Improved Markov Models for Terrestrial Free-Space Optical Links

Finite-state Markov chains are a useful tool for modelling communication channels with correlated fading and have recently also been applied with success to terrestrial free-space optical communication channels. However, the issue of how such Markov models should be optimised in order to accurately approximate the original continuous fading channel has not been addressed in a systematic manner. In this study, the authors improve on previous proposals by optimising the state space partitioning of the considered models. In particular, they investigate the properties and approximation accuracy of Markov models which are optimised according to information-theoretic considerations. They validate and evaluate their approach using a set of experimental measurements over a 12 km link distance. The obtained results confirm that optimised Markov models can provide better accuracy at lower state complexity, yet there remain shortcomings in capturing the autocovariance of the fading process

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

A Prospective Look: Key Enabling Technologies, Applications and Open Research Topics in 6G Networks

The fifth generation (5G) mobile networks are envisaged to enable a plethora of breakthrough advancements in wireless technologies, providing support of a diverse set of services over a single platform. While the deployment of 5G systems is scaling up globally, it is time to look ahead for beyond 5G systems. This is driven by the emerging societal trends, calling for fully automated systems and intelligent services supported by extended reality and haptics communications. To accommodate the stringent requirements of their prospective applications, which are data-driven and defined by extremely low-latency, ultra-reliable, fast and seamless wireless connectivity, research initiatives are currently focusing on a progressive roadmap towards the sixth generation (6G) networks. In this article, we shed light on some of the major enabling technologies for 6G, which are expected to revolutionize the fundamental architectures of cellular networks and provide multiple homogeneous artificial intelligence-empowered services, including distributed communications, control, computing, sensing, and energy, from its core to its end nodes. Particularly, this paper aims to answer several 6G framework related questions: What are the driving forces for the development of 6G? How will the enabling technologies of 6G differ from those in 5G? What kind of applications and interactions will they support which would not be supported by 5G? We address these questions by presenting a profound study of the 6G vision and outlining five of its disruptive technologies, i.e., terahertz communications, programmable metasurfaces, drone-based communications, backscatter communications and tactile internet, as well as their potential applications. Then, by leveraging the state-of-the-art literature surveyed for each technology, we discuss their requirements, key challenges, and open research problems

Advanced Trends in Wireless Communications

Physical limitations on wireless communication channels impose huge challenges to reliable communication. Bandwidth limitations, propagation loss, noise and interference make the wireless channel a narrow pipe that does not readily accommodate rapid flow of data. Thus, researches aim to design systems that are suitable to operate in such channels, in order to have high performance quality of service. Also, the mobility of the communication systems requires further investigations to reduce the complexity and the power consumption of the receiver. This book aims to provide highlights of the current research in the field of wireless communications. The subjects discussed are very valuable to communication researchers rather than researchers in the wireless related areas. The book chapters cover a wide range of wireless communication topics

A prospective look: key enabling technologies, applications and open research topics in 6G networks

The fifth generation (5G) mobile networks are envisaged to enable a plethora of breakthrough advancements in wireless technologies, providing support of a diverse set of services over a single platform. While the deployment of 5G systems is scaling up globally, it is time to look ahead for beyond 5G systems. This is mainly driven by the emerging societal trends, calling for fully automated systems and intelligent services supported by extended reality and haptics communications. To accommodate the stringent requirements of their prospective applications, which are data-driven and defined by extremely low-latency, ultra-reliable, fast and seamless wireless connectivity, research initiatives are currently focusing on a progressive roadmap towards the sixth generation (6G) networks, which are expected to bring transformative changes to this premise. In this article, we shed light on some of the major enabling technologies for 6G, which are expected to revolutionize the fundamental architectures of cellular networks and provide multiple homogeneous artificial intelligence-empowered services, including distributed communications, control, computing, sensing, and energy, from its core to its end nodes. In particular, the present paper aims to answer several 6G framework related questions: What are the driving forces for the development of 6G? How will the enabling technologies of 6G differ from those in 5G? What kind of applications and interactions will they support which would not be supported by 5G? We address these questions by presenting a comprehensive study of the 6G vision and outlining seven of its disruptive technologies, i.e., mmWave communications, terahertz communications, optical wireless communications, programmable metasurfaces, drone-based communications, backscatter communications and tactile internet, as well as their potential applications. Then, by leveraging the state-of-the-art literature surveyed for each technology, we discuss the associated requirements, key challenges, and open research problems. These discussions are thereafter used to open up the horizon for future research directions

6G Wireless Systems: Vision, Requirements, Challenges, Insights, and Opportunities

