Otimização do fronthaul ótico para redes de acesso de rádio (baseadas) em computação em nuvem (CC-RANs)

Doutoramento conjunto (MAP-Tele) em Engenharia Eletrotécnica/TelecomunicaçõesA proliferação de diversos tipos de dispositivos moveis, aplicações e serviços com grande necessidade de largura de banda têm contribuído para o aumento de ligações de banda larga e ao aumento do volume de trafego das redes de telecomunicações moveis. Este aumento exponencial tem posto uma enorme pressão nos mobile operadores de redes móveis (MNOs). Um dos aspetos principais deste recente desenvolvimento, é a necessidade que as redes têm de oferecer baixa complexidade nas ligações, como também baixo consumo energético, muito baixa latência e ao mesmo tempo uma grande capacidade por baixo usto. De maneira a resolver estas questões, os MNOs têm focado a sua atenção na redes de acesso por rádio em nuvem (C-RAN) principalmente devido aos seus benefícios em termos de otimização de performance e relação qualidade preço. O standard para a distribuição de sinais sem fios por um fronthaul C-RAN é o common public radio interface (CPRI). No entanto, ligações óticas baseadas em interfaces CPRI necessitam de uma grande largura de banda. Estes requerimentos podem também ser atingidos com uma implementação em ligação free space optical (FSO) que é um sistema ótico que usa comunicação sem fios. O FSO tem sido uma alternativa muito apelativa aos sistemas de comunicação rádio (RF) pois combinam a flexibilidade e mobilidade das redes RF ao mesmo tempo que permitem a elevada largura de banda permitida pelo sistema ótico. No entanto, as ligações FSO são suscetíveis a alterações atmosféricas que podem prejudicar o desempenho do sistema de comunicação. Estas limitações têm evitado o FSO de ser tornar uma excelente solução para o fronthaul. Uma caracterização precisa do canal e tecnologias mais avançadas são então necessárias para uma implementação pratica de ligações FSO. Nesta tese, vamos estudar uma implementação eficiente para fronthaul baseada em tecnologia á rádio-sobre-FSO (RoFSO). Propomos expressões em forma fechada para mitigação das perdas de propagação e para a estimação da capacidade do canal de maneira a aliviar a complexidade do sistema de comunicação. Simulações numéricas são também apresentadas para formatos de modulação adaptativas. São também considerados esquemas como um sistema hibrido RF/FSO e tecnologias de transmissão apoiadas por retransmissores que ajudam a alivar os requerimentos impostos por um backhaul/fronthaul de C-RAN. Os modelos propostos não só reduzem o esforço computacional, como também têm outros méritos, tais como, uma elevada precisão na estimação do canal e desempenho, baixo requisitos na capacidade de memória e uma rápida e estável operação comparativamente com o estado da arte em sistemas analíticos (PON)-FSO. Este sistema é implementado num recetor em tempo real que é emulado através de uma field-programmable gate array (FPGA) comercial. Permitindo assim um sistema aberto, interoperabilidade, portabilidade e também obedecer a standards de software aberto. Os esquemas híbridos têm a habilidade de suportar diferentes aplicações, serviços e múltiplos operadores a partilharem a mesma infraestrutura de fibra ótica.The proliferation of different mobile devices, bandwidth-intensive applications and services contribute to the increase in the broadband connections and the volume of traffic on the mobile networks. This exponential growth has put considerable pressure on the mobile network operators (MNOs). In principal, there is a need for networks that not only offer low-complexity, low-energy consumption, and extremely low-latency but also high-capacity at relatively low cost. In order to address the demand, MNOs have given significant attention to the cloud radio access network (C-RAN) due to its beneficial features in terms of performance optimization and cost-effectiveness. The de facto standard for distributing wireless signal over the C-RAN fronthaul is the common public radio interface (CPRI). However, optical links based on CPRI interfaces requires large bandwidth. Also, the aforementioned requirements can be realized with the implementation of free space optical (FSO) link, which is an optical wireless system. The FSO is an appealing alternative to the radio frequency (RF) communication system that combines the flexibility and mobility offered by the RF networks with the high-data rates provided by the optical systems. However, the FSO links are susceptible to atmospheric impairments which eventually hinder the system performance. Consequently, these limitations prevent FSO from being an efficient standalone fronthaul solution. So, precise channel characterizations and advanced technologies are required for practical FSO link deployment and operation. In this thesis, we study an efficient fronthaul implementation that is based on radio-on-FSO (RoFSO) technologies. We propose closedform expressions for fading-mitigation and for the estimation of channel capacity so as to alleviate the system complexity. Numerical simulations are presented for adaptive modulation scheme using advanced modulation formats. We also consider schemes like hybrid RF/FSO and relay-assisted transmission technologies that can help in alleviating the stringent requirements by the C-RAN backhaul/fronthaul. The propose models not only reduce the computational requirements/efforts, but also have a number of diverse merits such as high-accuracy, low-memory requirements, fast and stable operation compared to the current state-of-the-art analytical based approaches. In addition to the FSO channel characterization, we present a proof-of-concept experiment in which we study the transmission capabilities of a hybrid passive optical network (PON)-FSO system. This is implemented with the real-time receiver that is emulated by a commercial field-programmable gate array (FPGA). This helps in facilitating an open system and hence enables interoperability, portability, and open software standards. The hybrid schemes have the ability to support different applications, services, and multiple operators over a shared optical fiber infrastructure

