Apparatus and method of compensating for I/Q imbalance in direct up-conversion system
An apparatus and a method of compensating for an I/Q imbalance in a direct up-conversion system prevents the performance of the system from being deteriorated by efficiently compensating for an I/Q timing skew, an I/Q phase imbalance, and an I/Q gain imbalance by using a characteristic of an OFDM scheme in an Orthogonal Frequency Domain Multiple (Access) (OFDM(A)) system using a direct up-conversion scheme. According to the apparatus and the method of compensating for an I/Q imbalance in the direct up-conversion system of the present invention, an OFDM(A) system using a direct up-conversion scheme may efficiently compensate for I/Q timing skew, I/Q phase imbalance, and I/Q gain imbalance by using a characteristic of an OFDMA scheme, so that a performance of the system is prevented from being deteriorated.
1. An In-phase/Quadrature-phase (I/Q) imbalance compensation apparatus in a direct up-conversion system, the I/Q imbalance compensation apparatus comprising:
a pre-distortion factor calculator for fetching pre-measured I/Q imbalance factors and calculating pre-distortion factor A, B, C and D; and
a pre-distortion executer for executing a pre-distortion for each subcarrier through the distortion factors A, B, C and D corresponding to each subcarrier,
wherein the pre-distortion factors A, B, C and D are calculated by the following equations,
Phase
timing
_
skew
=
2
*
π
*
(
subcarrier
index
-
(
N
2
-
1
)
)
*
Δ
f
*
τ
N
2
≤
subcarrier
index
<
N
point
FFT
A
=
0.5
*
[
(
1
+
g
)
*
cos
(
ϕ
)
+
(
1
-
g
)
+
cos
(
ϕ
)
*
cos
(
Phase
timing
_
skew
)
-
(
1
+
g
)
*
sin
(
ϕ
)
*
sin
(
Phase
timing
_
skew
)
]
B
=
0.5
*
[
(
g
-
1
)
*
sin
(
ϕ
)
+
(
1
-
g
)
+
cos
(
ϕ
)
*
sin
(
Phase
timing
_
skew
)
+
(
1
+
g
)
*
sin
(
ϕ
)
*
cos
(
Phase
timing
_
skew
)
]
C
=
0.5
*
[
(
1
+
g
)
*
cos
(
ϕ
)
-
(
1
-
g
)
+
cos
(
ϕ
)
*
cos
(
Phase
timing
_
skew
)
-
(
1
+
g
)
*
sin
(
ϕ
)
*
sin
(
Phase
timing
_
skew
)
]
D
=
0.5
*
[
(
g
-
1
)
*
sin
(
ϕ
)
+
(
1
-
g
)
+
cos
(
ϕ
)
*
sin
(
Phase
timing
_
skew
)
-
(
1
+
g
)
*
sin
(
ϕ
)
*
cos
(
Phase
timing
_
skew
)
]
in which τ indicates I/O timing skew, g indicates an I/O gain imbalance, φ indicates an I/O phase imbalance, Δf indicates sampling rate/FFT size, and N indicates a positive integer equal to an FFT size.
2. The I/Q imbalance compensation apparatus as claimed in claim 1 , wherein the direct up-conversion system is applied to an Orthogonal Frequency Domain Multiple (OFDM) system.
3. The I/Q imbalance compensation apparatus as claimed in claim 2 , further comprising a pre-distortion factor storage unit for storing the pre-distortion factors calculated by the pre-distortion factor calculator,
wherein the pre-distortion executer fetches the pre-distortion factors calculated by the pre-distortion factor calculator to execute the pre-distortion.
4. The I/Q imbalance compensation apparatus as claimed in claim 2 , wherein the pre-distortion executer performs the pre-distortion for respective subcarriers bilaterally symmetric to each other with respect to a DC subcarrier, and the pre-distortion is performed by
Predistorted — I ( f m )= A ( f m )· I ( f m )+ B ( f m )· Q ( f m )+ C ( f m )· I ( f −m )+ D ( f )· Q ( f −m )
Predistorted — Q ( f m )= A ( f m )· Q ( f m )− B ( f m )· I ( f m )+ D ( f m )· I ( f −m )− C ( f )· Q ( f −m )
and
Predistorted — I ( f −m )= A ( f −m )· I ( f −m )+ B ( f −m )· Q ( f −m )+ C ( f −m )· I ( f m )+ D ( f −m )· Q ( f m )
Predistorted — Q ( f −m )= A ( f −m )· Q ( f −m )− B ( f −m )· I ( f −m )+ D ( f −m )· I ( f m )− C ( f −m )· Q ( f m ),
in which f m indicates a predetermined subcarrier index, and f −m indicates a subcarrier index symmetric to f m .
