Fiber-optic code division multiple access : multi-class optical orthogonal codes, optical power control, and polarization encoding

Ever since the mid- 1980s when the single-mode fiber-optic media were believed to become the main highways of future telecommunications networks for transporting high-volume high-quality multipurpose information, the need for all-optical multi-access networking became important. An all-optical multi-access network is a collection of multiple nodes where the interconnection among various nodes is via single- or multi-mode fiber optics and for which they perform all their essential signal processing requirements such as switching, add-drop, multiplexing/demultiplexing and amplification in the optical domain. Optical CDMA networking is one possible technique that allows multiple users in local area networks to access the same fiber channel asynchronously with no delay or scheduling. Optical CDMA networks are not without their own problems. Search for codes suitable to the optical domain is one of the important topics addressed in the literature on optical CDMA. Existing codes developed in the late 80's are limited to single class traffic or can support multiclass traffic but with restrictions on code lengths and weights. Also the number of generated codes is severely limited due to orthogonality issues. In this thesis, we pay particular attention to propose new codes that can support multiclass traffic with arbitrary code weights and lengths. Therefore, data sources with varying traffic demands can be accommodated by optical CDMA networks using the proposed codes. We also present a simple generation technique for the proposed multiclass codes and analyze their performance. The number of users supported by the proposed multiclass codes will be limited since it is an extension of existing code designs with such limitation. We then propose the use of polarization dimension in order to double the number of supported users. On the other hand, incoherent optical CDMA systems are considered as positive systems meaning that only unipolar codes can be considered for such systems. Therefore, multiple access interference will be quite high in optical CDMA due to the nature of incoherent power detection. Reducing the effect of the interference on the performance of optical CDMA is an important topic. We propose the use of power control to decrease the effects of interference in optical star networks in which users' fiber lengths and data rates are not equal. We consider the case of optically amplified network with amplifier noise as the main source. We then elaborate by considering the nonlinearity in the photodetection process and propose the use of an iterative algorithm to find the solution of the non-linear optical power control problem. Finally, we propose an optical CDMA system based on polarization encoding. Since the encoding is performed in the spatial domain, therefore, positive and negative levels can be realized. This approach leads to increasing the number of supported users of optical CDMA by the use of known codes, such as Gold and Hadamard codes, with enhanced performance.reviewe

Fast Decoder for Overloaded Uniquely Decodable Synchronous CDMA

We consider the problem of designing a fast decoder for antipodal uniquely decodable (errorless) code sets for overloaded synchronous code-division multiple access (CDMA) systems where the number of signals K_{max}^a is the largest known for the given code length L. The proposed decoder is designed in a such a way that the users can uniquely recover the information bits with a very simple decoder, which uses only a few comparisons. Compared to maximum-likelihood (ML) decoder, which has a high computational complexity for even moderate code length, the proposed decoder has a much lower computational complexity. Simulation results in terms of bit error rate (BER) demonstrate that the performance of the proposed decoder only has a 1-2 dB degradation at BER of 10^{-3} when compared to ML

Convergencia de tecnologías ópticas y Ethernet en LAN, MAN y SAN: nuevas arquitecturas, análisis de prestaciones y eficiencia energética