Povećanje informacionog kapaciteta i pouzdanosti u bežičnim optičkim komunikacijama primenom algoritama adaptivnog prenosa

This dissertation considers adaptive transmission algorithms and their influence towards increasing information capacity and system reliability. Theses algorithms are applied to optical wireless communications, as well as to radio-frequency system with diversity receiver. System of adaptive transmission is described in detail, and different algorithms of adaptation are applied: ORA, OPRA, CIFR, and TIFR. Derived probability density function combines influence of atmospheric turbulence, atmospheric losses and positioning errors that consist of laser pointing precision and jitter. FSO system without adaptation is also considered. For this case, bit-error rate and outage probability are determined when signal is exposed to different strengths of atmospheric turbulence, when there is atmospheric attenuation and positioning errors due to both main causes, when IM/DD and heterodyne detection are used. Power penalty is also determined for such system. Bit-error rate is also determined for the case when SIM is used in combination with binary modulation and quadrature amplitude modulation. Capacity analysis is given for the use of ORA, OPRA, CIFR and TIFR adaptive transmission algorithms, and for IM/DD and heterodyne detection cases. Algorithms used for transmitter power adaptation are additionally analyzed and improved with an aim to obtain realistic and functional systems. Based on analytical expression, simulation model is developed for adaptive transmission systems, which is used to validate numerical results. Approximate expressions are also given for used adaptive algorithms. Finally, adaptive transmission is considered also for a class of wireless RF systems. Fading model used is based on α-κ-μ distribution. Outage probability and amount of fading are given as representative performance measures. Besides adaptive transmission algorithms used in FSO systems, analysis of space diversity reception is also presented for the general case of using L antennas

Optical Communication

Optical communication is very much useful in telecommunication systems, data processing and networking. It consists of a transmitter that encodes a message into an optical signal, a channel that carries the signal to its desired destination, and a receiver that reproduces the message from the received optical signal. It presents up to date results on communication systems, along with the explanations of their relevance, from leading researchers in this field. The chapters cover general concepts of optical communication, components, systems, networks, signal processing and MIMO systems. In recent years, optical components and other enhanced signal processing functions are also considered in depth for optical communications systems. The researcher has also concentrated on optical devices, networking, signal processing, and MIMO systems and other enhanced functions for optical communication. This book is targeted at research, development and design engineers from the teams in manufacturing industry, academia and telecommunication industries

Performance of wireless optical telecommunication systems in the presence of fading and interference

