BER Estimation of Dual Hop QAM OFDM ROFSO over Exponentially Modeled Turbulence and Optical Fiber with Nonlinear Clipping

The radio on free space optical (RoFSO) communication systems are gaining popularity due to their high data rates, license free spectrum and adequate reliability at installation and operational costs which are much lower than comparable technologies. A significant disadvantage of these systems concerns the randomly time varying characteristics of the propagation path mainly caused by the turbulence effect. On the other hand, the optical fiber communication systems offer links with higher data rates but with expensive infrastructure and installation cost. In this work, we study the BER performance of an optical communication system which consists of a RoFSO link that is connected with an optical fiber link through a regenerator node. The signal propagates, in both links, using the OFDM technique with QAM format and the dominant impairments which have been taken into account are the atmospheric turbulence, modelled with the negative exponential distribution, the nonlinear responsivity of the laser diode which can be modelled with a Volterra series and the biasing with nonlinear clipping noise. For this setup, closed form mathematical expression for the estimation of system's BER has been derived and the corresponding numerical results are presented for common link parameters

K Modeled Turbulence and Nonlinear Clipping for QAM OFDM with FSO and Fiber Serial Linked

The free space optical (FSO) and more specifically the radio on free space optical (RoFSO) communication systems are becoming very popular because they can achieve high bit rate transmission with low operational and installation cost. The main disadvantage of such systems is their dependence on the atmospheric conditions and more specifically the randomly time varying characteristics of the propagation path through the atmosphere which is, mainly, caused by the turbulence effect and affects significantly the system's availability and performance. On the other hand, the use of optical fiber systems, offer higher bit-rates and security level but their cost is larger. In this work, the performance of a hybrid dual hop optical communication system is investigated. This system consists of a RoFSO communication link which is connected with an optical fiber link part using a regenerator between them. In both links, the modulation technique that is used is the orthogonal frequency division multiplexing (OFDM) with either a 4 or 16 QAM format. The main phenomena that are taken into account are the atmospheric turbulence, which is modelled with K distribution, the nonlinearities of the laser diode which could be modelled by Volterra series and the biasing with the nonlinear clipping at the optical fiber segment. For this system, closed form mathematical expression for the estimation of its BER is derived and numerical results are presented for realistic parameter values

OFDM Wireless Optical Communication Systems with Serial Relays Over Exponentially Modeled Turbulence Channels

The optical wireless communication systems attract very significant research and commercial interest, the last years, due to their high availability, and performance characteristics with relatively low installation and operational cost. On the other hand, due to the fact that the optical channel which is used in this technology is the atmosphere, their performance depends strongly on the atmospheric conditions and the link’s characteristics. Thus, the effective distance that these systems can cover is relatively short. Thus, in many cases where the long distance signal propagation using wireless optical systems, is necessary, relay nodes are used. In this work, we study the performance of a wireless optical system, which is using the orthogonal frequency division multiplexing (OFDM) technique and relay nodes which decode and retransmit the received signal, over atmospheric turbulence channels modeled with the negative exponential distribution. For this system, we derive closed form mathematical expressions for its outage probability and average bit error rate (BER). Finally, we present the corresponding numerical results for realistic cases with common parameter values

Coded Free-Space Optical Links over Strong Turbulence and Misalignment Fading Channels

The performance of optical wireless systems deteriorates to a large extent from the presence of turbulence and pointing error effects. To meet the typical bit error rate (BER) targets for reliable communications within the practical ranges of signal-to-noise ratio, error control coding schemes are often proposed. This paper investigates the error performance for convolutional coded on-off keying free-space optical systems through symbol by symbol interleaved channels characterized by strong turbulence and/or pointing error effects. We consider several channel types and derive exact analytical expressions for the pairwise error probability. These expressions are applied to obtain upper bounds on the BER performance using the transfer function technique

Experimental Characterisation and Modelling of Atmospheric Fog and Turbulence in FSO

Free space optical (FSO) communication uses visible or infrared (IR) wavelengths to broadcast high-speed data wirelessly through the atmospheric channel. The performance of FSO communications is mainly dependent on the unpredictable atmospheric channel such as fog, smoke and temperature dependent turbulence. However, as the real outdoor atmosphere (ROA) is time varying and heterogeneous in nature as well as depending on the magnitude and intensity of different weather conditions, carrying out a proper link assessment under specific weather conditions becomes a challenging task. Investigation and modelling the ROA under diverse atmospheric conditions is still a great challenge in FSO communications. Hence a dedicated indoor atmospheric chamber is designed and built to produce controlled atmosphere as necessary to mimic the ROA as closely as possible. The experimental results indicate that the fog attenuation is wavelength dependent for all visibility V ranges, which contradicts the Kim model for V < 0.5 km. The obtained result validates that Kim model needs to be revised for V < 0.5 km in order to correctly predict the wavelength dependent fog attenuation. Also, there are no experimental data and empirical model available for FSO links in diverse smoke conditions, which are common in urban areas. Therefore, a new empirical model is proposed to evaluate the wavelength dependent fog and smoke attenuation by reconsidering the q value as a function of wavelength rather than visibility. The BER performance of an FSO system is theoretically and experimentally evaluated for OOK- NRZ, OOK-RZ and 4-PPM formats for Ethernet line data-rates from light to dense fog conditions. A BER of 10-6 (Q-factor ≈ 4.7) is achieved at dense fog (transmittance, T = 0.33) condition using 4-PPM than OOK-NRZ and OOK-RZ modulation schemes due to its high peak-to-average power ratio albeit at the expense of doubling the bandwidth. The effects of fog on OOK-NRZ, 4-PAM and BPSK are also experimentally investigated. In comparison to 4-PAM and OOK-NRZ signals, the BPSK modulation signalling format is more robust against the effects of fog. Moreover, the effects of using different average transmitted optical communication powers Popton the T and the received Q-factor using the OOK-NRZ modulation scheme are also investigated for light and dense fog conditions. The results show that for an FSO system operating at a Q-factor of 4.7 (for BER = 10-6), the required Q-factor is achieved at T of 48% under the thick fog condition by increasing Popt to 1.07 dBm, whereas the values of T are 55% and ~70% for the transmit power of 0.56 dBm and -0.7 dBm, respectively. The experimental characterisation and investigation of the atmospheric turbulence effect on the Ethernet and Fast-Ethernet FSO link is reported using different modulation schemes. The experiment is carried out in a controlled laboratory environment where turbulence is generated in a dedicated indoor atmospheric chamber. The atmospheric chamber is calibrated to mimic an outdoor turbulence conditions and the measured data are verified against the theoretical predictions. The experiment also demonstrates methods to control the turbulence levels and determine the equivalence between the indoor and outdoor FSO links. The results show that the connectivity of Ethernet and Fast-Ethernet links are highly sensitive to atmospheric turbulence. The results also show that the BPSK and OOK-NRZ modulation signalling formats are more robust against the weak atmospheric turbulence conditions than PAM signal

