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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Ray-optics based propagation tools for future wireless communication networks

Sheikh, Muhammad Imran January 1997 (has links)
The thesis is concerned with the current use of ray-optics based propagation prediction tools for the determination of wideband characteristics of the wireless urban propagation channel. Reliable prediction of propagation channel parameters such as delay spread, coherence bandwidth etc. is important for the deployment of wideband systems. Deterministic propagation models, based on ray-optical techniques make use of site-specific topographic databases, and aimed to yield reliable area coverage and wideband propagation channel parameters. Such models determine wideband wireless channel characteristics by Fourier transforming the radio channel impulse response at a center frequency. This can lead to errors in the derived wideband parameters. A three-dimensional ray-optics based propagation prediction tool is developed. The model uses image theory, Geometrical Optics (GO) and the Uniform Theory of Diffraction (UTD) to determine wideband characteristics of the urban wireless communication channel. Higher order diffractions which can contribute significantly in shadow region are incorporated in addition to first order diffractions. The model is thoroughly validated with single frequency and swept frequency measurements in scaled down controlled environment. Systematic errors are identified in deriving wideband response. Errors (up to 27 dB and 4.1 rad) in the lit regions corresponds to the individual tiny deep nulls which are quite insignificant. In the shadow region average errors gradually increases away from the center frequency and rises up to 1 dB and 0.05 rad. Error sign reversal is observed in the transition regions. Errors corresponding to realistic bandwidths are less than 0.5 dB and 0.03 rad. Errors in frequency correlation function corresponds to shadow regions and frequency separation of 0.25 GHz to 7 GHz. It is concluded that the coherence bandwidth provides more information of the dispersive nature of the channel compared to delay spread.
2

Novel antenna design for future mobile systems

Song, Peter C. T. January 2001 (has links)
User requirements for access to a diversified range of wireless telecommunication services have resulted in a rapid technological push to unify these several different systems on a common platform. The objective of this work is to develop novel antenna design solutions not only for existing systems but also any future generation wireless communication systems. This thesis titled "Novel Antenna Design for Future Mobile Systems" describes the work done for the period from 15th January 1998 to 14th January 2001, under the studentship agreement between British Telecommunications and the University of Birmingham. Three topics are investigated in this thesis. They are: firstly, the family of multiband planar inverted F antenna (PIFA); secondly, the gain enhancement and packaging concepts of electrically small antennas and finally, the family of multiband fractal antennas. In the first topic, two novel PIFA antennas were designed. The first is a triple band antenna where a dual band single feed element using a reactive L shape spur line load is housed within a lower frequency antenna element. The frequency ratio for the reactively loaded dual band single feed antenna is tuneable between 1.28 to over 3.5. The second antenna is designed with two PBFA housed within a quarter wave patch antenna. The antenna is tuned for operation in the GSM, DECT and WLAN bands. The second includes a discussion of a method of integrating electrically small antenna with the transceiver chip on a single package, where the poor efficiency of small integrated antenna can be overcome by the use of a parasitic radiator. Experimental investigation of the characteristics of this parasite, including circular polarisation techniques, are addressed. The final topic is the family of multiband fractal antennas where five novel designs are proposed. The first two designs address feeding solutions for a perturbed Sierpinski gasket monopole antenna, using a microstrip and an angular feed. These techniques enable the closely spaced, tuned bands of the perturbed Sierpinski gasket to be matched without additional matching circuits. The third design is a shorted half sized Sierpinski gasket monopole antenna, similar to the design of the inverted L antenna development. Tuning of bands are also demonstrated by structural perturbation. The fourth design is a set of multiple rings, similar to the multi-level design of the Sierpinski gasket monopole antenna. The bandwidth of each band is well over 40%, which is much larger than the Sierpinski gasket monopole. Finally, a pair of Sierpinski carpet monopoles are fed in parallel and demonstrate a very large impedance bandwidth.
3

