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In some emerging satellite applications, it is of paramount importance to accurately measure the distance between satellites in formations. Range measurement systems based on femtosecond lasers have been suggested as very precise solutions that could be incorporated into satellites. It would be fortuitous to exploit the same laser link for both the distance measurement and communication, thus achieving two functions with considerably reduced payload weight and size compared with that could be achieved if the two tasks were performed separately. In this letter, we propose to modulate the ranging femtosecond pulses to carry communication information. The proposed modulation is in the frequency domain, benefiting from the wideband property of femtosecond pulses. The method hinges on the special characteristics of the femtosecond pulse, which has a very broad frequency spectrum alongside its ultrashort time duration. We analyze the multiple-inverse-pulse-position modulation (MIPPM) method. The main idea of the MIPPM is the ON–OFF modulation of each spectral component, while each modulated symbol includes all spectral components and only few spectral components are in the OFF state at any given time. We analyzed the performance of the proposed system in terms of symbol error rate for a given set of parameters.

Security is an increasingly important issue in modern communication. We propose a concept for secure detection of vortex beams in free space optics (FSO). The transmitted signal is composed of a Gaussian beam together with a low powered vortex beam. For each vortex mode, we select the detectors with the maximum received power for the particular signal, depending on the spatial distribution probability of the signal beam. Eavesdroppers have no way of knowing which patterns are relevant for the transmitted signal. We perform a numerical calculation for maximal signal detection and security. Applications include l free space communication.

Over the last few years, there has been an exponential increase in the amount of communication network traffic, where the data center (DC) is a major building block of this network. However current DCs face various problems in the light of current demands, such as high power consumption, low scalability and low flexibility. It is necessary to build a new high speed data center which could support this exponential growth. One of the technologies that could scale up the performance of the data center is free space optical (FSO) communication. FSO communication could provide an adaptive, flexible and dynamic network that could meet the performance requirements of future DCs. However, no one has characterized the optical communication channel in DC. In DC there is an HVAC system that causes non-homogeneous changes in temperature and air velocity that can affect the performance of the optical signal. In this work, we demonstrate that by using deep learning algorithms for channel estimation and signal detection, without knowledge of the channel model, we can improve the signal detection and increase the performance of the optical communication in DC environment.

In the last decade data centers have become a crucial element in modern human society. However, to keep pace with internet data rate growth, new technologies supporting data center should develop. Integration of optical wireless communication (OWC) in data centers is one of the proposed technologies as augmented technology to the fiber network. One implementation of the OWC technology is deployment of optical wireless transceiver on top of the existing cable/fiber network as extension to the top of rack (TOR) switch; in this way, a dynamic and flexible network is created. Optical wireless communication could reduce energy consumption, increase the data rate, reduce the communication latency, increase flexibility and scalability, and reduce maintenance time and cost, in comparison to extra fiber network deployment. In this paper we review up to date literature in the field, propose an implementation scheme of OWC network, discuss ways to reduce energy consumption by parallel link communication and report preliminary measurement result of university data center environment.

This paper discusses the most relevant aspects of the practical implementation of a long-range Quantum Key Distribution (QKD) link with trusted nodes, achieving the highest possible secret key rate generation within the security and system level constraints. To this purpose, the implementation of an end-to-end QKD system will be discussed, including implementation aspects from physical transmission of photon states through a standard telecommunications grade optical fiber, to consideration of device imperfections, information reconciliation protocols. In addition, since there are circumstances when a fiber optical link may not be available, we will also discuss a test bench implementation of a Free Space Optics (FSO) QKD link. Furthermore, in spite of the fact that Discrete Variable QKD (DV-QKD) systems have reached a maturity level that allows their potential full realization and implementation for creation of a secure network backbone for key distribution in nations, in realistic links DV-QKD is really limited by technology and physical constraints associated with construction of reliable high rate single photon (or at least low photon count) sources, and of fast and reliable single photon detectors with very low dark count rates. In these cases, the use of Continuous Variable QKD (CV-QKD) schemes may be advantageous. For this reason the paper also discusses the problem of information reconciliation in CVQKD scenarios, showing that in long distance links the sign of the received Gaussian samples contains the largest fraction of information, leading to the design of an Unequal Error Protection (UEP) reverse reconciliation scheme.

Optical wireless communication (OWC) systems provide many advantages over radio frequency (RF) wireless technologies in some scenarios, including significantly higher data rates and a large amount of available license-free frequency spectrum. Recently, OWC has also been proposed in fifth generation 5G standard as a tool to augment capacity due to RF spectrum crunching challenges. Various forms of OWC can be used to augment RF capacity at both access and backhaul/fronthaul levels. Despite the major advantages and various application areas, the widespread deployment of OWC is delayed by several challenges, such as the demand to maintain strict line-of-sight alignment between transmitter and receiver in long range outdoor applications; the need to combat attenuation due to adverse weather conditions such as fog, cloud, and turbulence; to modulate light emitting diodes at high frequencies without distortion at indoor visible light communication (VLC) applications and retaining power levels within the eye safety limits for laser transmitters and comfortable illumination levels for LED transmitters.

