Adaptive peak windowing for crest factor reduction in a communication system transmitter
View Patent ↗In a communication system transmitter, a baseband processor is configured for coupling via an upconverter to an input of a power amplifier of the transmitter. The baseband processor comprises a crest factor reduction element implementing an adaptive peak windowing algorithm for application to an input signal, the adaptive peak windowing algorithm comprising a function that is adaptively adjustable for reducing interference among different peaks of the input signal.
1. An apparatus for use in a communication system transmitter, the apparatus comprising:
a baseband processor having an output configured for coupling via an upconverter to an input of a power amplifier of the transmitter;
the baseband processor comprising a crest factor reduction element implementing an adaptive peak windowing algorithm for application to an input signal, the adaptive peak windowing algorithm comprising a windowing function that is adaptively adjustable for reducing interference among different peaks of the input signal;
wherein, after a complete peak pulse of the input signal is observed in a current window, a corresponding sequence of samples is saved as a first vector;
wherein the first vector is processed to generate a second vector which corresponds to the first vector prefixed and suffixed with one or more values, the prefixing and suffixing being based at least in part on the length of the first vector and the length of the current window; and
wherein the windowing function is adaptively adjustable based at least in part on the second vector.
2. The apparatus of claim 1 wherein the adaptive peak windowing algorithm multiplies the input signal with the following function, which comprises a predefined windowing function w(n):
b
(
n
)
=
1
-
∑
k
=
-
∞
∞
δ
·
[
1
-
c
′
(
k
)
]
w
(
n
-
k
)
,
where δ denotes a scaling factor and c′(k) denotes a modification of a kth clipping factor c(k) of the input signal.
3. The apparatus of claim 2 wherein the scaling factor δ changes adaptively based on information associated with one or more peaks of the input signal observed within the current window.
4. The apparatus of claim 2 wherein the modified clipping factor c′(k) changes adaptively based on information associated with one or more peaks of the input signal observed within the current window.
5. The apparatus of claim 1 wherein the adaptive peak windowing algorithm comprises an initialization phase wherein an initial window is determined, prior to arrival of a first peak of the input signal, utilizing a predetermined value of a scaling factor of the windowing function.
6. The apparatus of claim 5 wherein the predetermined value is approximately one.
7. The apparatus of claim 5 wherein in conjunction with arrival of the first peak in the input signal, the adaptive peak windowing algorithm updates the scaling factor and utilizes the updated scaling factor to scale the windowing function.
8. The apparatus of claim 7 wherein if a given peak does not end, the corresponding scaling factor continues to be applied in calculating b(n).
9. The apparatus of claim 8 wherein once the given peak ends and in conjunction with arrival of a complete new peak, the adaptive peak windowing algorithm again updates the scaling factor and utilizes the updated scaling factor to scale the windowing function.
10. The apparatus of claim 1 wherein the crest factor reduction element is implemented at least in part in software running on the baseband processor.
11. The apparatus of claim 1 wherein the baseband processor further comprises a multi-carrier combining element and a digital pre-distortion element, and the crest factor reduction element is coupled between an output of the multi-carrier combining element and an input of the pre-distortion element.
12. The apparatus of claim 1 wherein the baseband processor is implemented in the form of an integrated circuit.
13. An apparatus for use in a communication system transmitter, the apparatus comprising:
a baseband processor having an output configured for coupling via an upconverter to an input of a power amplifier of the transmitter;
the baseband processor comprising a crest factor reduction element implementing an adaptive peak windowing algorithm for application to an input signal, the adaptive peak windowing algorithm comprising a function that is adaptively adjustable for reducing interference among different peaks of the input signal;
wherein the adaptive peak windowing algorithm multiplies the input signal with the following function, which comprises a predefined windowing function w(n):
b
(
n
)
=
1
-
∑
k
=
-
∞
∞
δ
·
[
1
-
c
′
(
k
)
]
w
(
n
-
k
)
,
where δ denotes a scaling factor and c′(k) denotes a modification of a kth clipping factor c(k) of the input signal;
wherein the scaling factor δ changes adaptively based on information associated with one or more peaks of the input signal observed within a current window; and
wherein after a complete peak pulse of the input signal is observed in the current window, a corresponding sequence of c(k) samples is saved as a vector P , where P is processed to generate a vector {tilde over (C)} which corresponds to the vector P prefixed and suffixed with 1 values until its length is the same as the length of the current window and a minimum value of P is positioned in the center of the current window.