Mobile communications have been undergoing a generational change every ten years or so. However, the time difference between the so-called "G's" is also decreasing. While fifth-generation (5G) systems are becoming a commercial reality, there is already significant interest in systems beyond 5G, which we refer to as the sixth-generation (6G) of wireless systems. In contrast to the already published papers on the topic, we take a top-down approach to 6G. We present a holistic discussion of 6G systems beginning with lifestyle and societal changes driving the need for next generation networks. This is followed by a discussion into the technical requirements needed to enable 6G applications, based on which we dissect key challenges, as well as possibilities for practically realizable system solutions across all layers of the Open Systems Interconnection stack. Since many of the 6G applications will need access to an order-of-magnitude more spectrum, utilization of frequencies between 100 GHz and 1 THz becomes of paramount importance. As such, the 6G eco-system will feature a diverse range of frequency bands, ranging from below 6 GHz up to 1 THz. We comprehensively characterize the limitations that must be overcome to realize working systems in these bands; and provide a unique perspective on the physical, as well as higher layer challenges relating to the design of next generation core networks, new modulation and coding methods, novel multiple access techniques, antenna arrays, wave propagation, radio-frequency transceiver design, as well as real-time signal processing. We rigorously discuss the fundamental changes required in the core networks of the future that serves as a major source of latency for time-sensitive applications. While evaluating the strengths and weaknesses of key 6G technologies, we differentiate what may be achievable over the next decade, relative to what is possible.Comment: Accepted for Publication into the Proceedings of the IEEE; 32 pages, 10 figures, 5 table

Advanced adaptive compensation system for free-space optical communications

Massive amounts of information are created daily in commercial fields like earth observation, that must be downloaded to earth ground stations in the short time of a satellite pass. Today, much of the collected information must be dropped due to lack of bandwidth, and laser downlinks can offer tenths of gigabits throughput solving this bottleneck limitation. In a down-link scenario, the performance of laser satellite communications is limited due to atmospheric turbulence, which causes fluctuations in the intensity and the phase of the received signal leading to an increase in bit error probability. In principle, a single-aperture phase-compensated receiver, based on adaptive optics, can overcome atmospheric limitations by adaptive tracking and correction of atmospherically induced aberrations. However, under strong-turbulence situations, the effectiveness of traditional adaptive optics systems is severely compromised. In such scenarios, sensor-less techniques offer robustness, hardware simplicity, and easiness of implementation and integration at a reduced cost, but the state-of-the-art approaches still require too many iterations to perform the correction, exceeding the temporal coherence of the field and thus falling behind the field evolution. This thesis proposes a new iterative AO technique for strong turbulence compensation that reduces the correction time, bridging the limitation of similar systems in lasercom applications. It is based on the standard sensor-less system design, but it additionally uses a short-exposure focal intensity image to accelerate the correction. The technique combines basic principles of Fourier optics, image processing, and quadratic signal optimization to correct the wave-front. This novel approach directly updates the phases of the most intense focal-plane speckles, maximizing the power coupled into a single-mode fiber convexly. Numerical analyses show that this method has a robust and excellent performance under very strong turbulence. Laboratory results confirm that a focal speckle pattern can be used to accelerate the iterative compensation. This technique delivers nearly twofold bandwidth reduction compared with standard methods, and sufficient signal gain and stability to allow high throughput data transmission with nearly error-free performance in emulated satellite downlink scenarios. A property highlight is the in-advance knowledge of the required number of iterations, allowing on-demand management of the loop bandwidth in different turbulent regimes. Besides remaining conceptually and technically simple, it opens a new insight to iterative solutions that may lead to the development of new methods. With further refinement, this technique can surely contribute to making possible the use of iterative solutions in laser communicationsSatélites de observación de la tierra diariamente generan gigantescas cantidades de datos que deben ser enviados a estaciones terrenas. La mayoría de la información recolectada debe desecharse debido al reducido tiempo visible de un satélite en movimiento y el limitado ancho de banda de transmisión. Enlaces ópticos pueden solucionar esta limitación ofreciendo multi-gigabit de ancho de banda. Sin embargo, el desempeño de un downlink laser está limitado por la turbulencia atmosférica, la cual induce variaciones en la intensidad y la fase de la señal recibida incrementando la probabilidad de error en los datos recibidos. En principio, un receptor basado en una apertura simple utilizando óptica adaptativa puede corregir las aberraciones de fase inducidas por la atmósfera, mejorando el canal de transmisión. Sin embargo, la eficiencia de los sistemas de óptica adaptativa tradicionales se ve seriamente reducida en situaciones de turbulencia fuerte. En tales escenarios, técnicas iterativas ofrecen mayor robustez, simplicidad de diseño e implementación, así como también facilidad de integración a un costo reducido. Desafortunadamente, dicha tecnología aún requiere demasiadas iteraciones para corregir la fase distorsionada, excediendo el tiempo de coherencia del frente de onda. Esta tesis propone una nueva técnica iterativa de óptica adaptativa capaz de reducir el tiempo de convergencia en escenarios de turbulencia fuerte. La técnica utiliza el diseño tradicional de los sistemas de corrección iterativos, agregando el uso de una imagen focal de intensidad para acelerar el proceso de corrección del campo distorsionado. En dicha técnica se combinan principios básicos de óptica de Fourier, procesamiento de imagen, y optimización cuadrática de la señal para corregir el frente de onda. De esta forma, la fase de los puntos focales de mayor intensidad (speckles) puede modificarse directamente y con ello maximizar de forma convexa la potencia acoplada en fibra. Los análisis numéricos demuestran robustez y un excelente desempeño en escenarios de turbulencia fuerte. Los resultados de laboratorio confirman que el moteado de intensidad puede utilizarse para acelerar la corrección iterativa. Esta técnica utiliza la mitad del ancho de banda requerido con la técnica tradicional, al mismo tiempo que ofrece suficiente ganancia y estabilidad de la señal para lograr enlaces ópticos con muy baja probabilidad de error. Al mismo tiempo, la técnica propuesta permite conocer con anticipación el número total de iteraciones y posibilita la administración bajo demanda del ancho de banda requerido en diferentes escenarios de turbulencia. Esta tesis ofrece una mirada diferente a los métodos iterativos, posibilitando el desarrollo de nuevos conceptos y contribuyendo al uso de soluciones iterativas en comunicaciones laser por espacio libre.Postprint (published version