Due to copyright restrictions, the access to the full text of this article is only available via subscription. New data services and applications are emerging continuously and enhancing the mobile broadband experience. The ability to cope with these varied and sophisticated services and applications will be a key success factor for the highly demanding future network infrastructure. One such technology that could help address the problem would be optical wireless communications (OWC), which presents a growing research interest in the last few years for indoor and outdoor applications. This paper is an overview of the OWC systems focusing on visible light communications, free space optics, transcutaneous OWC, underwater OWC, and optical scattering communications. European Commission ; National Key Basic Research Program of China ; National Natural Science Foundation of China ; Shenzhen Peacock Plan

The growing demand for high interconnection speed in next-generation computers is driving the technology shift for communication from the electronic to the optic domain. One of the favored interconnection technologies for this task is the free-space optical interconnect (FSOI). FSOI technology uses laser links between computer components and provides a lower bound on propagation delay due to the low index of refraction of air, when compared with the indexes common in waveguide technologies. FSOIs based on dc-biased optical orthogonal frequency-division multiplexing (DCO-OFDM) may provide excellent data throughput in intensity modulation/direct detection systems. However, the main drawback limiting the implementation of FSOIs is the inevitable tradeoff between interconnection density and the crosstalk level, resulting from the diffraction effect and from optical misalignment. The purpose of this paper is to promote improved interconnection density of such FSOIs by use of inherent DCO-OFDM resource allocation capabilities. The crosstalk-resulted interference was formulated as joint multilink bit-and-power allocation optimization. The theoretical analysis reveals general guidelines for dense FSOI. Further, a reduced-complexity numerical suboptimal algorithm for joint multilink bit-and-power allocation was proposed. The simulation results show that the proposed suboptimal algorithm outcome is close to the theoretical optimal performance.

Confining light to nanoscale dimensions has become possible with surface plasmons. However, active control of plasmonic responses remains a hurdle for building plasmonic optical devices. In this letter, we analytically derive a model describing the gap between the local surface plasmon (LSP) modes of a spherical nanoparticle in anisotropic media as a function of applied external electric field. Anisotropic ferroelectric materials exhibit birefringence that can be controlled through the electro-optic effect. Hence, the splitting of LSP frequencies in ferroelectrics embedded with nanoparticles can be controlled. In other words, by applying an external voltage a transition between the anisotropic and the isotropic phases can be achieved. As a result, the gap created by anisotropy can be tuned by the applied voltage. We derive and provide an explanation to the required field intensity according to the coercive field of ferroelectric materials. This simple approach is the basis for a method to actively tune the amplitude and the polarizability of light. The method can be considered for photonic applications, such as optical switches and biomedical sensors.

There is a growing need for accurate, high-resolution intersatellite ranging, for instance for gravitational mapping, which has led to extensive research on optical methods that can augment or replace existing spaceborne microwave techniques. In this letter, we outline a top-level design concept for intersatellite ranging based on measurements using a homodyne Mach–Zehnder interferometer housed on one satellite. The phase degradation in the reference arm of the interferometer on the local satellite is maintained at a minimal level to yield useful interference with the measuring arm that traverses hundreds of kilometers to the remote satellite and back. In this preliminary study, the feasibility of the concept in terms of link budget and noise level is validated. Ranging resolution is expected to be similar to the current heterodyne state-of-the-art.

In recent years, an increasing number of data center networks (DCNs) have been built to provide various cloud applications. Major challenges in the design of next generation DC networks include reduction of the energy consumption, high flexibility and scalability, high data rates, minimum latency and high cyber security. Use of optical wireless communication (OWC) to augment the DC network could help to confront some of these challenges. In this paper we present an OWC multi channels communication method that could lead to significant energy reduction of the communication equipment. The method is to convert a high speed serial data stream to many slower and parallel streams and vies versa at the receiver. We implement this concept of multi channels using optical orthogonal frequency division multiplexing (O-OFDM) method. In our scheme, we use asymmetrically clipped optical OFDM (ACO-OFDM). Our results show that the realization of multi channels OFDM (ACO-OFDM) methods reduces the total energy consumption exponentially, as the number of channels transmitted through them rises.