Postojeći komunikacioni sistem u domenu RF elektromagnetnog spektra nije u mogućnosti da zadovolji sve potrebe brzog i obimnog prenosa podataka, koje se javljaju usled ekspanzije i sve veće upotrebe IoT uređaja, 5G i B5G mreža, kao i raznovrsnih aplikacija i multimedijalnog sadržaja. Optička bežična komunikacija (OWC), koja koristi veliki opseg nelicenciranog dela spektra, se pokazala kao dobra alternativa za ublažavanje nedostataka konvencionalnog sistema za prenos podataka koji radi u RF domenu. FSO (Free Space Optics) tehnologija predstavlja jednu od vrsta optičkih bežičnih komunikacija, ima veliku upotrebu u LAN i MAN mrežama , bežičnom video nadzoru, koristi se u medicinske svrhe, u svemirskoj komunikaciji, za rešavanje problema poslednje milje itd. Primena bežičnih komunikacija, znatno može da doprinese performansama sistema, i to u smislu spektralne i energetske efikasnosti kao i u smislu pouzdanosti. U disertaciji je u cilju utvrđivanja optimalnog scenarija prijema signala, kao i određivanja optimalnih vrednosti parametara takvog prenosa, izvršena analiza karakteristika bežičnog optičkog prenosa signala u prisustvu turbulencije i efekta greške pozicioniranja, koji se odvija pod kompozitnim uticajem navedenih smetnji. Za posmatrane scenarije prenosa razmatrane su standardne mere performansi sistema, kao što su srednja verovatnoća greške po bitu, posmatrana za odgovarajuće modulacione formate, kao i verovatnoća otkaza. Predstavljena su analitička i numerička rešenja problema, a uticaji pojedinih parametara sistema na performanse bežičnog optičkog prenosa prikazani su i grafički

Single and Multi-Hop Vehicular Visible and Infrared Light Communications

Visible light communications (VLC) have been proposed as a complementary technology in vehicular networks due to its several merits including high security, high scalability than RF technology. Notably, the RF technology established for vehicular networks best known as the dedicated short-range communications, supports many applications but doubts still exist on the capability of this technology to meet the low latency (where not more than 20 ms is required for pre-crash sensing and cooperative collision mitigation) and high reliability requirements in intelligent transport systems (ITS), when considering issues such as network outages as well as security issues. Of interest is the wide increase in the use of light emitting diode (LED)-based vehicle and traffic lights, and cameras in vehicles (rear and dashcams), traffic and security cameras, hence opening more opportunities for the VLC technology as part of ITS. Remarkably, camera-based VLC (i.e., optical camera communications) offers even further capabilities such as vehicle localization, motion and scene detection and pattern recognition. However, the VLC system has few challenges that needs addressing for the practical implementation of this technology as part of ITS. Consequently, this thesis focuses on addressing the key challenges and proposing novel technical analytical and experimental solutions. Firstly, increasing the robustness to sunlight induced noise is one of the major challenges in vehicular VLC, hence this thesis proposes an infrared (IR) transmission, as the amount of solar irradiance is lesser in the IR band than in the visible band. Performance of the proposed scheme is validated through numerical simulations with realistic emulated sunlight noise from empirical measurement. Investigations on the effects of turbulence with aperture averaging and fog on vehicular VLC is also carried out via experiments. Secondly, increasing the communication range is another major challenge, consequently the feasibility of using different vehicle taillights (TLs) as the VLC transmitter are evaluated via simulations based on empirical measurements of the radiation characteristics and transmit powers of the TLs. Results obtained indicate that, only a very low link span of 89 m at the forward error correction (FEC) bit error rate (BER) limit of 3.8 × 10-3, compared to 4.5, 5.4, and 6.3 m for the BMW vehicle-based TL at data rates of 10, 6, and 2 Mbps are achieved under realistic sunlight conditions. While, to increase the communication distance of camera-based VLC links, reducing the spatial bandwidth of the camera in its out of focus regions is proposed, mathematically analysed, and experimentally demonstrated where up to a 400 m link span at a 100 % success reception rate is achieved at a data rate of 800 bps, which is the longest so far reported. Relay-assisted links are also investigated using amplify-and-forward (AF) and decode-and-forward (DF) relaying schemes under the emulated sunlight noise. A mathematical and simulation-based system model is developed, where different transmitter/receiver geometries are considered and AF and DF schemes. Results obtained via simulations shows that the DF scheme is a suitable candidate for vehicular VLC connectivity under emulated sunlight noise, offering at the FEC BER limit of 3.8 × 10-3 up to 150 % increase in the link distance by the end of the 2nd hop. Proof of concept experimental demonstration of AF and DF schemes for vehicular VLC are also carried out showing that DF is the preferred option. Moreover, insights are provided into the impact of various system parameters on the relay-assisted links. Finally, increasing the mobility of the vehicular VLC system is another major challenge, hence analysis on the required angular field of view (AFOV) for vehicular links considering necessary geometry parameters is investigated. Mathematical expressions to determine the required AFOV based on key system parameters are also derived. Furthermore, the relevance of the choice of the receiver parameters for an enhanced AFOV is also analysed, consequently a means to mitigate the effects of beam spot offset induced power losses at the photodiode caused by the misalignment of the transmitter and imaging receiver is proposed

