Pseudorandom noise code acquisition in direct sequence spread spectrum systems
View Patent ↗A method for receiving spread spectrum signals and for initial code acquisition from the received signals comprises de-spreading the received signals, based on code phase synchronization, to produce a de-spread signal; and performing a modulation detection based on a change of time-domain characteristic of the de-spread signal.
1. An initial code acquisition method, comprising:
receiving a spread spectrum signal including symbols;
de-spreading the received spread spectrum signal based on code phase synchronization to produce a de-spread signal; and
detecting a modulation format of the received spread spectrum signal based on a change of a time-domain characteristic of the de-spread signal,
wherein the change of a time-domain characteristic comprises a magnitude variation in a despread symbol in the de-spread signal.
2. A method as recited in claim 1 , wherein the de-spreading is based on frequency offset and code phase synchronization.
3. A method as recited in claim 2 , wherein:
a range of code phase to be searched is determined by a propagation delay of the spread spectrum signal; and
a range of frequency offset to be searched is determined by a Doppler frequency shift of the spread spectrum signal.
4. A method as recited in claim 1 , where the de-spreading includes de-scrambling.
5. A method as recited in claim 4 , wherein the de-scrambling employs a scrambling sequence that is a function of a code phase.
6. A method as recited in claim 1 , wherein the de-spreading includes de-spreading at one of a set of code channels, each of the code channels being based on a respective one of a set of orthogonal functions.
7. A method as recited in claim 6 , wherein:
the set of orthogonal functions includes a set of Walsh functions, each of the orthogonal functions comprising a respective one of the Walsh functions; and
the de-spreading includes de-scrambling and de-spreading at a predetermined Walsh code channel that corresponds with the respective one of the Walsh functions.
8. A method as recited in claim 7 , wherein the de-spreading includes:
generating a scrambling sequence based on a code phase and a frequency offset;
multiplying the received signal by the scrambling sequence to produce a product thereof; and
multiplying the product by the respective one of the Walsh functions.
9. A method as set forth in claim 1 , wherein detecting the modulation format of the received spread spectrum signal includes
recognizing a modulation format from among a plurality of possible modulation formats for the spread spectrum signal; and
detecting a threshold activity at the recognized modulation format.
10. A method as recited in claim 9 , wherein recognizing the modulation format includes recognizing based on Walsh code channels.
11. A method as recited in claim 1 , wherein:
the received spread spectrum signal is modulated with one of a plurality of possible modulation formats; and
detecting the modulation format of the received spread spectrum signal includes:
(i) calculating a metric for each of the possible modulation formats,
(ii) comparing the calculated metric for the modulation format to a threshold, and
(iii) detecting the presence of a given one of the modulation formats based on the comparison with the threshold.
12. A method as recited in claim 11 , wherein the calculating a metric includes calculating the magnitude variation in the de-spread signal.
13. An initial code acquisition system, comprising:
de-spreading circuitry for de-spreading a received spread spectrum signal based on code phase synchronization to produce a de-spread signal comprising symbols; and
a modulation detector for detecting a modulation format of the received spread spectrum signal based on a change of a time-domain characteristic of the de-spread signal,
wherein the change of a. time-domain characteristic comprises a magnitude variation in a despread symbol in the de-spread signal.
14. A system as recited in claim 13 , wherein the de-spreading circuitry includes circuitry that is operable based on frequency offset and code phase synchronization.
15. A system as recited in claim 14 , wherein:
a range of code phase to be searched is determined by a propagation delay of the spread spectrum signal; and
a range of frequency offset to be searched is determined by a Doppler frequency shift of the spread spectrum signal.
16. A system as recited in claim 13 , where the de-spreading circuitry includes de-scrambling circuitry,
17. A system as recited in claim 16 , wherein the de-scrambling circuitry employs a scrambling sequence that is a function of a code phase.
18. A system as recited in claim 13 , wherein the de-spreading circuitry includes de-spreading circuitry for de-spreading at one of a set of code channels, each of the code channels being based on a respective one of a set of orthogonal functions.
19. A system as recited in claim 18 , wherein:
the set of orthogonal functions includes a set of Walsh functions, each of the orthogonal functions comprising a respective one of the Walsh functions; and
the de-spreading circuitry includes circuitry for de-scrambling and de-spreading at a predetermined Walsh code channel that corresponds with the respective one of the Walsh functions.
20. A system as recited in claim 19 , wherein the de-spreading circuitry includes:
scrambling sequence generating circuitry that is operable based on a code phase and a frequency offset;
first multiplying circuitry for multiplying the received signal by the scrambling sequence to produce a product thereof; and
second multiplying circuitry for multiplying the product by the respective one of the Walsh functions.
21. A system as set forth in claim 13 , wherein the modulation detector includes:
circuitry for recognizing a modulation format from among a plurality of possible modulation formats for the spread spectrum signal; and
circuitry for detecting a threshold activity at the recognized modulation format.
22. A system as recited in claim 21 , wherein the circuitry for recognizing includes circuitry for recognizing based on Walsh code channels.
23. A system as recited in claim 21 , wherein:
the received spread spectrum signal is modulated with one of a plurality of possible modulation formats; and
the modulation detection circuitry includes:
(i) circuitry for calculating a metric for each of the possible modulation formats,
(ii) circuitry for comparing the calculated metrics to a threshold, and
(iii) circuitry for detecting the presence of a given one of the modulation formats based on the comparison with the threshold.
24. A system as recited in claim 23 , wherein the circuitry for calculating a metric includes circuitry for calculating the magnitude variation in the de-spread signal.
25. A system as recited in any of claims 13 - 24 , wherein circuitry recited therein is implemented as a processor and software for performing the operation of the recited circuitry.
26. A non-transitory computer-readable medium encoded with a computer program, for directing a processing system to perform initial code acquisition for received spread spectrum signals, the computer-readable medium comprising:
software program code, provided on the computer-readable medium, for directing the processing system to perform a method as recited in any of claims 1 - 12 .