Pulse or digital communications – Receivers – Interference or noise reduction
Reexamination Certificate
1999-01-22
2002-08-06
Chin, Stephen (Department: 2734)
Pulse or digital communications
Receivers
Interference or noise reduction
C342S361000, C342S373000, C342S383000, C343S726000, C343S756000, C343S893000
Reexamination Certificate
active
06430239
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention concerns a process of cyclic detection in diversity of polarization of digital cyclostationary radioelectric signals. Notably, the invention concerns the detection of such radioelectric signals by two antennas of radiation diagram sensitive to orthogonal polarization of the signals. The invention is applicable in the field of radiocommunications.
DESCRIPTION OF THE PRIOR ART
In the field of radiocommunications, the utilization of digital signals is rapidly increasing. During a radio communication a transmitter emits radioelectric signals in one or more frequency channels each characterized by a frequency bandwidth B
max
and a central frequency. The signals transmitted in these frequency channels are characterized by a number of parameters such as the binary bit rate, the modulation index and the carrier frequency. Appendix A contains a glossary of terms used in the text. There are known techniques for calculating the spectral correlation or the cyclic correlation of cyclostationary signals to determine these parameters and detect these signals. Appendix B contains some definitions and properties concerning cyclostationary signals, 2nd order cyclic statistics and the spectral correlation of such signals. Known systems implementing these techniques perform this detection using a signal received on a single antenna. Notably, the known FAM algorithm (meaning Fast Fourier Transform Accumulation Method) has been the object of numerous publications, in particular those of the colloquium “4th ASSP Workshop on spectrum modeling, August 1988”, such as “Digital implementation of spectral correlation analyzers” by W. A. Brown, H. H. Loomis, and “Computationally efficient algorithms for cyclic spectral analyzers' by R. S. Roberts, W. A. Brown, H. H. Loomis. The FAM algorithm enables rapid calculation of the estimators of spectral correlation by means of FFT in the whole cyclic frequency/harmonic frequency (&agr;,f) space.
The estimator of the first moment E[x(f
k
)x(f
m
)*] is expressed as:
γ
^
x1
(
α
0
,
f
0
)
=
∑
t
=
1
K
x
(
f
k
,
t
)
x
(
f
m
,
t
)
*
exp
{
-
j2πδα
t
}
with
f
0
=
f
k
+
f
m
2
and
α
0
=
f
k
-
f
m
+
δ
α
(
1
)
The estimator of the second moment E[x(f
k
)x(−f
m
)] is expressed as:
γ
^
x2
(
α
0
,
f
0
)
=
∑
t
=
1
K
x
(
f
k
,
t
)
x
(
-
f
m
,
t
)
exp
{
-
j2πδα
t
}
with
f
0
=
f
k
-
f
m
2
and
α
0
=
f
k
+
f
m
+
δα
(
2
)
The signals x(f,t) are calculated by a bank of band filters B
canal
and central frequency filters denoted f
k
, f
m
. The signal x(f,t) therefore has a band equal to B
canal
. To obtain the exact frequency representation of the signal x(t), B
canal
must be very much smaller than the band B of the signal x(t). Since x(f) is calculated in a non-null band, this signal x(f) evolves with time and is therefore a function of f and t: x(f,t). In a first stage, the signals x(f
k
,t) and x(f
m
,t)* are intercorrelated after having been displaced to the base band. These signals being in a non-null band B
canal
, the signal z(t) obtained evolves with time in a band of width 2×B
canal
. Given that z(t) is in a non-null band and that the correlation peak in cyclic frequency does not necessarily lie in f
k
-f
m
, it is necessary to perform a Fourier transform of z(t) to obtain the offset &dgr;&agr; of this correlation peak.
To enable a rapid calculation of the estimators of spectral correlation, {circumflex over (&ggr;)}
x1
and {circumflex over (&ggr;)}
x2
, the FAM algorithm is implemented in several stages.
