Comparative Analysis of Different Modulation Techniques for Optical Wireless Communication under Atmospheric Turbulence

As an alternative to the access network bottleneck, wireless optical communications offers transmission rates far beyond possible by RF technology. However, its performance is sensitive to the surrounding channel conditions, which may vary widely and frequently due to fading and dispersion. Among all the impairments to wireless optical communication links, atmospheric turbulence-induced scintillation is a major one[1]. In particular, the impact of scintillation is to cause random variations in signal intensity, leading to large signal fades. Furthermore, the fast intensity variations caused by scintillation also increase the difficulty of signal intensity estimation, which is necessary to a widely used intensity modulation with direct detection (IM/DD) transmission system using on-off-keying (OOK) modulation. Thus appropriate modulation techniques are required to combat the adverse channel conditions. In this work, differentially encoded phase-shift keying (DPSK) is considered as a means of improving the detection sensitivity. In this paper, the error rate performance of wireless optical  DPSK system over atmospheric turbulence channel are compared with that of a most commonly used OOK system under the same channel conditions and then related numerical results are further analyzed. The error probability expression over atmospheric channels is derived under the assumption of log normal distributed scintillation, and then the performance of DPSK system is compared with that of a most commonly used on-off keying (OOK) system under then same channel conditions. Theoretical analysis and numerical results illustrate that with the same bandwidth, DPSK system has a higher sensitivity than OOK, DPSK format is very suitable for atmosphere channels and has a broad prospect in wireless optical communications