Tato práce se zabývá problematikou optických bezkabelových spojů (FSO). V úvodní kapitole jsou diskutovány přednosti, základní principy a dílčí komponenty FSO spojů. Druhá kapitola se zabývá atmosférou z pohledu šířícího se optického svazku, jejím složením, základními veličinami a jevy v ní nastávajícími. V následující kapitole je popsáno několik metod ke zmírnění jevů majících negativní vliv na kvalitu spoje. Druhá část práce se zabývá návrhem podpůrného systému pro FSO, založeném na softcore mikroprocesoru MicroBlaze, návrhem jednoduchého síťového přepínače a síťového rozhraní. Závěr práce pojednává o implementaci webového serveru a tvorbě webové prezentace umožňující vzdálenou správu FSO a jeho komponent.

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Free-space optical communication (FSO) has been proposed as an attractive alternative to radio frequency communication in the sense that it provides wide bandwidth and high capacity without requirement of license. However, the scalability of FSO link is limited by pointing error, atmospheric turbulence, and loss. Especially, when it comes to the FSO link between moving platforms, it is imperative works to analyze the statistical channel model considering accurate pointing errors and atmospheric turbulence at the same time. In this paper, we analyze performance of FSO links over various atmospheric situations with pointing errors. First, we assume strong turbulence and obtain a unified approximation of the composite probability density function (PDF) of channel gain, which embraces generalized pointing error models. This approximation leads to new unified formulas for the bit error rate (BER) and outage capacity of a FSO link, which account for the two possible detection mechanisms of intensity modulation/direct detection (IM/DD) and heterodyne detection. Secondly, we statistically derive the unified composite PDF containing all possible pointing error models based on weak turbulence model. In addition, we analyze BER performance in FSO communication with IM/DD and heterodyne detection technology based on the derived unified composite PDF results. Finally, we investigate the ergodic capacity of unmanned aerial vehicle (UAV)-based FSO links over random foggy channel. More specifically, we derive composite PDF and close approximation for the moments of the composite PDF using the statistical model of a UAV-based 3D pointing error and a random foggy channel. With it, we obtain upper bound and asymptotic approximation of the ergodic capacity for the two possible detection techniques of IM/DD and heterodyne detection at high and low signal-to-noise ratio (SNR) regimes.

<p>Free-space optical (FSO) communication has witnessed rapid development recently in response to ever-increasing demands for greater bandwidth. FSO links provide fiberspeed with the flexibility of wireless. Commercially available systems offer transmission speeds up to 2.5 Gbps, 5 Gbps and 10 Gbps, and demonstration systems report data rates as high as 160 Gbps. Its advantages also include license free operation, high immunity to interference, and ease of deployment. However, FSO systems are sensitive to adverse weather conditions such as fog, rain and snow.</p> <p>In order to improve the availability of FSO channels degraded by atmospheric turbulence and varying weather conditions, the effects of channel gain variations must be compensated. In this thesis, two rate-adaptive techniques, punctured low-density parity-check (LDPC) codes and Raptor codes, are studied using experimental data measured over a1.87 km terrestrial FSO link.</p> <p>Rate-adaptive performances with punctured LDPC codes and Raptor codes are evaluated in terms of outage probability and throughput. In comparison to uncoded system, rate-adaptive systems with both techniques demonstrate significant improvement of throughput and mitigation of outage probability especially in rainy weather. Due to its flexible rate-adaptation and simple hardware implementation, Raptor coded systems are judged more applicable to be implemented in field-programmable gate array (FPGA) based hardware. A dedicated decoding structure is proposed and tested, showing remarkable improvement in resource efficiency as compared to traditional Gauss-Jordan (GJ) decoding structures.</p> / Master of Applied Science (MASc)

<p> Wireless LAN mesh networks (WMNs) are a cost effective way of deploying wireless LAN (WLAN) coverage over extended areas. As WMNs become more populated, scalability issues may arise due to the co-channel interference which is inherent in publicly available RF (radio frequency) channels. This co-channel interference can severely degrade network capacity and link reliability and may eventually make it impossible to operate with the frequency channels for which the network was originally designed. In this thesis, this problem is addressed by selectively installing supplementary free-space optical (FSO) links when RF link performance has deteriorated. The frequency assignment problem is solved using a heuristic technique based on a genetic algorithm. In order to determine the quality of the results, the proposed algorithm is compared with a lower bound solution obtained using an Integer Linear Programming (ILP) formulation.</p> <p> Another advantage of FSO links is that they may reduce node power consumption compared with conventional RF links. This may be an important consideration in cases where power consumption at the nodes is important, such as in solar powered mesh networks. Power consumption estimates of RF and FSO links are obtained and compared for different data rates. This data is then used along with historical solar insolation data to estimate the solar panel and battery sizes required to guarantee a given node outage probability. The results show that no extra provisioning is required for replacing the deployed wireless nodes with new FSO links.</p> / Thesis / Master of Applied Science (MASc)