5. A direct up-conversion system comprising:
the I/Q imbalance compensation apparatus for performing a pre-distortion so as to compensate for an I/Q imbalance for baseband digital I signals and baseband digital Q signals as claimed in claim 1 ;
a baseband processor comprising an IFFT (Inverse Fast Fourier Transformation) unit for performing an IFFT on I signals and Q signals pre-distorted by the I/Q imbalance compensation apparatus;
a D/A converter for converting I signals and Q signals which have undergone the IFFT to analog data; and
an I/Q modulator for I/Q modulating the analog data.
6. A method of compensating for an I/O imbalance in a direct up-conversion system, the method comprising the steps of:
(a) calculating pre-distortion factors A, B, C and D for each subcarrier by using pre-measured I/O gain imbalance g, I/O phase imbalance φ, and I/O timing skew τ(timing_skew);
(b) execute a pre-distortion for each subcarrier through the calculated pre-distortion factors A, B, C and D for each subcarrier,
(c) performing an IFFT on pre-distorted I/O signals, and
(d) converting I signals and Q signals which have undergone and IFFT to analog data and I/O modulating the analog data,
wherein the pre-distortion factors A, B, C and D are calculated by the following equations,
Phase
timing
_
skew
=
2
*
π
*
(
subcarrier
index
-
(
N
2
-
1
)
)
*
Δ
f
*
τ
N
2
≤
subcarrier
index
<
N
point
FFT
A
=
0.5
*
[
(
1
+
g
)
*
cos
(
ϕ
)
+
(
1
-
g
)
+
cos
(
ϕ
)
*
cos
(
Phase
timing
_
skew
)
-
(
1
+
g
)
*
sin
(
ϕ
)
*
sin
(
Phase
timing
_
skew
)
]
B
=
0.5
*
[
(
g
-
1
)
*
sin
(
ϕ
)
+
(
1
-
g
)
+
cos
(
ϕ
)
*
sin
(
Phase
timing
_
skew
)
+
(
1
+
g
)
*
sin
(
ϕ
)
*
cos
(
Phase
timing
_
skew
)
]
C
=
0.5
*
[
(
1
+
g
)
*
cos
(
ϕ
)
-
(
1
-
g
)
+
cos
(
ϕ
)
*
cos
(
Phase
timing
_
skew
)
-
(
1
+
g
)
*
sin
(
ϕ
)
*
sin
(
Phase
timing
_
skew
)
]
D
=
0.5
*
[
(
g
-
1
)
*
sin
(
ϕ
)
+
(
1
-
g
)
+
cos
(
ϕ
)
*
sin
(
Phase
timing
_
skew
)
-
(
1
+
g
)
*
sin
(
ϕ
)
*
cos
(
Phase
timing
_
skew
)
]
in which τ indicates I/O timing skew, g indicates an I/O gain imbalance, φ indicates an I/O phase imbalance, Δf indicates sampling rate/FFT size, and N indicates a positive integer equal to an FFT size.
7. The method as claimed in claim 6 , further comprising a step of storing the pre-distortion factors calculated in the step of (a) before the step of (b),
wherein the step of (b) is performed by fetching the stored pre-distortion factors.
8. The method as claimed in claim 7 , wherein the step of (b) is performed for respective subcarriers bilaterally symmetric to each other with respect to a DC subcarrier, and the pre-distortion is performed by
Predistorted — I ( f m )= A ( f m )· I ( f m )+ B ( f m )· Q ( f m )+ C ( f m )· I ( f −m )+ D ( f )· Q ( f −m )
Predistorted — Q ( f m )= A ( f m )· Q ( f m )− B ( f m )· I ( f m )+ D ( f m )· I ( f −m )− C ( f )· Q ( f −m )
and
Predistorted — I ( f −m )= A ( f −m )· I ( f −m )+ B ( f −m )· Q ( f −m )+ C ( f −m )· I ( f m )+ D ( f −m )· Q ( f m )
Predistorted — Q ( f −m )= A ( f −m )· Q ( f −m )− B ( f −m )· I ( f −m )+ D ( f −m )· I ( f m )− C ( f −m )· Q ( f m ),
in which f m indicates a predetermined subcarrier index, and f −m indicates a subcarrier index symmetric to f m .
9. The method as claimed in claim 6 , wherein the method is applied to an OFDM system.
10. The method as claimed in claim 7 , wherein the method is applied to an OFDM system.
11. The method as claimed in claim 8 , wherein the method is applied to an OFDM system.