FREE SPACE OPTICAL COMMUNICATION

The DARPA TeraHertz Operational Reachback (THOR) program developed and assessed a suite of technologies for application in a mobile ad-hoc free space optical communication system. The notional THOR system includes airborne and ground based full duplex active terminals and passive optical terminals that form the physical layer of a high speed ad hoc mobile optical network. THOR consists of airborne terminals networked to form a communications backbone transmitting and receiving data at 2.5Gbps as well as passive optical terminals connected as high speed edge devices with a data rate of 2OOMbps. The operational objective is to enable high data rate communications between a combat theater command post and an edge terminal linked to the Global Information Grid (GIG) located outside of theater. DARPA investments were made in compact electro-optic beam steering devices to decrease the size and weight of optical terminals, high-speed high sensitivity (-47dBm) detectors, and high power (SOW) modulating laser sources. Passive optical terminal technology investments focused on highspeed modulating quantum well focal plane arrays for integration with wide field of view +/-I5 degree Cat's Eye optical systems to enable high speed modulating retro reflectors. Networking technologies included protocol assessments and quality of service algorithms to maintain low latency data transfer over unreliable optical data channels while maintaining data rates in the 2.5Gbps range. Analysis and network simulation results demonstrated that with multiple redundant. links an all-optical network could operate with atmospheric weather conditions consisting of 80% broken cloud cover.

In this paper experimentally investigated error-free underwater wireless optical communication (UWOC) system over 7m path in laboratory tap water with up to 46.808dB and BER less than 2.487×10 -6 . The laser diode source of wavelength 532 nm with 50mW has modulated by intensity modulation/direct detection (IM/DD) technique, BER and S/N have inspected in an underwater optical wireless communication channel with five different water channels types. These are tap water, different concentration of Maalox (Mg(OH)2 and Al(OH)3) in order to obtain high turbid water and salt with Maalox. The analysis of BER has achieved for pulse width modulation (PWM) to transmit a text from optical transmitter to receiver. Results shows that salt and Maalox content decreases the received power, S/N and increase in BER. Also, that 532nm wavelength is the suitable choice for a clear water channel. © 2017 Elixir All rights reserved. Elixir Elec. Engg. 113 (2017) 49051-49053 Electrical Engineering Available o...

Active medical implants and other contemporary medical applications need a dependable, high-speed communication channel between external and internal transceivers. Optical wireless communications have demonstrated advantages over widely used radio frequency technology in terms of data speeds, bandwidth abundance, and immunity to interference. Regretfully, this advantage implies strict alignment requirements for both sending and receiving ends. This study focuses on the effects of using multiple transmitters or receivers under the influence of pointing error on the transcutaneous link's overall performance measured by the outage probability and outage rate. Spatial diversity techniques have demonstrated their viability in increasing the link's reliability in free space optical communications. This drives the investigation of improvement transdermal communication system by adding numerous transmitters or receivers. Various misalignment severities are used to represent different operating circumstances, and these analyses result in explicit closed-form formulas for the relevant metrics. The findings clearly show the benefits of employing multiple transmitters and receivers on the link's outage performances. A notable improvement in the average signal-to-noise ratio values for the outage probability and outage rate compared to the single input single output setup was achieved. Furthermore, the theoretical conclusions are subsequently confirmed by MATLAB-based Monte-Carlo simulation for several instructive cases.

Free Space Optical Communications (FSO) has become a magnet for a variety of applications in the communication field. Signal attenuation was highlighted in data transfer, particularly in the multimedia e.g. high-speed video camera network requires sufficient throughput. This paper focuses on finding an approach to improve the signal received through the use of bi-directional FSO multiple access channel with high transmission power to avoid signal attenuation in short distances over atmospheric turbulence channels. Comparison of the extracted results proved that the performance of the proposed system is effective to overcome these effects.

The clustering of satellites in cooperative fly formation constellations produces a lightweight, low cost, highly reliable and supraperformance space system. One way to communicate between the satellites is by a laser beam with a narrow divergence angle. To maintain line of sight between the transmitter and the receiver during the communication time, a steering device has to be incorporated into the communication system. We adopt a 2-D optical phased array (OPA) antenna. An OPA antenna is an attractive option for a steering device as it is lightweight, compact, and provides agile and inertia-free 2-D beam steering. We integrate two fields of study-OPA and laser communication theory-to attain a new perspective on communication system design. We theoretically analyze and model a laser intersatellite link using a 2-D OPA antenna and illustrate the model by a numerical simulation. The microsatellite antenna gain distribution statistics are derived from the OPA theory using the Monte Carlo method. We apply the results to evaluate system performance, particularly the effect of OPA phase errors.

