Analog signal processor for nonlinear predistortion of radio-frequency signals
View Patent ↗RF predistortion apparatus for making linear the output signal of non-linear components such as RF power amplifiers. The apparatus comprises an RF input line for carrying an RF signal connected to an envelope detector for finding the envelope of the RF signal, a power detector for finding the power of the RF signal and a quadrature modulator. The apparatus also comprises a coefficient vector input line for carrying an input signal that carries one or more coefficients to a digitally controlled analog subsystem (DCAS). The DCAS having circuitry for processing both the output of the envelope detector and the output of the power detector by selecting one or more coefficients from the coefficient vector input line for generating a weighted summation of the power of the RF signal and a weighted summation of the envelope voltage of the RF signal that are output to the quadrature modulator. The quadrature modulator has circuitry for mixing the RF input signal with the output of the DCAS to generate a signal for predistorting the RF input signal feeding the power amplifier.
1. An RF predistortion apparatus comprising:
an RF input line for carrying an RF signal;
a coefficient vector input bus for carrying a coefficient vector comprising a plurality of coefficient signals each representing a coefficient;
an RF predistortion processor, comprising:
(a) a digitally controlled analog subsystem (DCAS) receiving an envelope signal representing a signal envelope of the RF signal and the coefficient vector; the DCAS generating first and second weight signals, V p (t) and V q (t), each representing a polynomial function of the RF input signal; and
(b) a quadrature modulator coupled to the RF signal line and the DCAS to receive the RF input signal and the first and second weight signals, the quadrature modulator providing a predistortion signal representing a sum of in-phase and quadrature signals derived from the RF input signal, respectively weighted by the first and second weight signals; and
an RF delay element coupled to the RF input line to provide a delayed RF input signal; an RF coupler coupling the predistortion signal and the delayed RF input signal to provide a predistorted input signal to a power amplifier.
2. The RF predistortion apparatus as in claim 1 , wherein the predistortion processor further comprising an envelope detector coupled to the RF input line for providing the envelope signal.
3. The predistortion apparatus according to claim 2 wherein the envelope detector, the DCAS and the quadrature modulator reside on an integrated circuit.
4. The RF predistortion apparatus according to claim 2 wherein the envelope detector comprises a limiting amplifier, a current-mode Gilbert multiplier and a trans-impedance amplifier.
5. The RF predistortion apparatus according to claim 2 wherein the envelope detector is an approximate envelope detector followed by a soft limiting amplifier.
6. The RF predistortion apparatus according to claim 1 , wherein the DCAS further receives a power signal representing the power of the RF signal.
7. The RF predistortion apparatus according to claim 6 , further comprising a power detector providing the power signal, wherein the power detector comprises a current-mode Gilbert followed by a trans-impedance amplifier.
8. The RF predistortion apparatus according to claim 2 wherein the first and second weight signals, V p (t) and V q (t), are generated according to the following equations:
V
p
(
t
)
=
∑
m
=
0
M
-
1
∑
k
=
1
K
a
mk
r
m
(
t
-
τ
k
)
(
1
)
V
q
(
t
)
=
∑
m
=
0
M
-
1
∑
k
=
1
K
b
mk
r
m
(
t
-
τ
k
)
(
2
)
where a mk and b mk are polynomial coefficients from the coefficient vector, τ k are delays from one or more memory delay circuits, M, K are integers, and r(t) is the envelope signal.
9. An RF predistortion apparatus according to claim 7 wherein the quadrature modulator outputs at least one time-varying signal, y(t) , according to the following equation:
y ( t )= V p ( t ) r ( t )cos [2 πft +φ( t )]+ V q ( t ) r ( t )sin [2 πft +φ( t )] (3)
where f is the frequency, r(t) is an envelope signal, and φ(t) is a phase of the RF input signal.
