Analysis of a FSO link for different modulation scheme under various turbulence conditions

Abstract

Currently, the technologies used in access network include the copper and coaxial cable, optical fiber, wireless internet access and broadband radio frequency. These technologies, in particular copper/coaxial cables and RF based have limitations such as congested spectrum, a lower data rate, an expensive licensing, security issues,etc. With the rapid development and maturity of optoelectronic devices, license free operation and easy installation, free space optical (FSO) communication became a viable alternative for addressing the last mile bottleneck and meets the increasing bandwidth demand of emerging applications and end users. The free-space medium is utilized by FSO system for the transmission of data. As a result, the FSO communication faces challenges from atmospheric channel turbulence or turbidity resulting signal scattering, absorption, temporal and spatial fluctuation of light intensity, which ultimately degrade the link performance substantially. In the last one decade, many research works were carried out to mitigate and improve the performance of FSO link. Advanced data modulation formats can compensate the transmission impairments of the atmospheric turbulence and enhance link sustainability significantly. Generally, most commercially deployed FSO systems uses the intensity modulation direct detection (IM/DD) technique for simplicity but the coherent detection scheme assures higher receiver sensitivity than IM/DD. In this research work, the performance of a FSO link will be evaluated using coherent detection scheme for different modulation formats under various atmospheric turbulence conditions.The performance will be evaluated in terms of signal to noise ratio (SNR) and bit error rate (BER).The modulation techniques used in the analysis areOOK, BPSK,DPSK and QPSK. To evaluate the system error performance in turbulence regimes from weak to strong, the probability density function (pdf) of the received irradiance after traversing in the atmosphere is modeled using the gamma-gamma distribution. In the study, it is found that BPSK gives best BER performance in the proposed system in different turbulence strength compared toany other modulation techniques