Apparatus and method for robust BPSK/QPSK blind modulation classification against errors in satellite control system
View Patent ↗Provided are an apparatus and method for robust Binary Phase Shift Keying/Quadrature Phase Shift Keying (BPSK/QPSK) blind modulation classification. The apparatus includes first and second likelihood value calculators for calculating a likelihood value of a received baseband signal each for of BPSK and QPSK modulation modes. A maximum setting unit derives a maximum value of the likelihood values or i th likelihood ratios calculated by the first and second likelihood value calculators. A flag is set for the maximum value to “1” and a flag for the remaining value is set to “0”. First and second flag combining units combine the flags for the modulation modes. A modulation mode flag setting unit selects a maximum value from the flags combined by the first and second flag combining units.
1. An apparatus for Binary Phase Shift Keying/Quadrature Phase Shift Keying (BPSK/QPSK) blind modulation classification in a satellite communication system, comprising:
a first and a second likelihood value calculating means for calculating a likelihood value of a received baseband signal for each of BPSK and QPSK modulation modes, respectively;
a maximum value setting means for obtaining a maximum value of the likelihood values or ith likelihood ratios calculated by the first and second likelihood value calculating means and setting a flag corresponding to the maximum value to have a first flag value and a flag corresponding to a remaining value to have a second flag value, wherein i is a positive integer;
a first and a second flag combining means for combining the flags for the modulation modes, respectively;
a modulation mode flag setting means for setting a corresponding modulation mode flag based upon the combining result; and
a modulation mode deciding means for combining the modulation mode flags obtained by repeating a modulation classification processes in all of said means L times and deciding an actual modulation mode based upon combining the modulation mode flags result, wherein L is an integer greater than 1.
2. The apparatus as recited in claim 1 , wherein the modulation mode deciding means decides a number of minimally required symbols to meet a desired Correct Classification Probability (CCP) based on the resultant values outputted by repeating the modulation classification processes L times.
3. The apparatus as recited in claim 1 , wherein the first flag value is set to “1” and the second flag value is set to “0” in the maximum value setting means.
4. A method for BPSK/QPSK blind modulation classification in a satellite communication system, comprising the steps of:
a) calculating a likelihood ratio of a received baseband signal for each of BPSK and QPSK modulation modes;
b) deriving a maximum value from the calculated likelihood ratios or ith likelihood ratios, setting a flag for the maximum value to have a first flag value and a flag for the remaining value to have a second flag value, wherein i is a positive integer;
c) combining the flags for each of the modulation modes and setting a corresponding modulation mode flag based upon the combining result; and
d) combining the modulation mode flags obtained by repeatedly performing a modulation classification processes in the steps a) to c) L times and deciding an actual modulation mode based upon combining the modulation mode flags result, wherein L is an integer greater than 1.
5. The method as recited in claim 4 , wherein the combined flags are obtained by combining flags for the modulation modes when a residual that is derived by dividing i by N is 0, which are calculated based on:
Y
j
=
∑
i
=
1
N
X
ji
,
j
=
1
,
2
Eq
.
(
1
)
wherein N is the number of samples to be observed and Xji is the flag of the jth modulation mode at ith time.
6. The apparatus as recited in claim 4 , wherein the first flag value is set to “1” and the second flag value is set to “0” in the step b.
7. The method as recited in claim 4 , wherein the modulation mode deciding step decides a number of minimally required symbols to meet a desired Correct Classification Probability (CCP) based on the resultant values provided by repeating the modulation classification processes L times.
8. The method as recited in claim 7 , wherein the CCP is calculated by Eq. (2), and if L is sufficiently large, P c is represented by Eq. (3):
P
c
=
∑
i
=
L
/
2
L
(
L
i
)
p
0
i
(
1
-
p
0
)
L
-
i
Eq
.
(
2
)
wherein p o indicates a CCP at a specific frequency offset obtained by observing a short N symbol block, and
P
c
≅
1
-
Φ
(
L
/
2
-
Lp
0
Lp
0
(
1
-
p
0
)
)
wherein
Φ
(
x
)
=
∫
-
∞
x
1
2
π
ⅇ
p
/
2
ⅆ
t
.
Eq
.
(
3
)
9. The method as recited in claim 8 , wherein the number of minimally required symbols to meet the CCP, which is defined as:
Q
=
NL
≥
4
p
0
(
1
-
p
0
)
(
1
-
2
p
0
)
2
(
Φ
-
1
(
1
-
P
c
0
)
)
2
N
wherein N is the number of samples to be observed and Pco is a threshold probability to decide the modulation mode (BPSK/QPSK).