However,
due to the temperature change, component aging, manufacturing process, carrier frequency, and so
forth, the characteristics change. Hence, an adaptive predistortion technique is a good solution. There,
the PA output signal is sampled and fed back so as to track time-varying characteristics [18].
The second technique of Cartesian feedback continuously examines the transmitter PA output and
then corrects for any nonlinearity in the transmit chain [19]. The performance of this technique is
highly dependent on the overall delay in the loop, as well as the gain and phase imbalance in the
feedback??”not to mention that the PA characteristics also vary with time. Here the PA output signal
MODULATION THEORY 59
??/4-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
FIGURE 2.13 Spectral regrowth for the /4-QPSK modulation scheme.
is sampled and spectrally down converted in order to observe the distortion present on the signal and
then a correction algorithm is applied to reduce the spectral regrowth. In Fig. 2.16, we plot the simplified
Cartesian feedback linearization technique.
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