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Multipulse pulse position modulation (MPPM) has been widely proposed to improve data rate over the traditional pulse position modulation (PPM) in free-space optical communication systems. However, there is no known efficient method of encoding MPPM codewords. Furthermore, MPPM is not the optimal coding scheme (in terms of data rate) given the two main constraints of optical systems (duty cycle and zero runlength). In this work, an improved encoding technique for MPPM is provided as well as an analysis of regions where significant rate gain over MPPM is achievable. A new coding technique based on constrained coding is introduced that allows construction of codes which achieves considerable rate gain over comparable MPPM systems. In addition, our new codes allow for convenient concatenation with an outer-code and are suitable for iterative decoding. Simulation results show that these codes can achieve a $6$ dB coding gain over comparable MPPM systems.

Multipulse pulse position modulation (MPPM) has been widely proposed to improve data rate over the traditional pulse position modulation (PPM) in free-space optical communication systems. However, there is no known efficient method of encoding MPPM codewords. Furthermore, MPPM is not the optimal coding scheme (in terms of data rate) given the two main constraints of optical systems (duty cycle and zero runlength). In this work, an improved encoding technique for MPPM is provided as well as an analysis of regions where significant rate gain over MPPM is achievable. A new coding technique based on constrained coding is introduced that allows construction of codes which achieves considerable rate gain over comparable MPPM systems. In addition, our new codes allow for convenient concatenation with an outer-code and are suitable for iterative decoding. Simulation results show that these codes can achieve a $6$ dB coding gain over comparable MPPM systems.

Στα οπτικά τηλεπικοινωνιακά συστήματα μεγάλων αποστάσεων το κύριο μέλημα είναι η καταπολέμηση των φαινομένων διάδοσης στις οπτικές ίνες. Από το 2004 και μετά το ενδιαφέρον της ερευνητικής κοινότητας έχει στραφεί προς τα οπτικά τηλεπικοινωνιακά συστήματα με δέκτες σύμφωνης φώρασης, καθώς οι τελευταίοι επιτρέπουν την αντιμετώπιση των φαινομένων διάδοσης με ηλεκτρονικά μέσα. Οι σύμφωνοι δέκτες έχουν κατά πολύ αυξημένη πολυπλοκότητα και κόστος σε σχέση με τους δέκτες άμεσης φώρασης, κάνοντας χρήση επιπλέον διατάξεων όπως laser τοπικού ταλαντωτή και προσαρμοσμένες φωτοδιόδους. Όμως, οι δέκτες σύμφωνης φώρασης διατηρούν όλη την πληροφορία (πλάτος και φάση) κατά τη μετατροπή του σήματος από οπτικό σε ηλεκτρονικό, σε αντίθεση με τους δέκτες άμεσης φώρασης που διατηρούν μόνο την πληροφορία που μεταφέρεται από τη στιγμιαία ισχύ του σήματος. Το μεγάλο αυτό πλεονέκτημα των σύμφωνων δεκτών δίνει τη δυνατότητα της αντιμετώπισης των φαινομένων διάδοσης, όπως ο θόρυβος φάσης που θα αναλυθεί σε αυτήν την εργασία, με χρήση συστημάτων ψηφιακής επεξεργασίας σήματος. Ο θόρυβος φάσης είναι ένας από τους σημαντικότερους περιοριστικούς παράγοντες της απόδοσης ενός σύμφωνου οπτικού τηλεπικοινωνιακού συστήματος. Η εκτίμηση και η απαλοιφή του αποτελεί πάντα αναγκαίο και θεμελιώδες βήμα της σωστής ανάκτησης των δεδομένων στο δέκτη, αλλά και σημαντικότατο παράγοντα της επιτυχούς σχεδίασης ενός οπτικού τηλεπικοινωνιακού συστήματος. Παραδοσιακά, για τον προσδιορισμό του θορύβου φάσης, χρησιμοποιείται ένα κύκλωμα PLL (Phase Locked Loop). Το κύκλωμα αυτό όμως παρουσιάζει αυξημένη πολυπλοκότητα, όσον αφορά την υλοποίησή του σε hardware. Έτσι, το επιστημονικό ενδιαφέρον έχει στραφεί πια στη χρήση προσαρμοστικών αλγορίθμων προσδιορισμού θορύβου φάσης. Για την εκτίμηση του θορύβου φάσης χρησιμοποιούνται κυρίως τυφλοί προσαρμοστικοί εξισωτές πρόσθιας τροφοδότησης, αλλά έχουν προταθεί στη βιβλιογραφία και εξισωτές με ανατροφοδότηση απόφασης. Στην πράξη όμως, μόνο οι εξισωτές πρόσθιας τροφοδότησης είναι υλοποιήσιμοι σε hardware αφού η ανάδραση στα οπτικά δίκτυα εισάγει σημαντική καθυστέρηση. Στην παρούσα εργασία μελετάμε με προσομοίωση την απόδοση πέντε διαφορετικών προσαρμοστικών αλγορίθμων εκτίμησης του θορύβου φάσης σε σύμφωνα συστήματα δεκαεξαδικής ορθογωνικής διαμόρφωσης πλάτους 16-QAM (Quadrature Amplitude Modulation). Πρόκειται για τον αλγόριθμο του Pfau, τον αλγόριθμο του Fatadin, τον αλγόριθμο ελαχίστων μέσων τετραγώνων (LMS, Least Mean Square), τον κανονικοποιημένο αλγόριθμο ελαχίστων μέσων τετραγώνων (NLMS, Normalized LMS) και τον αλγόριθμο ελαχίστων μέσων τετραγώνων με περιορισμούς (Constrained LMS). / The subject of this Diploma Thesis is the phase noise estimation in long-haul terrestrial coherent optical communications systems, using 16-QAM modulation (Quadrature Amplitude Modulation). Five adaptive equalization algorithms are studied by simulation. The phase noise is a limiting factor of coherent optical communications systems efficiency. The phase noise etsimation is necessary for the right data recovery at the receiver and for the successful design of a coherent optical communications system. Traditionally, a PLL (Phase Locked Loop) circuit is used for the phase noise estimation. However, the implementation of PLL in hardware show increased complexity. Thus, scientific interest is focused to the adaptive algorithms for the phase noise estimation. Blind adaptive feed forward equalizers are used for the phase noise estimation, while decision feedback equalizers have been also proposed in the literature. In practice, however, only the feed forward equalizers are achievable in hardware since the feedback introduces significant delay in optical networks. In this Diploma Thesis we study, by simulation, the performance of five different adaptive phase noise estimation algorithms in 16-QAM (Quadrature Amplitude Modulation) coherent systems. These algorithms are: the Pfau algorithm , Fatadin algorithm, the Least Mean Square Algorithm (LMS Algorithm), the Normalized LMS (NLMS) and the Constrained LMS.

