Adaptive predistortion device and method using digital receiver
An advanced adaptive baseband/RF predistorting device, which advantageously uses the concept of digital receiver technology into power amplifier (PA) linearization area. The predistorting device performs an instantaneous characterization of the PA using two digital receivers to supply its dynamic AM-AM and AM-PM transfer functions used to synthesize Look-Up Tables (LUT) which implement the complex predistortion function in order to compensate for any non-linearity and memory effects.
1. An adaptive method for predistorting an RF modulated signal, to be transmitted, supplied by a signal source to an input of a power amplifier having an output for delivering an amplified output signal, said method comprising the steps of:
predistorting the RF modulated signal to be transmitted using an I/Q modulator interposed between the signal source and the input of the power amplifier, and controlled by means of amplitude and phase look-up tables stored in a distorting generator;
producing, via a first digital receiver, a first feedback signal in response to the RF predistorted signal;
producing, via a second digital receiver, a second feedback signal in response to the RF amplified output signal from the power amplifier;
modeling the power amplifier in response to the first and second feedback signals; and
updating the predistortion amplitude and phase look-up tables in response to said modeling of the power amplifier,
wherein said second feedback signal includes the complex envelope of the RF amplified output signal, and
wherein said modeling step includes the discrimination of the complex envelope of the first feedback signal referenced to the complex envelope of the second feedback signal to yield a predistortion function correlated to a behaviour of the power amplifier including nonlinearities and memory effects.
2. An adaptive method as recited in claim 1 , wherein said modeling step is done in real time.
3. An adaptive method for predistorting an RF modulated signal, to be transmitted, supplied by a signal source to an input of a power amplifier having an output for delivering an amplified output signal, said method comprising the steps of:
predistorting the RF modulated signal to be transmitted using an I/Q modulator interposed between the signal source and the input of the power amplifier, and controlled by means of amplitude and phase look-up tables stored in a distorting generator;
producing, via a first digital receiver, a first feedback signal in response to the RF predistorted signal;
producing, via a second digital receiver, a second feedback signal in response to the RF amplified output signal from the power amplifier;
modeling the power amplifier in response to the first and second feedback signals; and
updating the predistortion amplitude and phase look-up tables in response to said modeling of the power amplifier,
wherein said updating step is done when a linearity metric adjacent channel power ratio (ACPR) measurement sub-step indicates that the predistorting step is not adequate to meet predetermined ACPR standards.
4. An adaptive method as recited in claim 3 , wherein said linearity metric ACPR measurement sub-step is done via a digital receiver that includes a first channel tuned to a mean frequency and a second channel that is tuned to a predetermined offset frequency, said linearity metric ACPR measurement sub-step including comparing an average power at the mean frequency and at the predetermined offset frequency.
5. An adaptive device for predistorting an RF modulated signal to be transmitted, supplied by a signal source to an input of a power amplifier having an output for delivering an amplified output signal, said adaptive device comprising:
an I/Q modulator interposed between the signal source and the input of the power amplifier;
a distorting generator including predistortion amplitude and phase look-up tables; said distorting generator controlling said I/Q modulator to predistort the RF modulated signal to be transmitted in amplitude and in phase;
a first digital receiver producing a first feedback signal in response to the RF predistorted signal from said I/Q modulator;
a second digital receiver producing a second feedback signal in response to the RF amplified output signal from the power amplifier;
a control module receiving said first and second feedback signals from said first and second digital receivers; said control module being so configured as to model the power amplifier in response to the first and second feedback signals and to update said amplitude and phase look-up tables of said distorting generator in response to a dynamic modeling of the power amplifier,
wherein said look-up tables of said distorting generator are indexed by an envelope detector that detects the envelope of the signal to be transmitted before predistortion.
6. An adaptive device as recited in claim 5 , wherein said envelope detector indexes the distorting generator via an analog to digital converter.
7. An adaptive device for predistorting an RF modulated signal to be transmitted, supplied by a signal source to an input of a power amplifier having an output for delivering an amplified output signal, said adaptive device comprising:
an I/Q modulator interposed between the signal source and the input of the power amplifier;
a distorting generator including predistortion amplitude and phase look-up tables; said distorting generator controlling said I/Q modulator to predistort the RF modulated signal to be transmitted in amplitude and in phase;
a first digital receiver producing a first feedback signal in response to the RF predistorted signal from said I/Q modulator;
a second digital receiver producing a second feedback signal in response to the RF amplified output signal from the power amplifier;
a control module receiving said first and second feedback signals from said first and second digital receivers; said control module being so configured as to model the power amplifier in response to the first and second feedback signals and to update said amplitude and phase look-up tables of said distorting generator in response to a dynamic modeling of the power amplifier,
wherein said look-up tables of said distorting generator are indexed by data from a third digital receiver that down-converts the signal to be transmitted to baseband.
8. An adaptive device as recited in claim 7 , wherein the data from said third digital receiver is supplied to said control module that indexes said distorting generator accordingly.
9. An adaptive device for predistorting an RF modulated signal to be transmitted, supplied by a signal source to an input of a power amplifier having an output for delivering an amplified output signal, said adaptive device comprising:
an I/Q modulator interposed between the signal source and the input of the power amplifier;
a distorting generator including predistortion amplitude and phase look-up tables; said distorting generator controlling said I/Q modulator to predistort the RF modulated signal to be transmitted in amplitude and in phase;
a first digital receiver producing a first feedback signal in response to the RF predistorted signal from said I/Q modulator;
a second digital receiver producing a second feedback signal in response to the RF amplified output signal from the power amplifier;
a control module receiving said first and second feedback signals from said first and second digital receivers; said control module being so configured as to model the power amplifier in response to the first and second feedback signals and to update said amplitude and phase look-up tables of said distorting generator in response to a dynamic modeling of the power amplifier,
wherein said control module is so configured as to update said amplitude and phase look-up tables when an adjacent channel power ratio (ACPR) measurement indicates that the predistortion made by said predistorting generator is not adequate to meet predetermined ACPR standards.
10. An adaptive device as recited in claim 9 , wherein said ACPR measurement is done via said second digital receiver that includes a first channel tuned to a mean frequency and a second channel that is tuned to a predetermined offset frequency, said ACPR measurement including comparing an average power at the means frequency and at the predetermined offset frequency.
11. An adaptive device for predistorting an RF modulated signal to be transmitted, supplied by a signal source to an input of a power amplifier having an output for delivering an amplified output signal, said adaptive device comprising:
an I/Q modulator interposed between the signal source and the input of the power amplifier;
a distorting generator including predistortion amplitude and phase look-up tables; said distorting generator controlling said I/Q modulator to predistort the RF modulated signal to be transmitted in amplitude and in phase;
a first digital receiver producing a first feedback signal in response to the RF predistorted signal from said I/Q modulator;
a second digital receiver producing a second feedback signal in response to the RF amplified output signal from the power amplifier;
a control module receiving said first and second feedback signals from said first and second digital receivers; said control module being so configured as to model the power amplifier in response to the first and second feedback signals and to update said amplitude and phase look-up tables of said distorting generator in response to a dynamic modeling of the power amplifier,
wherein said control module is further configured as to insert an adequate delay between the first feedback signal and the second feedback signal.