Rapid signal acquisition by spread spectrum transceivers

• US 6,757,323 B1
• Filed: 11/22/2000
• Issued: 06/29/2004
• Est. Priority Date: 05/16/2000
• Status: Active Grant

• Current Assignee: Cherie Elaine Kushner, Robert Alan Fleming
• Original Assignee: Cherie Elaine Kushner, Robert Alan Fleming
• Inventors: Fleming, Robert Alan, Kushner, Cherie Elaine
• Primary Examiner(s): PHU, PHUONG M
• Application Number:
  US09/721,116
• Time in Patent Office: 1,315 Days
• Field of Search: 375/142, 375/130, 375/133, 375/134, 375/140, 375/141, 375/145, 375/147, 375/149, 375/150, 375/152, 370/330, 370/347
• US Class Current: 375/142
• CPC Class Codes: H04B 1/70755 Setting of lock conditions, e.g. threshold
• CPC Class Codes: H04B 1/70775 Multi-dwell schemes, i.e. multiple accumulation times
• CPC Class Codes: H04B 2201/70701 featuring pilot assisted reception
• CPC Class Codes: H04B 2201/70709 with discontinuous detection

Abstract

Spread spectrum transceivers communicate using code sequences having low cross-correlations and well-peaked autocorrelations. The initial communications involve broadcasting a beacon signal consisting of a beacon packet repeated at regular intervals (the cycle time). The code sequences may be period-(2ⁿ−1) Small Kasami sequences; the beacon packet is a repeated series of (2^n/2+1) period-(2^n/2−1) progenitor maximal sequences, and behaves like a member of the Kasami family. The acyclic autocorrelation of the beacon packet has regularly-spaced sharp peaks modulated by a pyramidal envelope. The initial communications involve calculating the correlation between the received signal and delayed versions of an internally-generated beacon packet. The length of the initial communications is proportional to the square of the cycle time divided by the width of the acyclic autocorrelation. Synchronization involves locating the peak of the pyramidal envelope, and has a length related to the cycle time times the number of peaks in the acyclic autocorrelation.

Claims

1. A method for signal acquisition between a first spread spectrum transceiver and a second spread spectrum transceiver, comprising the steps of:

transmission by said first spread spectrum transceiver of a beacon signal which includes beacon packets each having a beacon packet length, said beacon packets being transmitted at regularly spaced beacon transmission times, each of said beacon transmission times being separated by a beacon cycle interval, said beacon packet length being shorter than said beacon cycle interval so that there is a period of nontransmission between each of said beacon packets, each of said beacon packets having an acyclic auto-correlation function with sidelobes having sidelobe values that are not small relative to a peak value of said acyclic auto-correlation function, said sidelobes having a structure which allows a sampling of a number of said sidelobe values to be used to provide an estimate of a location of said peak value of said acyclic auto-correlation function;

initial-stage reception by said second spread spectrum transceiver over an initial-stage reception window which begins at an initial-stage reception time which is a first integer multiple of said beacon cycle interval augmented in a first direction by a first fraction of twice said beacon packet length later than a previous initial-stage reception time of said second spread spectrum transceiver if initial-stage test values from a previous execution of said initial-stage reception are less than a cut-off value, said initial-stage reception being performed by calculation of correlations between time delayed versions of said beacon packet and a received signal to produce said initial-stage test values;

first intermediate-stage reception by said second spread spectrum transceiver over a first intermediate-stage reception window which begins at a first intermediate-stage reception time which is a second integer multiple of said beacon cycle interval augmented in a second direction by a second fraction of twice said beacon packet length later than said previous initial-stage reception time of said second spread spectrum transceiver if any of said initial-stage test values from said previous execution of said initial-stage reception are greater than said cut-off value, said first intermediate-stage reception being performed by calculation of correlations between said time delayed versions of said beacon packet and said received signal to produce first intermediate-stage reception values;

second intermediate-stage reception by said second spread spectrum transceiver if all said first intermediate-stage reception values from said first intermediate-stage reception are less than said cut-off value, second intermediate-stage reception being performed over a second intermediate-stage reception window which begins at a second intermediate-stage reception time which is a third integer multiple of said beacon cycle interval augmented in a third direction, opposite said second direction, by said second fraction of twice said beacon packet length later than said first intermediate-stage reception time of said second spread spectrum transceiver, said second intermediate-stage reception being performed by said calculation of said correlations between said time delayed versions of said beacon packet and said received signal to produce second intermediate-stage reception values, some of said second intermediate-stage reception values being greater than said cut-off value; and

determination of a location of a maximum value of a beacon packet isolated-sequence autocorrelation using said initial-stage test values which are greater than said cut-off value, using said first intermediate-stage reception values if some of said first intermediate-stage reception values are greater than said cut-off value, and using said second intermediate-stage reception values if some of said second intermediate-stage reception values are greater than said cut-off value.

2. The method of claim 1 wherein each of said beacon packets includes consecutive repeated beacon segments each having a segment length, said beacon segments having a beacon segment cyclic autocorrelation function with sidelobes which are substantially smaller than a maximum value of said beacon segment cyclic autocorrelation function, so that said beacon packet acyclic autocorrelation function includes a series of peaks which are separated by said segment length and decrease linearly in magnitude with distance from said maximum value of said beacon packet acyclic autocorrelation function.

