Digital pre-distortion technique using nonlinear filters
A system and method for stabilizing a coefficient set used by a digital predistortion (DPD) engine to apply pre-distortion to a transmit signal and cancel distortion generated by a distorting element or distorting system when transmitting the transmit signal, including obtaining an initial coefficient set; rotating the initial coefficient set to maintain a phase of fundamental components (w 10 (t), . . . , w 1Q (t)) of the initial coefficient set as a constant value; averaging in the time domain the rotated coefficient set to obtain an averaged coefficient set; applying the averaged coefficient set to the DPD engine, the initial coefficient set expressed in a first equation [27]; computing the phase of the fundamental components of the initial coefficient set with a second equation [28]; and computing the rotated coefficient set with a third equation [29].
1. A method for stabilizing a coefficient set used by a digital predistortion (DPD) engine to apply pre-distortion to a transmit signal and cancel distortion generated by a distorting element or distorting system when transmitting the transmit signal, the method comprising:
obtaining an initial coefficient set;
rotating the initial coefficient set to maintain a phase of fundamental components (w 10 (t), . . . , w 1Q (t)) of the initial coefficient set as a constant value;
averaging in the time domain the rotated coefficient set to obtain an averaged coefficient set;
applying the averaged coefficient set to the DPD engine,
the initial coefficient set expressed as
W
(
t
)
=
[
w
10
(
t
)
w
11
(
t
)
…
w
1
Q
(
t
)
w
20
(
t
)
w
21
(
t
)
…
w
2
Q
(
t
)
…
…
…
…
w
K
0
(
t
)
w
K
1
(
t
)
…
w
KQ
(
t
)
]
;
computing the phase of the fundamental components of the initial coefficient set as
θ
1
(
t
)
=
angle
(
∑
q
=
0
Q
w
1
q
(
t
)
)
;
and
computing the rotated coefficient set as
W
2
(
t
)
=
[
w
10
(
t
)
w
11
(
t
)
…
w
1
Q
(
t
)
w
20
(
t
)
w
21
(
t
)
…
w
2
Q
(
t
)
…
…
…
…
w
K
0
(
t
)
w
K
1
(
t
)
…
w
KQ
(
t
)
]
ⅇ
-
jθ
1
(
t
)
.
2. The method of claim 1 , further comprising averaging the rotated coefficient set by computing an average of a current rotated coefficient set and a previous rotated coefficient set.
3. The method of claim 1 , further comprising computing a current weighted averaged rotated coefficient set W 3 (t+1) from the current rotated coefficient set and the previous weighted averaged rotated coefficient set according to the equation
W 3 ( t+ 1)=(1−α) W 3 ( t )+α W 2 ( t+ 1),
wherein W 2 (t+1) is the current rotated coefficient set, W 3 (t) is the previous weighted averaged rotated coefficient set, and α is a weighting factor, 0≦α≦1.
4. A system for stabilizing a coefficient set used by a digital predistortion (DPD) engine to apply pre-distortion to a transmit signal and cancel distortion generated by a distorting element or distorting system when transmitting the transmit signal, the system comprising:
means for obtaining an initial coefficient set;
means for rotating the initial coefficient set to maintain a phase of fundamental components (w 10 (t), . . . , w 1Q (t)) of the initial coefficient set as a constant value;
means for averaging in the time domain the rotated coefficient set to obtain an averaged coefficient set;
means for applying the averaged coefficient set to the DPD engine,
the initial coefficient set expressed as
W
(
t
)
=
[
w
10
(
t
)
w
11
(
t
)
…
w
1
Q
(
t
)
w
20
(
t
)
w
21
(
t
)
…
w
2
Q
(
t
)
…
…
…
…
w
K
0
(
t
)
w
K
1
(
t
)
…
w
KQ
(
t
)
]
;
means for computing the phase of the fundamental components of the initial coefficient set as
θ
1
(
t
)
=
angle
(
∑
q
=
0
Q
w
1
q
(
t
)
)
;
and
means for computing the rotated coefficient set as
W
2
(
t
)
=
[
w
10
(
t
)
w
11
(
t
)
…
w
1
Q
(
t
)
w
20
(
t
)
w
21
(
t
)
…
w
2
Q
(
t
)
…
…
…
…
w
K
0
(
t
)
w
K
1
(
t
)
…
w
KQ
(
t
)
]
ⅇ
-
j
θ
1
(
t
)
.
5. The system of claim 4 , further comprising means for averaging the rotated coefficient set by computing an average of a current rotated coefficient set and a previous rotated coefficient set.
6. The system of claim 4 , further comprising means for computing a current weighted averaged rotated coefficient set W 3 (t+1) from the current rotated coefficient set and the previous weighted averaged rotated coefficient set according to the equation
W 3 ( t+ 1)=(1−α) W 3 ( t )+α W 2 ( t+ 1),
wherein W 2 (t+1) is the current rotated coefficient set, W 3 (t) is the previous weighted averaged rotated coefficient set, and α is a weighting factor, 0≦α≦1.
7. A method for generating a coefficient set with improved accuracy, the coefficient set used by a digital predistortion (DPD) engine to apply pre-distortion to a transmit signal and cancel distortion generated by a distorting element or distorting system when transmitting the transmit signal, the method comprising:
selecting samples from a pre-distorted transmit signal from the DPD engine exceeding a predetermined threshold;
zeroing the selected samples from the transmit signal and the corresponding row of the “H” matrix of the distorting element or distorting system;
computing a coefficient set from the transmit signal and the delayed feedback signal with the selected samples removed,
the pre-distorted transmit signal having samples S represented as
S=[s (0) s (1) s (2) s (3) . . . s ( U− 1)] T ;
defining a soft factor vector as
G=[g (0) g (1) g (2) g (3) . . . g ( U− 1)] T
where
g(i)=1 for |s(i)|<T 2 , g(i)=0 otherwise, and T 2 is the predetermined threshold; and
if g(i)=0, zeroing an ith sample of S and zeroing an ith row of the H matrix.
8. A system for generating a coefficient set with improved accuracy, the coefficient set used by a digital predistortion (DPD) engine to apply pre-distortion to a transmit signal and cancel distortion generated by a distorting element or distorting system when transmitting the transmit signal, the system comprising:
means for selecting samples from a pre-distorted transmit signal from the DPD engine exceeding a predetermined threshold;
means for zeroing the selected samples from the transmit signal and the corresponding row of the “H” matrix of the distorting element or distorting system;
means for computing a coefficient set from the transmit signal and the delayed feedback signal with the selected samples removed,
the pre-distorted transmit signal having samples S represented as
S=[s (0) s (1) s (2) s (3) . . . s ( U− 1)] T ;
means for defining a soft factor vector as
G=[g (0) g (1) g (2) g (3) . . . g ( U− 1)] T
where
g(i)=1 for |s(i)|<T 2 , g(i)=0 otherwise, and T 2 is the predetermined threshold; and
if g(i)=0, zeroing an ith sample of S and zeroing an ith row of the H matrix.