Intereference management in cognitive radio networks

Zarrebini-Esfahani, Azar January 2013 (has links)
Radio spectrum is becoming increasingly scarce as more and more devices go wireless. Meanwhile, studies indicate that the assigned spectrum is not fully utilised. Cognitive radio technology is envisioned to be a promising solution to address the imbalance between spectrum scarcity and underutilisation. Cognitive radio enables the unlicensed (secondary) user to establish a communication link in licensed (primary) spectrum on the condition that there is no or minimal interference to the primary user. The interference management has become an important topic in cognitive radio in order to manage and fulfill the regulatory constraints. The management of interference is, unquestionably, required to treat and quantify all the interference produced by the cognitive transmission at the primary users. In order to manage this interference, the secondary users must be able to adjust their parameters to fulfill these constraints. In addition, the performance of contemporary multicell wireless networks is limited by intercell interference (ICI), due to cochannel transmission in other cells. This per¬formance degradation is especially severe for users close to the cell-edge. As a solution, in this thesis different cognitive beamfomring techniques are proposed by deploying cognitive cells on the primacy cells boundaries to support the primary cell-edge users as well as servicing the secondary users. This thesis proposes interference management techniques based on cognitive beam-forming in a cellular network. We have identified conditions and proposed different techniques for optimal usage of radio spectrum, by allowing coexistence on the same spectrum resources between primary and cognitive users. Deploying cognitive cells on the primary cells borders results to ICI mitigation for primary cell-edge users within the cognitive cell and also leads to supporting cognitive users with the same allocated spectrum to the primary network. The aim is to minimise the total transmit power across the cognitive network while maintaining the required signal-to-interference-plus-noise ratio (SINK) for all primary cell-edge/cognitive users within the cognitive cell and to mitigate the interference caused by the cognitive system towards the primary users. It forms the fundamental basis for interference management in cognitive radio systems and consequently gives insights into the design and deployment of cognitive radio net¬works. At the end, we introduce robust cognitive beamforming based on imperfect channel state information for both primary and cognitive users’ channel. These different approaches to interference management at cognitive radio networks contribute to the increasing set of techniques that will make cognitive radio possible to deploy.
4

Measurement of propagation loss in trees at SHF frequencies

Adegoke, Adesoye Sikiru January 2015 (has links)
Measurements on single trees, group of trees and lines of trees have been undertaken at microwave frequencies (3.2-3.9 GHz and 4.9-5.9 GHz) in order to investigate the influence of trees on radio waves. Several factors that are thought to be influencing excess loss estimation in trees were considered, among which are canopy thickness, leaf density, operating frequency, states of foliation and antenna geometry. Efforts were made to carry out repeat experiments at different periods of the year (autumn, winter, spring and summer) in order to include seasonal effects of trees on radio waves in the investigation. Results show that attenuation as high as 30 dB in excess of free space were recorded across single isolated trees. For the woodland (group of trees) experiment, an overall increase in excess attenuation was noticed with increase in depth of vegetation. The trend shows variation from path to path. Antenna position relative the trees, path geometry and leaf density are all contributing factors that determine excess loss estimation in a typical woodland. Three standard empirical loss prediction models; the FITU-R, MED and COST 235 have been used to evaluate the measurement data. Generally, the FITU-R model, which is a derivative of ITU-R model gave a better fit to the experimental data. The MED and COST 235 repeatedly under-estimated and over-estimated the measured losses respectively. However, these two models (MED and COST 235) occasionally showed good fit when antenna positions relative to the trees are at trunk and canopy levels respectively. Findings in this study bear direct relevance to radio wave propagation in trees and will provide an impetus for accurate design of link budget by radio systems planners.
5

Generation of orbital angular momentum in a data-bearing 60-GHz wireless channel

Mahmouli, Fariborz Eslampanahi January 2014 (has links)
The increasing demand for wireless connectivity and subsequently larger bandwidth requirements for high capacity connections have stretched the physical limits of existing wireless technologies in terms of bandwidth availability across the wireless frequency spectrum. Numerous research methods have been investigated and introduced in current wireless devices to overcome the problems relating channel capacity restriction. Some of these solutions are antenna diversity techniques such as multiple-input multiple-output (MIMO), and frequency diversity methods such as orthogonal frequency-division multiplexing (OFDM). In this thesis, orbital angular momentum (OAM) is successfully generated to transmit uncompressed high-definition (HD) video content at 4 Gbps rate over a 60 GHz wireless channel. As a prediction method, MA TLAB simulation was employed to design and create holographic plate (HP) masks and spiral phase plates (SPP). The MATLAB code was further utilized to control a printed circuit board (PCB) router drill to generate any desired models on copper or dielectric plates for example, holographic masks. It is believed for the first time, an experimental setup is exploited for generation of a 4 Gbps uncompressed video link over 60 GHz OAM wireless channel, and results are confirmed in tern:1s of received signal power and data rate throughput to illustrate the existence of OAM in 60 GHz channel. The achievements of this research wi~l contribute to the field of knowledge and it is concluded that it is a feasible prospect for multi-gigabit wireless communications to be achieved over OAM channels while improving the spatial diversity and spatial multiplexing of the wireless channel.
6

Power allocation in OFDM based cognitive radio systems with arbitrary input distributions

