Adaptive digital predistortion device and method
An adaptive digital predistortion device and method, the adaptive digital predistortion device including a predistortion unit for predistorting an input signal according to a predistortion parameter stored in a look-up table; a cost function generation unit for generating a cost function; a fixed segment point determination unit for determining a fixed segment point; and an update unit for updating parameters (u 1 , u 2 , k a ) according to the cost function to update the look-up table based on the updated parameters (u 1 , u 2 , k a ), wherein k a is an updated adaptive segment point, u 1 and u 2 are slopes on two sides of the adaptive segment point ka; and for subsequently updating parameters (v 1 , v 2 ) according to the cost function to update the look-up table based on the updated parameters (v 1 , v 2 ) and the fixed segment point, wherein v 1 and v 2 are slopes on two sides of the fixed segment point. Therefore, the adaptive digital predistortion device according to the present invention can obtain a good performance within an acceptable convergence time by combining the adaptive segment with the fixed segment.
1. An adaptive digital predistortion device, comprising:
a predistortion unit for retrieving a predistortion parameter from a look-up table based on an input signal to be amplified by an amplifier, predistorting the input signal according to the predistortion parameter to generate a predistorted signal to be amplified by the amplifier;
a cost function generation unit for generating a cost function based on an output signal of the amplifier;
a fixed segment point determination unit for determining a fixed segment point;
an update unit for updating parameters (u 1 , u 2 , k a ) according to the cost function to update the look-up table based on the updated parameters (u 1 , u 2 , k a ), wherein k a is an adaptive segment point, u 1 is a slope of a straight line on one side of the adaptive segment point k a , and u 2 is a slope of a straight line on another side of the adaptive segment point k a ; and for updating parameters (v 1 , v 2 ) according to the cost function to update the look-up table based on the updated parameters (v 1 , v 2 ) and the fixed segment point, wherein v 1 is a slope of a straight line on one side of the fixed segment point, and v 2 is a slope of a straight line on another side of the fixed segment point,
wherein the update unit updates the look-up table based on the updated parameters (u 1 , u 2 , k a ) by:
L
(
k
)
=
{
u
1
×
(
L
′
(
k
)
-
L
′
(
0
)
)
+
L
′
(
0
)
k
≤
k
a
u
2
×
(
L
′
(
k
)
-
L
′
(
k
a
)
)
+
L
′
(
k
a
)
k
>
k
a
,
wherein, L′(●) is a look-up table before updating, L(●) is the updated look-up table, and k is an index of the look-up table, and
wherein the update unit updates the look-up table based on the updated parameters (v 1 , v 2 ) and the fixed segment point by:
L
(
k
)
=
{
v
1
×
(
L
′
(
k
)
-
L
′
(
k
b
)
)
+
L
′
(
k
b
)
k
≤
k
b
v
2
×
(
L
′
(
k
)
-
L
′
(
k
b
)
)
+
L
′
(
k
b
)
k
>
k
b
wherein, k b is the fixed segment point, L′(●) is a look-up table before updating. L(●) is the updated look-up table, and k is an index of the look-up table.
2. The adaptive digital predistortion device according to claim 1 , wherein the cost function generation unit comprises:
an outband power computation part for computing an outband power based on the output signal of the amplifier;
a comb filter for filtering the outband power and outputting the filtered outband power to the update unit as the cost function.
3. The adaptive digital predistortion device according to claim 2 , wherein the comb filter has a transfer function:
H
(
Z
)
=
0.05
1
+
z
-
8
1
-
0.9
Z
-
8
.
wherein z is the outband power.
4. The adaptive digital predistortion device according to claim 1 , wherein the fixed segment point determination unit determines the fixed segment point based on the adaptive segment point k a .
