**Digital predistortion**

In order to obtain undistorted signal at the transmitter output, the input signal can be intentionally distorted before being fed to a nonlinear power amplifier. The predistorter generates nonlinearities which operate in the opposite way to the nonlinearities generated by the power amplifier, so that the overall response at the PA-output is linear (see Fig. 8). The linearization is done in the digital regime using FPGA which makes the system very flexible and adaptive for changes in power device over time to ensure linear output. As computational power of FPGA is continuously increasing, linearization over larger bandwidth can be realized with this technique. In the literature, linearization with digital predistortion technique which can cope with dynamic nonlinearity caused by electrical memory effects has also been reported (Lee et al, 2009).

Fig. 7. Block diagram of a feedforward transmitter.

Fig. 8. Principle and block diagram of digital predistortion for linearization of power amplifiers.

In comparison to other techniques, digital predistortion offers higher efficiency and greater flexibility at low cost and represents a mature linearization technique for mobile base stations. Due to the mentioned flexibility and simple architecture, digital predistortion has gained it popularities in the power amplifier design community. In most of the cases where no extremely large bandwidth is required, high efficiency amplifiers e.g. Doherty and switched-mode amplifiers are combined with digital predistortion to improve the linearity.