Διάδοση Σήματος στις Επίγειες Οπτικές Ασύρματες Ζεύξεις με Πολυπλεξία και Τεχνικές Διαφορικής Λήψης

Οι ασύρματες οπτικές επικοινωνίες ελευθέρου χώρου (Free-Space Optical, FSO) έχουν κερδίσει σημαντικό εμπορικό και ερευνητικό ενδιαφέρον τα τελευταία χρόνια ως αποτέλεσμα των διαφόρων πλεονεκτικών χαρακτηριστικών τους. Είναι σε θέση να ανταποκριθούν στις σημαντικά αυξανόμενες ανάγκες μεταφοράς τεράστιου όγκου πληροφοριακών δεδομένων στα υφιστάμενα και μελλοντικά τηλεπικοινωνιακά δίκτυα. Τα συστήματα FSO λειτουργούν στη ζώνη συχνοτήτων μεταξύ 300 GHz - 430 THz η οποία δεν απαιτεί ειδικές άδειες για τη χρήση της, προσφέροντας ένα σημαντικό οικονομικό πλεονέκτημα σε σύγκριση με τα αντίστοιχα συστήματα ραδιοσυχνοτήτων (RF). Τα FSO συστήματα δεν επηρεάζονται από ηλεκτρομαγνητικές παρεμβολές και παρουσιάζουν υψηλό επίπεδο ασφάλειας λόγω των στενών οπτικών δεσμών laser. Επίσης, θεωρούνται φιλικές προς το περιβάλλον λόγω της χαμηλής κατανάλωσης ηλεκτρικής ενέργειας κατά τη λειτουργία τους. Σε αντίθεση με τα ευεργετικά χαρακτηριστικά τους, οι επίγειες οπτικές ασύρματες ζεύξεις είναι ευάλωτες στις ατμοσφαιρικές επιδράσεις. Το φαινόμενο της ατμοσφαιρικής τυρβώδους ροής (atmospheric turbulence) είναι ένας από τους σημαντικότερους επιβλαβείς παράγοντες κατά τη διάδοση του οπτικού ηλεκτρομαγνητικού κύματος διαμέσου της ατμόσφαιρας. Η ατμοσφαιρική τυρβώδης ροή δημιουργείται ως αποτέλεσμα των ανομοιογενειών στον δείκτη διάθλασης μεταξύ των αέριων μαζών στην ατμόσφαιρα, οδηγώντας σε διακυμάνσεις της λαμβανόμενης έντασης και φάσης και τελικώς σε απώλεια ισχύος στην πλευρά του δέκτη. Λόγω των ραγδαίων διακυμάνσεων που προκαλούνται στο λαμβανόμενο οπτικό σήμα, η επίδραση της ατμοσφαιρικής τυρβώδους ροής μελετάται μέσω στατιστικών μοντέλων για την συνάρτηση της πυκνότητας πιθανότητας της λαμβανόμενης οπτικής έντασης, φαινόμενο γνωστό και ως σπινθηρισμός. Τα συστήματα FSO συνήθως εγκαθίστανται στις στέγες υψηλών κτιρίων ή σε μεγάλα υψόμετρα πάνω από το έδαφος. Έτσι, αυτά τα συστήματα είναι ευάλωτα σε ριπές ανέμου, σε πιθανή ταλάντευση των κτιρίων π.χ. λόγω μικρών σεισμών και σε θερμικές συστολές και διαστολές. Κατά αυτό το τρόπο, μπορούν να προκληθούν επιπρόσθετες διακυμάνσεις στο οπτικό σήμα. Αυτό το φαινόμενο είναι γνωστό στην τεχνική βιβλιογραφία ως σφάλματα σκόπευσης (pointing errors) και μελετάται με κατάλληλα στατιστικά μοντέλα σε σύνδεση με το φαινόμενο της ατμοσφαιρικής τυρβώδους ροής. Αξίζει να σημειωθεί ότι στην πλειονότητα των περιπτώσεων χρησιμοποιείται ένα προσεγγιστικό μοντέλο της κατανομής του Beckmann, η οποία λαμβάνει υπόψη την πιθανή σταθερή μη μηδενική απόκλιση του κέντρου της οπτικής δέσμης από το κέντρο του δέκτη και διαφορετικές τυπικές αποκλίσεις για την ακτινική μετατόπιση στους κατακόρυφους άξονες στο επίπεδο του δέκτη. Εκτός από τα προαναφερθέντα στατιστικά φαινόμενα, οι FSO ζεύξεις υποφέρουν από διάφορες προκαθοριστικές επιπτώσεις, όπως ο θόρυβος περιβάλλοντος, απώλειες οπτικής ισχύος λόγω διαφόρων ατμοσφαιρικών συστατικών (σωματιδίων, μορίων) και από ποικίλες καιρικές συνθήκες όπως ομίχλη, βροχή, χαλάζι κλπ., και απώλειες διάδοσης ελευθέρου χώρου. Όλα τα μαθηματικά μοντέλα που περιγράφουν την επίδραση αυτών των φαινομένων, με πολύ υψηλή ακρίβεια, περιλαμβάνονται στη διατριβή και ο αντίκτυπός τους μελετάται στην τελική αξιολόγηση των επιδόσεων των ασύρματων οπτικών ζεύξεων. Οι τεχνικές διαφορικής λήψης έχουν αποδειχθεί πολύ αποτελεσματικές στην καταπολέμηση διαλείψεων και εξασθενίσεων στα RF τηλεπικοινωνιακά συστήματα. Στην παρούσα διατριβή μελετάται η εφαρμογή διαφορικής λήψης στα FSO