5. An adaptive digital predistortion device, comprising:
a processor for retrieving a predistortion parameter from a look-up table based on an input signal to be amplified by an amplifier, predistorting the input signal according to the predistortion parameter to generate a predistorted signal to be amplified by the amplifier; for generating a cost function based on an output signal of the amplifier; for determining a fixed segment point; for updating parameters (u 1 , u 2 , k a ) according to the cost function to update the look-up table based on the updated parameters (u 1 , u 2 , k a ), wherein k a is an adaptive segment point, u 1 is a slope of a straight line on one side of the adaptive segment point k a , and u 2 is a slope of a straight line on another side of the adaptive segment point k a ; and for updating parameters (v 1 , v 2 ) according to the cost function to update the look-up table based on the updated parameters (v 1 , v 2 ) and the fixed segment point, wherein v 1 is a slope of a straight line on one side of the fixed segment point, and v 2 is a slope of a straight line on another side of the fixed segment point;
a converter for converting the predistorted signal into a radio signal; and
an amplifier for amplifying the radio signal,
wherein the processor updates the look-up table based on the updated parameters (u 1 , u 2 , k a ) by:
L
(
k
)
=
{
u
1
×
(
L
′
(
k
)
-
L
′
(
0
)
)
+
L
′
(
0
)
k
≤
k
a
u
2
×
(
L
′
(
k
)
-
L
′
(
k
a
)
)
+
L
′
(
k
a
)
k
>
k
a
wherein, L′(●) is a look-up table before updating, L(●) is the updated look-up table, and k is an index of the look-up table, and
wherein the processor updates the look-up table based on the updated parameters (v 1 , v 2 ) and the fixed segment point by:
L
(
k
)
=
{
v
1
×
(
L
′
(
k
)
-
L
′
(
k
b
)
)
+
L
′
(
k
b
)
k
≤
k
b
v
2
×
(
L
′
(
k
)
-
L
′
(
k
b
)
)
+
L
′
(
k
b
)
k
>
k
b
wherein, k b is the fixed segment point, L′(●) is a look-up table before updating, L(●) is the updated look-up table, and k is an index of the look-up table.
6. An adaptive digital predistortion method, comprising:
retrieving a predistortion parameter from a look-up table based on an input signal to be amplified by an amplifier; and
predistorting the input signal according to the predistortion parameter to generate a predistorted signal to be amplified by the amplifier;
wherein the look-up table is automatically updated by:
during an adaptive segment phase, generating a cost function based on an output signal of the amplifier, updating parameters (u 1 , u 2 , k a ) according to the cost function to update the look-up table based on the updated parameters (u 1 , u 2 , k a ), wherein k a is an adaptive segment point, u 1 is a slope of a straight line on one side of the adaptive segment point k a , and u 2 is a slope of a straight line on another side of the adaptive segment point k a ; and
during a fixed segment phase after the adaptive segment phase, re-generating a cost function based on an output signal of the amplifier, updating parameters (v 1 , v 2 ) according to the cost function and determining a fixed segment point, to update the look-up table based on the updated parameters (v 1 , v 2 ) and the fixed segment point, wherein v 1 is a slope of a straight line on one side of the fixed segment point, and v 2 is a slope of a straight line on another side of the fixed segment point,
wherein during the adaptive segment phase, the look-up table is updated based on the updated parameters (u 1 , u 2 , k a ) by:
L
(
k
)
=
{
u
1
×
(
L
′
(
k
)
-
L
′
(
0
)
)
+
L
′
(
0
)
k
≤
k
a
u
2
×
(
L
′
(
k
)
-
L
′
(
k
a
)
)
+
L
′
(
k
a
)
k
>
k
a
,
wherein, L′(●) is a look-up table before updating, L(●) is the updated look-up table, and k is an index of the look-up table, and
wherein during the fixed segment phase, the look-up table is updated based on the updated parameters (v 1 , v 2 ) and the fixed segment point by:
L
(
k
)
=
{
v
1
×
(
L
′
(
k
)
-
L
′
(
k
b
)
)
+
L
′
(
k
b
)
k
≤
k
b
v
2
×
(
L
′
(
k
)
-
L
′
(
k
b
)
)
+
L
′
(
k
b
)
k
>
k
b
wherein, k b is the fixed segment point, L′(●) is a look-up table before updating, L(●) is the updated look-up table, and k is an index of the look-up table.
7. The adaptive digital predistortion method according to claim 6 , wherein the cost function is generated by:
computing an outband power based on the output signal of the amplifier; and
filtering the computed outband power by a comb filter and outputting the filtered outband power as the cost function.
8. The adaptive digital predistortion method according to claim 7 , wherein the comb filter has a transfer function:
H
(
Z
)
=
0.05
1
+
z
-
8
1
-
0.9
z
-
8
.
wherein z is the outband power.
9. The adaptive digital predistortion method according to claim 6 , wherein the fixed segment point is determined based on the adaptive segment point k a .