12 Spectral regrowth for the OQPSK modulation scheme.
FIGURE 2.11 Spectral regrowth for the QPSK modulation scheme.
QPSK NLA Output PSD
??“75
??“70
??“65
??“60
??“55
??“50
??“45
??“40
??“35
??“30
??“25
??“20
??“4 ??“3 ??“2 ??“1 0 1
Frequency (Hz)
Magnitude (dB)
PA
Input
PA
Output
2 3 4
Compensation Techniques. In this subsection, we will present some commonly used PA linearization
techniques [13]. As discussed above, spectral regrowth occurs when a modulation
scheme with envelope variations enters a PA with a nonlinear transfer function. In the latter sections,
we will discuss options communications system designers have when choosing a modulation
scheme. So, in this section, we will leave the modulation schemes and concentrate on the PA transfer
characteristics [15??“17].
The transmit PA nonlinearities are typically modeled as AM-AM and AM-PM transfer characteristics.
Above, we used AM-AM to represent the input AM power versus the output PA signal, and
AM-PM to represent the input AM power versus the output phase signal. As discussed above, operating
in the highly efficient region is desirable, hence either another type of PA is chosen or a linearization
technique is performed to reduce the out-of-band emissions.
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