Subcarrier intensity modulated free-space optical communication systems

This thesis investigates and analyses the performance of terrestrial free-space optical communication (FSO) system based on the phase shift keying pre-modulated subcarrier intensity modulation (SIM). The results are theoretically and experimentally compared with the classical On-Off keying (OOK) modulated FSO system in the presence of atmospheric turbulence. The performance analysis is based on the bit error rate (BER) and outage probability metrics. Optical signal traversing the atmospheric channel suffers attenuation due to scattering and absorption of the signal by aerosols, fog, atmospheric gases and precipitation. In the event of thick fog, the atmospheric attenuation coefficient exceeds 100 dB/km, this potentially limits the achievable FSO link length to less than 1 kilometre. But even in clear atmospheric conditions when signal absorption and scattering are less severe with a combined attenuation coefficient of less than 1 dB/km, the atmospheric turbulence significantly impairs the achievable error rate, the outage probability and the available link margin of a terrestrial FSO communication system. The effect of atmospheric turbulence on the symbol detection of an OOK based terrestrial FSO system is presented analytically and experimentally verified. It was found that atmospheric turbulence induced channel fading will require the OOK threshold detector to have the knowledge of the channel fading strength and noise levels if the detection error is to be reduced to its barest minimum. This poses a serious design difficulty that can be circumvented by employing phase shift keying (PSK) pre-modulated SIM. The results of the analysis and experiments showed that for a binary PSK-SIM based FSO system, the symbol detection threshold level does not require the knowledge of the channel fading strength or noise level. As such, the threshold level is fixed at the zero mark in the presence or absence of atmospheric turbulence. Also for the full and seamless integration of FSO into the access network, a study of SIM-FSO performance becomes compelling because existing networks already contain subcarrier-like signals such as radio over fibre and cable television signals. The use of multiple subcarrier signals as a means of increasing the throughput/capacity is also investigated and the effect of optical source nonlinearity is found to result in intermodulation distortion. The intermodulation distortion can impose a BER floor of up to 10-4 on the system error performance. In addition, spatial diversity and subcarrier delay diversity techniques are studied as means of ameliorating the effect of atmospheric turbulence on the error and outage performance of SIM-FSO systems. The three spatial diversity linear combining techniques analysed are maximum ratio combining, equal gain combining and selection combining. The system performance based on each of these combining techniques is presented and compared under different strengths of atmospheric turbulence. The results predicted that achieving a 4 km SIM-FSO link length with no diversity technique will require about 12 dB of power more than using a 4 × 4 transmitter/receiver array system with the same data rate in a weak turbulent atmospheric channel. On the other hand, retransmitting the delayed copy of the data once on a different subcarrier frequency was found to result in a gain of up to 4.5 dB in weak atmospheric turbulence channel

Performance evaluation of FSO communication systems over weak atmospheric turbulence channel for eastern coast of South Africa.

Masters Degree. University of KwaZulu-Natal, Durban.Free space optical (FSO) communication, otherwise known as optical wireless communication (OWC), is an established line-of-sight telecommunication technique which utilises an optical signal carrier to propagate modulated signals in the form of a light wave (visible or infrared) over the atmospheric medium. It has numerous advantages, including ease of deployment, large bandwidth, cost effective, full duplex high data rate throughput, protocol independence, highly secured data rate transmission, unregulated frequency spectrum, limited electromagnetic interference, and minimum amount of power consumption. With all the inherent advantages in FSO systems, the technology is impaired by atmospheric turbulence. Atmospheric turbulence occurs due to the persistent random changes of the refractive index as a result of variations in atmospheric temperature and pressure. This results in fluctuations in the irradiance of the laser (simply referred to as scintillation), which may lead to attenuation of optical signals in the FSO communication system. Thus, atmospheric attenuation and turbulent conditions have negative effects on the performance and ease of deployment of FSO communication systems. In this dissertation, we examine the performance of FSO systems over weak atmospheric turbulence channel for the eastern coast of South Africa. We evaluate the feasibility of the FSO link and how to improve the reliability by estimating the link margin, probability of attenuation exceedance, power scintillation index, overall power loss due to attenuation and turbulence, link budget estimate for different link lengths and wavelengths. The FSO system availability estimated for the eastern coast of South Africa is above 99% for link distances ranging from 1 km-4 km at 850 nm, 950 nm and 1550 nm. It is also observed that the FSO link availability increases with corresponding increase in wavelengths. Adopting the Kim model to estimate the atmospheric attenuation at 850 nm wavelength, the attenuation due to scattering contributes 9.47% to the absolute atmospheric losses while the atmospheric turbulence loss contributes 90.53% to the overall power loss at a link range of 4 km. Using the Ferdinandov model for a link range of 4 km at 950 nm wavelength, the attenuation due to scattering contributes 8.81% to the total power loss while the atmospheric turbulence loss contributes 91.19% to the overall power loss. It is observed that the attainable link distance increases with increase in atmospheric visibility status. The FSO system availability reduces with increase in the propagation link distance. Furthermore, it is found that the fading loss from scintillation effects strongly depends on the power scintillation index. An increase in the power scintillation index, causes an increase in the fading loss. Thus, the power scintillation index also increases per unit increase in transmission link length and refractive index. The compensation margin for such atmospheric fading loss increases with decrease in accessible FSO system bound probability. Therefore, for a highly reliable FSO system link, extra margin must be incorporated to compensate for fading loss caused by scintillation