συστήματα. Συγκεκριμένα, διερευνάται η διαφορική λήψη στο δέκτη μαζί με τη βέλτιστη περίπτωση χρήσης του συνδυαστή μέγιστης αναλογίας (MRC). Η διαφορική λήψη δεκτών μελετάται για ένα FSO σύστημα μονής εισόδου πολλαπλής εξόδου (SIMO) με χρήση τεχνικών ψηφιακής διαμόρφωσης. Μελετώνται οι πιο συχνά εφαρμοζόμενες τεχνικές ψηφιακής διαμόρφωσης στα συστήματα οπτικών επικοινωνιών, όπως η κωδικοποίηση on-off (OOK), η διαμόρφωση πλάτους παλμού (PAM ) και η διαμόρφωση θέσης παλμού (PPM). Η απόδοση της SIMO FSO ζεύξης με διαφορική λήψη εκτιμάται με βάση την μέτρηση του μέσου ρυθμού σφάλματος δυαδικών ψηφίων (average BER), υπό την επίδραση της ατμοσφαιρικής τυρβώδους ροής που μοντελοποιείται είτε μέσω της Gamma-Gamma (GG) κατανομής είτε μέσω της εκθετικής κατανομής (NE). Ο ρυθμός σφάλματος μπλοκ πληροφορίας (BLER) αποτελεί μια βασική μετρική απόδοσης για κάθε τηλεπικοινωνιακή ζεύξη που λειτουργεί σε σχετικά υψηλούς ρυθμούς μετάδοσης. Είναι μια μετρική που έχει ερευνηθεί κυρίως στις RF επικοινωνίες. Στο πλαίσιο της παρούσας διδακτορικής διατριβής, διερευνάται η μέση απόδοση BLER ενός OOK FSO συστήματος σε συνθήκες ατμοσφαιρικής τυρβώδους ροής που μοντελοποιείται μέσω των κατανομών GG και NE με σφάλματα σκόπευσης μη-μηδενικής απόκλισης. Η τεχνική αναλογικής διαμόρφωσης έντασης (AIM) έχει διερευνηθεί εκτενώς στις επικοινωνίες οπτικών ινών μέσω των πεδίων της μικροκυματικής φωτονικής (MWP) και των ραδιοσυχνοτήτων μέσω οπτικών ινών (RoF). Ωστόσο, η εφαρμογή της στις ασύρματες οπτικές συνδέσεις βρίσκεται ακόμη σε πρώιμο στάδιο. Σε αυτή τη διατριβή διεξάγεται εκτενής έρευνα για την εφαρμογή των τεχνικών AIM στις FSO ζεύξεις και ειδικότερα στη τεχνική μεταφοράς RF σήματος μέσω των FSO συστημάτων, μια τεχνική γνωστή ως Radio-over-FSO (RoFSO). Έτσι, οι συνδέσεις RoFSO εξετάζονται για τη μετάδοση σημάτων με πολυπλεξία όπως OFDM και CDMA σε κανάλια ατμοσφαιρικής τυρβώδους ροής με σφάλματα σκόπευσης. Αξίζει να σημειωθεί ότι για την περίπτωση της CDMA RoFSO μετάδοσης, η απόδοση ενός τέτοιου συστήματος διερευνάται για πρώτη φορά στις κατευθύνσεις της εμπρόσθιας και της αντίστροφης ζεύξης σε συνθήκες τυρβώδους ροής που μοντελοποιούνται από το ενοποιητικό μοντέλο της M(alaga) κατανομής. Μια από τις πιο ελπιδοφόρες λύσεις, προκειμένου να βελτιωθεί η απόδοση, να ξεπεραστούν οι επιβλαβείς ατμοσφαιρικές επιπτώσεις και να επεκταθεί τελικά η απόσταση κάλυψης των FSO συστημάτων, είναι η χρήση αρχιτεκτονικών αναμετάδοσης. Εξετάζεται η εφαρμογή αρχιτεκτονικής πολλαπλών αλμάτων με σειριακούς κόμβους αποκωδικοποίησης και προώθησης (DF) για ένα σύστημα OFDM RoFSO. Οι συγκεκριμένοι DF κόμβοι δρουν ως αναγεννητές για το σήμα πληροφορίας και έτσι επιτυγχάνεται μια βέλτιστη απόδοση. Η βελτίωση της απόδοσης για το σύστημα πολλαπλών αλμάτων αξιολογείται μέσω του μέσου ρυθμού σφάλματος δυαδικών ψηφίων και της εκτίμησης της πιθανότητας διακοπής. Τέλος, μελετάται η διαφορική λήψη δεκτών για OFDM και CDMA RoFSO ζεύξεις, όπου οι συγκεκριμένες χωρικά ποικιλόμορφες ζεύξεις χρησιμοποιούν πολλαπλές πηγές laser. Σε αυτό το σύστημα διαφορικής λήψης, κάθε μία από τις πηγές laser συνδέεται με ένα συγκεκριμένο δέκτη, μέσω της χρήσης πολύ στενών οπτικών δεσμών. Τα αποτελέσματα που προκύπτουν αποκαλύπτουν την αποτελεσματικότητα αυτής της διαμόρφωσης καθιστώντας αυτά τα συστήματα RoFSO με διαφορική λήψη δεκτών πολύ αξιόπιστα ακόμη και στις πιο δυσμενείς συνθήκες λειτουργίας τους υπό ισχυρή επίδραση της ατμοσφαιρικής τυρβώδους ροής, των σφαλμάτων σκόπευσης και μη γραμμικών φαινομένων που σχετίζονται με τα συστήματα RoFSO.Free-Space Optical (FSO) communication systems