Mención Internacional en el título de doctorThe development of Information Technologies in the last decades, especially the last two, together with the introduction of computing devices to the mainstream consumer market, has had the logical consequence of the generalisation of the Internet access. The explosive development of the smartphone market has brought ubiquity to that generalisation, to the point that social interaction, content sharing and content production happens all the time. Social networks have all but increased that trend, maximising the diffusion of multimedia content: images, audio and video, which require high network capacities to be enjoyed quickly. This need for endless bandwidth and speed in information sharing brings challenges that affect mainly optical Metropolitan Area Networks (MANs) and Wide Area Networks (WANs). Furthermore, the wide spreading of Ethernet technologies has also brought the possibility to achieve economies of scale by either extending the reach of Ethernet Local Area Networks (LANs) to the MAN and WAN environment or even integrating them with Storage Area Networks (SANs). Finally, this generalisation of telecommunication technologies in every day life has as a consequence an important rise in energy consumption as well. Because of this, providing energy efficient strategies in networking is key to ensure the scalability of the whole Internet. In this thesis, the main technologies in all the fields mentioned above are reviewed, its core challenges identified and several contributions beyond the state of the art are suggested to improve today’s MANs andWANs. In the first contribution of this thesism, the integration between Metro Ethernet and Wavelength Division Multiplexion (WDM) optical transparent rings is explored by proposing an adaptation architecture to provide efficient broadcast and multicast. The second contribution explores the fusion between transparent WDM and OCDMA architectures to simplify medium access in a ring. Regarding SANs, the third contribution explores the challenges in SANs through the problems of Fibre Channel over Ethernet due to buffer design issues. In this contribution, analysis, design and validation with FCoE traces and simulation is provided to calculate buffer overflow probabilities in the absence of flow control mechanisms taking into account the bursty nature of SAN traffic. Finally, the fourth and last contribution addresses the problems of energy efficiency in Plastic Optical Fibres (POF), a new kind of optical fibre more suitable for transmission in vehicles and for home networking. This contribution suggests two packet coalescing strategies to further improve the energy effiency mechanisms in POFs.El desarrollo de las Tecnologías de la Información en las últimas décadas, especialmente las últimas dos, junto con la introducción de dispositivos informáticos al mercado de masas, ha tenido como consecuencia lógica la generalización del acceso a Internet. El explosivo desarrollo del mercado de teléfonos inteligentes ha añadido un factor de ubicuidad a tal generalización, al extremo de que la interacción social, la compartición y producción de contenidos sucede a cada instante. Las redes sociales no han hecho sino incrementar tal tendencia, maximizando la difusión de contenido multimedia: imágenes, audio y vídeo, los cuales requieren gran capacidad en las redes para poder obtenerse con rapidez. Esta necesidad de ancho de banda ilimitado y velocidad en la compartición de información trae consigo retos que afectan principalmente a las Redes de Área Metropolitana (Metropolitan Area Networks, MANs) y Redes de Área Extensa (Wide Area Networks, WANs). Además, la gran difusión de las tecnologías Ethernet ha traído la posibilidad de alcanzar economías de escala bien extendiendo el alcance de Ethernet más allá de las Redes de Área Local (Local Area Networks, LANs) al entorno de las MAN y las WAN o incluso integrándolas con Redes de Almacenamiento (Storage Area Networks, SANs). Finalmente, esta generalización de las tecnologías de la comunicación en la vida cotidiana tiene también como consecuencia un importante aumento en el consumo de energía. Por tanto, desarrollar estrategias de transmisión en red eficientes energéticamente es clave para asegurar la escalabilidad de Internet. En esta tesis, las principales tecnologías de todos los campos mencionados arriba serán estudiadas, sus más importantes retos identificados y se sugieren varias contribuciones más allá del actual estado del arte para mejorar las actuales MANs y WANs. En la primera contribución de esta tesis, se explora la integración entre Metro Ethernet y anillos ópticos transparentes por Multiplexión en Longitud de Onda (Wavelength Division Multiplex, WDM) mediante la proposición de una arquitectura de adaptación para permitir la difusión y multidifusión eficiente. La segunda contribución explora la fusión entre las arquitecturas transparentes WDM y arquitecturas por Accesso Dividido Múltiple por Códigos Ópticos (OCDMA) para simplificar el acceso en una red en anillo. En lo referente a las SANs, la tercera contribución explora los retos en SANs a través de los problemas de Fibre Channel sobre Ethernet debido a los problemas en el diseño de búferes. En esta contribución, se provee un análisis, diseño y validación con trazas FCoE para calcular las probabilidades de desbordamiento de buffer en ausencia de mecanismos de control de flujo teniendo en cuenta la naturaleza rafagosa del tráfico de SAN. Finalmente, la cuarta y última contribución aborda los problemas de eficiencia energética en Fibras Ópticas Plásticas (POF), una nueva variedad de fibra óptica más adecuada para la transmisión en vehículos y para entornos de red caseros. Esta contribución sugiere dos estrategias de agrupamiento de paquetes para mejorar los mecanismos de eficiencia energética en POFs.Programa Oficial de Posgrado en Ingeniería TelemáticaPresidente: Luca Valcarenghi.- Secretario: Ignacio Soto Campos.- Vocal: Bas Huiszoo

