Electrical and Electronic Engineering - Theses
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ItemEnergy efficient wireless system design( 2015)The demand for telecommunication networks is increasing rapidly. Wireless access is a major contributor to this trend. On the other hand, wireless is considered as a least energy efficient transmission medium mainly due to its unguided nature. The general focus of increasing wireless system energy efficiency is on reduction of the transmit power. However, this strategy may not save energy in short distance communication systems as the processing energy in hardware becomes more significant compared to the transmit radio energy. This thesis focuses on looking at the energy consumption of wireless systems by modeling the energy consumption as a function of several parameters such as receiver SNR, RF bandwidth, information rate, modulation scheme and code rate. We propose energy models for synchronization systems and other digital signal processing modules by considering the computational complexity of the algorithm and the required circuitry. Initially we focus on the synchronization aspects of wireless receivers. We study various algorithms on symbol timing recovery, carrier frequency recovery and carrier phase recovery and compare the performance in order to identify the suitable algorithms to operate at different SNR regions. We then develop energy models for those synchronization sub-systems by analyzing the computational complexity of circuitries based on a number of arithmetic, logic and memory operations. We define a new metric - energy consumption to achieve a given performance as a function of SNR - in order to compare the energy efficiency of different estimation algorithms. Next, we investigate the energy-efficiency trade-offs of a point-to-point wireless system by developing energy models of both the transmitter and receiver that include practical aspects such as error control coding, synchronization and channel equalization. In our system, a multipath Rayleigh-fading channel model and a low-density parity check (LDPC) coding scheme are chosen. We then develop a closed-form approximation for the total energy consumption as a function of receiver SNR and use it to find a minimum-energy transmission configuration. The results reveal that low SNR operation (i.e. low transmit power) is not always the most energy efficient strategy, especially in short distance communication. We present an optimal-SNR concept which can save a significant amount of energy mainly in short-range transmission systems. We then focus on cooperative relay systems. We investigate the energy efficiency trade-offs of single--relay networks by developing energy models for two relay strategies: amplify-and-forward (AF) and detect-and-forward (DF). We then optimize the location and power allocation of the relay to minimize the total energy consumption. The optimum location is found in two-dimensional space for constrained and unconstrained scenarios. We then optimize the total energy consumption over the spectral efficiency and derive expressions for the optimal spectral efficiency values. We use numerical simulations to verify our results. Finally, we focus on energy efficiency of multi-relay systems by considering a dual-relay cooperative system using DF protocol with full diversity. We propose a location-and-power-optimization approach for the relays to minimize the transmit radio energy. We then minimize the total system energy from spectral efficiency perspective for two scenarios: throughput-constrained and bandwidth-constrained configurations. Our proposed approach reduces the transmit energy consumption compared to an equal-power allocated and equidistant-located relay system. Finally, we present an optimal transmission scheme as a function of distance by considering single-hop and multi-hop schemes. The overall results imply that more relays are required as the transmission distance increases in order to maintain a higher energy efficiency.
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ItemFundamental energy requirements of information processing and transmission( 2015)This thesis investigates fundamental limits on the energy required to process and transmit information. By combining physical laws, such as the second law of thermodynamics, with information theory, we present novel limits on the efficiency of systems that track objects, perform stochastic control, switch communication systems and communicate information. This approach yields results that apply regardless of how the system is constructed. While the energy required to perform an ideal measurement of a static state has no known lower bound, this thesis demonstrates that this is not true for noisy measurements or if the state is evolving stochastically. We derive new lower bounds on the energy required to perform such tracking tasks, including Kalman filtering. The goal of stochastic control is usually to reduce the entropy of the controlled system. This is also the task of a Maxwell demon, a thought experiment in which a device or being reduces the thermodynamic entropy of a closed system, violating the second law of thermodynamics. We demonstrate that the same arguments that `exorcise' Maxwell's demon can be used to find lower bounds on the energy consumption of stochastic controllers. We show that the configuration of a switching system in communications, that directs input signals to the desired outputs, can be used to store information. Reconfiguring the switch therefore erases information, and must have an energy cost of at least $k_B T \ln(2)$ per bit due to Landauer's principle. We then calculate lower bounds on the energy required to perform finite-time switching in a one-input, two-output MEMS (microelectromechanical system) mirror switch subject to Brownian motion, demonstrating that the shape of the potential that the switch is subject to affects both the steady-state noise and the energy required to change the configuration. Finally, by modifying Feynman's ratchet and pawl heat engine in order to perform communication instead of doing work, we investigate the efficiency of communication systems that operate solely using the temperature difference between two thermal reservoirs. The lower bound for the energy consumption of any communication system operating between two thermal reservoirs, with no channel noise and using equiprobable partitions of heat energy taken from these reservoirs, is found to be $\frac{T_H T_C}{T_H-T_C} k_B \ln(2)$, where $T_H$ and $T_C$ are the temperatures of the hot and cold reservoir, and $k_B$ is Boltzmann's constant.
