There are essentially two
types of linearization techniques available: feed forward and feed backward.
An example of a feed forward technique is called predistortion. Here, the baseband signal is predistorted
using a priori information about the characteristics of the PA. Hence, additional distortion
is intentionally inserted into the signal such that the PA output signal will have less spectral regrowth.
The benefits of this technique are that it is not complex and can be implemented in a cost-effective
manner. The disadvantage is that if the PA nonlinear characteristics are assumed time variant, then the
linearization technique may not be able to track the changes that occur with aging, temperature, manufacturing
sampling, and so forth. Hence, some feedback information path should be used to track the
slow time-varying nature of the component. In Fig. 2.14, we provide a block diagram depicting the
predistortion technique.
In Fig. 2.15, we provide an example using 16-QAM modulation. These symbols are intentionally
distorted in both amplitude and phase, in order to compensate for the PA nonlinear characteristic. The
desired output 16-QAM constellation is shown in the figure.
Note that the PA characteristics are typically stored in a form of a lookup table (LUT).
Pages:
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123