope of said correlation signal and generating an envelope detected correlation signal thereby;
storing at least a portion of said envelope detected correlation signal;
summing from a plurality of test signal locations an energy level of said envelope detected correlation signal, and generating a summation signal thereby;
comparing said summation signal with a first threshold signal and generating an alert signal in response thereto;
comparing the energy level at each of said test signal locations with a second threshold signal and generating a plurality of comparison signals thereby; and
generating a confirm signal in response to said plurality of comparison signals. - View Dependent Claims (7, 8, 9, 10, 11, 12)
generating an average background noise signal from said envelope detected correlation signal; and
deriving said first threshold signal and said second threshold signal from said average background noise signal.
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8. The method of claim 7 wherein said step of generating an average background noise signal from said envelope detected correlation signal comprises the step of summing the energy from a portion of envelope detected correlation signal between two of said test signal locations.
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9. The method of claim 6 wherein said test signal locations are separated by a period of one of said plurality of subcodes.
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10. The method of claim 6 wherein said plurality of subcodes are four in number.
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11. The method of claim 6 wherein said subcodes are selected from a family of codes selected from the group consisting of the following:
- Gold codes, Kasami codes, minimum peak sidelobes codes, Barker codes, Boztas codes, and Matsufuji codes.
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12. The method of claim 6 wherein said plurality of subcodes comprise quadrature codes.
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13. An alert/confirm preamble code detector comprising:
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a matched filter;
a correlation signal output from said match filter;
a shift register connected to said correlation signal and having a plurality of test signal taps separated from each other by a period of a subcode in a repeated code to be detected;
an alerting circuit connected to said test signal taps and responsive to a total energy of said correlation signal at said test signal taps;
an alert signal output from said alerting circuit and switching states when said total energy of said correlation signal at said test signal taps exceeds a first threshold signal;
a confirming circuit connected to said test signal taps; and
a confirm signal output from said confirming circuit and switching states when a correlation energy level at a predetermined number of said test signal taps exceeds a second threshold signal. - View Dependent Claims (14, 15, 16, 17)
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Specification