Transmitter, communication unit and method for reducing harmonic distortion in a training mode

A communication unit comprises a transmitter comprising a transmitter configured to operate in a training mode of operation and a normal transmission mode of operation, the transmitter comprising:

• a signal generator arranged to generate a digital baseband signal that is representative of a signal for transmission;
• a digital predistortion, DPD, circuit configured to predistort the digital baseband signal;
• a first frequency shift circuit operably coupled to the DPD circuit and configured to frequency shift the signal for transmission in a first frequency direction;
• a power amplifier arranged to amplify the predistorted signal and provide a power amplified output thereof;
• a feedback path coupled to an output of the power amplifier and configured to route a portion of the power amplified predistorted signal to a second frequency shift circuit that is configured to frequency shift a fed back power amplified predistorted signal portion in a second frequency direction opposite to the first frequency direction;
• and
• a calibration engine, located in the feedback path and coupled to the signal generator and arranged to receive and compare the digital baseband signal representative of a signal for transmission with the fed back power amplified predistorted signal portion shifted in the second frequency direction, wherein the calibration engine is configured to determine at least one DPD compensation value during a training mode of operation and wherein the first frequency shift circuit and second frequency shift circuit apply the frequency shift in a first direction and a second frequency direction opposite to the first frequency direction only in the training mode of operation.

A method for limiting harmonic distortion of a transmitter signal that uses a digital predisortion circuit, the method comprising:

• generating a digital baseband signal representative of a signal for transmission;
• predistorting the digital baseband signal by a digital predistortion, DPD, circuit;
• applying a first frequency shift to frequency shift the signal for transmission in a first frequency direction, and the frequency-shifted signal is then power-amplified;
• feeding back a portion of the power amplified predistorted frequency shifted signal;
• applying a second frequency shift to a fed back power amplified predistorted frequency shifted signal portion in a second frequency direction opposite to the first frequency direction;
• receiving and comparing the digital baseband signal representative of a signal for transmission with the fed back power amplified predistorted signal shifted in the second frequency direction;
• and
• determining at least one DPD compensation value to be applied in a DPD circuit, such that the determined at least one DPD compensation value compensates for harmonic distortion of the transmitter signal identified using the first frequency shift and the second frequency shift and wherein applying of a first frequency shift and a second frequency shift is performed solely during a training mode of operation.