Optical Communication Through the Turbulent Atmosphere with Transmitter and Receiver Diversity, Wavefront Control, and Coherent Detection

Thesis Supervisor: Vincent W. S. Chan Title: Joan and Irwin M. Jacobs Professor of Electrical Engineering and Computer ScienceFree space optical communication through the atmosphere has the potential to provide secure, low-cost, rapidly deployable, dynamic, data transmission at very high rates. However, the deleterious e ects of turbulence can severely limit the utility of such a system, causing outages of up to 100 ms. For this thesis, we investigate an architecture that uses multiple transmitters and multiple coherent receivers to overcome these turbulence-induced outages. By controlling the amplitude and phase of the optical eld at each transmitter, based on turbulence state information fed back from the receiver, we show that the system performance is greatly increased by exploiting the instantaneous structure of the turbulence. This architecture provides a robust highcapacity free-space optical communication link over multiple spectral bands, from visible to infrared. We aim to answer questions germane to the design and implementation of the diversity optical communication architecture in a turbulent environment. We analyze several di erent optical eld spatial modulation techniques, each of which is based on a di erent assumption about the quality of turbulence state information at the transmitter. For example, we explore a diversity optical system with perfect turbulence state information at the transmitter and receiver that allocates transmit power into the spatial modes with the smallest propagation losses in order to decrease bit errors and mitigate turbulence-induced outages. Another example of a diversity optical system that we examine is a diversity optical system with only a subset of the turbulence state information: this system could allocate all power to the transmitter with the smallest attenuation. We characterize the system performance for the various spatial modulation techniques in terms of average bit error rate (BER), outage probability, and power gain due to diversity. We rst characterize the performance of these techniques in the idealized case, where the instantaneous channel state is perfectly known at both the receiver and transmitter. The time evolution of the atmosphere, as wind moves tur- 3 bules across the propagation path, can limit the ability to have perfect turbulence state knowledge at the transmitter and, thus can limit any improvement realized by optical eld spatial modulation techniques. The improvement is especially limited if the latency is large or the feedback rate is short compared to the time it takes for turbules to move across the link. As a result, we make successive generalizations, until we describe the optimal system design and communication techniques for sparse aperture systems for the most general realistic case, one with inhomogeneous turbulence and imperfect (delayed, noisy, and distorted) knowledge of the atmospheric state

Atmospheric compensation experiments on free-space optical coherent communication systems

