Method and apparatus for robust acquisition of spread spectrum signals
View Patent ↗A Direct Sequence Spread Spectrum receiver is described in which a complex spread spectrum signal is cross correlated by four digitally matched filters. The filters and subsequent multipliers correlate the in-phase and quadrature components as well as the current and previous symbols. The result is a more robust signal acquisition at very low signal to noise ratios.
1. A spread spectrum receiver, comprising:
a correlator to cross correlate in-phase and quadrature direct sequence spread spectrum (DSSS) signals (a) for a current symbol, by:
correlating the in-phase DSSS signal for the current symbol relative to a stored replica of the in-phase DSSS signal;
correlating the quadrature DSSS signal for the current symbol relative to a stored replica of the quadrature DSSS signal;
correlating the in-phase DSSS signal for the current symbol relative to a stored replica of the quadrature DSSS signal;
correlating the quadrature DSSS signal for the current symbol relative to a stored replica of the in-phase DSSS signal; and
(b) for a previous symbol, by:
correlating the in-phase DSSS signal for the previous symbol relative to a stored replica of the in-phase DSSS signal;
correlating the quadrature DSSS signal for the previous symbol relative to a stored replica of the quadrature DSSS signal;
correlating the in-phase DSSS signal for the previous symbol relative to a stored replica of the quadrature DSSS signal; and
correlating the quadrature DSSS signal for the previous symbol relative to a stored replica of the in-phase DSSS signal; and
a mixer to mix each of the four cross correlations for the current symbol with corresponding ones of the four cross correlations for the previous symbol; and
an adder to combine the associated current and previous symbol cross correlations into a detection statistic.
2. A spread spectrum receiver according to claim 1 , wherein:
the correlator includes a digitally matched filter.
3. A spread spectrum receiver according to claim 2 , wherein:
the correlator includes a plurality of digitally matched filters, two for each of the in-phase and quadrature DSSS signals.
4. A spread spectrum receiver according to claim 3 , wherein:
the two matched filters for the in-phase DSSS signals correlate the in-phase DSSS signals to, respectively, the in-phase and quadrature DSSS signals; and
the two matched filters for the quadrature DSSS signals correlate the quadrature DSSS signals to, respectively, the in-phase and quadrature DSSS signals.
5. A spread spectrum receiver according to claim 1 , wherein:
the mixer includes storage to hold the cross correlations for the previous symbols for the in-phase and quadrature DSSS signals.
6. A spread spectrum receiver according to claim 5 , wherein:
the mixer includes multipliers receiving the outputs of the correlator and the storage and to output a multiplication value of (1) the cross correlations for the current symbols for the in-phase and the quadrature DSSS signals from the correlator and (2) the cross correlations for the previous symbols for the in-phase and quadrature DSSS signals from the storage.
7. A spread spectrum receiver according to claim 6 , wherein:
the adder receives and sums the outputs of the multipliers.
8. A spread spectrum receiver according to claim 1 , wherein:
(a) the correlator includes:
a first digitally matched filter receiving an in-phase symbol stream and correlating said in-phase symbol stream by referencing the in-phase symbol stream;
a second digitally matched filter receiving a quadrature symbol stream and correlating said quadrature symbol stream by referencing the quadrature symbol stream;
a third digitally matched filter receiving the quadrature symbol stream and correlating said quadrature symbol stream by referencing the in-phase symbol stream;
a fourth digitally matched filter receiving the in-phase symbol stream and correlating said in-phase symbol stream by referencing the quadrature symbol stream;
(b) the mixer includes:
a first storage location to receive an output of the first digitally matched filter corresponding to a previous symbol in the in-phase symbol stream;
a second storage location to receive an output of the second digitally matched filter corresponding to a previous symbol in the quadrature symbol stream;
a third storage location to receive an output of the third digitally matched filter corresponding to a previous symbol in the quadrature symbol stream;
a fourth storage location to receive an output, of the fourth digitally matched filter corresponding to a previous symbol in the in-phase symbol stream; and
multipliers to respectively mix the contents of the respective first through fourth storage locations with corresponding outputs of the first through fourth digitally matched filters; and
(c) the adder includes:
summers to add the outputs of the multipliers.
9. A method of processing spread spectrum signals, comprising:
cross correlating in-phase and quadrature direct sequence spread spectrum (DSSS) signals for a current symbol, by:
correlating the in-phase DSSS signal for the current symbol relative to a stored replica of the in-phase DSSS signal;
correlating the quadrature DSSS signal for the current symbol relative to a stored replica of the quadrature DSSS signal;
correlating the in-phase DSSS signal for the current symbol relative to a stored replica of the quadrature DSSS signal;
correlating the quadrature DSSS signal for the current symbol relative to a stored replica of the in-phase DSSS signal; and
(b) for a previous symbol, by:
correlating the in-phase DSSS signal for the previous symbol relative to a stored replica of the in-phase DSSS signal;
correlating the quadrature DSSS signal for the previous symbol relative to a stored replica of the quadrature DSSS signal;
correlating the in-phase DSSS signal for the previous symbol relative to a stored replica of the quadrature DSSS signal; and
correlating the quadrature DSSS signal for the previous symbol relative to a stored replica of the in-phase DSSS signal; and
multiplying the current cross correlations for the current symbols with corresponding ones of the cross correlations for the previous symbols; and
adding the multiplied cross correlations into a detection statistic.
10. A method according to claim 9 , wherein:
the cross correlating includes digitally matched filtering.
11. A method according to claim 10 , wherein:
the cross correlating includes correlating the in-phase DSSS signals respectively to the in-phase and quadrature DSSS signals; and
the cross correlating includes correlating the quadrature DSSS signals respectively to the in-phase and quadrature DSSS signals.
12. A method according to claim 11 , wherein:
the multiplying step includes the step of storing the cross correlations for previous symbols for the in-phase and quadrature DSSS signals.
13. A method according to claim 12 , wherein:
the multiplying step includes the step of multiplying the cross correlations for the current symbols for the in-phase and the quadrature DSSS signals with corresponding ones of the cross correlations for the previous symbols for the in-phase and quadrature DSSS signals.
14. A method according to claim 9 , wherein:
the cross correlating includes:
correlating the in-phase symbol stream at a first symbol location by referencing the in-phase symbol stream;
correlating the quadrature phase symbol stream at a second symbol location by referencing the in-phase symbol stream;
correlating the quadrature phase symbol stream at the second symbol location by referencing the quadrature phase symbol stream;
correlating the in-phase symbol stream at the first symbol location by referencing the quadrature phase symbol stream;
repeating the correlating steps for other symbols in each of said symbol streams;
multiplying the correlations for said first and second symbols with corresponding ones of said correlations for said other symbols; and
adding the results of said multiplying step.