The increasing demand on user rates in the fifth generation (5G) requires network architectures that can support high data rates with acceptable reliability. In order to increase the data rates in the presence of the current spectrum crisis, shrinking cells and reusing the spectrum is a proposed solution. Conventional implementation of dense cells requires a large number of expensive BSs to locally process and decode users’ signals. Another limiting factor that degrades the performance in a dense network is the inter-cell interference. A cloud radio access network (CRAN) is a promising solution to those cost, complexity, and interference challenges. A typical CRAN architecture consists of simplified low-cost base stations (BSs), termed radio units (RUs), that collect the radio frequency (RF) user equipments’ (UEs) signals and forward them over the fronthaul links to the central office (CO) where signal processing is done over shared resources. Besides the reduced cost and complexity of a CRAN, the joint processing at the CO enables joint interference mitigation techniques. However, the performance of CRANs depends critically on the availability of reliable fronthaul links with large bandwidth that may be expensive. Analog optical fronthaul links provide high data rates at lower cost and complexity since UEs’ signals are optically analog-modulated without digitalization, however, they suffer from other channel impairments and nonlinearities. In this thesis, analog optical fronthaul topologies are considered in which radio signals are forwarded over free-space optical (FSO) links, termed radio-over-free-space optical (RoFSO) links, and optical fiber (OF) links, termed radio-over-fiber (RoF) links. Firstly, a CRAN with mixed RF/RoFSO fronthaul is considered to investigate the performance improvement when RF fronthaul links are replaced one-by-one by RoFSO links. A novel joint optimization problem is introduced for the given architecture in which the weighted sum of UEs’ rates is maximized by jointly designing RF and RoFSO links. The optimization problem is solved over different numbers of RF and RoFSO links and under various weather conditions. Under favorable weather conditions, the replacement of 1 RF link by a RoFSO link is shown to increase the 50th percentile of UEs’ rates by 7 times. Secondly, the reliability of a CRAN with two-hop RoFSO/RoF fronthaul links is derived along with other performance metrics such as the average bit-error rate and the cumulative distribution function of UEs’ rates. For the given architecture, the Gaussian noise model of fiber nonlinearity is applied and an optimal OF average optical power is derived to minimize the outage probability. Using the optimal power, and under favorable weather conditions, the 50th percentile of user rate exceeds 1:5 Gbps. Finally, a CRAN with passive all-optical two-hop fronthaul links is considered where optical signals from the first RoFSO fronthaul hop are passively coupled into the RoF fronthaul link. The fronthaul outage probability is derived in the context of network planning to provide guidance on designing a set of system parameters. Those parameters include coverage area radius, density of RUs, RoFSO gain, RoFSO optical power and RoF length. / Thesis / Doctor of Philosophy (PhD) / The upcoming generation of wireless communications, termed fifth generation (5G), promises faster data rates and lower latency. In order to achieve this, more base stations (BSs) have to be deployed which increases the cost and complexity of the network. A solution to this challenge is to install simple BSs, i.e. radio units (RUs), that collect signals from users and forward them to a central office (CO) for joint processing which is referred to as a cloud radio access network (CRAN). The fronthaul network in a CRAN connects the RUs to the CO and it can be implemented using different kinds of links. While there are several fronthaul media (e.g., radio frequency (RF), free-space optical (FSO) links, copper lines, satellite communications, and optical fiber (OF)), optical links provide high data rates that are promising to achieve the 5G requirements. In this thesis, a novel architecture of a CRAN is considered in which analog optical links, namely FSO links and OF links, are used for fronthauling. Performance improvement in terms of rate and reliability is investigated and optimized through different design tools. In response to the challenges introduced by the proposed architecture, such as the nonlinearities of analog FSO and OF links, various design parameters are proposed in the optimization problems to tackle those challenges. Furthermore, a network planning framework is introduced to provide guidance and insights on designing the network.