In the last years free-space optical communications systems for wireless links have been proposed, studied, and implemented mainly due to the higher bandwidth that this technology is able to provide. Still, radio frequency (RF) systems have been maintained in practical wireless communications systems due to the improvement of the microwave sources and the development of high speed electronics. Nowadays the circumstances are changing as a consequence of the increasing data-rate needed in terrestrial and outer space communications. The shift from RF systems to optical communication systems in the free space applications provide a wide set of advantageous characteristics that are motivating the use of these optical technologies in detriment of the RF systems. One of the key reasons is the advantage of working with optical wavelengths in compare to the RF spectral band. As well as the already mentioned increase in the available bandwidth due to the fact that higher optical frequencies directly mean wider bandwidths, the use of optical frequencies lead to a better performance in terms of the received power: for equal antenna sizes the received signal goes inversely as the square of the wavelength. Of the most interest, recent coherent optical communication systems address modulation and detection techniques for high spectral efficiency and robustness against transmission impairments. Coherent detection is an advanced detection technique for achieving high spectral efficiency and maximizing power or signal-to-noise (SNR) efficiency, as symbol decisions are made using the in-phase and quadrature signals, allowing information to be encoded in all the available degrees of freedom. In this context, the effects of Earth's atmosphere must be taken into account. Turbulenceinduced wavefront distortions affect the transmitted beam responsible for deterioration of the link bit error rate (BER). The use of adaptive optics to mitigate turbulence-induced phase fluctuations in links employing coherent (synchronous) detection is poised to reduce performance penalties enabling a more capable next generation of free-space optical communications. In this work, we describe the implementation of a free space optical coherent communication system using QPSK modulation and heterodyne downconvertion that uses adaptive optics techniques and digital signal processing to mitigate turbulenceinduced phase fluctuations and channel impairments in coherent receivers. A new method for generating atmospheric turbulence based on binary computer generated holography (BCGH) using binary arrays is presented and its performance is evaluated. The feasibility of FSO coherent systems working with adaptive optics is demonstrated and the system performance in terms of the BER is experimentally evaluated under the influence of atmospheric turbulence. The resulting system performance is compared against the theoretical models. The viability of the approach to improve the system efficiency and sensitivity of coherent receivers is experimentally demonstrated.En los últimos años las comunicaciones ópticas en el espacio libre han sido propuestas, analizadas e implementadas debido, principalmente, al gran ancho de banda disponible mediante esta tecnología. Aún así, en la práctica, los sistemas de radiofrecuencia (RF) han sido mantenidos en las aplicaciones comerciales debido a la mejora de los dispositivos utilizados y al desarrollo de equipos electrónicos con gran velocidad de procesado. Hoy en día la situación está cambiando como consecuencia de un incremento en la tasa de transmisión requerida en sistemas de comunicaciones terrestres y en el espacio exterior. El cambio de sistemas de RF hacia sistemas ópticos en el espacio libre implica una serie de ventajas clave que motiva la transición hacia estas tecnologías. La primera y gran ventaja de trabajar con frecuencias pertenecientes al espectro óptico es el aumento del ancho de banda disponible, ya que trabajar a alta frecuencia implica directamente un incremento en el ancho de banda. Además, la eficiencia en términos de potencia es incrementada, ya que, para un tamaño de antena fijo, la potencia de señal recivida es proporcional al inverso de la longitud de onda al cuadrado. De especial interés es el desarrollo de sistemas de comunicaciones ópticos que utilicen modulaciones complejas, lo que implica una mayor eficiencia espectral y una mayor robustez contra efectos perniciosos introducidos por el canal. La detección coherente es una avanzada técnica que permite un aumento en la eficiencia espectral y maximiza la eficiencia de la potencia recibida. Esto es debido a que los simbolos son demodulados utilizando las señales en fase y cuadratura, aumentando los grados de libertad del sistema. En este contexto, los efectos de la atmósfera sobre las comunicaciones ópticas coherentes deben ser analizadas en detalle. Las turbulencias atmosféricas distorsionan el frente de onda y son responsables del deterioro de la tasa de error en las comunicaciones ópticas en el espacio libre. El uso de óptica adaptativa para mitigar los efectos de turbulencia atmosphérica abre una ventana a la implementación de la próxima generación de sistemas de comunicaciones, basados en tecnologías coherentes. En este trabajo se describe la implementación de un sistema completo de comunicaciones ópticas coherentes utilizando una modulación coherente (QPSK) y detección heterodina. Un sistema de óptica adaptativa y algoritmos de procesado de señal son implementados con el objetivo de mitigar los diferentes efectos introducidos por el canal. Por otro lado, un nuevo método para generar frentes de onda distorsionados por el canal atmosférico es desarrollado y su eficiencia es analizada. Este método se basa en el uso de holografía binaria generada por computador (BCGH) junto con un dispositivo de modulación óptica binaria de bajo coste (DLP). El funcionamiento del sistema completo es verificado y su eficiencia, en términos de tasa de error, son analizados. La eficiencia obtenida experimentalmente es comparada contra los modelos teóricos propuestos en la literatura. La viabilidad del uso de óptica adaptativa para mitigar efectos en sistemas ópticos coherentes es experimentalmente demostrada