14. The apparatus of claim 13 wherein the vectors P and {tilde over (C)} are utilized to update the scaling factor δ.
15. The apparatus of claim 14 wherein the scaling factor δ is updated using the following equation:
δ
=
1
-
min
(
P
)
(
1
-
C
~
)
*
W
T
.
16. An apparatus for use in a communication system transmitter, the apparatus comprising:
a baseband processor having an output configured for coupling via an up converter to an input of a power amplifier of the transmitter;
the baseband processor comprising a crest factor reduction element implementing an adaptive peak windowing algorithm for application to an input signal, the adaptive peak windowing algorithm comprising a function that is adaptively adjustable for reducing interference among different peaks of the input signal;
wherein the adaptive peak windowing algorithm multiplies the input signal with the following function, which comprises a predefined windowing function w(n):
b
(
n
)
=
1
-
∑
k
=
-
∞
∞
δ
·
[
1
-
c
′
(
k
)
]
w
(
n
-
k
)
,
where δ denotes a scaling factor and c′(k) denotes a modification of a kth clipping factor c(k) of the input signal;
wherein the modified clipping factor c′(k) changes adaptively based on information associated with one or more peaks of the input signal observed within a current window; and
wherein after a complete peak pulse of the input signal is observed in the current window, a corresponding sequence of c(k) samples is saved as a vector P, and the modified clipping factor c′(k) is determined as an entry of a vector C′ which corresponds to the vector P prefixed with 1 values and suffixed with a single 1 value until its length is the same as the length of the current window.
17. The apparatus of claim 16 wherein the vector C′ is prefixed and suffixed with 1 values in order to eliminate effects of one or more other peaks positioned in the same window.
18. The apparatus of claim 16 wherein the vector C′ is utilized in computing a new function b(n).
19. An integrated circuit for use in a communication system transmitter, the integrated circuit implementing at least a portion of a baseband processor of the transmitter, the portion of the baseband processor implemented by the integrated circuit comprising a crest factor reduction element implementing an adaptive peak windowing algorithm for application to an input signal, the adaptive peak windowing algorithm comprising a windowing function that is adaptively adjustable for reducing interference among different peaks of the input signal, wherein, after a complete peak pulse of the input signal is observed in a current window, a corresponding sequence of samples is saved as a first vector; wherein the first vector is processed to generate a second vector which corresponds to the first vector prefixed and suffixed with one or more values, the prefixing and suffixing being based at least in part on the length of the first vector and the length of the current window; and wherein the windowing function is adaptively adjustable based at least in part on the second vector.
20. A communication system transmitter comprising:
a baseband processor;
an upconverter for upconverting an output signal from the baseband processor; and
a power amplifier having an input coupled to an output of the upconverter;
the baseband processor comprising a crest factor reduction element implementing an adaptive peak windowing algorithm for application to an input signal, the adaptive peak windowing algorithm comprising a windowing function that is adaptively adjustable for reducing interference among different peaks of the input signal;
wherein, after a complete peak pulse of the input signal is observed in a current window, a corresponding sequence of samples is saved as a first vector;
wherein the first vector is processed to generate a second vector which corresponds to the first vector prefixed and suffixed with one or more values, the prefixing and suffixing being based at least in part on the length of the first vector and the length of the current window; and
wherein the windowing function is adaptively adjustable based at least in part on the second vector.