Yes / Computer simulation is a powerful and convenient tool for the design and performance evaluation of free-space optical (FSO) communication systems. In this article, we present two simulation frameworks that incorporate not only the effects of atmospheric turbulence but also the impact of the angular fluctuations of the transmitter and receiver in FSO systems. In the first framework, the waveform of the optical signal is calculated sequentially from the transmitter to the receiver. Thus, it takes very long to run the simulation numerous times to obtain the statistical performance of the system. This is because the vast majority of simulation time is spent on the split-step beam propagation. In the second framework, we propose to isolate the beam propagation through atmospheric channel from the other effects. We compare the two frameworks in terms of accuracy and simulation time. We show that the second framework reduces the simulation time by more than a factor of 10 without sacrificing the accuracy under various conditions. / 10.13039/501100003725-National Research Foundation of Korea HR&D Center Funded by the Ministry of Science and ICT (Grant Number: 2022M1A3C2069728)

The physical-layer security of a line-of-sight (LOS) free-space optical (FSO) link using orbital angular momentum (OAM) multiplexing is studied. We discuss the effect of atmospheric turbulence to OAM-multiplexed FSO channels. We numerically simulate the propagation of OAM-multiplexed beam and study the secrecy capacity. We show that, under certain conditions, the OAM multiplexing technique provides higher security over a single-mode transmission channel in terms of the total secrecy capacity and the probability of achieving a secure communication. We also study the power cost effect at the transmitter side for both fixed system power and equal channel power scenarios.

Approximately, two-thirds of earth's surface is covered by water. There is a growing interest from the military and commercial communities in having, an efficient, secure and high bandwidth underwater wireless communication (UWC) system for tactical underwater applications such as oceanography studies and offshore oil exploration. The existing acoustic and radio frequency (RF) technologies are severely limited in bandwidth because of the strong frequency dependent attenuation of sound in seawater and the high conductivity of seawater at radio frequencies, respectively. Recently, underwater wireless optical communication (UWOC) has been proposed as the best alternative or complementary solution to meet this challenge. Taking advantage of the low absorption window of seawater in blue-green (400-550 nm) regime of the electromagnetic spectrum, UWOC is expected to establish secure, efficient and high data rate communication links over short and moderate distances (< 100 m) for versatile applications such as underwater oil pipe inspection, remotely operated vehicle (ROV) and sensor networks. UWOC uses the latest gallium nitrite (GaN) visible light-emitting diode (LED) and laser diode (LD) transmitters. Although some research on LED lased UWOC is being conducted, both the military and academic 5 research communities are favoring the use of laser beams, which potentially could enhance the available bandwidth by up to three orders of magnitude. However, the underwater wireless channel is optically very challenging and difficult to predict. The propagation of laser beams in seawater is significantly affected by the harsh marine environments and suffers from severe attenuation which is a combined effect of absorption and scattering, optical turbulence, and multipath effects at high transmission rates. These limitations distort the intensity and phase structure of the optical beam leading to a decrease in signal-to-noise ratio (SNR) which ultimately degrades the performance of UWOC links by increasing the probability of error. In this dissertation, we seek to experimentally demonstrate the feasibility of short range (≤ 20 m) UWOC systems over various underwater channel water types using different modulation schemes as well as to model and describe the statistical properties of turbulence-induced fading in underwater wireless optical channels using laser beam intensity fluctuations measurements.

Electroabsorption is the physical phenomenon by which the absorption of light in a medium can be controlled by applying an electric field. The Quantum–Confined Stark Effect, which makes the absorption band–edge in quantum wells very field–dependent, together with the strong absorption peak provided by excitons, are the physical foundations for the success of electroabsorption modulators based on quantum well structures in telecommunication networks. This thesis describes the design and fabrication of surface–normal electroabsorption modulation devices. The techniques needed to understand the design and fabrication of surface–normal multiple quantum well optical modulators are introduced, as are the various characterisation techniques used during and after the fabrication. Devices for several types of applications have been designed, fabricated, characterised and in some cases integrated into optical systems: – Two–dimensional arrays of 128´128 pixel amplitude modulators grown on GaAs substrates have been fabricated and characterised. Speeds of up to 11700 frames per second were demonstrated, limited by the output electronics of the computer interface. – Large–area modulators grown on GaAs substrates for free–space optical communication were developed, with an active area of 2cm2 and a modulation speed of several megahertz. Contrast ratios up to 5:1 on full modulator areas were measured. Problems limiting the yield and modulation speed of such devices have been studied, and solutions to overcome them have been demonstrated. – Large–area devices grown on InP substrates for free–space optical communication have been developed. Contrast ratios of up to 2:1 for transmissive types have been demonstrated. – Devices consisting of two rows of pixels, grown on GaAs substrates, with an active area of 22mm´5mm, divided into 64 or 128 pixels per row have been developed. These amplitude modulation devices were designed for optical signal processing applications. – One variant of these optical signal processing devices was also characterised as a ternary, binary amplitude and binary phase modulator array. – The use of GaAs multiple quantum well optical modulators in a free–space optical retro–communication system has been studied. An opto–mechanical design for a modulating retro–reflector is described, allowing a large field of view in one direction using reflecting, resonant–cavity modulators for high contrast ratios. / QC 20100802