A number of serious consortiums develop satellite communication networks. The objective of these communication projects is to service personal communication users almost everywhere on earth. The intersatellite links in those projects use microwave radiation as the carrier. Free-space optical communication between satellites networked together can make possible high-speed communication between different places on earth. Some advantages of an optical communication system over a microwave communication system in free space are 1) smaller size and weight, 2) less transmitter power, 3) larger bandwidth, and 4) higher immunity to interference. The pointing from one satellite to another is a complicated problem due to the large distance between the satellite, the narrow beam divergence angle, and vibration of the pointing system. Such vibration of the transmitted beam in the receiver plane decreases the average received signal, which increases the bit error rate. In this paper, we review 1) the present status of satellite networks, 2) developing efforts of optical satellite communication around the world, 3) performance results of vibration effects on different kinds of optical communication satellite networks, and 4) seven approaches to overcome the problems caused by transmitter pointing vibration.

In this paper, we analyze 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 satellite-to-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 D o is smaller than the turbulence coherence diameter d o . Our results indicate that BLISL can achieve a BEP of 10 -9 and data rate of 1Gbps with normalized pointing vibration of G T *σ 2 θ =0.05 and turbulence with σ X =0.3.

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.

We propose an adaptive transmission technique for free space optical (FSO) systems, operating in atmospheric turbulence and employing subcarrier phase shift keying (S-PSK) intensity modulation. Exploiting the constant envelope characteristics of S-PSK, the proposed technique offers efficient utilization of the FSO channel capacity by adapting the modulation order of S-PSK, according to the instantaneous state of turbulence induced fading and a pre-defined bit error rate (BER) requirement. Novel expressions for the spectral efficiency and average BER of the proposed adaptive FSO system are presented and performance investigations under various turbulence conditions and target BER requirements are carried out. Numerical results indicate that significant spectral efficiency gains are offered without increasing the transmitted average optical power or sacrificing BER requirements, in moderate-to-strong turbulence conditions. Furthermore, the proposed variable rate transmission technique is applied to multiple input multiple output (MIMO) FSO systems, providing additional improvement in the achieved spectral efficiency as the number of the transmit and/or receive apertures increases.

In some of the future laser communication satellites, it is plausible to assume that tracking and communication receivers will use the same detector array. The reason for dual use of the detector is to design simpler and less expensive satellites. Satellites vibrate continually because of their subsystems and environmental sources. The vibrations cause nonuniform spreading of the received energy on the detector array. In view of this, the information from the tracking system is used to adapt individually the communication signal gain of each of the detectors in the array. This adaptation of the gains improves communication system performance. It is important to emphasize that the communication performance improvement is achieved only by gain adaptation. Any additional vibrations decrease the tracking and laser pointing system performances, which decrease the return communication performances ͑two-way communication͒. A comparison of practical communication systems is presented. The novelty of this research is the utilization of natural satellite vibrations to improve the communication system performance.

Terrestrial optical wireless communication ͑OWC͒ is emerging as a promising technology, which makes connectivity possible between high-rise buildings and metropolitan and intercity communication infrastructures. A light beam carries the information, which facilitates extremely high data rates. However, strict alignment between the transmitter and the receiver must be maintained at all times, and a pointing error can result in a total severance of the communication link. In addition, the presence of fog and haze in the propagation channel hampers OWC as the small water droplets scatter the propagating light. This causes attenuation due to the resultant spatial, angular, and temporal spread of the light signal. Furthermore, the ensuing low visibility may impede the operation of the tracking and pointing system so that pointing errors occur. We develop a model of light transmission through fogs of different optical densities and types using Monte Carlo simulations. Based on this model, the performance of OWC in fogs is evaluated at different wavelengths. The handicap of a transceiver pointing error is added to the model, and the paradoxically advantageous aspects of the transmission medium are exposed. The concept of a variable field of view receiver for narrow-beam OWC is studied, and the possibility of thus enhancing communication system performance through fog in an inexpensive and simple way is indicated.

To establish optical communication between any two satellites, the lines of sight of their optics must be aligned for the duration of the communication. The satellite pointing and tracking systems perform the alignment. The satellite pointing systems vibrate because of tracking noise and mechanical impacts ͑such as thruster operation, the antenna pointing mechanism, the solar array driver, navigation noise, tracking noise͒. These vibrations increase the bit error rate ͑BER͒ of the communication system. An expression is derived for adaptive transmitter power that compensates for vibration effects in heterodyne laser satellite links. This compensation makes it possible to keep the link BER performance constant for changes in vibration amplitudes. The motivation for constant BER is derived from the requirement for future satellite communication networks with high quality of service. A practical situation of a two-low-Earth-orbit satellite communication link is given. From the results of the example it is seen that the required power for a given BER increases almost exponentially for linear increase in vibration amplitude.

