Robust Optical Wireless Links over Turbulent Media using Diversity Solutions

Free-space optic (FSO) technology, i.e., optical wireless communication (OWC), is widely recognized as superior to radio frequency (RF) in many aspects. Visible and invisible optical wireless links solve first/last mile connectivity problems and provide secure, jam-free communication. FSO is license-free and delivers high-speed data rates in the order of Gigabits. Its advantages have fostered significant research efforts aimed at utilizing optical wireless communication, e.g. visible light communication (VLC), for high-speed, secure, indoor communication under the IEEE 802.15.7 standard. However, conventional optical wireless links demand precise optical alignment and suffer from atmospheric turbulence. When compared with RF, they suffer a low degree of reliability and lack robustness. Pointing errors cause optical transceiver misalignment, adversely affecting system reliability. Furthermore, atmospheric turbulence causes irradiance fluctuations and beam broadening of transmitted light. Innovative solutions to overcome limitations on the exploitation of high-speed optical wireless links are greatly needed.Spatial diversity is known to improve RF wireless communication systems. Similar diversity approaches can be adapted for FSO systems to improve its reliability and robustness; however, careful diversity design is needed since FSO apertures typically remain unbalanced as a result of FSO system sensitivity to misalignment. Conventional diversity combining schemes require persistent aperture monitoring and repetitive switching, thus increasing FSO implementation complexities. Furthermore, current RF diversity combining schemes may not be optimized to address the issue of unbalanced FSO receiving apertures.This dissertation investigates two efficient diversity combining schemes for multi-receiving FSO systems: switched diversity combining and generalized selection combining. Both can be exploited to reduce complexity and improve combining efficiency. Unlike maximum ratio combing, equal gain combining, and selective combining, switched diversity simplifies receiver design by avoiding unnecessary switching among receiving apertures. The most significant advantage of generalized combining is its ability to exclude apertures with low quality that could potentially affect the resultant output signal performance.This dissertation also investigates mobile FSO by considering a multi-receiving system in which all receiving FSO apertures are circularly placed on a platform. System mobility and performance are analyzed. Performance results confirm improvements when using angular diversity and generalized selection combining.The précis of this dissertation establishes the foundation of reliable FSO communications using efficient diversity-based solutions. Performance parameters are analyzed mathematically, and then evaluated using computer simulations. A testbed prototype is developed to facilitate the evaluation of optical wireless links via lab experiments

On the Performance of Terrestrial Free-Space Optical (FSO) Links under the Presence of Generalized Pointing Errors

En ambos grupos se han obtenido expresiones matemáticas en forma cerrada que permiten evaluar la capacidad en todo el rango de valores de SNR (Signal-to-Noise Ratio) en algunos casos y, en otros, solo ha sido posible obtener su comportamiento asintótico debido a la dificultad matemática que presentaba el análisis. A la luz de los resultados obtenidos, podemos concluir que los sistemas MISO FSO son probablemente la solución más interesante en comparación a los sistemas SIMO y MIMO FSO. Al mismo tiempo, los resultados obtenidos en comunicaciones cooperativas permiten concluir que los sistemas cooperativos basados en retransmisión DF son capaces de aumentar la capacidad e incluso mejorar a la capacidad obtenida por un sistema basado en diversidad espacial para determinadas posiciones del nodo retransmisor. En el caso de las contribuciones realizadas en el modelado de errores por desapuntamiento generalizado, los cuales siguen una distribución Beckmann, podemos destacar la aproximación propuesta en esta tesis que nos permite incluir de una forma eficiente y sencilla dichos errores por desapuntamiento al análisis de prestaciones de cualquier sistema de comunicaciones FSO. La herramienta propuesta es válida para analizar cualquier sistema FSO en términos de BER y probabilidad de outage y nos permite detectar qué efecto es dominante, es decir, si la turbulencia atmosférica o los errores por desapuntamiento. El efecto de la correlación también ha sido contemplado, concluyendo que no puede ser ignorado.Los sistemas de comunicaciones ópticas en espacio libre (FSO, Free-Space Optical) para aplicaciones terrestres se presentan en la actualidad como una solución muy interesante para solventar el importante reto provocado por la escasez del espectro RF (Radio-Frequency) disponible. Además, los sistemas FSO se configuran como una seria alternativa frente a otras tecnologías de acceso y transporte como los sistemas de RF debido a las altas tasas de señalización potencialmente muy superiores que se pueden conseguir. Estas ventajas, entre otras, han intensificado la investigación en estos sistemas en las últimas décadas. Por tanto, el análisis de sus prestaciones en términos de probabilidad de error de bit (BER, Bit Error-Rate), probabilidad de outage y capacidad ergódica es de interés relevante, siendo estas altamente afectadas por la turbulencia atmosférica, los errores por desapuntamiento entre transmisor y receptor así como por la niebla densa. En esta tesis, el análisis de las prestaciones de los sistemas FSO ha sido abordado, presentando novedosos resultados para la comunidad científica e investigadora. Dicho análisis de prestaciones se ha dividido en dos grandes áreas de investigación: análisis de la capacidad ergódica, y modelado de errores por desapuntamiento generalizado entre transmisor y receptor. Las contribuciones realizadas dentro del análisis de la capacidad ergódica están divididas en dos grupos: por un lado, el análisis de la capacidad de sistemas FSO avanzados basados en diversidad espacial tales como los sistemas MISO (Multiple-Input/Single-Output), SIMO (Single-Input/Multiple-Output) y MIMO(Multiple-Input/Multiple-Output) FSO; por otro lado, el análisis de la capacidad de sistemas cooperativos basados en retransmisión DF (Detect-and-Forward)

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

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