The power amplier (PA) is one of the most critical subsystems in wireless (and wired) trans- mitters. Not only is it one of the most power hungry devices that accounts for most of the direct current (DC) power consumed in macro base stations, but also it is the main source of nonlinear distortion in the transmitter. When the information to be transmitted is included in both amplitude and phase, as is the case of modern digital communication modulations, the non-constant amplitude modulated waveforms present a certain peak-to-average power ratio (PAPR). As shown in Figure 1.1, in order to prevent the peaks of the RF modulated signal going into compression it is necessary to operate with certain input or output back-o (IBO and OBO, respectively), i.e., dBs of separation from the PA compression point. Therefore, to avoid generating nonlinear distortion (e.g., due to clipping the signal peaks) the PA has to be operated with back-o levels equivalent to the signal's PAPR. As depicted in Figure 1.1, this has a negative impact in the transmitter's mean power eciency, because the PA power eciency is always greater at high power levels. This inherent linearity versus eciency trade-o is more evident when considering orthogonal frequency division multiplexing (OFDM)-based waveforms (e.g., such as the ones considered in 4G, 5G New Radio) presenting high PAPR and operating linear but low-ecient class-A or class-AB PAs.

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