have been gaining significant commercial and research interest in the last few years as a result of their various advantageous features. They are capable of meeting the fast-paced growing needs for transferring huge amounts of data in the existing and future telecommunications networks. FSO systems operate in the unlicensed band of frequencies between 300 GHz – 430 THz, offering a significant economic advantage compared to their radio frequency (RF) counterparts. They are immune to electromagnetic interference and exhibit high-security level due to their narrow optical laser beams. Also, they are considered as environmental-friendly due to their low electrical energy consumption. Unlike their beneficial characteristics, the terrestrial FSO links are vulnerable to atmospheric effects. The atmospheric turbulence phenomenon is one of the main degradation factors for the electromagnetic optical-wave propagation in the atmospheric medium. Atmospheric turbulence arises as a result of inhomogeneities in the refractive index between air masses in the atmosphere, leading to intensity and phase fluctuations and eventually to amplitude loss on the receiver side. Due to the rapid fluctuations induced to the optical signal, the atmospheric turbulence effect is studied in a statistical manner through probability density functions for the characterization of irradiance fluctuations or the commonly referred to as scintillations. FSO systems are usually installed at the rooftops of tall buildings or at high altitudes above the ground. Thus, these systems are susceptible to gusts of wind, potential sway of the buildings e.g. due to small earthquakes and thermal contraction and expansion. In a similar vein, additional irradiance fluctuations can be provoked to the optical signal. This phenomenon is well-known in the technical literature as pointing errors and is studied statistically in conjunction with the atmospheric turbulence effect. It is worth noting that an approximation of the Beckmann’s distribution model is employed in most cases, which takes account of the potential fixed non-zero deviation of the optical beam centre from the receiver centre and different standard deviations for the radial displacement for the vertical axes at the receiver. Apart from the foregoing statistical phenomena, FSO links suffer from various deterministic effects such as background noise, optical power losses due to various atmospheric constituents and weather conditions such as fog, haze, rain, hail etc and free-space loss. All the mathematical models that describe the behavior of the aforementioned effects, with very high accuracy, are included in the thesis and their impact is studied to the final performance evaluation of the wireless optical links. Spatial diversity techniques have been proved very effective in combating fading in RF wireless communication systems. In the present thesis, the application of spatial diversity to the FSO systems is studied. Specifically, the spatial diversity of the receivers is investigated along with the optimum case of using the maximum ratio combiner (MRC). The spatial diversity of the receivers is studied for a single-input multiple-output (SIMO) FSO link employing some of the most widely used modulation schemes in optical communications, such as on-off keying (OOK), pulse amplitude modulation (PAM) and pulse position modulation (PPM). The performance of the link is assessed in terms of the average bit error rate (BER) metric estimation, under the influence of the atmospheric turbulence