Sohail, Ahmed January 2014 (has links)
Future demands for higher data-rate services is putting pressure on the current fixed radio spectrum assignment scheme adopted by regulators. The Cognitive Radio (CR) approach is one possible solution to these issues by dynamically assigning spectrum between a Primary User (PU) and Secondary User (SU) and thus sharing spectrum and using it more efficiently. In this thesis we consider various forms of CR (interweave, underlay and joint interweave/underlay) applied to OFDM based radio systems. As conventionally used power allocation of the OFDM modulated SU aims to dynamically control the transmit power on each subcarrier in order to maximize the achievable data rate while protecting the PU from the interference caused by the SUo However, assumptions currently made do not account for practical system considerations e.g., Gaussian input and an interference model that assumes an FDM modulated PU with perfect synchronization with the SU plus a static traffic model. This results in higher optimized power levels and lower SU achievable data rates and thus underestimates usefulness of the system. The thesis addresses these issues by deriving a more general interference model which takes into consideration the more realistic waveforms and imperfect synchronization as well as real traffic models. Based on the interference model, the power optimization problem is formulated under the condition of real (Finite Symbol Alphabet) input as used in practical systems, and evaluates, for the interweave and joint schemes, using single and multiple terminal antennas (SISO and MIMO). The more generalised model shows a saving of transmit power and achieves higher data rates compared to the previous Gaussian approximation. Motivated by the power saving results, a theoretical framework is further established for all cases by evaluating average allocated power for the Gaussian and real waveform model. This theoretical analysis provides guidance for the system design and gives a deeper insight into the choice of parameters affecting power saving and rate improvement. In addition, for given channel statistics, the theoretical analysis can be used to estimate the power saving without running timeconsuming Monte Carlo simulations. Finally, by taking into account more practical scenarios, optimal power and achievable data rate is also evaluated for the case of the dynamic (considering probability of PU arrival and departure) joint scheme. In comparison to the static scheme, the joint scheme protects the PU in case of PU arrivals and yields improved utilization of spectrum when the PU departures are considered.
7

Traffic distribution fairness and congestion in social opportunistic networks

Soelistijanto, Bambang January 2014 (has links)
Social opportunistic networks (SONs) are intermittently-connected networks that exploit unpredictable contacts between users' mobile devices. The connectivity of SONs exhibits a nonrandom structure with the existence of a few hub nodes and social-aware routing protocols favour these nodes as the best carriers for message transfers. As a result, the network suffers from unbalanced traffic distribution leading to traffic congestion in the hub nodes. In this thesis, strategies for improving traffic distribution fairness in SONs and reducing traffic congestion in hub nodes are considered. The thesis proposes three innovative contributions as follows. An Analysis of traffic distribution and network capacity in SONs is fIrst performed. It considers the traffic distribution in a SON, and characterizes the network as being scale-free. Several forwarding strategies are considered, based on the routing information required by a node, i.e. isolated, local and complete networks. A network capacity model for a SON is then derived as an upper-bound of network delivery performance, where hub nodes' resources become the limiting factor. The analytical study shows that unfair traffic distribution occurs in the network for all three forwarding strategies, because a few hub nodes process a large fraction of the traffic. A Traffic Distribution Aware (TraDA) routing protocol is therefore proposed, aimed at improving traffic distribution fairness in SONs. A novel computation of node global popularity (the TraDA routing metric) is proposed, comprising node intrinsic popularity and social-relations popularity calculations. TraDa-Comm, a community-aware variant of TraDA, is also presented and simulation results show that it achieves better traffic distribution fairness than the state-of-the-art Bubble Rap, without degrading network delivery performance.
8