Les longueurs d’onde optiques sont une alternative aux liens radio-fréquences pour lestransmissions satellite-sol du futur. Elles sont envisagées pour les futurs systèmes de télémesuresatellitaires (liens optiques descendants en provenance de satellites LEO) ou de communication(liens optiques bi-directionnels avec des satellites GEO). A sa traversée de l’atmosphère l’ondeoptique peut être profondément affectée par la turbulence atmosphérique. Elle subit desvariations spatiales et temporelles d’amplitude et de phase. Les variations d’amplitudesse traduisent par des variations de la puissance lumineuse collectée (scintillations). Lesperturbations de la phase affectent la distribution spatiale de la puissance au foyer du systèmede détection, qui n’est alors plus limitée par la diffraction. Des pertes peuvent en découler lorsdu couplage du flux incident à un détecteur optronique ou à une fibre optique monomode.Ces pertes se traduisent par des atténuations du signal reçu et donc par la perte d’informations.Pour s’en abstraire, les études de faisabilité les plus récentes mettent en avant l’utilisation desystèmes d’optique adaptative et de techniques numériques adaptées (codage/entrelacement).Pour limiter la complexité et le coût des systèmes de liens optiques, la définition des techniquesde compensation des atténuations peut être menée conjointement. C’est l’objectif principalde cette thèse. Il s’agit d’investiguer les complémentarités des techniques de compensationphysiques (optique adaptative) et numériques (entrelacement, codes correcteurs) pour disposerdes éléments permettant de définir les systèmes de correction les mieux adaptés. / Optical wavelengths are an alternative to radio-frequency links for future satellite-to-groundtransmissions. They are envisioned in the framework of payload/telemetry data transfer (opticaldownlinks from LEO satellites) or communication (bi-directional optical links with GEOsatellites). However, as it propagates through the atmosphere, the optical wave can be deeplyaffected by atmospheric turbulence which induces randomspatial and temporal variations ofits amplitude and phase. Variations in amplitude translate into fluctuations of the collectedpower (scintillation). The phase distortions affect the spatial distribution of the power at thefocal plane of the telescope causing deleterious losses when the incident flux needs to becoupled to an optoelectronic detector or to a single-mode optical fiber. Such losses result indynamical attenuations of the received signal -called fading- and hence potentially to the lossof information. The most recent feasibility studies highlight the use of two types of fadingmitigation techniques: adaptive optics systems and digital techniques (coding and interleaving).To limit the complexity and cost of such systems, the optimization of these mitigationtechniques should be conducted jointly. The main objective of this thesis is therefore theinvestigation of the complementarity of physical (adaptive optics) and digital data reliabilitymechanisms (interleaving, correcting and erasure codes in a cross-layer approach).

