Harilaos G. Sandalidis

Optical wireless links with spatial diversity over strong atmospheric turbulence channels

Optical wireless, also known as free-space optics, has received much attention in recent years as a cost-effective, license-free and wide-bandwidth access technique for high data rates applications. The performance of free-space optical (FSO) communication, however, severely suffers from turbulence-induced fading caused by atmospheric conditions. Multiple laser transmitters and/or receivers can be placed at both ends to mitigate the turbulence fading and exploit the advantages of spatial diversity. Spatial diversity is particularly crucial for strong turbulence channels in which single-input single-output (SISO) link performs extremely poor. Atmospheric-induced strong turbulence fading in outdoor FSO systems can be modeled as a multiplicative random process which follows the K distribution. In this paper, we investigate the error rate performance of FSO systems for K-distributed atmospheric turbulence channels and discuss potential advantages of spatial diversity deployments at the transmitter and/or receiver. We further present efficient approximated closed-form expressions for the average bit-error rate (BER) of single-input multiple-output (SIMO) FSO systems. These analytical tools are reliable alternatives to time-consuming Monte Carlo simulation of FSO systems where BER targets as low as 10-9 are typically aimed to achieve.

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.

BER Performance of FSO Links over Strong Atmospheric Turbulence Channels with Pointing Errors

In this letter, we investigate the error rate performance of free-space optical (FSO) links over strong turbulence fading channels together with misalignment (pointing error) effects. First, we present a novel closed-form expression for the distribution of a stochastic FSO channel model which takes into account both atmospheric turbulence-induced fading and misalignment-induced fading. Then, we evaluate the average bit-error rate in closed form of a FSO system operating in this channel environment, assuming intensity modulation/direct detection with on-off keying. Numerical examples are further provided to collaborate on the derived analytical expressions.

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.

An efficient evolutionary algorithm for channel resource management in cellular mobile systems

Modern cellular mobile communications systems are characterized by a high degree of capacity. Consequently, they have to serve the maximum possible number of calls while the number of channels per cell is limited. The objective of channel allocation is to assign a required number of channels to each cell such that both efficient frequency spectrum utilization is provided and interference effects are minimized. Channel assignment is therefore an important operation of resource management and its efficient implementation increases the fidelity, capacity, and quality of service of cellular systems. Most channel allocation strategies are based on deterministic methods, however, which result in implementation complexity that is prohibitive for the traffic demand envisaged for the next generation of mobile systems. An efficient heuristic technique capable of handling channel allocation problems is introduced as an alternative. The method is called a combinatorial evolution strategy (CES) and belongs to the general heuristic optimization techniques known as evolutionary algorithms (EAs). Three alternative allocation schemes operating deterministically, namely the dynamic channel assignment (DCA), the hybrid channel assignment (HCA), and the borrowing channel assignment (BCA), are formulated as combinatorial optimization problems for which CES is applicable. Simulations for representative cellular models show the ability of this heuristic to yield sufficient solutions. These results will encourage the use of this method for the development of a heuristic channel allocation controller capable of coping with the traffic and spectrum management demands for the proper operation of the next generation of cellular systems.

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 (OOK), 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.

Inverse Gaussian Modeling of Turbulence-Induced Fading in Free-Space Optical Systems

We propose the inverse Gaussian distribution, as a less complex alternative to the classical log-normal model, to describe turbulence-induced fading in free-space optical (FSO) systems operating in weak turbulence conditions and/or in the presence of aperture averaging effects. By conducting goodness of fit tests, we define the range of values of the scintillation index for various multiple-input multiple-output (MIMO) FSO configurations, where the two distributions approximate each other with a certain significance level. Furthermore, the bit error rate performance of two typical MIMO FSO systems is investigated over the new turbulence model; an intensity-modulation/direct detection MIMO FSO system with Q-ary pulse position modulation that employs repetition coding at the transmitter and equal gain combining at the receiver, and a heterodyne MIMO FSO system with differential phase-shift keying and maximal ratio combining at the receiver. Finally, numerical results are presented that validate the theoretical analysis and provide useful insights into the implications of the model parameters on the overall system performance.

New results on turbulence modeling for free-space optical systems

In this paper, we propose a statistical channel model, named as Double-Weibull, to describe the irradiance fluctuations in moderate and strong turbulence for free-space optical (FSO) systems. The proposed stochastic model is based on the scintillation theory and derived via the product of two Weibull random variables. Closed-form expressions of probability and cumulative density functions are provided in terms of Meijers G-function. We also compare the new model with the classical gamma-gamma model and assess their accuracy via a set of simulations when both plane and spherical waves are considered. We finally evaluate the performance of an FSO system over the Double-Weibull turbulence channel and derive closed-form expressions for the bit-error rate, assuming intensity-modulation/direct detection with On-Off keying, and the outage probability.

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.

WiMAX on FSO: Outage Probability Analysis

The transmission of multiple wireless signals over optical links has attained a great research interest nowadays. In case where optical fibers are difficult to be deployed, or installation cost is prohibited, optical wireless systems provide an efficient alternative means. In this paper, we consider the WiMAX (IEEE802.16) standard and construct a simple but adequate scenario to investigate radio signal transmission over terrestrial optical wireless channels. An appropriate system architecture is adopted and a channel model, which entails some of the most critical impairments of the optical channel, i.e., attenuation, turbulence, pointing error effects, as well as of the RF channel, i.e., path loss, shadowing, and Rayleigh fading, is taken into account. The overall link budget and a closed-form of the outage probability of the system are deduced. Several analytical results are depicted using a realistic set of parameter values, to lend a helpful insight to the performance of the proposed architecture.