ABSTRACT
Most practical OWC systems being currently deployed employ the IM/DD scheme for outdoor as well as indoor applications. Atmospheric conditions, in particular heavy fog, is the major problem, as the intensity of light propagating through a thick fog is reduced considerably. Therefore, intuitively, it appears that the best solution to high attenuation would be to pump more optical power or concentrate and focus more power into smaller areas. However, the eye safety introduces a limitation on the amount of optical power being transmitted. For indoor applications, the eye safety limit on transmit optical power is even more stringent. The optical channel differs significantly from the RF channels. Unlike RF systems where the amplitude, frequency and phase of the carrier signal are modulated, in optical systems, it is the intensity of the optical carrier that is modulated in most systems operating below 2.5 Gbps data rates. For data rates >2.5 Gbps, external modulation is normally adopted. Additionally, the use of photodetectors with a surface area many times larger than the optical wavelength facilitates the averaging of thousands of wavelength of the incident wave. In this chapter, a number of modulation techniques that are most popular, in terms of power efficiency and bandwidth efficiency, for both indoor and outdoor OWC applications are discussed. The analogue and digital baseband modulation techniques are discussed in Sections 4.2 and 4.3. The spectral properties, error probability as well as the power and bandwidth requirements are presented. Advanced modulation techniques such as the subcarrier intensity modulation and the polarization shift keying are presented in Sections 4.3 and 4.4.
