The advantage of this technique is that the performance can be very good. The disadvantage is that
additional components are required to sample the PA output signal and then spectrally down convert
them to baseband and the digitize them. However, this adaptive method can be used to compensate
for time-varying mechanisms. In order to minimize the many variables that can lead to performance
degradation, it is highly recommended that the entire technique be performed on a single IC [20, 21].
2.1.3 Gain and Phase Imbalance
In addition to the AM and PM distortion, there exists the possibility of the quadrature modulator being
less than perfect. These imperfections will take on the form of gain and phase distortion. Recall the discussion
on the complex envelope of a band pass signal in Chap. 1, where we connected the complex
60 CHAPTER TWO
Predistorter NLA
Input Signal Output Signal
AM-AM
Characteristic
Predistort
AM-AM
AM-PM
Characteristic
Predistort
AM-PM
Cascaded
Characteristic
In
Out
In
Out
In
Out
In
Out
In
Out
In
Out
?‡’
?‡’
?‡’
?‡’
I
Q
I
Q
PA
FIGURE 2.14 Predistortion linearization block diagram and example.
FIGURE 2.15 16-QAM predistortion linearization example.
envelop to the input of a quadrature modulator which performed the spectral shifting operations.
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