We experimentally demonstrate an eight-state continuous-variable quantum key distribution (CV-QKD) over atmospheric turbulence channels. The high secret key rate (SKR) is enabled by 4-D multiplexing of 96 channels, i.e., six-channel wavelength-division multiplexing, four-channel orbital angular momentum multiplexing, two-channel polarization multiplexing, and two-channel spatial-position multiplexing. The atmospheric turbulence channel is emulated by a spatial light modulator on which a series of azimuthal phase patterns yielding Andrews' spectrum are recorded. A commercial coherent receiver is implemented at Bob's side, followed by a phase noise cancellation stage, where channel transmittance can be monitored accurately and phase noise can be effectively eliminated. Compared to four-state CV-QKD, eight-state CV-QKD protocol potentially provides a better performance by offering higher SKR, better excess noise tolerance, and longer secure transmission distance. In our proposed CV-QKD system, the minimum transmittances of 0.24 and 0.26 are required for OAM states of 2 (or -2) and 6 (or -6), respectively, to guarantee the secure transmission. A maximum SKR of 3.744 Gb/s is experimentally achievable, while a total SKR of 960 Mb/s can be obtained in case of mean channel transmittances.

When using quantum key distribution (QKD), one of the trade-offs for security is that the generation rate of a secret key is typically very low. Recent works have shown that using a weak coherent source allows for higher secret key generation rates compared to an entangled photon source, when a channel with low loss is considered. In most cases, the system that is being studied is over a fiber-optic communication channel. Here a theoretical QKD system using the BB92 protocol and entangled photons over a free-space maritime channel with multiple spatial modes is presented. The entangled photons are generated from a spontaneous parametric down conversion (SPDC) source of type II. To employ multiple spatial modes, the transmit apparatus will contain multiple SPDC sources, all driven by the pump lasers assumed to have the same intensity. The receive apparatuses will contain avalanche photo diodes (APD), modeled based on the NuCrypt CPDS-1000 detector, and located at the focal point of the receive aperture lens. The transmitter is assumed to be located at Alice and Bob will be located 30 km away, implying no channel crosstalk will be introduced in the measurements at Alices side due to turbulence. To help mitigate the effects of atmospheric turbulence, adaptive optics will be considered at the transmitter and the receiver. An eavesdropper, Eve, is located 15 km from Alice and has no control over the devices at Alice or Bob. Eve is performing the intercept resend attack and listening to the communication over the public channel. Additionally, it is assumed that Eve can correct any aberrations caused by the atmospheric turbulence to determine which source the photon was transmitted from. One, four and nine spatial modes are considered with and without applying adaptive optics and compared to one another.

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.

The means of communication in oceanic environments is currently dominated by sonar. Although it is reliable for long-distance transmission, the vision of internet of underwater things (IoUT) requires an alternate means for high-data-rate transmission. It is also envisaged that a networked underwater and above-water objects, such as sensor nodes, and autonomous underwater vehicles will benefit seafloor exploration. The use of laser-based optical communication is poised to realize this dream while working hand-in-hand with acoustic and radio-frequency technologies from the littoral zone to deep blue sea. While blue and green lasers are typically utilized depending on the optical properties of the water, laser-based white light is attractive in a number of aspects. In this thesis, we proposed and realized the use of white light to model the channel and to provide the immediate decision for the preferred system configuration, which is critical for developing reliable communication links, particularly, in the presence of turbulence, which makes the alignment of underwater wireless optical communication (UWOC) links challenging. Temperature and salinity changes are among factors that change the refraction index, giving rise to beam wander. This thesis explores the dependence of underwater turbulence on the wavelength. After comparing the performance of red, green, and blue lasers, an ultra-fast comprehensive method that utilizes a white-light source that can produce a wide range of wavelengths is implemented. Experimental results show an 80%-decrease in the scintillation index as the wavelength is increased from 480 to 680 nm in weak turbulence caused by a 0.02-℃/cm temperature gradient with a 40-ppt salt concentration, which emulates conditions found in the Red Sea. The effect of turbulence on the bit error ratio (BER) is also investigated experimentally. Temperature gradients increased the BER especially for shorter wavelengths. The results along long-transmission distances were verified using Monte Carlo simulations. The correlation matrix between wavelengths was studied, which is important for designing multiple-input multiple-output systems. The results obtained show that as the difference in the wavelengths increases, the correlation decreases. Based on the interplay among scintillations, scattering, absorption, and the correlation between different wavelengths, it is possible to design a more reliable UWOC link.

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