In a first stage, the signals x(f
k
,t) and x(−f
m
,t) are calculated by a system of smooth FFTs. The difference &Dgr;&agr;=f
k+1
−f
k
between the frequencies f
k+1
and f
k
is constant and the sampling frequency of the signals x(f,t) is
Fe
canal
=
Fe
RE
,
where RE is the shift of the moving windows (expressed as a number of samples) and Fe is the sampling frequency of x(t).
In a second stage, the signal z(t)=x(f
k
,t)×x(f
m
,t)* is calculated for the first moment and the signal z(t)=x(f
k
,t)×x(−f
m
,t) is calculated for the second moment.
In a third stage, an FFT is performed on the signal z(t) on a number of samples.
On the first moment to calculate the spectral correlation in
f
=
(
f
k
+
f
m
)
2
and for &agr; lying between &agr;
min
=f
k
−f
m
−{fraction (&agr;/2)} and &agr;
max
=f
k
−f
m
+&Dgr;{fraction (&agr;/2)} on K samples, an FFT of the signal z(t)=x(f
k
,t)×x(f
m
,t)* must be performed on K samples. The K frequency sales of the resulting signal z(&agr;
k
) represent the spectral correlation in
f
=
(
f
k
+
f
m
)
2
with &agr;
k
lying between
α
min
=
f
k
-
f
m
-
Fe
canal
2
and
α
max
=
f
k
-
f
m
+
Fe
canal
2
.
Since Fe
canal
is greater than &Dgr;&agr;, only the cyclic frequencies &agr;
k
contained in the interval (f
k
−f
m
−&Dgr;{fraction (&agr;/2)}) . . . (f
k
−f
m
+&Dgr;{fraction (&agr;/2)} are retained. All the outputs X(f
k
,t) and x(f
m
,t) of the bank of filters are correlated to obtain the points of the spectral correlation in the zone of interest of the first moment, in other words for &agr;<B
max
.
On the second moment, the calculation of the spectral correlation is limited in harmonic frequency by B
max
.
The FAM algorithm described previously is applicable only to signals from a single antenna. However, in the field of radiocommunications signals propagate with a degree of polarization and the gain of all antennas is dependent on the polarization of the signals received. This polarization dependence means that a single antenna filters the sources and can even completely cancel out the signals received, which would make detection of the sources impossible. This remark is true whatever algorithm is used, in particular a FAM algorithm.
SUMMARY OF THE INVENTION
The object of the invention is to propose a solution to this problem. For this purpose, the invention is a process of cyclic detection in diversity of polarization of digital cyclostationary radioelectric signals of sampling frequency Fe transmitted in a frequency channel of bandwidth B
max
and received on a network of N antennas, N being at least two, whose radiation diagram has a maximum maximorum of sensitivity wherein, for any pair of antennas, said process consists in acquiring over an observation period T and in an acquisition band B
acq
the digital signals output by the antennas, in calculating, for each cyclic frequency of a determined cyclic frequency/harmonic frequency space limited by the bandwidth B
max
. of the frequency channel and the acquisition band B
acq
of the digital signals acquired, a cyclic covariance matrix on a first moment of a spectral correlation of the digital signals acquired, and a cyclic covariance matrix on a second moment of a spectral correlation of the digital signals acquired, and in detecting peaks of spectral correlation by comparing a likelihood ratio determined from each said cyclic covariance matrix with a detection threshold determined statistically.
The invention consists in a cyclic detection test of radioelectric signals received by any pair of antennas of a network of N antennas whose radiation diagram presents a maximum maximorum (maximum of the maxima) of sensitivity.
In a first embodiment of the process, the network includes N=2 antennas whose maximums maximorum of sensitivity point in orthogonal directions.
In a second embodiment of the process, the network includes N=2 antennas whose maximums maximorum of sensitivity point in the same direction.
The main advantage of the invention is that it performs a rapid test of detection of radioelec
"Thomson-CSF"
Chin Stephen
Ha Dac V.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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