les usagers d'un dispositif de formation en ligne, d'une conception de celui-ci permettant d'en saisir et d'en interpréter les buts, d'en dégager les implications en termes d'attitudes d'apprentissage, de modes d'échanges, d'interactions au sens le plus large. Dans cette perspective, le tutorat comme fonction formative se caractérise par l'exercice de rôles variés et parfois contradictoires (empathie paritaire / validation d'étapes, guidage général / adaptation aux spécificités). partage contribue à valider l'"ouverture" d'une formation et ce d'autant plus qu'il est conçu de manière à pouvoir évoluer au fil des échanges en ligne. On présentera donc les concepts clés de cette problématique : conception et développement des dispositifs, espace de communication, fonctionnalités offertes par les environnements, outils dédiés au tutorat, afin de mettre en évidence la spécificité de telles situations éducatives. Jean-François BOURDET, après avoir été enseignant de Fle et responsable pédagogique de l'Alliance Française de Paris, est à présent enseignant-chercheur (MCF, HDR) à l'Université du Maine, responsable du parcours de Master 2 "Didactique des langues et environnements informatiques". Membre du Laboratoire d'Informatique de cette université (spécialisé EIAH), il anime des travaux de recherche portant sur la formation à distance, le tutorat en ligne et les interfaces dédiées.

Following the success of the first -Sat mission, in 2020, the European Space Agency (ESA) announced the opportunity to present CubeSat-based ideas for the -Sat-2 mission as part of its initiative to promote the development of radically innovative technologies such as Artificial Intelligence (AI) capabilities onboard Earth Observation (EO) missions. Open Cosmos and CGI submitted a joint proposal for the -Sat-2 mission idea, which takes advantage of the latest research and developments in the European ecosystem. The proposed mission idea is a game-changing EO CubeSat capable of running AI Apps that can be developed, easily deployed on the spacecraft and updated during flight operations. The AI Apps can be operated from ground using a simple user interface. This approach allows continuous improvement of the AI model parameters using the very same images acquired by the satellite. The mission takes advantage of the latest research for mission operations of CubeSats and use the NanoSat MO Framework, a framework for small satellites that allows software to be deployed in space as simple Apps, in a similar fashion to Android apps. This framework was previously demonstrated in ESA's OPS-SAT mission, and supports the orchestration of on-board Apps. It fully decouples the App features from the underlying on-board hardware via an abstraction layer API in the form of services. Additionally, it includes a Software Development Kit with demo Apps, development tools, and tutorials to facilitate the development of Apps. By decoupling the data platform from the Apps, it is possible to distribute the development of specialized AI Apps to different partners within the -Sat-2 mission consortium. The mission will include a set of default AI Apps that will be able to do vessel detection, high compression of images, and roadmap transformation from satellite imagery. The framework allows more than just the set of default Apps and so, third-party Apps can be included on later stages of the mission lifecycle. This paper will present the NanoSat MO Framework, introduce the AI Apps that are part of the -Sat-2 mission, and how the free and opensource framework enables the creation of software-defined satellite missions via on-board Apps.

Free Space Optic (FSO) merupakan sistem komunikasi yang memanfaatkan atmosfer sebagai media propagasi. Dengan itu perlu dilakukan penelitian pengaruh kondisi atmosfer terhadap kualitas dari sistem komunikasi FSO. Penelitian ini melakukan analisa kinerja transmisi FSO di model kanal Kim dan Kruse menggunakan teknik modulasi 16-Quadrature Amplitude Modulation, On-Off-Keying (OOK)-RZ, dan OOK-NRZ di sisi pengirim dan APD photodetector sebagai penerima. Parameter yang diamati yaitu Bit Error Rate (BER) terhadap perubahan visibility 1-6 km, daya 1-5 watt, dan jarak link 1-10 km pada panjang gelombang 850 nm, 1310 nm, dan 1550 nm. Hasil dari simulasi menunjukan bahwa kanal Kim dan Kruse dapat bekerja pada link yang jauh dengan memadukan panjang gelombang dan daya kirim. Perbandingan menunjukan bahwa OOK-RZ memiliki kinerja lebih baik dari 16-QAM dan OOK-RZ sehinngga OOK-RZ dapat menempuh link lebih jauh.

Free Space Optics (FSO) shows a great alternative as the solution for last mile problem where fiber optics is unavailable due to deployment time and cost constraint.  However, the feasibility of using FSO system as communication link very much depends on local weather.  Since tropical region experiences heavy rainfall, rain with high intensity is expected to affect the FSO link severely. Few prediction models have been proposed by ITU-R based on France and Japan’s measurement. This paper has compared  rain attenuation predicted by  models and  data measured in Malaysia over Free Space Optical links for one year period. Prediction models are clearly unable to predict attenuation measured in tropical climate. ABSTRAK: Wayarles optik menjadi alternatif sebagai satu penyelesaian kepada masalah kesesakan terutama diakhir sesuatu talian dimana ianya tidak dapat diselesaikan dengan menggunakan gentian fiber akibat daripada kekangan masa pemasangan dan masalah kewangan.  Walaubagaimanapun, ...

