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

Availability, outage, and capacity of spatially correlated, Australasian free-space optical networks

Network capacity and reliability for free space optical communication (FSOC) is strongly driven by ground station availability, dominated by local cloud cover causing an outage, and how availability relations between stations produce network diversity. We combine remote sensing data and novel methods to provide a generalised framework for assessing and optimising optical ground station networks. This work is guided by an example network of eight Australian and New Zealand optical communication ground stations which would span approximately $60^\circ$ in longitude and $20^\circ$ in latitude. Utilising time-dependent cloud cover data from five satellites, we present a detailed analysis determining the availability and diversity of the network, finding the Australasian region is well-suited for an optical network with a 69% average site availability and low spatial cloud cover correlations. Employing methods from computational neuroscience, we provide a Monte Carlo method for sampling the joint probability distribution of site availabilities for an arbitrarily sized and point-wise correlated network of ground stations. Furthermore, we develop a general heuristic for site selection under availability and correlation optimisations, and combine this with orbital propagation simulations to compare the data capacity between optimised networks and the example network. We show that the example network may be capable of providing tens of terabits per day to a LEO satellite, and up to 99.97% reliability to GEO satellites. We therefore use the Australasian region to demonstrate novel, generalised tools for assessing and optimising FSOC ground station networks, and additionally, the suitability of the region for hosting such a network.Comment: Accepted in Journal of Optical Communications and Networking. 16 pages, 16 figure

MOBILE CUBESAT COMMAND AND CONTROL GROUND STATION ARCHITECTURE FOR FREE-SPACE OPTICAL COMMUNICATION RECEIVER

The United States military continues to encourage the need for robust satellite communications in order to successfully execute defense missions. CubeSats are a smaller-scale spacecraft, initially utilized to expand educational opportunities in the field of aerospace and satellite communications. This research explores both existing and potential ground station architecture options for integration of free-space optical communication downlinks from CubeSats. Future experimentation plans will focus on the application of this capability in more diverse environments to include expanded ground architecture opportunities. Systems engineering design and architecture methods are useful in understanding the current hardware and software options and limitations for future expansion opportunities. By considering a comparable planning approach, alternatives for architecture development can be organized to aid in the identification of control factors for sub-system and ground communication interfaces. As a well-established CubeSat communications system, the existing Mobile CubeSat Command and Control (MC3) architecture serves as an excellent candidate for experimental integration and eventual considerations for a planned proof of concept.Civilian, Naval Information Warfare CenterApproved for public release. Distribution is unlimited

Quality Metrics and Reliability Analysis of Laser Communication System

Beam wandering is the main cause for major power loss in laser communication. To analyse this prerequisite at our environment, a 155 Mbps data transmission experimental setup is built with necessary optoelectronic components for the link range of 0.5 km at an altitude of 15.25 m. A neuro-controller is developed inside the FPGA and used to stabilise the received beam at the centre of detector plane. The Q-factor and bit error rate variation profiles are calculated using the signal statistics obtained from the eye-diagram. The performance improvements on the laser communication system due to the incorporation of beam wandering mitigation control are investigated and discussed in terms of various communication quality assessment key parameters.

Modeling and estimation of scattering attenuation and scintillation effects on optical wireless communication systems in South Africa.

