Optimization models for misalignment fading mitigation in optical wireless links

We consider a free-space optical (FSO) channel model affected by misalignment fading (pointing error) effects. Assuming intensity modulation/direct detection (IM/DD) with on-off keying BOOK), new closed form expressions for the bit-error rate (BER) and the outage probability are presented. Furthermore, four optimization models are formulated and solved taking into account various metrics such as the beamwidth, the electrical signal-to-noise ratio, the normalized jitter, the BER, and the outage probability. The results obtained can be a useful outcome for FSO system designers in order to limit pointing error effects and achieve, thus, an optimum performance

Performance Analysis of a Laser Ground-Station-to-Satellite Link With Modulated Gamma-Distributed Irradiance Fluctuations

The performance of a ground-station-to-space laser uplink with a Gaussian-beam wave model subject to turbulence and beam wander effects is the topic of the present study. The modulated gamma distribution is used to describe the combined effect of the above two deteriorating factors. At first, a versatile expression of the above probability density function is deduced. We then derive novel closed-form expressions for its cumulative distribution function and the moment-generating function. The scintillation index and the probability of fade are hence readily evaluated. The analysis is completed with the evaluation of the bit error rate assuming heterodyne detection with differential phase-shift keying. In order to attain an adequate error rate target, we incorporate diversity at the satellite receiver. A proper simulation scenario is adopted, and numerical results are provided to verify the accuracy of the derived expressions

Coded Free-Space Optical Links over Strong Turbulence and Misalignment Fading Channels

The performance of optical wireless systems deteriorates to a large extent from the presence of turbulence and pointing error effects. To meet the typical bit error rate (BER) targets for reliable communications within the practical ranges of signal-to-noise ratio, error control coding schemes are often proposed. This paper investigates the error performance for convolutional coded on-off keying free-space optical systems through symbol by symbol interleaved channels characterized by strong turbulence and/or pointing error effects. We consider several channel types and derive exact analytical expressions for the pairwise error probability. These expressions are applied to obtain upper bounds on the BER performance using the transfer function technique

Outage probability and ergodic capacity of free-space optical links over strong turbulence

The performance of a free-space optical (FSO) communication system under strong turbulence regime that follows the K distribution is evaluated. Some useful channel statistics are derived in closed form and the outage probability and ergodic capacity for a single-input single-output FSO link are evaluated. Numerical examples are further presented to verify the accuracy of the derived mathematical expressions

Connectivity Issues for Ultraviolet UV-C Networks

This paper focuses on the connectivity issues of a non-line-of-sight (NLOS) optical wireless network operating in the ultraviolet UV-C spectral region. NLOS UV-C transmitters have a limited effective coverage and, hence, a dense node distribution is required in order to efficiently cover a large geographical area. Under this assumption, the concept of connectivity is more than important since it provides a strong indication of the network reliability and robustness. In the present study, we consider transmission with on-off keying and pulse position modulation schemes assuming both Gaussian and Poisson noise and adopt an effective experimental path loss model. Then, we evaluate the k-connectivity properties in terms of several network parameters. More precisely, we present and analyze the trade-off between node density and the degree of k-connectivity against other parameters (i.e., transmitted power, supported data rate, and error probability). The derived results are depicted using appropriate figures and tables and constitute the theoretical basis for the design and implementation of a reliable UV-C network in practice

Node Isolation Probability for Serial Ultraviolet UV-C Multi-hop Networks

Non-line-of-sight optical wireless transmission, operated in the unlicensed ultraviolet UV-C band, has been recently suggested as an alternative means of communication. However, due to limited coverage, relayed UV-C networks need to be deployed in order to supply communication services at large distances. In this paper, we consider a serial multi-hop UV-C network where the nodes are distributed at fixed positions on a given service interval. We adopt a suitable path loss model and derive analytical expressions for the node isolation probability assuming on-off keying and pulse position modulation formats. Moreover, we investigate the node density required to achieve connectivity for several geometrical transceiver configurations. The numerical results of this paper are of significant value for telecom researchers working toward a flexible UV-C network deployment in practice

Underwater Optical Wireless Networks: A k-Connectivity Analysis

In this paper, optical wireless has been addressed as a promising technology to provide high-bandwidth services for underwater communications. However, the significant attenuation degree due to high absorption and scattering of optical transmission in the water confines the achievable range of optical links to only few meters. One way to achieve transmission at long distances is to employ a dense network configuration where information can be transferred through a series of intermediate nodes acting as relays. In this study, we consider optical wireless network arrangements where nodes are floating at different depths into a service aquatic medium. We deploy an effective path loss model which incorporates the key factors that deteriorate the optical power, and we derive the achievable transmission range to satisfy connectivity criteria assuming intensity-modulation direct detection (IM/DD) with ON-OFF keying (OOK). A set of numerical results is presented in order to reveal the interaction between various parameters such as error probability, wavelength, node density, transmitted power, data rate, etc., in order to achieve k-connectivity. The proposed analysis could be the basis of deploying reliable underwater optical networks suitable to deliver broadband services at far distances

Weather Effects on FSO Network Connectivity

The use of relays is one of the most promising methods for mitigating impairments of the performance of free-space optical (FSO) systems and extending their limited transmission range. However, several factors contribute to significant link performance degradation. Most severe is the influence of the adverse atmospheric conditions that frequently appear, thus making the design of strongly connected networks a demanding issue. In this paper, we consider a multiple-hop FSO network, where the nodes are distributed at fixed positions on a given path-link. We take account of the most critical weather phenomena, i.e., fog, rain, and snow, and derive analytical expressions for the node isolation probability, assuming a suitable path loss model. Next, we find the number of transceivers for a given path-link in order to achieve reliable performance. We also examine the reverse case; i.e., we find the total service length for a known number of FSO transceivers. The effect of the prime FSO modulation formats is also considered. The addressed analytical framework offers significant insights into the main factors that degrade the performance of FSO networks. It constitutes a valuable tool for telecom researchers to design such networks in practice

Optical Wireless Communications With Heterodyne Detection Over Turbulence Channels With Pointing Errors

We study the error performance of an heterodyne differential phase-shift keying (DPSK) optical wireless (OW) communication system operating under various intensity fluctuations conditions. Specifically, it is assumed that the propagating signal suffers from the combined effects of atmospheric turbulence-induced fading, misalignment fading (i.e., pointing errors) and path-loss. Novel closed-form expressions for the statistics of the random attenuation of the propagation channel are derived and the bit-error rate (BER) performance is investigated for all the above fading effects. Numerical results are provided to evaluate the error performance of OW systems with the presence of atmospheric turbulence and/or misalignment. Moreover, nonlinear optimization is also considered to find the optimum beamwidth that achieves the minimum BER for a given signal-to-noise ratio value