We investigate the behavior of a Ge-on-Si light source and demonstrate its feasibility for polarization-encoded discrete-variable quantum key distribution following the BB84 protocol, enabling a potential "all-silicon" QKD scheme which can operate well below the necessary QBER limit and successfully generate secret keys.

Recently, a combination of unmanned aerial vehicles (UAVs) and free-space optics (FSO) has been investigated as a potential method for high data-rate front-haul communication links. The aim of this work was to address the performance of UAV-to-ground station-based FSO communications in terms of the symbol error rate (SER). The system proposes utilizing subcarrier intensity modulation and an avalanche photo-diode (APD) to combat the joint effects of atmospheric turbulence conditions and pointing error due to the UAV’s fluctuations. In the proposed system model, the FSO transmitter (Tx) is mounted on the UAV flying over the monitoring area, whereas the FSO receiver (Rx) is placed on either the ground or top of a high building. Unlike previous works related to this topic, we considered combined channel parameters that affect the system performance such as transmitted power, link loss, various atmospheric turbulence conditions, pointing error loss, and the total noise at the APD receive...

In this paper, a prototype of full-duplex indoor Visible Light Communication (VLC) transceiver is designed to demonstrate an optical wireless system based on Universal Serial Bus (USB) port for data transmissions using white Light Emitting Diode (LED). Based on VLC technology, the designed system consists of transmitters and receivers can achieve data rate of 161.2 Kbps error free at the distance of 92 cm between two computers for real-time text and image transmission over indoor environment. Experiment results show that for text transmission, symbol error rate (SER) does not vary data rate at some transmission distances, whereas for image transmission, minimum square error (MSE) and peak signal to noise ratio (PSNR) indicate that the received image and transmitted image were almost similar.

We theoretically study the performance of multiple-input multiple-output (MIMO) free-space optical (FSO) systems using subcarrier quadrature modulation (SC-QAM) signaling. The system average symbolerror rate (ASER) is derived taking into account the atmospheric turbulence effects on the MIMO/FSO channel, which is modeled by log-normal and the gamma-gamma distributions for weak and moderate-to-strong turbulence conditions. We quantitatively discuss the influence of index of refraction structure parameter, link distance, and different MIMO configurations on the system ASER. We also analytically derive and discuss the MIMO/FSO average (ergodic) channel capacity (ACC), which is expressed in terms of average spectral efficiency (ASE), under the impact of various channel conditions. Monte Carlo simulations are also performed to validate the mathematical analysis, and a good agreement between numerical and simulation results is confirmed.

In this work, we analyse pointing error effects on the performance of amplify-and-forward (AF) relaying multiple-input multiple-output (MIMO) free-space optical (FSO) communication system employing sub-carrier quadrature amplitude modulation (SC-QAM) signal over log-normal and gamma-gamma distributed atmospheric turbulence channels. We study the pointing error effects by taking into account the influence of beam-width, aperture size and jitter variance on the average symbol error rate (ASER), which is derived in closed-form expressions of MIMO/FSO and SISO/FSO systems. In addition, the number of relaying stations is taken into account in the statistical model of the combined channel including atmospheric loss, atmospheric turbulence and pointing error impairments. The numerical results show that by combining AF relaying stations and MIMO/FSO configurations, the link length can be extended because the transmitted power is reduced according to the amplifier gain. Moreover, the performance of AF relaying MIMO/FSO systems is better than that of AF relaying SISO/FSO systems at the same link length.

Apart from land and air, sea has also become very important from defense strategic point of view and is required to be integrated with network centric communication. In future it may play a very important role in network centric warfare. Free space laser communications provide wide bandwidth and high security capabilities to unmanned aircraft systems in order to successfully accomplish intelligence, surveillance, target acquisition, and reconnaissance missions(l) Laser based communication is emerging as a most promising solution for underwater communication. Most of laser cannot penetrate through the sea due to be absorbed by the sea, but the blue-green laser (the wavelength is about 470 ~ 570nm) has the minimum energy fading in the sea, whose fading rate is about 0.155~0.5dB/m, this feature of blue-green laser in the sea is said the window effect(2). Long- range links use directed laser beams to transmit data, and can be used for building-to-building, ground- to-aircraft, or ground...

