Usually, we use two modulation methods to produce optical
source, such as direct modulation and indirect modulation.
Indirect modulation has good modulating performance which
use modulator and has complicate structure, and it fits for
60GHz RoF system, In contrast, considering device complexity
and performance, direct modulation fits for a 3.5GHz WiMAX
system, which uses IMDD (Intensity Modulation Direct
Detection) and has a simple structure.