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RF Analog Impairments Modeling for Communication Systems Simulation: Application to OFDM-based Transceivers by Lydi Smaini

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4.5 Power Amplifier Linearization

4.5.1 Digital Predistortion Principle

One of the major issues with OFDM modulation is the high PAPR of the temporal signal, which dictates the linearity requirement of the transceiver chain. In transmission the signal distortion is especially due to PA nonlinearity, which degrades the system performance and also generates out-of-band emissions. The simplest solution to limit the transmission distortion is to back-off the PA in order to operate in a wide linear region at the detriment of the power efficiency. However, in modern integrated CMOS transceivers, PAs are sometimes also embedded on-chip for cost reasons and then cannot take advantage of large back-off due to a limited voltage supply. So a new design tendency is to allow the PA to operate in its nonlinear region and to compensate the distortion using digital predistortion (Cavers, 1990; Ding et al., 2004; Li et al., 2009). The basic principle of the compensation is to predistort the signal in DBB before the DAC, that is, ahead of the PA, with a function that is the inverse of the nonlinear transfer function of the PA, as represented in Figure 4.22. Whereas PA compresses at high amplitudes, the predistorter will compensate the compression with an expansion. Furthermore, because the PA transfer function can vary in time, for example, due to temperature changes or output impedance variation, adaptive digital predistortion can be introduced to track these variations. In that case the predistorter ...

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