effect modeled either by the gamma-gamma (GG) or the negative exponential (NE) distribution with pointing errors. The block error rate (BLER) constitutes an essential performance measure for every communication link operating at relatively high throughput conditions. It’s a metric which has been investigated mostly in RF communications. In the context of the current thesis, the average BLER performance of an OOK FSO link is investigated over atmospheric turbulent conditions modeled by the GG and NE distributions with non-zero boresight pointing errors. Analogue intensity modulation (AIM) technique has been extensively researched in optical fibre communications through the fields of microwave photonics (MWP) and radio over fibre (RoF). However, its application to the wireless optical links is at an immature stage. In this thesis, extensive research is conducted for the application of AIM techniques to the FSO links and especially of the RF signal transport scheme over FSO links, a technique known as Radio-over-FSO (RoFSO). Thus, RoFSO links are examined for transmission of OFDM and CDMA RF signals over atmospheric turbulence channels with pointing errors. It is worth mentioning that for the case of the CDMA RoFSO link, the performance is investigated for the first time for both directions of the forward and the reverse link over atmospheric turbulent conditions modeled by the M(alaga) distribution. One of the most promising solutions, in order to enhance the performance, overcome the harmful atmospheric effects and eventually extend the distance coverage of FSO systems, is the use of relay architectures. The application of multi-hop architecture with serial decode-and-forward (DF) relay nodes to an OFDM RoFSO system is investigated. The specific DF relay nodes act as regenerators for the information signal and thus an optimum performance is achieved. The performance improvement for the multi-hop system is evaluated through the average bit error rate and the outage probability estimation. Finally, the spatial diversity of the receivers is studied for OFDM and CDMA RoFSO links, where the specific spatially diverse links employ multiple laser sources. In this scheme, each one of the laser sources is linked to a specific receiver, through the use of very narrow optical beams. The derived results reveal the effectiveness of this configuration, rendering the links very reliable even in the most adverse operating conditions under the strong influence of the atmospheric turbulence and the pointing errors and the enhanced impact of the nonlinear distortion effects related to the RoFSO systems

1-D broadside-radiating leaky-wave antenna based on a numerically synthesized impedance surface

A newly-developed deterministic numerical technique for the automated design of metasurface antennas is applied here for the first time to the design of a 1-D printed Leaky-Wave Antenna (LWA) for broadside radiation. The surface impedance synthesis process does not require any a priori knowledge on the impedance pattern, and starts from a mask constraint on the desired far-field and practical bounds on the unit cell impedance values. The designed reactance surface for broadside radiation exhibits a non conventional patterning; this highlights the merit of using an automated design process for a design well known to be challenging for analytical methods. The antenna is physically implemented with an array of metal strips with varying gap widths and simulation results show very good agreement with the predicted performance