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ItemOptical wireless integration: network design challenges( 2013)Today, telecommunication networks are expected to provide high bandwidth services to anywhere and at anytime that the services are requested. Among the many solutions available, the optical-wireless converged network has emerged as a popular solution to enable these mandatory features through combining high capacity optical backhaul and flexible wireless access network. However, as the bandwidth demand in the wireless network increases drastically day by day, wireless access technological advances and new requirements continue to evolve. Therefore, to keep pace with the ever advancing wireless access networks and its services, rapid advancements in the optical backhaul network and a tighter integration of optical and wireless access networks are required in order to provide a quality-of-service (QoS) guaranteed ubiquitous access to end users. In particular, numerous challenges need to be addressed in order to deploy optical-wireless converged networks efficiently. These challenges include provisioning adequate backhaul capacities to guarantee QoS of essential services, facilitating efficient intercommunication between base stations to enable traffic diversions and coordination functions, achieving cost-effective deployment and improving energy efficiency of the converged network. This thesis explores efficient solutions to overcome these key challenges. In particular, approaches to achieve cost effective deployments, to guarantee QoS, and to preserve energy efficiency of the converged networks, are investigated. A denser small cell deployment is considered as a future proof solution to cope with the unabated growth of mobile traffic. However, the deployments of small cells are challenging due to the cost associated with the backhaul, powering, and real estate requirements. Therefore, these challenges need to be tackled effectively in order to achieve all the potential benefits of small cell deployments. To this end, this thesis explores cost-effective approaches that leverage the resources associated with the existing fibre infrastructure to provide fibre backhaul for small cells. By analysing the geographic information system (GIS) data of a real network, we show that the existing fibre terminals can be strategically exploited for the small-cell deployment. Moreover, to reduce the dark fibre usage associated with backhauling, we present an approach to select an optimal subset of fibre terminals for small-cell deployments, which yields maximum possible coverage. In addition, to further reduce the total deployment cost of small-cell backhauling, cost-optimal deployment of passive optical networks (PONs) on top of the existing infrastructure is also explored in this thesis. We demonstrate the applicability of our proposed approaches by using them to plan small-cell deployments in a portion of a large carrier's network in the USA. The results reveal that our proposed approaches can save half of the deployment costs associated with small-cell backhauling. Quality-of-service is an essential requirement for next-generation broadband services such as e-Health and Internet protocol television (IPTV). Therefore, in this thesis, we investigate how the characteristics of both optical and wireless networks can be exploited to provide tighter integration and hence, guaranteed QoS in optical-wireless converged networks. In particular, resource handling methods that strategically exploit the frame structures and resource allocation information retrieved from the wireless network to allocate bandwidth in the converged network, are investigated. Moreover, since the direct communication between base stations is identified as one of the major considerations in the next-generation wireless access networks, we investigate how the widely deployed tree topology-based PON can be used to enable these complementary features. To this end, we explore different PON-wireless converged network architectures. In addition, we also investigate an architecture discovery enabled resource allocation mechanism that can be implemented irrespective of the architecture used for the converged network deployment. Our simulation results indicate that the proposed approaches can significantly improve the QoS performance of both the uplink and inter-communications links between base stations. Moreover, as the energy consumption of telecommunication networks is identified as a major contributor towards the global energy consumption, this thesis also explores the energy efficient aspects of the converged networks. The energy consumption of proposed converged network architectures is investigated under various deployment scenarios by analytically modelling the energy consumption of each of the architectures. In addition, we also explore how the energy saving mechanisms can be implemented in the converged network without compromising the QoS. Overall, our studies reported in this thesis, provide insight into deployment strategies that can be used to realise cost-optimal, energy-efficient, and QoS rich next generation optical-wireless converged networks.
