Adaptive linearizer with narrowband feedback path
An ISI Replica unit is provided, which produces interference components similar to one due to LPF into the output of HPA MODEL unit. An HPA MODEL unit models the characteristic of HPA using the output signal of the predistorter and an error signal obtained by subtracting a feedback signal from the output of the ISI Replica so that the error signal approaches zero. The feedback signal is used after filtration by LPF, thereby including interference components. The error signal obtained by the feedback signal after LPF and the output signal of ISI Replica unit includes few interference components, thereby modeling by the HPA MODEL unit becomes accurate and the linearization performances become better.
1. An adaptive linearizer, comprising:
a feedback path, a pass bandwidth of which is limited compared to an output signal bandwidth of an amplifier;
an analog-to-digital converter that is placed in the feedback path and digitizes an output signal of the amplifier which is fed back by the feedback path;
a filter that is placed in the feedback path after the analog-to-digital converter and limits a bandwidth of an output signal of the analog-to-digital converter;
a replica unit that introduces interference components into an output signal modeling the characteristic of the amplifier, which have a similar characteristic of interference components induced by a limited pass bandwidth of the feedback path, by introducing interference components similar to those of the filter;
a training unit that calculates an inverse characteristic of a characteristic of the amplifier using an error signal between an output signal of the replica unit and the output signal of the amplifier; and
a predistorter that distorts a signal to be input to the amplifier according to the inverse characteristic calculated by the training unit.
2. The adaptive linearizer according to claim 1 , wherein
the filter is a first low-pass filter, and
the replica unit is a second low-pass filter which has a similar characteristic to the first low-pass filter.
3. The adaptive linearizer according to claim 1 , wherein
the training unit comprises:
an amplifier model unit that models a characteristic of the amplifier, and
an inverse unit that calculates an inverse characteristic of the characteristic of the amplifier and inputs the inverse characteristic into the predistorter.
4. The adaptive linearizer according to claim 1 , wherein
the characteristic of the amplifier is represented by a memory polynomial, and
the training unit models the characteristic of the amplifier by adjusting coefficients of the memory polynomial.
5. The adaptive linearizer according to claim 1 , wherein
the amplifier operates in an operation range extending to a non-linear region.
6. A transmitter, comprising:
an adaptive linearizer, including:
a feedback path, a pass bandwidth of which is limited compared to an output signal bandwidth of an amplifier;
an analog-to-digital converter that is placed in the feedback path and digitizes an output signal of the amplifier which is fed back by the feedback path;
a filter that is placed in the feedback path after the analog-to-digital converter and limits a bandwidth of an output signal of the analog-to-digital converter;
a replica unit that introduces interference components into an output signal modeling the characteristic of the amplifier, which have a similar characteristic of interference components induced by a limited pass bandwidth of the feedback path, by introducing interference components similar to those of the filter;
a training unit that calculates an inverse characteristic of a characteristic of the amplifier using an error signal between an output signal of the replica unit and the output signal of the amplifier; and
a predistorter that distorts a signal to be input to the amplifier according to the inverse characteristic calculated by the training unit.
7. An adaptive linearizing method for an adaptive linearizer provided with an amplifier and a feedback path, a pass bandwidth of which is limited compared to an output signal bandwidth of the amplifier, comprising:
introducing interference components into an output signal modeling the characteristic of the amplifier, which have a similar characteristic of interference components induced by a limited pass bandwidth of the feedback path, by introducing interference components similar to those of a filter that is placed in the feedback path after an analog-to-digital converter and limits a bandwidth of an output signal of the analog-to-digital converter, the analog-to-digital converter being placed in the feedback path and digitizing an output signal of the amplifier which is fed back by the feedback path;
calculating an inverse characteristic of a characteristic of the amplifier using an error signal between an output signal modeling the characteristic of the amplifier to which the interference components are introduced and the output signal of the amplifier; and
distorting a signal to be input to the amplifier according to the inverse characteristic of the characteristic of the amplifier.