A data center (DC) is a facility either physical or virtual, for running applications, searching, storage, management and dissemination of information known as cloud computing, which consume a huge amount of energy. A DC includes thousands of servers, communication and storage equipment and a support system including an air conditioning system, security, monitoring equipment and electricity regulator units. Data center operators face the challenges of meeting exponentially increasing demands for network bandwidth without unreasonable increases in operation and infrastructure cost. In order to meet the requirements of moderate increase in operation and infrastructure cost technology, a revolution is required. One way to overcome the shortcomings of traditional static (wired) data center architectures is use of a hybrid network based on fiber and optical wireless communication (OWC) or free space optics (FSO). The OWC link could be deployed on top of the existing cable/fiber network layer, so that live migration could be done easily and dynamically. In that case the network topology is flexible and adapts quickly to changes in traffic, heat distribution, power consumption and characteristics of the applications. In addition, OWC could provide an easy way to maintain and scale up data centers. As a result total cost of ownership could be reduced and the return on investment could be increased. In this talk we will review the main OWC technologies applicable for data centers, indicate how energy could be saved using OWC multichannel communication and discuss the issue of OWC pointing accuracy for data center scenario.

Data centers are a key building block in the rapidly growing area of internet technology. A typical data center has tens of thousands of servers, and communication between them must be flexible and robust. Vortex light beams have orbital angular momentum and can provide a useful and flexible method for optical wireless communication in data centers. Vortex beams can be generated with orbital angular momentum but independent of polarization, and used in a multiplexed system. We propose a multiplexing vortex system to increase the communication capacity using optical wireless communication for data center networks. We then evaluate performance. This paper is intended for use as an engineering guideline for design of vortex multiplexing in data center applications.

A data center (DC) is one of the main building blocks of modern information technology. Any intra DC network should strive to achieve high throughput, while continuing to be flexible in responding to changing traffic patterns. Optical wireless communication links on top of the fiber/wire infrastructure could offer flexible and dynamic reconfiguration of the network. In this paper we present the design of an optical wireless communication orbital angular momentum (OAM) receiver with detector array. The proposed receiver is described in detail, a numerical example is given, and alternate options for its construction are explored. For a given set of parameters we found that a three ring detector with optimized ring area is comparable to seven ring evenly spaced detector.

Data centers collect and process information with a capacity that has been increasing from year to year at an almost exponential pace, while many datacenter applications are provided at no cost. This faces datacenter operators with the challenges of meeting exponentially increasing demands for network bandwidth without unreasonable increases in operation and infrastructure cost. In order to meet the requirements of moderate increase in operation and infrastructure cost new technologies are desired. Optical wireless technology could a) reduce the power consumption, b) increase the flexibility and scalability and c) reduce the network overload.. The OWC link could be deployed on top of the existing cable/fiber network layer, such that live migration could be done easily and dynamically and network topology is flexible and adapts quickly to changes in traffic, heat distribution, power consumption and characteristics of the applications. In addition, OWC could provide an easy way to maintains and scale up data centers which would reduce total cost of ownership and increase the return on investment. In this talk we will review the main OWC technologies, algorithms concepts and configurations which improve the performance of next generation data centers.

This research comprises an end-to-end experimental comparison of the bit-rates and bit error-rates of four major adaptive bit-and-power allocation algorithms for dc biased optical orthogonal frequency-division multiplexing. The comparison includes studying different channel conditions and different numbers of subcarriers. The experimental results show that all the methods compared display similar performance without significant superiority of a single method, even though in theory some difference is expected. Our analysis of the experimental data showed that the main reason for the uniformity in performance is the high variance in the channel estimation, especially at higher frequency subcarriers.

Discrete multi-tone (DMT) modulation is known to be an efficient single-transmitter technique for visible-light communication. However, the use of this technique in a multiple transmitter environment requires effective subcarrier and power allocation design in order to exploit the full potential of spatial multiple-transmitter diversity. Spatial reuse of the subcarriers in the presence of interference and power constraints increases the efficiency of multiple access (MA) DMT communication. In this paper, we propose an algorithm that manages interference-constrained subcarrier reuse between different transmitters and power redistribution between different subcarriers in a heuristic manner. The algorithm simulation shows an improvement in the average bit-rate as compared with a conventional DMT method. Furthermore, the effectiveness of the proposed MA-DMT scheme increases with the number of users.

Intelligent transport systems have become the major technology for increasing road safety and traffic efficiency, as well as for reducing the number of road casualties. Radio frequency and optical wireless communication technologies have been proposed as a means for establishing communication between vehicular and road infrastructure, such as traffic lights, billboards and road infrastructures and for providing inter-vehicular communication. These technologies provide one-way or two-way short-range to medium-range wireless communication links that are specifically designed for the automotive sphere. The technology that is analysed in this paper is optical wireless communication. This technology modulates optical data on top of the illumination in the traffic lights and in the automotive lighting and detects the information using fast imaging sensors. Using knowledge of the amplitudes of the signal and the noise in each pixel, we tune adaptively and separately the gain of each individual pixel in the imaging sensor for communication signals. Tuning the gain is based on the mathematical model derived in this research. Comparison is made between the adaptive model and the standard model. From the mathematical analysis and the results of the comparison, it is clear that this model significantly improves communication system performance. In our numerical calculation for a given set of parameters, the signal-to-noise ratio of the standard system and the adaptive system changes from more than 7×10-4 to more than 7 and less than 2×10-3 to more than 30 for a change in transmitter power from 10 to 100watts respectively. Copyright © 2014 John Wiley & Sons, Ltd.

