Digital pre-distortion technique using non-linear filters
View Patent ↗A system, method and computer program product 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 a constant value; averaging the rotated coefficient set to obtain an averaged coefficient set; and applying the averaged coefficient set to the DPD engine.
1. A computer-readable medium comprising program code 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 program code for causing performance of a 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; and
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
)
]
,
the program code further for causing performance of a method comprising:
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
-
j
θ
1
(
t
)
.
2. The computer-readable medium of claim 1 , the program code further for causing performance of a method comprising averaging the rotated coefficient set by computing an average of a current rotated coefficient set and a previous rotated coefficient set.
3. The computer-readable medium of claim 1 , the program code further for causing performance of a method 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 computer-readable medium comprising program code 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 program code for causing performance of a 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; and
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 , the program code further for causing performance of a method comprising:
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.