Efficient Direct Detection of M-PAM Sequences with Implicit CSI Acquisition for The FSO System

Compared to on-off keying (OOK), M-ary pulse amplitude modulation (M-PAM, M>2) is more spectrally efficient. However, to detect M-PAM signals reliably, the requirement of accurate channel state information is more stringent. Previously, for OOK systems, we have developed a receiver that requires few pilot symbols and can jointly detect the data sequence and estimate the unknown channel gain implicitly. In this paper, using the same approach, we extend our previous work and derive a generalized receiver for M-PAM systems. A Viterbi-type trellis-search algorithm coupled with a selective-store strategy is adopted, resulting in a low implementation complexity and a low memory requirement. Therefore, the receiver is efficient in terms of energy, spectra, implementation complexity and memory. Using theoretical analysis, we show that its error performance approaches that of maximum likelihood detection with perfect knowledge of the channel gain, as the observation window length increases. Also, simulation results are presented to justify the theoretical analysis.Comment: 6 page

Subcarrier intensity modulated free-space optical communication systems

This thesis investigates and analyses the performance of terrestrial free-space optical communication (FSO) system based on the phase shift keying pre-modulated subcarrier intensity modulation (SIM). The results are theoretically and experimentally compared with the classical On-Off keying (OOK) modulated FSO system in the presence of atmospheric turbulence. The performance analysis is based on the bit error rate (BER) and outage probability metrics. Optical signal traversing the atmospheric channel suffers attenuation due to scattering and absorption of the signal by aerosols, fog, atmospheric gases and precipitation. In the event of thick fog, the atmospheric attenuation coefficient exceeds 100 dB/km, this potentially limits the achievable FSO link length to less than 1 kilometre. But even in clear atmospheric conditions when signal absorption and scattering are less severe with a combined attenuation coefficient of less than 1 dB/km, the atmospheric turbulence significantly impairs the achievable error rate, the outage probability and the available link margin of a terrestrial FSO communication system. The effect of atmospheric turbulence on the symbol detection of an OOK based terrestrial FSO system is presented analytically and experimentally verified. It was found that atmospheric turbulence induced channel fading will require the OOK threshold detector to have the knowledge of the channel fading strength and noise levels if the detection error is to be reduced to its barest minimum. This poses a serious design difficulty that can be circumvented by employing phase shift keying (PSK) pre-modulated SIM. The results of the analysis and experiments showed that for a binary PSK-SIM based FSO system, the symbol detection threshold level does not require the knowledge of the channel fading strength or noise level. As such, the threshold level is fixed at the zero mark in the presence or absence of atmospheric turbulence. Also for the full and seamless integration of FSO into the access network, a study of SIM-FSO performance becomes compelling because existing networks already contain subcarrier-like signals such as radio over fibre and cable television signals. The use of multiple subcarrier signals as a means of increasing the throughput/capacity is also investigated and the effect of optical source nonlinearity is found to result in intermodulation distortion. The intermodulation distortion can impose a BER floor of up to 10-4 on the system error performance. In addition, spatial diversity and subcarrier delay diversity techniques are studied as means of ameliorating the effect of atmospheric turbulence on the error and outage performance of SIM-FSO systems. The three spatial diversity linear combining techniques analysed are maximum ratio combining, equal gain combining and selection combining. The system performance based on each of these combining techniques is presented and compared under different strengths of atmospheric turbulence. The results predicted that achieving a 4 km SIM-FSO link length with no diversity technique will require about 12 dB of power more than using a 4 × 4 transmitter/receiver array system with the same data rate in a weak turbulent atmospheric channel. On the other hand, retransmitting the delayed copy of the data once on a different subcarrier frequency was found to result in a gain of up to 4.5 dB in weak atmospheric turbulence channel.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Optical Communication

Optical communication is very much useful in telecommunication systems, data processing and networking. It consists of a transmitter that encodes a message into an optical signal, a channel that carries the signal to its desired destination, and a receiver that reproduces the message from the received optical signal. It presents up to date results on communication systems, along with the explanations of their relevance, from leading researchers in this field. The chapters cover general concepts of optical communication, components, systems, networks, signal processing and MIMO systems. In recent years, optical components and other enhanced signal processing functions are also considered in depth for optical communications systems. The researcher has also concentrated on optical devices, networking, signal processing, and MIMO systems and other enhanced functions for optical communication. This book is targeted at research, development and design engineers from the teams in manufacturing industry, academia and telecommunication industries

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

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