System and method for classifying signal modulations
A method for pre-processing a signal prior to classification, where the signal includes non-contiguous segments; the method includes applying a coarse carrier frequency offset correction, applying a phase correction, applying a residual course frequency offset correction, and outputting a simulated continuous signal consisting of recorded continuous signal segments and modeled non-contiguous segments. A system for implementing the method is also disclosed.
1. A signal classifying unit comprising
a processor;
a signal receiver for receiving and recording a radio-frequency signal; wherein said receiver is interruptible such that segments of the recorded radio-frequency signal are non-contiguous segments;
a medium storing instructions executable by the processor for simulating contiguous segments by modeling said non-contiguous segments;
wherein said instructions include instructions to:
apply a coarse carrier frequency offset correction to said non-contiguous segments;
apply a phase correction to said non-contiguous segments after said coarse carrier frequency offset correction is applied;
apply a residual course frequency offset correction to said non-contiguous segments after said phase correction is applied to generate modeled contiguous segments; and,
output a simulated continuous signal consisting of recorded continuous signal segments and modeled contiguous segments;
a signal classifier receiving the outputted simulated continuous signal for classifying the simulated continuous signal.
2. A signal classifying unit according to claim 1 , wherein said coarse carrier frequency offset correction comprises a windowing algorithm, a power spectrum estimation algorithm and a frequency shift detection algorithm.
3. A signal classifying unit according to claim 2 , wherein said windowing algorithm applies a window function to each frame of the non-contiguous segments to suppress sudden amplitude changes at frame edges.
4. A signal classifying unit according to claim 3 , wherein said phase correction comprises an algorithm implementing the rotation of phases of each sample in a frame such that leading and trailing samples of adjacent frames have the same phase.
5. A signal classifying unit according to claim 4 , wherein the algorithm implementing the phase correction calculates the phase rotated samples y(n) by the following formulae:
y
(
n
)
=
x
(
n
)
ⅇ
-
j
Φ
(
⌊
n
w
+
1
+
1
⌋
)
where └ ┘ is the floor operator that maps its argument to the largest integer less than or equal to the argument, x(n) is the input signal containing discontinuities, n=1, . . . , N and N=mw is the total number of samples in m frames, w is the length of each non-contiguous frame, the phase correction to be applied to frame (k+1) is Φ(k+1), which is computed by
Φ( k+ 1)=Φ( k )+φ( kw+ 1)−φ( kw )
where k=1, . . . , m is the frame number, m is the total number of concatenated frames and
φ
(
·
)
=
tan
-
1
(
Im
(
x
(
·
)
)
Re
(
x
(
·
)
)
)
is the phase angle of sample x(.) and Φ(1)=φ(1) by definition.
6. A signal classifying unit according to claim 5 , wherein the residual coarse frequency offset is estimated using phase rotated samples.
7. A method for pre-processing a signal prior to classification, where the signal includes non-contiguous segments; the method comprising:
receiving and recording a radio-frequency signal by a signal receiver; wherein said receiver is interruptible such that segments of the recorded radio-frequency signal are non-contiguous segments;
applying by a computer processor a coarse carrier frequency offset correction to said non-contiguous segments;
applying by a computer processor a phase correction to said non-contiguous segments after the coarse carrier frequency offset correction is applied;
applying by a computer processor a residual course frequency offset correction to said non-contiguous segments after the phase correction is applied, to generate modeled contiguous segments; and,
outputting by a computer processor a simulated continuous signal consisting of recorded continuous signal segments and modeled contiguous segments.
8. A method according to claim 7 , wherein said coarse carrier frequency offset correction comprises a windowing algorithm, a power spectrum estimation algorithm and a frequency shift detection algorithm.
9. A method according to claim 8 , wherein said windowing algorithm applies a taper function to each frame of the non-contiguous segments to suppress sudden amplitude changes at frame edges.
10. A method according to claim 9 , wherein said phase correction comprises an algorithm implementing the rotation of phases of each sample in a frame such that leading and trailing samples of adjacent frames have the same phase.
11. A method according to claim 10 , wherein the algorithm implementing the phase correction calculates the phase rotated samples y(n) by the following formulae:
y
(
n
)
=
x
(
n
)
ⅇ
-
j
Φ
(
⌊
n
w
+
1
+
1
⌋
)
where └ ┘ is the floor operator that maps its argument to the largest integer less than or equal to the argument, x(n) is the input signal containing discontinuities, n=1, . . . , N and N=mw is the total number of samples in m frames, w is the length of each non-contiguous frame, the phase correction to be applied to frame (k+1) is Φ(k+1), which is computed by:
Φ( k+ 1)=Φ( k )+φ( kw+ 1)−φ( kw )
where k=1, . . . , m is the frame number, m is the total number of concatenated frames and
φ
(
·
)
=
tan
-
1
(
Im
(
x
(
·
)
)
Re
(
x
(
·
)
)
)
is the phase angle of sample x(.) and Φ(1)=φ(1) by definition.
12. A method according to claim 11 , wherein the residual course frequency offset is estimated using phase rotated samples.