In recent years, several space systems consisting of multiple satellites flying in close formation have been proposed for various purposes such as interferometric synthetic aperture radar measurement (TerraSAR-X and the TanDEM-X), detecting extra-solar earth-like planets (Terrestrial Planet Finder-TPF and Darwin), and demonstrating distributed space systems (DARPA F6 project). Another important purpose, which is the concern of this paper, is for improving radio frequency communication to mobile terrestrial and maritime subscribers. In this case, radio frequency signals from several satellites coherently combine such that the received/transmit signal strength is increased proportionally with the number of satellites in the formation. This increase in signal strength allows to enhance the communication data rate and/or to reduce energy consumption and the antenna size of terrestrial mobile users' equipment. However, a coherent combination of signals without aligning the phases of the individual communication signals interrupts the communication and outage link between the satellites and the user. The accuracy of the phase estimation is a function of the inter-satellite laser ranging system performance. This paper derives an outage probability model of a coherent combination communication system as a function of the pointing vibration and jitter statistics of an inter-satellite laser ranging system tool. The coherent combination probability model, which could be used to improve the communication to mobile subscribers in air, sea and ground is the main importance of this work.

In this paper, the free-space optics (FSO) quantum backscatter communication (QBC), inspired by the quantum illumination concept, is described. In this method, the transmitter generates entangled photon pairs. The signal photon is transmitted in the direction of the modulated retro reflector (MRR) or the tag antenna, and the idler photon is moved to the receiver. The MRR communicates by modulating the received photon and retro-reflecting it back to the receiver. QBC could provide performance improvement in comparison to conventional backscatter communications. In this work, a mathematical model of the FSO QBC systems is derived. In an FSO system, the pointing direction jitter is a stochastic process that reduces the communication performance. In this paper, optimization of FSO QBC performance is proposed to minimize the error probability. This could be done by adapting the telescope gain to jitter a standard deviation amplitude.

Data Center (DC) servers consume a huge amount of energy. However, a part of this energy (cooling and information technology devices) can be saved by shutting down racks containing idle servers. This can be achieved by rerouting workloads from partially loaded racks, and thus allowing additional racks to switch into idle mode. Workload rerouting requires additional wired links or by optical wireless communication (OWC) links, which can be deployed on top the existing network. In comparison to classic wired infrastructure, OWC deployment and maintenance is faster and simpler, and it offers dynamic configuration of the network. Moreover, OWC can be used to augment the performance of virtualization techniques, which allow load balancing. In this paper we developed a mathematical model and an optimization algorithm along with numerical simulations, which evaluate the energy consumption in DCs. Results show that for a uniform distributed load between 24–100% and a 100% deployment of OWC links, 33% of the IT and cooling energy consumed can be saved.

Visible light communication (VLC) is an emerging communication technology for short-range applications, especially indoors. This technology uses light, emitted from super-illuminating semiconductors for the primary purpose of illumination, to carry information in addition. One of the major challenges faced in implementing VLC technology is to find a way to achieve communication without affecting the illumination. In this paper, we describe an inverse pulse position modulation method that encapsulates communication data in the time domain without affecting the illumination property, which is conveyed in the amplitude domain. Due to the fact that this modulation method requires accurate synchronization, we have studied the effect of clock time shift and jitter on the system bit-error-rate performance and have derived models that could be used to define the minimum requirements of the synchronization system.

Data centers have become the major technology for collecting, storing, and processing information. The latency requirement for transferring information between servers has become more stringent from year to year as the speed of the new generation of servers increases at an almost exponential pace. In this paper, we propose a new method to compress switching information over data sequence packets using temporal cloaking. The main advantages of this method in comparison to electronic switching or the use of microelectromechanical system technology are that the compression and removal of the switching information are done entirely in the optical domain without increasing the packet duration time, changing or adding overhead bits, reducing the signal-to-noise ratio, or distorting the polarization or the phase of the signal. Only a small delay is added to the signal due to the cloaking process. This method requires tight synchronization; to address this issue, a mathematical model that describes the effect of clock jitter on the system bit error rate performance is derived in this paper. The same concept could also be used to increase the capacity of legacy communication systems as well as in coherent digital communication or analog communication, e.g., radio over fiber.

