ABSTRACT
This chapter analyses the effect of atmospheric turbulence on the different modulation techniques used in FSO. Atmospheric turbulence is known to cause signal fading in the channel, and the existing mathematical models for describing the fading have already been introduced in Chapter 3. There are many different types of modulation schemes that are suitable for optical wireless communication systems as discussed in Chapter 4; the emphasis in this chapter will, however, be on the effect of atmospheric turbulence-induced fading on the following techniques: on–off keying (OOK), pulse position modulation (PPM) and phase shift keying premodulated subcarrier intensity modulation. Since the average emitted optical power is always limited, the performance of modulation techniques is often compared in terms of the average received optical power required to achieve a desired bit error rate at a given data rate. It is very desirable for the modulation scheme to be power efficient, but this is, however, not the only deciding factor in the choice of a modulation technique. The design complexity of its transmitter and receiver and the bandwidth requirement of the modulation scheme are all equally important. Although it can be argued that the optical carrier has abundant bandwidth, the modulation bandwidth available for communication is ultimately limited by the optoelectronic devices.
