Abstract:
Free-space optical (FSO) communication is an attractive and cost-effective solution for high-rate image, voice, and data transmission than an RF channel. The last decades have witnessed a spectacular progress on FSO communication, which is very much promising for future optical networking where installation of fiber is very much limited. But the performance of FSO links are highly affected by beam dispersion, scattering, absorption, interference from background light sources including sun. Continuous researches are in progress to improve the performances of FSO communication under different turbulent conditions. Many research works were performed to improve the performances of FSO communication with different modulation and coding techniques. Even hybrid optical communication system that combines the OFDM and FSO are also investigated recently to develop a robust communication link in the presence of atmospheric turbulence. Most of the above research works are based on simulation and some experimental works are also reported in literature to analysis the performance of FSO communication. But the development of analytical approach to evaluate the performance of a FSO communication link is very limited. In this research work an analytical approach is developed for a RF subcarrier modulated FSO communication system for both non-orthogonal and orthogonal subcarriers considering the influence of atmospheric turbulence which is modeled by lognormal and gamma-gamma distribution for weak and strong turbulent respectively.
Primarily investigations are made to find the analytical approaches to approach in order to evaluate the carrier to noise ratio (CNR) and bit error rate (BER) of a FSO link with RF subcarrier modulation (SCM) followed by optical intensity modulation taking into account the effect of atmospheric turbulence (for both weak and strong) with coherent and direct detection receivers.
Analysis is carried out to find the carrier to noise ratio plus interference at the output of a direct/coherent optical detection receiver including the effect of FSO channel noise and receiver thermal noise, background channel noise and photo detector shot noise.
Analysis is also extended to find the output CNR for a given system parameters for LDPC and STBC Coded OFDM system under atmospheric turbulent conditions. The conditional BER and average BER analysis results are found numerically for several system and channel parameters. Performance results are numerically evaluated at a bit rate of 10 Gbps for different turbulent conditions, number of simultaneous user, link distance, code length etc.