Data centers have become the major technology for collecting and processing information. The data-rates required for transferring the information through the server's backplane has been increasing from year to year at an almost exponential pace. Therefore a technology-shift from the electronic to the optic domain has already been initiated in order to meet the required data-rate demands for next-generation servers. The future technology that is analyzed in the paper is free space optics (FSO). FSO technology uses laser links between the server cards and the backplane and provides a lower bound on propagation delay due to the low index of refraction of air, when compared with waveguide or fiber technologies. However, turbulence occurs due to the high temperature of the chips and to ventilation inside the server, which results in deterioration of the communication performance. In this paper, we examine the feasibility of wavelength diversity as a means to mitigate the effect of turbulence on the server communication link performance. It is shown that using the diversity scheme cuts the outage probability by half for the same total transmitter power by comparison with the single wavelength scheme. Therefore, the power consumption (and operational costs) of the data center could be reduced dramatically if a wavelength diversity scheme were adopted. The authors believe that this is the first time that a laboratory experiment has been performed modeling a server backplane with FSO communication using wavelength diversity.

Over recent decades, there have been increasing demands for high data-rate backplanes for enhanced performance in computers. The present technology of electronic backplanes cannot meet the required data-rate demands for next-generation computers, indicating a need for a technology shift from the electronic to the optic domain. In this paper, we develop a mathematical model to describe the effect of air turbulence on a short range free space optical communication link, such as would be found in computer backplane communication, and conduct experiments to validate the model. The air turbulence under discussion resembles that which would result from the high temperature of chips on the board together with the ventilation of the air by the chip fan. In our experiment, the communication performance, expressed as bit error rate (BER), is presented as a function of the location of the turbulence source and the log amplitude variance. The log amplitude variance was evaluated by two independent methods. One of the main results of this study is the indication that the performance of the backplane deteriorates even for very small values of air turbulence strength. This is exacerbated by the fact that extremely low BER performance is required at the backplane (e.g., from 10-14 to 10- 18 ). We also demonstrate that increasing the distance between the turbulence source and the optical link reduces the influence of the air turbulence in an exponential manner. This fact is important for the design of future-generation optical backplanes. It is noted that operation at wavelengths around 1550 nm yields communication performance that is less severely degraded by air turbulence effects than at 670 nm.

Energy-efficiency is an essential feature of wireless sensor networks (WSNs) where the longevity of autonomous sensor nodes is limited by their battery life and/or energy-harvesting capability. Base-station-initiated optical wireless communication with nodes equipped with a passive optical transmitter in the form of a corner cube retroreflector (CCR) provides sensor acquisition with no energy expenditure on the part of the sensor node itself and is therefore an attractive option for WSN. However, the return signal from an illuminated sensor node is a stochastic variable dependant on fabrication parameters, ambient conditions and receiver noise so that the sensor acquisition process is inherently error-prone. In this paper we propose an energy-aware, base station-initiated interrogation scheme based on exponentially increasing beam scan areas, that takes into consideration the error-prone trait of CCR-outfitted sensor nodes. We analyse the scheme performance subject to different values of signal variance and various cost functions. We extend the analysis to address the circumstance of a spatially-limited sensor-failure event, such as may be caused by deliberate tampering or by environmental factors. We show that agile beam-steering on the basis of accrued knowledge of contaminated sensor distributions promotes energy-conserving acquisition. The validity of a Poisson spatial distribution model for the sensor dispersion is discussed and the impact of this initial assumption on acquisition error is demonstrated.

Quantum key distribution (QKD) is a method for exchanging keys between two parties (Alice and Bob) with an absolute level of security in the ideal scenario. QKD systems are vulnerable to hacking through secondary emissions of photons by SPAD detectors, known as backflash. In this work, we derive an information leakage probability model for the scenario of QKD in a free space communication (FSO) system, and suggest a method for its minimization.

Multiple-pulse position modulation (MPPM) is an advanced modulation method for optical wireless communication (OWC), which could provide better performance when channel information is not available. We consider the evaluation of a symbol error rate (SER) expression for MPPM applied within OWC. In the proposed model, MPPM is realized by spectral-amplitude coding (SAC) with non-uniform spectral slot energies. The derived fading channel SER expression is applicable for an arbitrary fading distribution, which may be due to either atmospheric turbulence or transceiver pointing error. A detailed theoretical analysis of the SER is provided and the resulting SER expression is verified by simulation.

In this paper we seek a solution for robust broadband communication using high altitude platforms (HAPs). We address the requirements of the backhaul link and propose a hybrid millimeter-wave/optical solution to reduce weather-induced-outage probability and enhance throughput. We explore the concept of analogue-modulated wireless laser communication based on orthogonal frequency-division multiplexing (OFDM) in order to achieve a compact transparent payload onboard the HAP. Laser nonlinearity effects and the adverse effects of turbulence in the optical communication channel are identified as the major challenges to successful implementation of this solution and are analyzed. The dependence of normalized symbol error rate (SER) on signal power and turbulence strength is demonstrated for two modulation schemes. We show the severe performance degradation due to laser nonlinearity resulting from increasing signal power in sharp contrast to SER improvement with signal power when nonlinear distortion is not present.