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ItemScalable deployment of Video-on-Demand services( 2013)The Video-on-Demand (VoD) service is increasingly becoming popular. Unlike the conventional television services, the VoD service allows viewers to select the videos and the viewing times based on their personal preferences, and also provide the ability to control the video sessions. On one hand, the number of people using VoD service is rapidly increasing day by day. On the other, service providers are deploying many new VoD systems, and also improving the existing systems by enhancing the video libraries and providing videos with better quality. Consequently, the scalability and bandwidth efficiency of VoD systems have become important design considerations today. In addition, the energy efficiency is also now considered as a key concern in the deployment of VoD systems, with the Internet being identified as one of the main potential contributors towards global energy consumption and carbon footprint. In this thesis, we explore the fundamental question of how to realise scalable VoD systems that can sustain the sharp rise of number of VoD users, sizes of VoD libraries, and the quality of video content. In particular, we investigate the strategic exploitation of distributed storage resources to improve the scalability and bandwidth efficiency of VoD systems. To this end, we propose and evaluate architectures, delivery schemes, and techniques, that help VoD systems to efficiently scale and support customers as the number of users, library sizes, and streaming rates increase. In addition, we also analyse the distributed content placement in VoD systems from an energy conservation standpoint. Furthermore, our study also addresses the resulting sub-problems associated with our main objectives, such as estimating popularity of videos and understanding characteristics of VoD users. While unicast is a natural choice for delivering VoD services, unicast-based VoD systems scale very poorly with the system growth. We propose delivery schemes, which improve the scalability and bandwidth efficiency of the VoD systems by strategic exploitation of storage and multicast. The underlying principle behind these schemes is the pre-population of video content, which enables the user requests to be batched and served using multicast. We first use this concept to propose scalable delivery schemes for two-tier VoD systems, where we pre-populate the video content in the end-user's devices. Our simulations based performance evaluations confirm that the proposed schemes significantly improve the scalability and bandwidth efficiency of the VoD systems. We then extend this concept to hierarchical VoD systems and propose a novel scalable VoD delivery scheme, also taking into account the characteristics of VoD users. In this scheme, different portions of the videos are pre-populated in different levels of the network hierarchy. Our trace driven simulations verify the superior performance of our proposed scheme in terms of scalability and bandwidth efficiency. Recently, energy efficiency is also identified as one of the crucial requirements for network based services due to the rising contribution of the Internet towards global energy consumption and carbon footprint. In this thesis, we analyse the distributed content placement in VoD systems from an energy conservation standpoint. To this end, we formulate energy consumption models and numerically analyse those models using data from manufacturers' data sheets. We then use these models to design a replication scheme that improves the energy efficiency of VoD systems by strategic content placement and by switching ON/OFF content storages based on the demand. The popularity of videos is a valuable input for an efficient content placement. We propose a novel method for video popularity estimation in VoD systems. The proposed method uses the arrival times of video requests to derive a parameter called inverted pyramid distance, which captures both long term and short term popularities of videos. Our simulations indicate that the proposed video popularity estimation method performs extremely well despite the popularities of videos being stable or time-varying. The popularity of videos does not capture the personalised interests of an individual user. Therefore, a video popularity based content management strategy might not be the best option for a server that serves a small group of users. We investigate the effectiveness of collaborative filtering (CF) techniques for the content placement at end user's devices. To this end, we propose a bandwidth efficient personalised prefix placement scheme that uses CF techniques, and we analyse it using trace driven simulations. Throughout the thesis, our results provide insight into potential immediate and long term strategies that can significantly improve the scalability and/or energy efficiency of the VoD systems. We also point out possible future research, which will extend the work covered in this thesis while keeping up with the current technology trends.