Dark signalling and code division multiple access in an optical fibre LAN with a bus topology

This thesis describes an optical fibre network that uses a bus topology and Code Division Multiple Access (CDMA). Various potential configurations are analysed and compared and it is shown that a serious limitation of optical CDMA schemes using incoherent correlators is the effect of optical beating due to the presence of multiple incoherent optical signals at the receiver photodiode. The network proposed and analysed in this thesis avoids beating between multiple optical fields because it only uses a single, shared, optical source. It does this through the SLIM (Single Light-source with In-line Modulation) configuration in which there is a continuously-operating light source at the head-end of a folded bus, and modulators at the nodes to impose signals on the optical field in the form of pulses of darkness which propagate along the otherwise continuously bright bus. Optical CDMA can use optical-fibre delay-line correlators as matched filters, and these may be operated either coherently or incoherently.Coherent operation is significantly more complex than incoherent operation, but incoherent correlators introduce further beating even in a SLIM network. A new design of optical delay-line correlator, the hybrid correlator, is therefore proposed, analysed and demonstrated. It is shown to eliminate beating. A model of a complete network predicts that a SLIMbus using optical CDMA with hybrid correlators can be operated at TeraBaud rates with the number of simultaneous users limited by multiple access interference (MAI), determined only by the combinatorics of the code set

Enabling Technology in Optical Fiber Communications: From Device, System to Networking

This book explores the enabling technology in optical fiber communications. It focuses on the state-of-the-art advances from fundamental theories, devices, and subsystems to networking applications as well as future perspectives of optical fiber communications. The topics cover include integrated photonics, fiber optics, fiber and free-space optical communications, and optical networking

Journal of Telecommunications and Information Technology, 2018, nr 3

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Interference mitigation techniques for optical attocell networks

The amount of wireless data traffic has been increasing exponentially. This results in the shortage of radio frequency (RF) spectrum. In order to alleviate the looming spectrum crisis, visible light communication (VLC) has emerged as a supplement to RF techniques. VLC uses light emitting diodes (LEDs) for transmission and employs photodiodes (PDs) for detection. With the advancement of the LED technology, LEDs can now fulfil two functions at the same time: illumination and high-speed wireless communication. In a typical indoor scenario, each single light fixture can act as an access point (AP), and multiple light fixtures in a room can form a cellular wireless network. We refer to this type of networks as ‘optical attocell network’. This thesis focuses on interference mitigation in optical attocell networks. Firstly, the downlink inter-cell interference (ICI) model in optical attocell networks is investigated. The conventional ray-tracing channel model for non-line-of-sight (NLOS) path is studied. Although this model is accurate, it leads to time-consuming computer simulations. In order to reduce the computational complexity, a simplified channel model is proposed to accurately characterise NLOS ICI in optical attocell networks. Using the simplified model, the received signal-to-interference-plus-noise ratio (SINR) distribution in optical attocell networks can be derived in closed-form. This signifies that no Monte Carlo simulation is required to evaluate the user performance in optical attocell networks. Then, with the knowledge of simplified channel model, interference mitigation techniques using angle diversity receivers (ADRs) are investigated in optical attocell networks. An ADR typically consists of multiple PDs with different orientations. By using proper signal combining schemes, ICI in optical attocell networks can be significantly mitigated. Also, a novel double-source cell configuration is proposed. This configuration can further mitigate ICI in optical attocell networks in conjunction with ADRs. Moreover, an analytical framework is proposed to evaluate the user performance in optical attocell networks with ADRs. Finally, optical space division multiple access (SDMA) using angle diversity transmitters is proposed and investigated in optical attocell networks. Optical SDMA can exploit the available bandwidth resource in spatial dimension and mitigate ICI in optical attocell networks. Compared with optical time division multiple access (TDMA), optical SDMA can significantly improve the throughput of optical attocell networks. This improvement scales with the number of LED elements on each angle diversity transmitter. In addition, the upper bound and the lower bound of optical SDMA performance are derived analytically. These bounds can precisely evaluate the performance of optical SDMA systems. Furthermore, optical SDMA is shown to be robust against user position errors, and this makes optical SDMA suitable for practical implementations