We present the theory of a direct detection receiver for vortex beams for an optical wireless communication system. The proposed receiver has an array of annular detectors, which enables analysis of power distribution of the vortex beam. We give a detailed description, numerical optimization, and different options for its design. One possible application of this receiver could be high security communication systems, and another could be intra data center communication. Using a given set of parameters, we find that a seven-ring symmetrically spaced detector is comparable to a three-ring detector with an optimized ring area.

Optical wireless communication (OWC) systems provide many advantages over radio frequency (RF) wireless technologies in some scenarios, including significantly higher data rates and a large amount of available license-free frequency spectrum. Recently, OWC has also been proposed in fifth generation 5G standard as a tool to augment capacity due to RF spectrum crunching challenges. Various forms of OWC can be used to augment RF capacity at both access and backhaul/fronthaul levels. Despite the major advantages and various application areas, the widespread deployment of OWC is delayed by several challenges, such as the demand to maintain strict line-of-sight alignment between transmitter and receiver in long range outdoor applications; the need to combat attenuation due to adverse weather conditions such as fog, cloud, and turbulence; to modulate light emitting diodes at high frequencies without distortion at indoor visible light communication (VLC) applications and retaining power levels within the eye safety limits for laser transmitters and comfortable illumination levels for LED transmitters.

In this paper we address the task of locating and mapping sensor nodes equipped with passive optical transmitters deployed within a wireless sensor network. The passive transmitter is in the form of a modulatable corner cube retro-reflector (CCR) and is interrogated by a base station equipped with a variable sized laser beam prior to data harvesting using optical wireless communication. We propose and discuss an efficient acquisition algorithm based on the binary search concept and evaluate its performance in locating a solitary sensor node. We demonstrate the use of an analytical tool for assessing the acquisition cost for different error probability distributions and search regimes. The common line-of-sight (LOS) requirement for both sensor location and data harvesting renders the need for individual sensor node identification redundant and thus yields a simplified search regime by comparison with previously published research on node localisation.

We develop an adaptive power control algorithm to facilitate satellite to ground laser communication through turbulence fading channels. Adaptive control is a powerful tool that improves energetic gain of the satellite laser. We use discrete laser transmitter intensity levels in our control algorithm. A novel recursive technique defines optimal laser transmitting intensity levels so that the laser-transmitted power is adapted according to instant changes of the signal-to-noise ratio caused by channel fading. The algorithm's performance is first investigated in a general form suitable for any channel fading statistics. Then it is specified for a typical lognormal fading channel. The results indicate an improvement of up to 10dB in energetic gain. Copyright © 2007 John Wiley & Sons, Ltd.

Nowadays, there is a renaissance in the field of space exploration. Current and future missions depend on astronauts and a swarm of robots for reconnaissance. In order to reduce the power consumption, weight, and size of the robots, an asymmetric communication system may be used. This is achieved by installing modulating retroreflectors (MRRs) on one side of the link and an interrogating laser on the other side. In this paper, we theoretically study an innovative device that can serve as an MRR in the infrared range of the spectrum. The device is based on a ferroelectric PZT thin film containing TiOsub2/subcoated Ag nanoparticles, which exhibit strong plasmonic resonance in the infrared range. After intensive analyses, which included calculations and simulations, we were able to design the device to operate at the 1550 nm wavelength. This is of great importance since the design of devices operating at 1550 nm as this wavelength is a mature technology widely used in free-space optics. Hence, this MRR can serve in asymmetric communication links relying on 1550 nm transmissions, which are also eye-safe. To the best of our knowledge, this is the first time coated metal nanoparticles have been proposed to modulate light in the infrared region. The performance of this device is unique, reaching a 17.5 dB modulation contrast with only a ±2 V operating voltage. This modulator may also be used for terrestrial communication such as fiber optics and optical interconnects in future data centers.

Data centers collect and process information with a capacity that has been increasing from year to year at an almost exponential pace. Traditional fiber/cable data center network interconnections suffer from bandwidth overload, as well as flexibility and scalability issues. Therefore, a technology-shift from the fiber and cable to wireless has already been initiated in order to meet the required data-rate, flexibility and scalability demands for next-generation data center network interconnects. In addition, the shift to wireless reduces the volume allocated to the cabling/fiber and increases the cooling efficiency. Optical wireless communication (OWC), or free space optics (FSO), is one of the most effective wireless technologies that could be used in future data centers and could provide ultra-high capacity, very high cyber security and minimum latency, due to the low index of refraction of air in comparison to fiber technologies. In this paper we review the main concepts and configurations for next generation OWC for data centers. Two families of technologies are reviewed: the first technology regards interconnects between rack units in the same rack and the second technology regards the data center network that connects the server top of rack (TOR) to the switch. A comparison between different network technologies is presented.