Free Space Optical (FSO) communication systems have extensively invaded the speed of smart city evolution due to the current surge in demand for wireless communication spots that can match recent challenges due to high technical leaps in smart city evolution. As the number of users is vastly increasing throughout all networks in the form of machines, devices, and variously distinct objects, FSO is a hugely recommended robust communication system that mitigates a lot of RF disadvantages on the field with no need for licensing, fast rollout time, and low cost. This paper shows an exploit of a Low Power Field Programmable Gate Array (FPGA) based FSO communication system designed for Line of Sight (LOS) Building to Building Communication over a distance of 12 m using a 650 nm Visible Light (VL) red laser source via On-Off Keying (OOK) and higher-level Intensity Modulation (IM)/Pulse Width Modulation (PWM) schemes. The implemented system reached a doubled data rate than OOK of 230 kbps u...

Free space optics (FSO) systems are the terrestrial point-to-point optical wireless communications (OWC) systems, also known as one of the most advanced wireless technologies. FSO communication is one of the best cost saving ways to send and receive data with high data transmission capacity and security. In this paper, a channel distance of 1 km at the wavelength of 1550 nm is utilized. Different modulation schemes are analyzed to improve OWC system performance under different weather conditions. The effect of the number of transmitters and receivers on the performance of the proposed FSO system is investigated and evaluated through the evaluation parameters including the quality factor (Q-factor), power received and bit error rate (BER). The effect of the pointing error is also studied under different weather conditions using multiple input multiple output (MIMO) systems with different modulation schemes. Among these schemes, we focus on: on-off keyingnon-return to zero (OOK-NRZ), carrier suppressed non-return to zero (CSNRZ), duobinary non-return to zero (DBNRZ) and modified duobinary non-return to zero (MDBNRZ)). The obtained results reveal that, the received signal power of both CSNRZ and DBNRZ in MIMO systems is improved by 21% compared to single input single output (SISO) systems. It is also found that, the MDBNRZ received signal power increases by ~ 22% in MIMO system compared to SISO system at a beam divergence angle of 1 mrad. The performance the OOK-NRZ is enhanced by 23% in SISO system and by 25% in MIMO system than that in literature.

This paper contains some interesting experimental results of the study carried out on anomalous spectral behavior due to coherence and polarization change on polychromatic focused dark hollow Gaussian beam (DHGB). On the basis of experimental and numerical analysis, the possibility and significance of DHGB based free-space optical (FSO) links for indoor and outdoor optical communications are explored.

We consider solutions of quasilinear equations u t = ∆u m + u p in R N with the initial data u 0 satisfying 0 < u 0 < M and lim |x|→∞ u 0 (x) = M for some constant M > 0. It is known that if 0 < m < p with p > 1, the blow-up set is empty. We find solutions u that blow up throughout R N when m > p > 1.

Free space optics (FSO) is a promising technology for high data rate transmission where data is transmitted wirelessly from one place to another. It has a variety of applications nowadays including indoor, outdoor, underwater, and deep space communications. However, the availability of link under various atmospheric conditions is a major concern. In this study, we evaluate the transmission of 128 Gb/s optical quadrature phase shift keying (QPSK) signal under different weather conditions and pointing errors. Simulation parameters such as FSO link range, atmospheric attenuation, pointing loss, and wavelength are taken into consideration. The system performance evaluation parameters including eye diagram, constellation, and received signal to noise ratio (SNR) are compared to find the best performance under different weather conditions. Numerical simulations show that the pointing errors up to 10 µrad and bad weather conditions up to 15 dB attenuation has sever effects on the system performance and we should use digital signal processing and equalization to enhance it.

The growing deployment and advanced development of high-speed train (HST) systems, coupled with the reliance on and demand for constant Internet connectivity anytime anywhere, have necessitated the imminent provisioning of broadband Internet services in HSTs. Ground-to-train free space optical (FSO) communications suffer from frequent handovers due to high mobility of HSTs, thus shortening the connection time between the train and ground, and greatly impacting passengers' user experience. To provision high-speed Internet services in HSTs, we propose the RotAting TranscEiver scheme (RATE) to mitigate the impact of handover processes with steerable FSO transceivers in a ground-to-train communications system. Owing to the rotating feature of the scheme, the on-roof transceiver of a train can maintain a reliable link with a base station (BS) for a longer time, and remarkably reduce the number of handover processes, thus provisioning higher data rates of the system. Meanwhile, since the separation distances between BSs are extended, the total number of BSs required to be deployed along the track is also reduced, thus reducing the deployment cost for service providers. Moreover, a new handover method is designed to mitigate the impact of each handover delay, especially on live streaming applications. Finally, the performance improvement of the proposed scheme is demonstrated via simulation results. Index Terms-FSO, handover, ground-to-train communications, high-speed train. H IGH-SPEED Internet connectivity is becoming an inte- gral and indispensable part in our daily lives. Broadband data services are expected to be provisioned in high-speed trains (HSTs). Thus, providing Internet access in HSTs is one of the main incentives for the railway operators to attract more passengers. Currently, radio frequency wireless technologies are used to provide Internet access to passengers [1-3]. However, owing to the limitation of radio wave bandwidth and interference [4], [5], the existing infrastructure based on radio frequency technology such as Wi-Fi/WiMAX can provide peak data rates of up to 54/75 Mbps theoretically, which drop to lower than 10 Mbps in real scenarios [6],

