Technique for searching for a preamble signal in a spread spectrum using a fast hadamard transform
View Patent ↗In one embodiment, a method for demodulating and searching for a preamble signal containing a complex phasor signal is disclosed. The complex phasor is demodulated using a phasor-rotated fast transformer. A received signal is correlated with a spreading code to produce a correlated signal. The correlated signal is coherently accumulated to produce a coherently accumulated signal. A first phasor-rotated signal transformation is performed on a real component of the coherently accumulated signal, and a second phasor-rotated signal transformation is performed on an imaginary component of the coherently accumulated signal. Finally, the signal power of the transformed real and imaginary components of the coherently accumulated signal is determined.
1. A method for signal processing in a spread-spectrum communication system, comprising:
correlating a received signal with a spreading code to produce a correlated signal;
coherently accumulating the correlated signal to produce a coherently accumulated signal;
performing a first phasor-rotated signal transformation on a real component of the coherently accumulated signal;
performing a second phasor-rotated signal transformation on an imaginary component of the coherently accumulated signal; and
determining signal powers of the transformed real and imaginary components of the coherently accumulated signal, wherein:
the received signal is a phasor-rotated signal; and
the first and second phasor-rotated signal transformations perform phasor-derotation of the phasor-rotated signal.
2. The method according to claim 1 , further comprising sorting the signal powers to identify at least one of (1) a timing offset of a preamble transmission within the received signal, (2) a spreading code of the preamble transmission, and (3) a signature sequence of the preamble transmission.
3. The method according to claim 1 , further comprising:
noncoherently accumulating the signal powers of the transformed real and imaginary components of the noncoherently accumulated signal; and
sorting the noncoherently accumulated signal powers to identify at least one of (1) a timing offset of a preamble transmission within the received signal, (2) a spreading code of the preamble transmission, and (3) a signature sequence of the preamble transmission.
4. The method of claim 1 , wherein:
the received signal includes real and imaginary components; and
correlating the received signal with the spreading code comprises altering the signs of the real and imaginary components of the received signal, based on the value of the spreading code.
5. The method according to claim 1 , wherein:
the phasor-rotated signal is a transmission signal modulated by a complex phasor signal, and
the first and second phasor-rotated signal transformations are operative to demodulate the phasor-rotated signal to remove the complex phasor signal.
6. The method according to claim 5 , wherein the receiver performs the first and second phasor-rotated signal transformations to demodulate the phasor-rotated signal without multiplying the phasor-rotated signal by an inverse of the complex phasor signal.
7. The method according to claim 5 , wherein:
the complex phasor signal is
ⅇ
j
(
π
2
n
+
π
4
)
,
where n=0 corresponds to the chip transmitted first in time; and
an inverse of the complex phasor signal is
ⅇ
-
j
(
π
2
n
+
π
4
)
.
8. The method according to claim 1 , wherein the first and second phasor-rotated signal transformations are operative both (i) to de-rotate the phasor-rotated signal and (ii) to remove a signature code from the phasor-rotated signal.
9. The method according to claim 1 , wherein the first and second phasor-rotated signal transformations are operative simultaneously (i) to de-rotate the phasor-rotated signal and (ii) to remove the signature code from the phasor-rotated signal.
10. A signal processor, comprising:
a correlation unit configured to correlate a received signal with a spreading code to produce a correlated signal;
a coherent accumulator configured to coherently accumulate the correlated signal to produce a coherently accumulated signal;
one or more transform processors configured to (i) perform a first phasor-rotated signal transformation on a real component of the coherently accumulated signal and (ii) perform a second phasor-rotated signal transformation on an imaginary component of the coherently accumulated signal; and
an energy calculator configured to determine signal powers of the transformed real and imaginary components of the coherently accumulated signal, wherein:
the received signal is a phasor-rotated signal; and
the first and second phasor-rotated signal transformations perform phasor-derotation of the phasor-rotated signal.
11. The signal processor of claim 10 , further comprising a sort engine configured to sort the signal powers to identify at least one of (1) a timing offset of a preamble transmission within the received signal, (2) a spreading code of the preamble transmission, and (3) a signature sequence of the preamble transmission.
12. The signal processor of claim 10 , further comprising:
a noncoherent accumulator configured to noncoherently accumulate the signal powers of the real and imaginary components of the transformed signal; and
a sort engine configured to sort the noncoherently accumulated signal powers to identify at least one of (1) a timing offset of a preamble transmission within the received signal, (2) a spreading code of the preamble transmission, and (3) a signature sequence of the preamble transmission.
13. The signal processor of claim 10 , wherein:
the received signal includes real and imaginary components; and
the correlation unit comprises at least one sign alteration unit configured to alter the signs of the real and imaginary components of the received signal, based on the value of the spreading code.
14. The signal processor of claim 10 , wherein:
the phasor-rotated signal is a transmission signal modulated by a complex phasor signal, and
the first and second phasor-rotated signal transformations are operative to demodulate the phasor-rotated signal to remove the complex phasor signal.
15. The signal processor of claim 14 , wherein the signal processor performs the first and second phasor-rotated signal transformations to demodulate the phasor-rotated signal without multiplying the phasor-rotated signal by an inverse of the complex phasor signal.
16. The signal processor of claim 14 , wherein:
the complex phasor signal is
ⅇ
j
(
π
2
n
+
π
4
)
,
where n=0 corresponds to the chip transmitted first in time; and
an inverse of the complex phasor signal is
ⅇ
-
j
(
π
2
n
+
π
4
)
.
17. The signal processor of claim 10 , wherein the first and second phasor-rotated signal transformations are operative both (i) to de-rotate the phasor-rotated signal and (ii) to remove a signature code from the phasor-rotated signal.
18. The signal processor of claim 10 , wherein the first and second phasor-rotated signal transformations are operative simultaneously (i) to de-rotate the phasor-rotated signal and (ii) to remove the signature code from the phasor-rotated signal.