10. An RF predistortion apparatus according to claim 1 wherein the DCAS comprises memoryless polynomial circuits output at least one time-varying signal, V(t), according to the following equation:
V
(
t
)
=
a
1
r
(
t
)
+
∑
j
=
1
N
a
2
j
r
2
j
(
t
)
(
4
)
where r(t) is the envelop signal, N is a predetermined integer greater than or equal to 2, r 2j (t) are exponentials of the envelope signal, and a l and a 2j are weights from the coefficient vector input signal.
11. An RF predistortion apparatus as in claim 10 wherein the at least one time-varying signal, V (t) , is implemented according to the following equation:
V ( t )= a 1 r ( t )+ a 2 r 2 ( t )+ a 4 r 4 ( t )+ a 6 r 6 ( t )+ a 8 r 8 ( t ) (5)
where r(t) is the envelope signal, r n (t) are exponentials of the envelop signal, and a n are weights from the coefficient vector input signal.
12. An RF predistortion apparatus as in claim 10 , wherein N is greater than or equal to 4.
13. An RF predistortion apparatus as in claim 1 , further comprising a second RF delay element coupled between the RF input signal line and the quadrature modulator, such that the RF input signal received into the quadrature modulator is delayed by the second RF delay element.
14. An RF predistortion apparatus as in claim 1 , further comprising: a second predistortion processor substantially the same as the first predistortion processor providing a second predistortion signal; and a combiner combining the first and second predistortion signal providing a combined predistortion signal to the RF coupler.
15. An RF predistortion apparatus as in claim 14 , wherein the second predistortion processor receives as an RF input signal an output signal of the power amplifier.
16. An RF predistortion apparatus as in claim 14 , wherein the first and second predistortion processors are provided on the same integrated circuit.
17. A telecommunications system comprising an RF receiving antenna, an RF linear power amplifier, an RF receiving mixer, an RF receiving oscillator and an RF receiver; the RF linear power amplifier further comprising
an RF input line for carrying an RF input signal connected to a power amplifier, a quadrature modulator, an envelope detector and a power detector; and
a digitally controlled analog subsystem (DCAS) connected to an output of the envelope detector, an output of the power detector, and an output of a coefficient vector generator, wherein the DCAS has circuitry for processing the signals from the envelope detector, power detector and coefficient generator and wherein said circuitry selects one or more of coefficients from the coefficient vector generator and generates a weighted summation of the signals from the power detector and the envelope detector that are output to the quadrature modulator; and wherein the quadrature modulator has circuitry for mixing the DCAS output signals with the RF input signal to generate a signal output by the quadrature modulator that is connected to the RF input line for predistorting the RF input signal feeding the power amplifier.
18. A telecommunications system according to claim 17 further comprising an RF transmitter, an RF transmitting mixer, an RF transmitting oscillator and an RF transmitting antenna.
19. A linear power amplifier apparatus comprising:
an RF input line for carrying an RF input signal connected to a power amplifier, a quadrature modulator, an envelope detector and a power detector; and
a digitally controlled analog subsystem (DCAS) connected to an output of the envelope detector, an output of the power detector, and an output of a coefficient vector generator, wherein the DCAS has circuitry for processing the signals from the envelope detector, power detector and coefficient generator and wherein said circuitry selects one or more of coefficients from the coefficient vector generator and generates a weighted summation of the signals from the power detector and the envelope detector that are output to the quadrature modulator; and wherein the quadrature modulator has circuitry for mixing the DCAS output signals with the RF input signal to generate a signal output by the quadrature modulator that is connected to the RF input line for predistorting the RF input signal feeding the power amplifier.
20. A linear power amplifier according to claim 19 further comprising at least one RF coupler interposed between an output of the quadrature modulator and an input of the power amplifier.
21. A linear power amplifier according to claim 19 further comprising at least one RF delay element located in the RF input line to the power amplifier and at least one RF delay element located in the RF input line to the quadrature modulator.
22. A linear power amplifier according to claim 19 wherein the envelope detector, the power detector, the DCAS and the quadrature modulator reside on one or more integrated circuits.