A commonly used measurement of amplitude variations is the peak-to-average ratio (PAR). For this
particular example, the PAR 2.86 dB for the roll-off factor 0.2, and 1.11 dB for roll-off factor
0.9. The reason behind presenting this statistic is to address the effects of encountering a
nonlinearity. The larger the amplitude variation, the more linear operating range is required in the
transmit PA. For example, consider the typical PA characteristic in Fig. 2.26, where we have outlined
a pair of operating points A and B. When operating the PA at point A, there is a linear gain across
the dynamic range of the input signal. The signal was not distorted in the time domain, so the frequency
domain exhibits no distortion. However, when operating at point B, the gain is not linear
across the dynamic range of the input signal. In fact, there is distortion for the high-amplitude cases,
shown in the form of clipping. The signal is distorted in the time domain, and hence, we would expect
distortion in the frequency domain.
The PA designers would like to push the operating point as close as possible to the compression
region since this is the highly powered-efficient area. However, the communication system designers
f(t) tan 1 cb(t)
a(t) d
A(t) 2a2(t) b2(t)
j tan1Cb(t)
a(t)D a(t) jb(t) 2a2(t) b2(t) # e
must trade off out-of-band emissions for PA efficiency.
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