Input/Output Transfer Characteristics. In Fig. 2.9, we show a model used to include the effects of
the transmit PA nonlinearity into the system simulations. First, the complex-valued signal is converted
to the polar coordinate system (without any loss of information); the amplitude component
enters the two functions. The first function takes the input amplitude and transforms it to an output
amplitude based on the PA transfer function. This phenomenon is typically called AM-AM distortion.
Similarly, the input amplitude signal enters the phase component and outputs the amount of phase that
needs to be added to or subtracted from the input phase. This phenomenon is typically called AM-PM
distortion. Lastly, as the distortion is inserted into the symbol stream, the waveform is converted back
into the Cartesian coordinate system [12].
Next we discuss some typical characteristics of the transmit PA (see Fig. 2.10). We will perform
this operation not only in terms of the input and output power, but also in terms of the in-band and
out-of-band emissions.
Some noteworthy comments should be made concerning the PA operating point. The larger the
desired output power is, the closer we are operating to the nonlinear region of compression.
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