High speed energy efficient incoherent optical wireless communications

The growing demand for wireless communication capacity and the overutilisation of the conventional radio frequency (RF) spectrum have inspired research into using alternative spectrum regions for communication. Using optical wireless communications (OWC), for example, offers significant advantages over RF communication in terms of higher bandwidth, lower implementation costs and energy savings. In OWC systems, the information signal has to be real and non-negative. Therefore, modifications to the conventional communication algorithms are required. Multicarrier modulation schemes like orthogonal frequency division multiplexing (OFDM) promise to deliver a more efficient use of the communication capacity through adaptive bit and energy loading techniques. Three OFDM-based schemes – direct-current-biased OFDM (DCO-OFDM), asymmetrically clipped optical OFDM(ACO-OFDM), and pulse-amplitude modulated discrete multitone (PAM-DMT) – have been introduced in the literature. The current work investigates the recently introduced scheme subcarrier-index modulation OFDM as a potential energy-efficient modulation technique with reduced peak-to-average power ratio (PAPR) suitable for applications in OWC. A theoretical model for the analysis of SIM-OFDMin a linear additive white Gaussian noise (AWGN) channel is provided. A closed-form solution for the PAPR in SIM-OFDM is also proposed. Following the work on SIM-OFDM, a novel inherently unipolar modulation scheme, unipolar orthogonal frequency division multiplexing (U-OFDM), is proposed as an alternative to the existing similar schemes: ACO-OFDMand PAM-DMT. Furthermore, an enhanced U-OFDMsignal generation algorithm is introduced which allows the spectral efficiency gap between the inherently unipolar modulation schemes – U-OFDM, ACO-OFDM, PAM-DMT – and the conventionally used DCO-OFDM to be closed. This results in an OFDM-based modulation approach which is electrically and optically more efficient than any other OFDM-based technique proposed so far for intensity modulation and direct detection (IM/DD) communication systems. Non-linear distortion in the optical front-end elements is one of the major limitations for high-speed communication in OWC. This work presents a generalised approach for analysing nonlinear distortion in OFDM-based modulation schemes. The presented technique leads to a closed-form analytical solution for an arbitrary memoryless distortion of the information signal and has been proven to work for the majority of the known unipolar OFDM-based modulation techniques - DCO-OFDM, ACO-OFDM, PAM-DMT and U-OFDM. The high-speed communication capabilities of novel Gallium Nitride based μm-sized light emitting diodes (μLEDs) are investigated, and a record-setting result of 3.5Gb/s using a single 50-μm device is demonstrated. The capabilities of using such devices at practical transmission distances are also investigated, and a 1 Gb/s link using a single device is demonstrated at a distance of up to 10m. Furthermore, a proof-of-concept experiment is realised where a 50-μm LED is successfully modulated using U-OFDM and enhanced U-OFDM to achieve notable energy savings in comparison to DCO-OFDM