Visible light communication (VLC) is an evolving communication technology for short-range applications. Exploiting recent advances in the development of high-power visible-light emitting LEDs, VLC offers an energy-efficient, clean alternative to RF technology, enabling the development of optical wireless communication systems that make use of existing lighting infrastructure. Drawing on the expertise of leading researchers from across the world, this concise book sets out the theoretical principles of VLC, and outlines key applications of this cutting-edge technology. Providing insight into modulation techniques, positioning and communication, synchronisation, and industry standards, as well as techniques for improving network performance, this is an invaluable resource for graduate students and researchers in the fields of visible light communication, optical wireless communication, and industrial practitioners in the field of telecommunications.

We propose an indoor wireless infrared downlink scheme for high-data-rate multiuser connectivity with diffuse channels. The scheme is based on synchronous code-division multiple access with unipolar Hadamard codes. The orthogonality of unipolar Hadamard codes enables multiuser operation with relatively short codes. Thus, practical downlink rates of tens of Mb/s for each user can be obtained. However, multipath reflections in diffuse channels cause strong multipath dispersion and, consequently, severe distortion. This distortion becomes even more severe in a multiuser environment, as the dispersed incoherent infrared radiation of all users aggregates together. To mitigate this distortion, we use a novel adaptive multilevel serial composite decision feedback and feedforward equalizer. We investigate the system's performance with the proposed equalizer, and compare it with the performance of the same system, both composite decision-feedback and feedforward equalizers, and with a conventional decision-feedback equalizer (DFE). Our results show that the proposed scheme enables a high-data-rate multiaccess link and eliminates most of the multiuser distortion. Furthermore, it improves system performance in a multiaccess environment, as compared with the other composite equalizers and DFE for the same complexity. We also compare other coding schemes, and show that Hadamard codes are on top of the other codes

Quantum key distribution (QKD) systems for use in future free-space optical communication links restrict communication to a low bit rate. In this paper, increasing the QKD bit rate using a communication system that includes a multiple-input-multiple-output quantum key distribution (MQKD) is proposed. Such a system can enable a number of receivers to communicate simultaneously with a number of transmitter elements, which consequently increases the overall QKD bit rate. However, due to scattering and turbulence in the atmospheric channel, this method may introduce interference effects when a photon transmitted from one transmitter element reaches the wrong detector element in the receiver. These interference effects reduce the system bit rate and increase the quantum bit error rate. A novel mathematical model for analyzing the effect of crosstalk and interference on the MQKD system performance is derived. The results of the analysis indicate that atmospheric effects impair performance. In order to mitigate the atmospheric effect, using several wavelengths simultaneously is suggested. Criteria to define the number of wavelengths that are required to achieve a given performance are also provided

The development of optical wireless communication systems is accelerating as a cost-effective alternative to fibre optic links and wireless technologies. The optical wireless technology is used mostly in wide bandwidth data transmission applications. This technology can also be useful for cellular applications. The evolution of cellular networks, for increased coverage and capacity and for the evolution towards 3G, leads towards the necessity of increasing the number of base stations. The idea of using wireless optical links to implement additional base stations is introduced. A detailed analysis and measured results of optical wireless links utilised to transmit cellular signals are presented.

The crosstalk effect of aerosol backscatter on the performance of a wavelength-division-multiplexed (WDM) optical wireless communication (OWC) system is investigated, analyzed, and quantified. An OWC link could be a segment within a metropolitan area network (MAN) or a ground-station-to-space link of a satellite communication system. In these cases, a WDM transmitter and receiver are housed in one transceiver unit with parallel, or near-parallel, optic axes. The crosstalk at the receiver is caused by light from the transmitted signal of the same transceiver, which has been backscattered by molecules and aerosols in the atmosphere. This is exacerbated in the presence of fog and haze, in which case both the desired signal from another transceiver is attenuated by scattering and the backscatter-induced crosstalk increases. A bit-error-rate (BER) model is derived that takes into consideration the dominant noise sources, including backscatter-induced crosstalk and signal mixing with amplified stimulated emission (ASE) from an optical preamplifier at the receiver. The numerical calculations in this paper indicate that, in moderate fog, the BER may increase by an order of magnitude or more due to backscatter, depending upon the atmospheric extinction coefficient.

Free-space optical communication between satellites networked together can enable a high data rate between the satellites. Coherence multiplexing (CM) is an attractive technique for satellite networking due to its ability to cope with the asynchronous nature of communication traffic and the dynamic changes taking place in the satellite constellation. The use of optical radiation for intersatellite links creates very narrow beam divergence angles. Due to the narrow beam divergence angle, the vibration of the pointing system, the movement of the satellite, and the large distance between them the pointing from one satellite to another is a complicated task. The vibration of the pointing system is caused by two stochastic fundamental mechanisms: 1) tracking noises created by the electrooptic tracker and 2) vibrations created by internal satellite mechanical mechanisms and external environments. We derive mathematical models of signal, noise, the approximate signal-to-noise ratio, and the approximate bit-error rates of optical communication satellite networks as functions of the system's parameters, the number of satellites, and the vibration amplitude for frequency-shift keying coherence multiplexing (FSK-CM). Based on these models, we can calculate the negative impact of both the number of satellites and the optical terminal vibration on the system's performance.

