Radio transmitters usually use power amplifiers that are mostly affected by non-linear effects that cause spectral re-growth and impairments of the quality of the transmitted signals. As a counter-measure against such impairments, the signals to be amplified are predistorted, in particular digitally predistorted, to compensate for the non-linear effects. The predistortion is performed on the basis of a non-linear characteristic, which depends on the non-linear effects of the amplifier.
To estimate the non-linear characteristic of the predistorter, so called open-loop and closed-loop techniques may be deployed.
As to the open-loop methods, a non-linear characteristic for the predistorter is determined in advance and applied fixedly during operation. As to the closed-loop methods, the non-linear characteristic is adaptively varied in order to track the variation of the non-linear parameters due to temperature, aging and frequency changes of the amplifier. However, in the known methods, the output signal of the amplifier, which is usually a modulated radio frequency signal, is provided to a down-conversion chain comparable to that of a radio frequency receiver. For example, various local oscillator signals have to be generated for down-conversion, and the down-converted signal may have to be demodulated for obtaining baseband I- and Q-components.
U.S. Pat. No. 4,291,277 describes an amplifier with a predistorter, wherein an output signal of the amplifier is down-converted by mixing with a local oscillator signal and demodulated in a quadrature demodulator. The demodulated signal is used to update parameters of the predistorter. However, the effort for providing such a down-conversion chain for a transmitter may be considerable according to some implementations.
EP 2117115 A1 describes a transmission system in which a transmitter comprises a digital predistorter. Parameters of the predistorter are updated in a receiver, which provides these parameters to the predistorter via a return link. Since the receiver provides the parameters of the predistorter, the efficiency and the flexibility of the predistorter may be reduced.