Thesis (Master)--İzmir Institute of Technology, İzmir, 2004 / Includes bibliographical references (leaves. 77).

Thesis (Ph.D.) - University of St Andrews, May 2009.

No description available.

Thesis (Ph. D.)--Ohio State University, 2005. / Title from first page of PDF file. Document formatted into pages; contains xvii, 176 p.; also includes graphics (some col.). Includes bibliographical references (p. 171-176). Available online via OhioLINK's ETD Center

A wireless sensor network (WSN) consists of a large number of networked sensor nodes deployed to sense and report a particular phenomenon to a base station. Currently, most WSNs use radio frequency (RF) communications, and this accounts for a significant amount of energy expended. Free space optical (FSO) communications using modulating retroreflectors is potentially attractive for WSNs, due to the lower communications energy required. However, for FSO communications, line of sight (LOS) is required between the transmitter and the receiver. In this thesis, a hybrid Radio Frequency/Free Space Optical (RF/FSO) WSN is proposed. FSO links are used for communications, with RF links providing backup in the absence of LOS. This network has the potential to lower the overall energy consumption of a traditional RF-only WSN. Chapter 1 introduces the WSN and outlines the motivation for the RF/FSO WSN. Chapters 2 and 3 describe the RF and FSO link models used for the RF/FSO WSN. Chapter 4 describes how the WSN networks are configured. The energy model for the sensor node is discussed in Chapter 5. Chapter 6 discusses how network traffic and energy consumption are modelled. The results of the RF/FSO WSN simulations are presented in Chapter 7. Chapter 8 discusses the conclusions from the thesis and suggests areas for future work. Simulations show that for the wide range of scenarios considered, the RF/FSO WSN consumes less energy and has a lifetime at least twice as long as the RF-only WSN. For low and average optical blocking conditions, the RF/FSO WSN is also able to offer at least the same level of network coverage as the RF-only WSN.

We optimize secret key rates (SKRs) of weak coherent pulse (WCP)-based quantum key distribution (QKD) over time-varying free-space optical channels affected by atmospheric turbulence. The random irradiance fluctuation due to scintillation degrades the SKR performance of WCP-based QKD, and to improve the SKR performance, we propose an adaptive scheme in which transmit power is changed in accordance with the channel state information. We first optimize BB84 and decoy state-based QKD protocols for different channel transmittances. We then present our adaptation method, to overcome scintillation effects, of changing the source intensity based on channel state predictions from a linear autoregressive model while ensuring the security against the eavesdropper. By simulation, we demonstrate that by making the source adaptive to the time-varying channel conditions, SKRs of WCP-based QKD can be improved up to over 20%.

Free-space optical (FSO) communication offers significant technical and operational advantages such as higher bandwidth capacity, robustness to electromagnetic interference, a high degree of spatial confinement (bringing virtually unlimited reuse and inherent security), low power requirements, and unregulated spectrum. FSO communication can be deployed as an efficient solution for a wide range of applications such as last-mile access, fiber backup, back-haul for wireless cellular networks, and disaster recovery among others. Although FSO system have many appealing features, they have rather disappointing performance for long links due to the degrading effects of atmospheric turbulence-induced fading. In this dissertation, we investigate different diversity techniques to boost the performance of FSO systems in the presence of the atmospheric turbulence-induced fading. In Chapter 3, we investigate receive diversity in coherent FSO systems considering both turbulence-induced amplitude and phase fluctuations under weak turbulence regime. To mitigate the wavefront phase distortion effect, modal compensation is deployed at the receiver. Under the assumption of Rician channel that models the combined effects of the atmospheric fading and modal compensation, we derive outage probability and diversity- multiplexing tradeoff of such systems. Our results show that, at high signal to noise ratio (SNR) regime, the diversity gain as great as the number of receiving apertures is achieved. Moreover, it is found that the modal compensation provides finite-SNR diversity advantages in coherent receivers. In Chapter 4, we investigate multi-hop transmission (serial relaying) as a form of diversity technique to combat atmospheric fading in coherent FSO systems. Utilizing the same channel model as in Chapter 3 and considering decode-and-forward relaying strategy, we quantify the outage probability and the finite-SNR diversity-multiplexing tradeoff of this relaying scheme. Exploiting the fact that fading variance is distance-dependent in the atmospheric channel, our results demonstrate that the multi-hop transmission takes advantage of the resulting shorter hops and yields significant performance improvements in the presence of fading. In Chapter 5, we study hybrid-ARQ protocols in coherent FSO communications over Gamma-Gamma atmospheric fading channels. We investigate and compare the performance of three hybrid-ARQ protocols in terms of the outage probability and throughput. Furthermore, we characterize the outage performance at high-SNR regime by diversity and coding gains. Our results provide insight into the performance mechanisms of different hybrid-ARQ protocols in coherent FSO systems and demonstrate that hybrid-ARQ significantly improves the outage performance of a coherent FSO system particularly in strong turbulence regime. In Chapter 6, we investigate parallel relaying in an intensity modulation/direct detection (IM/DD) FSO system. Assuming Gamma-Gamma fading model, we analyze both decode-and-forward and amplify-and-forward modes of cooperation. Focusing on high SNR regime, we investigate the outage probability and characterize it by the diversity and coding gains. The diversity-multiplexing tradeoff expression of each cooperation mode is also derived. Our performance analysis reveals that large energy savings can be achieved through the use of parallel relaying in FSO systems.