Mitigation techniques through spatial diversity combining and relay-assisted technology in a turbulence impaired and misaligned free space optical channel.

Doctor of Philosophy in Electronic Engineering. University of KwaZulu-Natal, Durban, 2018.In recent times, spectrum resource scarcity in Radio Frequency (RF) systems is one of the biggest and prime issues in the area of wireless communications. Owing to the cost of spectrum, increase in the bandwidth allocation as alternative solution, employed in the recent past, does no longer offer an effective means to fulfilling high demand in higher data rates. Consequently, Free Space Optical (FSO) communication systems has received considerable attention in the research community as an attractive means among other popular solutions to offering high bandwidth and high capacity compared to conventional RF systems. In addition, FSO systems have positive features which include license-free operation, cheap and ease of deployment, immunity to interference, high security, etc. Thus, FSO systems have been favoured in many areas especially, as a viable solution for the last-mile connectivity problem and a potential candidate for heterogeneous wireless backhaul network. With these attractive features, however, FSO systems are weather-dependent wireless channels. Therefore, it is usually susceptible to atmospheric induced turbulence, pointing error and attenuation under adverse weather conditions which impose severe challenges on the system performance and transmission reliability. Thus, before widespread deployment of the system will be possible, promising mitigation techniques need to be found to address these problems. In this thesis, the performance of spatial diversity combining and relay-assisted techniques with Spatial Modulation (SM) as viable mitigating tools to overcome the problem of atmospheric channel impairments along the FSO communication system link is studied. Firstly, the performance analysis of a heterodyne FSO-SM system with different diversity combiners such as Maximum Ratio Combining (MRC), Equal Gain Combining (EGC) and Selection Combining (SC) under the influence of lognormal and Gamma-Gamma atmospheric-induced turbulence fading is presented. A theoretical framework for the system error is provided by deriving the Average Pairwise Error Probability (APEP) expression for each diversity scheme under study and union bounding technique is applied to obtain their Average Bit Error Rate (ABER). Under the influence of Gamma-Gamma turbulence, an APEP expression is obtained through a generalized infinite power series expansion approach and the system performance is further enhanced by convolutional coding technique. Furthermore, the performance of proposed system under the combined effect of misalignment and Gamma-Gamma turbulence fading is also studied using the same mathematical approach. Moreover, the performance analysis of relay-assisted dual-hop heterodyne FSO-SM system with diversity combiners over a Gamma-Gamma atmospheric turbulence channel using Decode-and-Forward (DF) relay and Amplify-and-Forward (AF) relay protocols also is presented. Under DF dual-hop FSO system, power series expansion of the modified Bessel function is used to derive the closed-form expression for the end-to-end APEP expressions for each of the combiners under study over Gamma-Gamma channel, and a tight upper bound on the ABER per hop is given. Thus, the overall end-to-end ABER for the dual-hop FSO system is then evaluated. Under AF dual-hop FSO system, the statistical characteristics of AF relay in terms of Moment Generating Function (MGF), Probability Density Function (PDF) and Cumulative Distribution Function (CDF) are derived for the combined Gamma-Gamma turbulence and/or pointing error distributions channel in terms of Meijer-G function. Based on these expressions, the APEP for each of the under studied combiners is determined and the ABER for the system is given by using union bounding technique. By utilizing the derived ABER expressions, the effective capacity for the considered system is then obtained. Furthermore, the performance of a dual-hop heterodyne FSO-SM asymmetric RF/FSO relaying system with MRC as mitigation tools at the destination is evaluated. The RF link experiences Nakagami-m distribution and FSO link is subjected to Gamma-Gamma distribution with and/or without pointing error. The MGF of the system equivalent SNR is derived using the CDF of the system equivalent SNR. Utilizing the MGF, the APEP for the system is then obtained and the ABER for the system is determined. Finally, owing to the slow nature of the FSO channel, the Block Error Rate (BLER) performance of FSO Subcarrier Intensity Modulation (SIM) system with spatial diversity combiners employing Binary Phase Shift Keying (BPSK) modulation over Gamma-Gamma atmospheric turbulence with and without pointing error is studied. The channel PDF for MRC and EGC by using power series expansion of the modified Bessel function is derived. Through this, the BLER closed-form expressions for the combiners under study are obtained

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

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