Doctoral Degree. University of KwaZulu-Natal, Durban.Optical wireless communication (OWC) is a viable complementary solution for next-generation communication networks saddled with meeting the great demands of high data rates and fast internet connectivity. Its numerous advantages include: high data throughput; secure transmission; license-free spectrum; relative low cost of deployment; flexible network connectivity; etc. However, OWC system performance is severely degraded by atmospheric conditions such as fog and scintillation. Most of the proposed FSOC and hybrid FSOC systems in the literature are limited in their capacity to predict the extent to which atmospheric disturbances will impact on the performance of FSOC links in each location where they are to be deployed. This is because of the complexities involved in accessing and analyzing the information on the unique meteorological and climatic characteristics of the locations of interest prior to FSOC link deployment. This important information is necessary for determining the fade margin required by FSOC systems to withstand atmospheric disturbances in various locations of deployment. The effects of other atmospheric conditions such as gas absorption, molecular scattering, and aerosol absorption on the transmission wavelengths of interest (850 and 1550 nm) are negligible, and as such, were not considered in this study. This research, therefore, focuses on the investigation and modeling of scattering attenuation and irradiance fluctuations based on the unique climatic peculiarities of nine major cities in each of the provinces of South Africa where OWC links are to be deployed. These cities are Bloemfontein, Cape Town, Durban, Johannesburg, Kimberley, Mafikeng, Mbombela, Polokwane, and Port Elizabeth. Meteorological data of visibility, wind speed, relative humidity, temperature, fractional sunshine, and atmospheric pressure from 1st January 2010 till 30th June 2018, for each of the locations of interest, are statistically processed and used in the investigation, estimation, and modeling of atmospheric phenomena affecting the performance of OWC signals. To achieve this, visibility modeling and prediction for OWC systems are performed using regression analysis. The results obtained show that various simple and multiple linear regression models reliably forecast visibility from other meteorological parameters considered in this study. The model's selection may be influenced not only by its performance but also by the parameters' availability. While caution is taken to avoid model over-specification, multiple linear regression models are preferable over simple regression models. The significance of the results obtained is the validated alternatives the simple and multiple linear regression models provide while saving costs and avoiding the complexities of measuring FSO visibility in the investigated locations. The relationship between atmospheric visibility and aerosol scattering attenuation has been established by various aerosol scattering models based on the Mie scattering theory. This is made possible because the radii of aerosol particles in the atmosphere are approximately equal to the infra-red wavelengths of optical signals. Thus, the cumulative distribution of visibility and aerosol scattering attenuations based on the Ijaz fog and Kim models for transmission wavelengths of 850 and 1550 nm in nine cities in South Africa are presented. The Ijaz fog and Kim models are also used in computing the probabilities of exceedance, deceedance, and encountering of different aerosol scattering attenuations for 850 and 1550 nm. The impact of these specific attenuations on free space optical communication (FSOC) link performance are investigated for all the various locations of interest. The results show that during foggy weather, the optical signals transmitted at 1550 nm encounter more scattering attenuation than those transmitted at the 850 nm wavelength. The reverse is the case during clear weather periods. Modeling of the minimum required visibility cumulative distribution functions (CDF) during foggy and clear weather conditions for both optical wavelengths is also presented. These CDFs are employed in evaluating the FSOC link availabilities in various cities in South Africa

Advanced Technique and Future Perspective for Next Generation Optical Fiber Communications

Optical fiber communication industry has gained unprecedented opportunities and achieved rapid progress in recent years. However, with the increase of data transmission volume and the enhancement of transmission demand, the optical communication field still needs to be upgraded to better meet the challenges in the future development. Artificial intelligence technology in optical communication and optical network is still in its infancy, but the existing achievements show great application potential. In the future, with the further development of artificial intelligence technology, AI algorithms combining channel characteristics and physical properties will shine in optical communication. This reprint introduces some recent advances in optical fiber communication and optical network, and provides alternative directions for the development of the next generation optical fiber communication technology

Directional Link Management using In-Band Full-Duplex Free Space Optical Transceivers for Aerial Nodes

Free-space optical (FSO) communication has become very popular for wireless applications to complement and, in some cases, replace legacy radio-frequency for advantages like unlicensed band, spatial reuse, and enhanced security. Even though FSO can achieve very high bit-rate (tens of Gbps), range limitation due to high attenuation and weather dependency has always restricted its practical implementation to indoor application like data centers and outdoor application like Project Loon. Building-to-building communication, smart cars, and airborne drones are potential futuristic wireless communication sectors for mobile ad-hoc FSO networking. Increasing social media usage demands high-speed mobile connectivity for applications like video call and live video stream on the go. For these scenarios, implementation of in-band full-duplex FSO (IBFD-FSO) transceivers will potentially double the network capacity to improve performance and reliability of the communication link. In this work, we focus on implementing prototypes of FSO transceivers on mobile platform using both off-the-shelf and customized components. Current goal is to implement a prototype of IBFD-FSO transceiver using VCSEL as transmitter and PIN photodiode as receiver at 900 nm wavelength. We are considering atmospheric attenuation, FSO beam propagation model, geometry, and tiling of the components to optimize the link performance while keeping the package low-cost and mobile, ensuring connectivity to mass population. Eventually, our goal is to have communication between multiple airborne drones through IBFD-FSO transceivers by discovering each other and maintaining established link. Applications of this research is not only limited to the conceived idea of smart cities, but it can also have real impact on disaster management in times of wildfire or hurricane