3. The method of claim 2 wherein an initial portion of an initial one of said beacon segments in said time delayed versions of said beacon packet is omitted to produce a truncated , beacon packet, such that off-peak values of an acyclic correlation between a complete one of said beacon segments and said truncated beacon packet are reduced.

4. The method of claim 2 wherein a final portion of a final one of said beacon segments in said time delayed versions of said beacon packet is omitted to produce a truncated beacon packet, such that off-peak values of an acyclic correlation between a complete one of said beacon segments and said truncated beacon packet are reduced.

5. The method of claim 2 wherein a final portion of one of said beacon segments is appended to a beginning of an initial one of said beacon segments in said time delayed versions of said beacon packet to produce an extended beacon packet, such that off-peak values of an acyclic correlation between a complete one of said beacon segments and said extended beacon packet are reduced.

6. The method of claim 2 wherein a initial portion of one of said beacon segments is appended to an end of a final one of said beacon segments in said time delayed versions of said beacon packet to produce an extended beacon packet, such that off-peak values of an acyclic correlation between a complete one of said beacon segments and said extended beacon packet are reduced.

7. The method of claim 2, 3, 4, 5 or 6 wherein a phase of said beacon segments is selected such that off-peak values of a beacon segment acyclic autocorrelation function are reduced.

8. The method of claim 2 wherein said code sequences are Small Kasami sequences generated by a product of a degree-(n) polynomial h(x) and a degree-(n/2) polynomial h′

(x) whose roots are (2^n/2−

1)th powers of roots of said degree-(n) polynomial h(x), and each of said beacon segments is a maximal sequence generated by said degree-(n/2) polynomial h′

(x).

9. The method of claim 1 further comprising the step of:

a third intermediate-stage reception if any of said first intermediate-stage reception values, from said first intermediate-stage reception are greater than said cut-off value, said third intermediate-stage reception being performed by said second spread spectrum transceiver over a third intermediate-stage reception window which begins at a third intermediate-stage reception time which is a fourth integer multiple of said beacon cycle interval augmented in a fourth direction by a third fraction of twice said beacon packet length later than said first intermediate-stage reception time, said third intermediate-stage reception being performed by said calculation of said correlation of said time delayed versions of said beacon packet with said received signal to produce third intermediate-stage reception values, and wherein said determination of said location of said maximum value of said beacon packet acyclic autocorrelation also uses said third intermediate-stage reception values; and

a fourth intermediate-stage reception if all of said first intermediate-stage reception values from said first intermediate-stage reception are less than said cut-off value, said fourth intermediate-stage reception being performed by said second spread spectrum transceiver over a fourth intermediate-stage reception window which begins at a fourth intermediate-stage reception time which is a fifth integer multiple of said beacon cycle interval augmented in a fifth direction by a fourth fraction of twice said beacon packet length later than said first intermediate-stage reception time, said fourth intermediate-stage reception being performed by said calculation of said correlation of said time delayed versions of said beacon packet with said received signal to produce fourth intermediate-stage reception values, and wherein said determination of said location of said maximum value of said beacon packet acyclic autocorrelation also uses said fourth intermediate-stage reception values.

10. The method of claim 9 wherein said third fraction is less than said second fraction, said fourth fraction is less than said second fraction, said fourth direction is opposite said second direction, and said fifth direction is opposite said third direction.

11. The method of claim 1 wherein said initial-stage reception window includes a plurality of initial-stage reception window frames which, modulo said beacon cycle interval, are substantially contiguous, wherein said first intermediate-stage reception window includes a plurality of first intermediate-stage reception window frames which, modulo said beacon cycle interval, are contiguous, and wherein said second intermediate-stage reception window includes a plurality of second intermediate-stage reception window frames which, modulo said beacon cycle interval, are contiguous.

12. The method of claim 1 wherein said beacon packet length is substantially shorter than said beacon cycle interval.

13. The method of claim 12 wherein said beacon packet length is at least ten times shorter than said beacon cycle interval.

14. The method of claim 1 wherein said first and second fractions are less than one and greater than zero.

15. The method of claim 14 wherein said first fraction is greater than said second fraction.

16. The method of claim 1 wherein said first integer multiple is unity, said second integer multiple is unity, and said third integer multiple is unity.

17. The method of claim 1 wherein said sidelobes of said beacon packet acyclic auto-correlation function have values which are roughly of the same magnitude as said peak value of said beacon packet isolated-sequence auto-correlation function.

18. The method of claim 1 wherein said first spread spectrum transceiver and said second spread spectrum transceiver are in the vicinity of a number of additional spread spectrum transceivers which communicate with each other using a class of code sequences which have low cross-correlations and have auto-correlations with small off-peak values.

19. The method of claim 1 wherein said code sequences have a code sequence bit length, and said cross-correlations and said off-peak values of said cyclic auto-correlations have a maximum value which is less than a small multiple of a square root of said code sequence bit length.

20. The method of claim 19 wherein said small multiple is less than 5.

21. The method of claim 19 wherein said small multiple is less than 4.

22. The method of claim 19 wherein said small multiple is less than 3.

23. The method of claim 19 wherein said small multiple is less than 2.