The growing deployment and advanced development of high-speed train (HST) systems, coupled with the reliance on and demand for constant Internet connectivity anytime anywhere, have necessitated the imminent provisioning of broadband Internet services in HSTs. Ground-to-train free space optical (FSO) communications suffer from frequent handovers due to high mobility of HSTs, thus shortening the connection time between the train and ground, and greatly impacting passengers' user experience. To provision high-speed Internet services in HSTs, we propose the RotAting TranscEiver scheme (RATE) to mitigate the impact of handover processes with steerable FSO transceivers in a ground-to-train communications system. Owing to the rotating feature of the scheme, the on-roof transceiver of a train can maintain a reliable link with a base station (BS) for a longer time, and remarkably reduce the number of handover processes, thus provisioning higher data rates of the system. Meanwhile, since the separation distances between BSs are extended, the total number of BSs required to be deployed along the track is also reduced, thus reducing the deployment cost for service providers. Moreover, a new handover method is designed to mitigate the impact of each handover delay, especially on live streaming applications. Finally, the performance improvement of the proposed scheme is demonstrated via simulation results. Index Terms-FSO, handover, ground-to-train communications, high-speed train. H IGH-SPEED Internet connectivity is becoming an inte- gral and indispensable part in our daily lives. Broadband data services are expected to be provisioned in high-speed trains (HSTs). Thus, providing Internet access in HSTs is one of the main incentives for the railway operators to attract more passengers. Currently, radio frequency wireless technologies are used to provide Internet access to passengers [1-3]. However, owing to the limitation of radio wave bandwidth and interference [4], [5], the existing infrastructure based on radio frequency technology such as Wi-Fi/WiMAX can provide peak data rates of up to 54/75 Mbps theoretically, which drop to lower than 10 Mbps in real scenarios [6],

This study deals with the communication capabilities of two kinds of semiconductor lasers emitting in one of the atmosphere transparency windows, around 4 µm. One of these two lasers is a quantum cascade laser and the other one is an interband cascade laser. With the quantum cascade laser, a subsequent attenuation is added to the optical path in order to mimic the attenuation of free-space transmission of several kilometers. Direct electrical modulation is used to transmit the message and two-level formats, non-return-to-zero and return-to-zero, are used and compared in terms of maximum transmission data rate. The sensitivity to optical feedback is also analyzed, as well as the evolution of the error rate when reducing the optical power at the level of the detector. This work provides a novel insight into the development of future secure free-space optical communication links based on midinfrared semiconductor lasers and sheds the light on improvements required to achieve multi-Gbits/s communication with off-the-shelf components.

We discuss the design of an acousto-optic cell based free space optical communication link where the data beam is made secure through chaos encryption. Using external signal modulation of the diffracted light from a hybrid acousto-optic cell chaos (or directly via incorporation in the sound-cell driver's bias voltage) encryption of data is possible. We have shown numerically that decryption of the encoded data is possible by using an identical acousto-optic system in the receiver.

The papers included in this volume were part of the technical conference cited on the cover and title page. Papers were selected and subject to review by the editors and conference program committee. Some conference presentations may not be available for publication. The papers published in these proceedings reflect the work and thoughts of the authors and are published herein as submitted. The publisher is not responsible for the validity of the information or for any outcomes resulting from reliance thereon. Please use the following format to cite material from this book: Author(s), "Title of Paper," in Atmospheric Propagation V, edited by G.

We report an eye-safe, 1550 nm, retromodulated 1 infrared data link which supports real-time color video. The retromodulated link was 2 meters in the lab and supported 3 Mbps and 30 frames per second using wavelet compression. The device consumes about 75-100 mW and is 10 grams, mounted. We will show a video sequence from the test. We will also discuss future plans for the device.

In this paper, we summarize progress in free space laser propagation research at the U.S. Naval Research Laboratory, specifically in the context of propagating and detecting signals through the atmosphere in a maritime environment. Transmission through the atmosphere over large bodies of water presents different challenges than transmission through the atmosphere over land. Our paper reports some of these findings as well as progress in our collaborative efforts to mitigate turbulence to enhance our data links.