Title from PDF of title page (University of Missouri--Columbia, viewed on March 10, 2010). The entire thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file; a non-technical public abstract appears in the public.pdf file. Thesis advisor: Dr. Gregory E. Triplett Jr. Includes bibliographical references.

Terrestrial free-space optical (FSO) communication uses visible or infrared wavelengths to broadcast high speed data wirelessly through the atmospheric channel. The performance of terrestrial FSO channel mainly depends on the local atmospheric conditions. Ad hoc networks offer cost-effective solutions for communications in areas where infrastructure is unavailable, e.g., intelligent transport system, disaster recovery and battlefield scenarios. Traditional ad hoc networks operate in the radio frequency (RF) spectrum, where the available bandwidth faces the challenge of rapidly increasing demands. FSO is an attractive alternative for RF in ad-hoc networks because of its high bandwidth and interference-free operation. This thesis investigates the influencing factors for routing traffic from given s-d pair while satisfying certain Quality of Services in terrestrial FSO ad hoc mesh networks under the effect of stochastic atmospheric turbulence. It starts with a comprehensive review of FSO technology, including the history, application, advantages and limitations. Subsequently the principle of operation, the building blocks and safety of FSO communication systems are discussed. The physics of atmosphere is taken into account to investigate how propagation of optical signals is affected in terrestrial FSO links. A propagation model is developed to grade the performance and reliability of the FSO ad hoc links in the network. Based on that model and the K-th shortest path algorithm, the performance of the path with highest reliability, the path with a second highest possible reliability and an independent path with no common links shared with the former two paths, were compared according to the simulation scenarios in node-dense area and node-sparse area. Matlab simulation shows that the short/long range dependent transmission delay are positively proportional to number of hops of the paths. Lower path reliability only dominate the cause of severe delay when traffic flow approaches near its upper link capacity in node-sparse area. In order to route traffic from given s-d pairs with satisfying certain Quality of Services, the path with highest reliability may not be the best choices since they may hold more hops which will degrade the QoS. Meanwhile, in case of exponential traffic congestion, it is recommended that both traffic demand and traffic flow propagating through the links should be pressed below a value close to the effective capacity, where the nonlinearity of the transmission delay curve starts to obviously aggravate.

Free-space optical (FSO) communication offers significant technical and operational advantages such as higher bandwidth capacity, robustness to electromagnetic interference, a high degree of spatial confinement (bringing virtually unlimited reuse and inherent security), low power requirements, and unregulated spectrum. FSO communication can be deployed as an efficient solution for a wide range of applications such as last-mile access, fiber backup, back-haul for wireless cellular networks, and disaster recovery among others. Although FSO system have many appealing features, they have rather disappointing performance for long links due to the degrading effects of atmospheric turbulence-induced fading. In this dissertation, we investigate different diversity techniques to boost the performance of FSO systems in the presence of the atmospheric turbulence-induced fading. In Chapter 3, we investigate receive diversity in coherent FSO systems considering both turbulence-induced amplitude and phase fluctuations under weak turbulence regime. To mitigate the wavefront phase distortion effect, modal compensation is deployed at the receiver. Under the assumption of Rician channel that models the combined effects of the atmospheric fading and modal compensation, we derive outage probability and diversity- multiplexing tradeoff of such systems. Our results show that, at high signal to noise ratio (SNR) regime, the diversity gain as great as the number of receiving apertures is achieved. Moreover, it is found that the modal compensation provides finite-SNR diversity advantages in coherent receivers. In Chapter 4, we investigate multi-hop transmission (serial relaying) as a form of diversity technique to combat atmospheric fading in coherent FSO systems. Utilizing the same channel model as in Chapter 3 and considering decode-and-forward relaying strategy, we quantify the outage probability and the finite-SNR diversity-multiplexing tradeoff of this relaying scheme. Exploiting the fact that fading variance is distance-dependent in the atmospheric channel, our results demonstrate that the multi-hop transmission takes advantage of the resulting shorter hops and yields significant performance improvements in the presence of fading. In Chapter 5, we study hybrid-ARQ protocols in coherent FSO communications over Gamma-Gamma atmospheric fading channels. We investigate and compare the performance of three hybrid-ARQ protocols in terms of the outage probability and throughput. Furthermore, we characterize the outage performance at high-SNR regime by diversity and coding gains. Our results provide insight into the performance mechanisms of different hybrid-ARQ protocols in coherent FSO systems and demonstrate that hybrid-ARQ significantly improves the outage performance of a coherent FSO system particularly in strong turbulence regime. In Chapter 6, we investigate parallel relaying in an intensity modulation/direct detection (IM/DD) FSO system. Assuming Gamma-Gamma fading model, we analyze both decode-and-forward and amplify-and-forward modes of cooperation. Focusing on high SNR regime, we investigate the outage probability and characterize it by the diversity and coding gains. The diversity-multiplexing tradeoff expression of each cooperation mode is also derived. Our performance analysis reveals that large energy savings can be achieved through the use of parallel relaying in FSO systems.