Performance analysis of ultraviolet communication in the presence of turbulence

Abstract

Optical wireless communication (OWC) is gaining considerable research atten tion due to the scarcity of RF spectrum, and increasing demand for multi-rate multi media services. The conventional OWC systems, including free-space optics (FSO) and visible light communication (VLC), requires line-of-sight (LOS) along with strict pointing, acquisition, and tracking (PAT) requirements. These constraints limit FSO and VLC system’s applicability in scenarios where it is not feasible to obtain LOS. Ultraviolet communication (UVC), on the other hand, overcomes such limitations and finds a prominent role in establishing the reliable communication link. UVC has the unique ability to operate in non-LOS (NLOS) mode. This ability is due to the extremely small operating wavelength of ultraviolet (UV) waves ranging from 200 nm to 280 nm (commonly referred to as UV-C band), which results in the strong interaction of UV waves with atmospheric particles, thereby giving rise to scattering phenomenon. Additionally, the deep UV band is solar blind due to the absorption of UV-C waves by the ozone layer, which results in almost negligible background noise near to the earth surface. Due to the exceptionally low background noise, a wide field of view (FOV) can be used at the receivers, enhancing the communication system’s performance. NLOS UVC suffers from high path-loss due to atmospheric absorption and turbulence-induced fading caused by inhomogeneous atmospheric conditions. These impairments severely degrade NLOS UVC system’s performance and limit its com munication range to a short distance. Further, in-spite of the availability of huge license free band, the data rate of NLOS UVC is rather limited due to low modu lation speed of UV LEDs. The goal of this thesis is to address these challenges of NLOS UVC through the use of spatial diversity, cooperative relaying, and higher order modulation schemes, and making it suitable for data-intensive applications, and long distance outdoor communication. In this thesis, initially the problem of turbulence induced fading is addressed through the use of a single-input-multiple-output (SIMO) NLOS UVC system em ploying selection combining (SC) at the receiver. The multiple receiver branches are assumed to be correlated, and the system’s performance is analyzed in terms of outage probability, ergodic capacity, and average symbol error rate (ASER) for higher order modulation formats including square quadrature amplitude modulation (SQAM), rectangular QAM (RQAM), cross QAM (XQAM), and hexagonal QAM (HQAM). Numerical study is conducted and it is demonstrated that the proposed system model effectively mitigates the effect of fading, and use of higher order mod ulation schemes significantly improves the data rate.