MIMO downlink transmitter design for mobile communication networks

Hou, Jiancao January 2014 (has links)
According to its distinctive feature of spatial orthogonality, multiple-input multipleoutput (MIMO) transmitter beamforming can improve spatial multiplexing gain, spatial diversity gain, or signal-to-noise ratio (SNR)/signal-to-interference-plus-noise ratio (SINR) without requiring extra channel bandwidth and transmission power comparing with single-antenna based scenarios. In the literature, MIMO transmitter beamforming has been quite well investigated. However, it is realized that the existing schemes can achieve the requirements only when the transmitter-side channel state information is assumed, or, with the limited channel feedback, the number of users in the cell should be much larger than the number of transmit antennas, where the multi user diversity can be well exploited. Otherwise, significant performance loss will be observed. Furthermore, in dense cellular networks, multi-cell coordinated beamforming plays a prominent role for interference pre-cancellation. However, most of coordinated beamforming schemes can only handle the optimization problems with single constraints, and the optimization process is significantly complicated if the objective functions involve the increased number of constraints. Motivated by these, the major contributions of this thesis are in two folds: A joint space-frequency user scheduling approach to enhance opportunistic beamforming with a small number of users in the cell is proposed. With the limited channel feedback to be assumed, two complexity reduced joint space-frequency user scheduling algorithms are proposed in order to maximize the system throughput and the number of active users. Simulation results show that the proposed SINR-based algorithm can largely improve the system throughput of conventional opportunistic beamforming up to 80 more, when the individual user's rate constraint is set up to 1 bit/s/Hz. Moreover, the performances of proposed SNRbased algorithm is even better with the pay of relatively increased computational complexity and signaling overhead. A novel coordinated beamforming approach for multi-cell MIMO downlink communication networks is proposed with the aim to maximize the system sum-rate subject to per transmitter power and individual user's rate constraints. The idea is basically a two-step iterative optimization procedure. Numerical results show that the proposed approach outperforms the existing schemes up to 3.2 bits/s/Hz per transmitter in terms of spectral efficiency. Moreover, by comparing with the optimal solution for finding the initial state at the first step, the proposed solution can exhibit close performance with faster convergence and lower computational complexity.
9

Real-time MIMO detection : algorithm and synthesis technology

Wu, Y. January 2014 (has links)
MIMO strategy is a key component of modern high throughput wireless communication. Robust detection algorithms are required to retrieve the signals at receiver in MIMO systems. However, these detection algorithms are of remarkable computational complexity and pose a substantial real-time embedded computing problem. Though the real-time solutions on FPGA have enabled such systems, large scale complex architectures are required to do so, demanding detailed manual desigli of circuit architectures. This thesis solves this design problem by addressing synthesis tools for 'Iarge scale detectors on FPGA. A cooperative actor-oriented synthesis (cooperative synthesis) approach is presented, enabling real-time implementations (480 Mbps for IEEE 802.11 n) of the FSD and SSFE detectors, using massively parallel softcore processor networks. This approach has also been applied to a BSS-EFE detector, which eliminates the restrictions and computational redundancies in SSFE to enhance the robustness of algorithm. More than 1 dB SNR gains is achieved with computational complexity reduced by 16:2%. With an alternative ordering approach, complexity may be reduced by 53 :26%. Applying the cooperative synthesis approach, the only recorded realization of this algorithm for IEEE 802.11n is presented.
10

Access network selection in heterogeneous networks

Alkhawlani, Mohammed Mohssen January 2008 (has links)
The future Heterogeneous Wireless Network (HWN) is composed of multiple Radio Access Technologies (RATs), therefore new Radio Resource Management (RRM) schemes and mechanisms are necessary to benefit from the individual characteristics of each RAT and to exploit the gain resulting from jointly considering the whole set of the available radio resources in each RAT. These new RRM schemes have to support mobile users who can access more than one RAT alternatively or simultaneously using a multi-mode terminal. An important RRM consideration for overall HWN stability, resource utilization, user satisfaction, and Quality of Service (QoS) provisioning is the selection of the most optimal and promising Access Network (AN) for a new service request. The RRM mechanism that is responsible for selecting the most optimal and promising AN for a new service request in the HWN is called the initial Access Network Selection (ANS). This thesis explores the issue of ANS in the HWN. Several ANS solutions that attempt to increase the user satisfaction, the operator benefits, and the QoS are designed, implemented, and evaluated. The thesis first presents a comprehensive foundation for the initial ANS in the H\VN. Then, the thesis analyses and develops a generic framework for solving the ANS problem and any other similar optimized selection problem. The advantages and strengths of the developed framework are discussed. Combined Fuzzy Logic (FL), Multiple Criteria Decision Making (MCDM) and Genetic Algorithms (GA) are used to give the developed framework the required scalability, flexibility, and simplicity. The developed framework is used to present and design several novel ANS algorithms that consider the user, the operator, and the QoS view points. Different numbers of RATs, MCDM tools, and FL inference system types are used in each algorithm. A suitable simulation models over the HWN with a new set of performance evolution metrics for the ANS solution are designed and implemented. The simulation results show that the new algorithms have better and more robust performance over the random, the service type, and the terminal speed based selection algorithms that are used as reference algorithms. Our novel algorithms outperform the reference algorithms in- terms of the percentage of the satisfied users who are assigned to the network of their preferences and the percentage of the users who are assigned to networks with stronger signal strength. The new algorithms maximize the operator benefits by saving the high cost network resources and utilizing the usage of the low cost network resources. Usually better results are achieved by assigning the weights using the GA optional component in the implemented algorithms.

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