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Free Space Optical (FSO) communications is a practical solution for creating a three dimensional global broadband communications grid, offering bandwidths far beyond possible in Radio Frequency (RF) range. However, attributes of atmospheric turbulence (scintillation) and obscurants such as clouds impose perennial limitations on availability and reliability of optical links. To design and evaluate optimum transmission techniques that operate under realistic atmospheric conditions, a good understanding of the channel behavior is necessary. In most prior works, Monte-Carlo Ray Tracing (MCRT) algorithm has been used to analyze the channel behavior. This task is quite numerically intensive. The focus of this paper is on investigating the possibility of simplifying this task by a direct extraction of state transition matrices associated with standard Markov modeling from the MCRT computer simulations programs. We show that by tracing a photon's trajectory in space via a Markov chain model, the angular distribution can be calculated by simple matrix multiplications. We also demonstrate that the new approach produces results that are close to those obtained by MCRT and other known methods. Furthermore, considering the fact that angular, spatial, and temporal distributions of energy are inter-related, mixing time of Monte-Carlo Markov Chain (MCMC) for different types of aerosols is calculated based on eigen-analysis of the state transition matrix and possibility of communications in scattering media is investigated. [ABSTRACT FROM AUTHOR]

Free space optical (FSO) communications is a cost-effective and high bandwidth access technique, which has been receiving growing attention with recent commercialization successes. A major impairment in FSO links is the turbulence- induced fading which severely degrades the link performance. To mitigate turbulence-induced fading and, therefore, to improve the error rate performance, spatial diversity can be used over FSO links which involves the deployment of multiple laser transmitters/receivers. In this paper, we investigate the bit error rate (BER) performance of FSO links with spatial diversity over log- normal atmospheric turbulence fading channels, assuming both independent and correlated channels among transmitter/receiver apertures. Our analytical derivations build upon an approximation to the sum of correlated log-normal random variables. The derived BER expressions quantify the effect of spatial diversity and possible spatial correlations in a log-normal channel. [ABSTRACT FROM PUBLISHER]

We propose a wireless optical-mesh network based on optical code-division multiple-access (OCDMA) -coded free-space optical links. The performance of the proposed network under intermediate and strong turbulence channel conditions is invest gated for synchronous and asynchronous OCDMA. We show that synchronous OCDMA using complementary Walsh-Jladamard codes operating at a raw bit rate of 622 Mb/s per user can achieve a bit-error rate of io-9. This is achievable in channels with strong turbulence and high temporal correlation, or in channels with intermediate turbulence conditions; reasonable conditions based on channel characteristics inferred from the refractive index spectrum. For asynchronous OCDMA using optical orthogonal codes (OOCs), using a lower bound, we show that asynchronous OCDMA using OOCs cannot achieve acceptable performance for the same channel conditions, due to interference limitations. [ABSTRACT FROM AUTHOR]

With proper system design, infrared multispot diffusing (MSD) configuration communications links promise several orders of magnitude higher bit rates than radio links. Essential to the communications system are the optical subsystems: transmitter and receiver optics. Preliminary experiments on fabrication of beamshaping optical elements for the transmitter and receiver optical front-end have been conducted. The impact of optical subsystems on channel characteristics is investigated, and the results undoubtedly prove the great potential of MSDC. Use of holographic optical elements at both transmitter and receiver increases the signal-to-noise ratio by at least 11 dB and at the same time significantly improves the power budget of the system by reducing path loss by more than 6 dB. [ABSTRACT FROM PUBLISHER]