Preamble code structure and detection method and apparatus

• US 6,363,107 B1
• Filed: 09/12/2000
• Issued: 03/26/2002
• Est. Priority Date: 11/15/1996
• Status: Expired due to Term

• Current Assignee: Intel Corporation
• Original Assignee: Xircom Wireless Incorporated
• Inventors: Scott, Logan
• Primary Examiner(s): CORRIELUS, JEAN B
• Application Number:
  US09/660,491
• Time in Patent Office: 560 Days
• Field of Search: 375/130, 375/136, 375/140, 375/147, 375/150, 375/152, 375/142, 375/153, 375/367, 375/139, 375/137, 375/145, 375/149, 370/515, 370/503, 370/517, 708/314, 708/422, 708/426
• US Class Current: 375/150
• CPC Class Codes: H04B 1/70755 Setting of lock conditions, e.g. threshold
• CPC Class Codes: H04B 1/7077 Multi-step acquisition, e.g. multi-dwell, coarse-fine or validation
• CPC Class Codes: H04B 1/7093 Matched filter type
• CPC Class Codes: H04J 13/0022 PN, e.g. Kronecker
• CPC Class Codes: H04J 13/107 by concatenation

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).

Claims

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.

2. The code detector of claim 1 wherein each delay block has a delay length of said subcode minus one chip interval, and each test signal location has a delay length of one chip interval.

3. The code detector of claim 1 wherein said correlation filter comprises a matched filter.

4. The code detector of claim 1 wherein said correlation filter comprises a mismatched filter.

5. The code detector of claim 1 further comprising:

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.

6. A method of detecting a repeated codeword, comprising the steps of:

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.

7. The method of claim 6 further comprising the steps of:

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.

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.

9. The method of claim 6 wherein said test signal locations are separated by a period of one of said plurality of subcodes.

10. The method of claim 6 wherein said plurality of subcodes are four in number.

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.

12. The method of claim 6 wherein said plurality of subcodes comprise quadrature codes.

13. An alert/confirm preamble code detector comprising:

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.

14. The alert/confirm preamble code detector of claim 13 wherein said alerting circuit comprising a summer connected to said test signal taps and a comparator connected to an output of said summer and said first threshold signal.

15. The alert/confirm preamble code detector of claim 14 wherein said first threshold signal comprises a product of a threshold coefficient signal and a signal indicative of an average background noise level.

16. The alert/confirm preamble code detector of claim 13 wherein said confirming circuit comprises a plurality of comparators, each comparator connected to one of said test signal taps and said second threshold signal, and a decision circuit connected to an output from each comparator.

17. The alert/confirm preamble code detector of claim 16 wherein said second threshold signal comprises a product of a threshold coefficient signal and a signal indicative of an average background noise level.