Urban optical wireless communication (UOWC) is rapidly gaining popularity as an effective means of transferring data at high rates over short distances. The UOWC terminal includes an optical transmitter and a receiver positioned, for example, on high-rise buildings separated by several hundred meters. Light beams propagating through the atmosphere carry the information from the transmitter to the receiver. UOWC boasts many advantages over its rivals. Notably, UOWC facilitates rapidly deployable, lightweight, high-capacity communication without licensing fees and tariffs. However, UOWC still faces many challenges, including how to improve communication performance in adverse weather conditions or during building sway. We present and evaluate some of the exciting new research approaches that have been suggested to deal with these issues, including optimization of telescope gain, new technologies for pointing systems, and solutions at the network level.

Urban optical wireless communication systems are considered a "last mile" technology. An optical wireless communication system uses the atmosphere as a propagation medium. In order to provide line-of-sight (LOS), the transceivers are placed on high-rise buildings. However, dynamic wind loads, thermal expansion, and weak earthquakes cause buildings to sway. These sways require the designer to increase the transmitter beam divergence angle so as to maintain LOS between the transmitter and the receiver. It is clear that an overly wide divergence angle increases the required laser power, and, as a result, terminal cost and complexity increase. On the other hand, an overly narrow beam divergence angle may result in cutoff in communication when there is building sway. In this paper, we derive a mathematical model to minimize transmitter power and optimize transmitter gain (divergence angle) as a function of the building-sway statistics, the communication system parameters, and the required bit-error probability (BEP). Reduction in laser power could improve overall system performances and cost. For example, for BEP of 10/sup -9/, we can attain at least a 4-dB reduction of the required transmitter power in comparison to a system with both half and twice the optimum beam divergence angle.

This work analyzes laser communication between a cluster of nanosatellites, which is a concentrated formation of small lightweight satellites and a ground station. The scenario under consideration is a cluster of nanosatellites communicating by means of a laser beam with a detector array receiver that is located on the earth's surface and equipped with a common optical system for all incoming beams. The beams are concentrated to spots over the detector plane by the receiver's optics. The detector array enables the ground station to communicate with a tight concentration of the nanosatellites, which reduces system complexity and cost. A critical parameter that determines the successful receipt and subsequent decoding of a transmitted signal for a given configuration is the angular separation between the satellites within the cluster. This separation must be retained to prevent critical overlapping of the spots on the detector's surface. The maximum allowable overlapping is calculated in terms of given bit-error rate. The spatial spreading of the beams, caused by scattering from aerosols in different layers of the atmosphere, is calculated for the case of single scattering. A stratified model of the atmosphere is used. Turbulence influences the beam width, especially for the case of short exposure, and is primarily caused by temperature changes, which result in fluctuations in the refractive index. In this research, a new approach is adopted for analyzing communication network performance through the atmosphere by applying optical-transfer function (OTF) concepts used in imaging and remote sensing. We evaluate the effectiveness of this new approach in applications where spatial spread between the users is very important.

A comparative study of the performance characteristics of five multiple access configurations for a wireless communication network is presented and evaluated within the context of the medical field of application. A universal variable has been defined for comparison between the alternative configurations, embodying the data rate, number of channels, power consumption, and bandwidth requirements. Special medical specifications have been accommodated, while the broader applications of this approach have been addressed. CDMA is indicated as the preferred method, over ALOHA, slotted ALOHA, CSMA, and polling.

SUMMARY In this paper, we analyse the effects of vibrations and the atmosphere on the performance of a broadband laser inter-satellite link (BLISL) which was studied within the framework of the BLISL joint Israeli– German applied research project. The use of optical radiation as a carrier between satellites and in satelliteto-ground links enables transmission using very narrow beam divergence angles. Due to the narrow beam divergence angle and the large distance between the satellite and the ground station or airplane the pointing is a complicated process. Further complication results from vibration of the pointing system caused by two fundamental mechanisms of a stochastic nature: (1) tracking noise created by the electro-optic tracker and (2) vibrations caused by internal satellite mechanical mechanisms. Additionally an inhomogeneity in the temperature and pressure of the atmosphere leads to variations of the refractive index along the transmission path. These variations of refractive index as well as pointing vibrations can cause fluctuations in the intensity and the phase of the received signal leading to an increase in link error probability. In this paper, we develop a bit error probability (BEP) model that takes into account both pointing vibrations and turbulence-induced log amplitude fluctuations (i.e. signal intensity fading) in a regime in which the receiver aperture D0 is smaller than the turbulence coherence diameter d0: Our results indicate that BLISL can achieve a BEP of 10 � 9 and data rate of 1Gbps with normalized pointing vibration of GT *s 2 ¼ 0:05 and turbulence of sX ¼ 0:3: Copyright # 2003 John Wiley & Sons, Ltd.