An adaptive thresholding method is presented for optimum detection for optical receivers with large multiplicative noise. The technique uses low frequency sampling of the detected current that enables calculation of the bit means and variances and estimation of the optimum detection threshold. The regime in which this holds is when the sampling frequency is lower than the bit rate but higher than atmospheric turbulence frequency content. Simulations are done with data obtained from the NRL Chesapeake Bay Lasercomm Testbed. The results of simulations comparing BER performance versus sample rate and parameter estimation error will be presented. If the system parameters are characterized in advance with reasonable accuracy, the BER obtained will typically be an order of magnitude improvement over the equal variance threshold (depending on the signal to noise ratio).

An optical communications link is used to transmit important navigation and range information between two spacecraft platforms to aid in the spacecraft docking process. This experiment uses NRL's multiple quantum well modulating retroreflector (MRR) array as the light payload optical communications transmitter on the target spacecraft. Spacecraft navigation information is determined by using the MRR array to impose five distinct modulation frequencies on the optical transmit beam, and analyzing the received signal on the pursuer spacecraft to extract target attitude and range information. Design, architecture, and status of the experiment at NRL are discussed.

Free space optics (FSO) can provide high data rates with efficient use of power. However, small platforms may not be able to support the payload requirements of a conventional FSO terminal. An alternative FSO terminal uses a modulating retro-reflector (MRR). MRRs shift most of the power, weight, and pointing requirements to one end of the link. With a MRR configuration, it is possible to establish a two-way FSO link using a single laser transmitter. The MRR terminal of these systems can be small, lightweight, and low power. The MRR maintains the small beam divergence of a conventional optical communications link, but gains the loose pointing advantage of an RF link, reducing the pointing requirements. Communication needs in space present many asymmetric scenarios in which a MRR architecture could be beneficial. This paper describes some of the current capabilities and limitations of MRR systems, as well as applications to space links. An evaluation of the radiation tolerance of modulators is presented.

Modulating retro-reflectors provide a means for free space optical communication without the need for a laser, telescope or pointer tracker on one end of the link. These systems work by coupling a retro-reflector with an electro-optic shutter. The modulating retro-reflector is then interrogated by a cw laser beam from a conventional optical communications system and returns a modulated signal beam to the interrogator. Over the last few years the Naval Research Laboratory has developed modulating retro-reflector based on corner cubes and large area Transmissive InGaAs multiple quantum well modulators. These devices can allow optical links at speeds up to about 10 Mbps. We will discuss the critical performance characteristics of such systems including modulating rate, power consumption, optical contrast ratio and operating wavelength. In addition a new modulating retro-reflector architecture based upon cat's eye retroreflectors will be discussed. This architecture has the possibility for data rates of hundreds of megabits per second at power consumptions below 100 mW.

Characterizing the fundamental response and operational parameters of a deformable mirror is a critical first step in the design of an adaptive optics system. This paper describes the characterization and training of a piezoelectric deformable mirror (PDM) to be implemented on a low order Adaptive Optics (AO) system at Short Wave Infrared (SWIR) wavelengths for free-space optical communications systems. The data were analyzed using commercial and customized software.

Based on the wavelet transformation and adaptive Wiener Filtering, a new method was presented by the authors to perform the synchronization and detection of the binary data from the Free-Space Optical (FSO) signal . It was shown in that the Haar wavelet with a fixed scale is an excellent choice for this purpose. The output of the filter is zero-mean and is closely related to the derivative of the binary data. In this effort an analysis of the work in [1] is presented to obtain the probability of bit error using a Bayesian ternary hypotheses testing. The analysis also results in determining optimum thresholds for the detection of binary data.

Photoreceiver array configurations for enhancing the sensitivities of free space optical terminals down to the quantum limits are described. Small format arrays at 1550nm have been implemented and shown to improve sensitivity by 1.8 dB. The concept of large format arrays based on optically preamplified photoreceivers in a summed configuration for is elucidated and shown to be capable of sensitivities as low as -53 dBm at 2.5 Gbps bit rate and 10 -9 BER for (8 X 8) arrays.

Atmospheric turbulence causes severe performance degradation in free space optical communication links. However, by using a reliable low bandwidth wireline or radio frequency link in parallel, the performance of such links can be improved significantly. This is achieved by using error correction codes at the transmitter, and decoding the received data jointly over the two links at the receiver. Our work describes the feasibility study of such a hybrid link. It is observed that a 10% use of the wireline link can result in more than 8 dB performance gain in the optical link.

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Nowadays, transmitting and receiving data with high speed and a high level of security are the main demands. So, a new model of spectral amplitude coding optical code division multiple access (SAC-OCDMA) is suggested in this paper, based on a free space optical (FSO) communication system using coherent sources. Three different codes: enhanced double weight (EDW), modified double weight (MDW) and multi-diagonal (MD) codes are assigned to our proposed model with the direct detection (DD) technique. Furthermore, the model is simulated under different weather conditions including clear air (CA), light mist (LM), very light fog, and light fog (LF). The system performance is evaluated through its bit error rate (BER), Q-factor, received power, and signal to noise ratio (SNR). Moreover, classification of the information received by the three different SAC-OCDMA using three different codes (EDW, MDW, and MD) is still challenging. So, two different machine learning (ML) algorithms are used, ...

Code classification is essential nowadays, as determining the transmission code at the receiver side is a challenge. A novel algorithm for fixed right shift (FRS) code may be employed in embedded next-generation optical fiber communication (OFC) systems. The code aims to provide various quality of services (QoS): audio, video, and data. The Q-factor, bit error rate (BER), and signal-to-noise ratio (SNR) are studied to be used as predictors for machine learning (ML) and used in the design of an embedded QoS classifier. The hypothesis test is used to prove the ML input data robustness. Pearson’s correlation and variance-inflation factor (VIF) are revealed, as they are typical detectors of a data multicollinearity problem. The hypothesis testing shows that the statistical properties for the samples of Q-factor, BER, and SNR are similar to the population dataset, with p-values of 0.98, 0.99, and 0.97, respectively. Pearson’s correlation matrix shows a highly positive correlation between...

A new hybrid polarization division multiplexing (PDM) spectral amplitude coding optical code division multiple access (SAC-OCDMA) is proposed for free space optical (FSO) for capacity enhancement. Two polarization signals are utilized; one is x-polarization and carries three different channels at 0° azimuthal angle while the other is y-polarization at 90° azimuthal angle, and carries the same three channels. Each channel is assigned with a diagonal permutation shift (DPS) code and carries 10 Gbps. The suggested system is simulated, and its performance is evaluated in terms of maximum allowable number of users, propagation range, bit error rate (BER), Q-factor, and received power for the different channels under various fog, dust storm, and rain scenarios. The reported results indicate that the system can support a signal travelling up to 2, 0.9, and 1.3 km, respectively, under light fog (LF), light dust (LD), and light rain (LR). As the level of these weather conditions is increased...

Smart and sustainable cities require a network that can ensure many exchanges of information. In this sense, the deployment of optical fiber seems essential in order to guarantee urban interconnection. In this work, a new algorithm for the right shift code to be used in optical fiber communication (OFC) is proposed. The algorithm was designed to support different quality of services (audio, video, and data) for smart cities. This algorithm was based on varying the code weight for a certain number of users and transmitting a service to them. This algorithm was used for a spectral amplitude-coding optical code division multiple-access (SAC-OCDMA) system. Two detection scheme approaches were used, which are the single photodiode (SPD) detection technique and the direct detection (DD) technique. The performance was analytically studied for the proposed system in terms of BER, SNR, and the maximum number of users. The obtained results, when our system used SPD, showed an improvement in t...

Free space optics (FSO) communication is a very recent mechanism. It is mainly needed when physical connection between the transmitter and the receiver is practically impossible and when high bandwidth data transmission is expected. This paper walks through the investigation of the performance of FSO communication adopting spectral amplitude coding optical code division multiple access (SAC-OCDMA) systems at different weather scenarios. The suggested system uses enhanced double weight codes as signature codes by 3 clients, simultaneously. To eliminate multiple access interference and phase induced intensity noise, two different detection schemes are used single photodiode (SPD) and modified-AND subtraction detection schemes. Furthermore, a comparison between PIN and APD detectors is performed. The performance is evaluated through its propagation distance, received power, Q-factor and bit error rate (BER). The obtained numerical results reveal both Q-factor and BER are modified by 30%. Accordingly, the system performance is enhanced by 30%, when using SAC-OCDMA-FSO with SPD detection technique compared with modified-AND subtraction technique.

This contribution investigates the effectiveness of optical communication links in enabling high-speed data transfer from deep-space (DS) probes directly to Earth ground stations. In particular, the propagation impairments induced by clouds are estimated by exploiting long-term radiosonde observation data collected in some European sites. The impact of different cloud types on optical links operating at 1.55 μm is first quantified in terms of total path attenuation, and afterward, the implementation of multisite diversity schemes is discussed to counteract the extremely high attenuation levels caused by clouds. Results show that a three-site diversity system with target availability of 90% allows reduction of the link margin to counteract cloud attenuation from at least 40 dB to ∼6 dB, which makes optical communications a viable option also for DS missions.

An OFDM transmission system is reported based on a directly modulated blue LASER diode, for high bit rate underwater optical communication applications. The 256 subcarriers 16-QAM signal is transmitted over a total distance of 2.4 m underwater with an EVM lower than -28.5 dB for a 250 MHz bandwidth and -16.5 dB for a 2 GHz bandwidth, the BER being lower than the forward error corrector limit. At the maximum bandwidth of 2 GHz a transmission rate of 5.36 Gbit/s is achieved.