Advances in micro-embedded computing systems, coupled with developments in wireless technology have enabled the deployment of large scale wireless and sensor networks for many important applications. These networks are characterized by local geographic connectivity among nodes and by very little computational and storage capabilities at each node. Moreover, data transfer is mainly through packet forwarding by intermediary nodes. Due to the nature of their connectivity, nodes may have extremely limited information about their network, possibly only of their one-hop neighbors. In such a scenario where the nodes may have limited/erroneous network state information, we study the two basic network primitives: (i) point-to-point routing and (ii) broadcasting. First, we study the problem of point-to-point routing in a network of nodes where each node has a corresponding destination to send/receive data. We consider geographic routing (routing based on the position of the nodes), as this routing scheme is scalable and of low complexity and well suited to operate over sensor networks. We study the effect of imperfect routing information on the path lengths of the individual routes. We provide error models for the routing errors and demonstrate routing strategies that achieve order-wise optimal delays even when only a small fraction of the nodes have any (possibly imperfect) geographic information. We characterize the throughput capacity of the network and show that for a class of progressive routing strategies with limited routing data, the throughput capacity is order-wise optimal. While much of the current research focuses on greedy routing in uniform sensor networks, we study routing in imperfect (anisotropic) networks where greedy geographic forwarding fails due to holes (nodes without any neighbors that are closer to the destination). We develop routing strategies in such networks that operate with geographic location at the nodes to achieve order-wise optimal delays while maximizing the network throughput capacity. These algorithms inherit the beneficial properties of geographic routing algorithms such as scalability and low complexity while providing near-optimal throughput and delay in a robust manner. We also study routing strategies in networks where the traffic demand may be non-uniform. Routing schemes such as geographic routing that minimize some metric of routing distance cause local points of congestion as they do not consider the traffic demands across different parts of the network and may concentrate traffic along some paths that lie across regions of higher demand. We design randomized routing schemes based on geographic routing that are shown to be able to support any traffic demand that is achievable (i.e. achievable by any other scheme). Second, we study the issue of broadcasting in networks with limited local information. We analyze broadcast schemes where nodes have little geographic information or state information (memory of transmitted packets). We demonstrate randomized broadcast algorithms that utilize the limited information and perform broadcasting with minimal transmission overheads. Further, we also study branching random walks in R[superscript d], in the context of broadcasting a message over a spatial network to understand the asymptotic distribution of the broadcast. We derive analytic results on the density of these branching processes / text

Wireless communication continues to make a profound impact upon our daily lives. The oft-touted benefits of high data rates and improved reliability via wireless communication are limited by its inherent drawbacks, including path loss, fading and interference. One promising strategy for overcoming these problems is to deploy nodes in the region between a transmitter and its intended receiver. These intermediate nodes can improve communication for this transmitter-receiver pair by receiving a transmitted message, processing it and relaying the processed output to the receiver. This transmission strategy, known as relay-assisted communication, can be especially beneficial when the transmitter-receiver pair are either separated by a large distance or when a large obstruction blocks the path between them. In a reasonably dense network, several relays may be available to assist a particular transmitter-receiver pair. Deciding which relays should forward the transmitted message is actually quite difficult. For example, the relay with the best physical-layer channel gain to the destination may also be running low on battery power. Another relay may have a good physical-layer channel gain to the destination and a reasonable amount of remaining battery power, but its queue may be full of messages from other transmitters, so it cannot forward a newly arrived message within a given delay constraint. Thus, optimal relay selection entails carefully balancing all system parameters, which is prohibitively complex in current wireless systems. This dissertation provides novel results for dealing with the relay selection problem in two distinct types of wireless systems. First, several selection algorithms are designed for single-antenna wireless networks, including a decentralized random access-based strategy and centralized methods that are based on throughput maximization and downlink user scheduling. Second, selection algorithms based on transmission hop length are designed for multipleantenna wireless networks. The presented strategies for both single-antenna and multiple-antenna relaying are highly intuitive, as they allow for concise descriptions, making them amenable to practical implementation. Also, the presented strategies illustrate the importance of application-specific design, since each of them yields good performance by focusing on a small set of system parameters. For example, observed latency is of paramount importance for wireless networks that support a significant level of video traffic. / text

published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

The increasing demand for high-mobility and high data rate in wireless communications results in constraints and problems in the limited radio spectrum, multipath fading, and delay spread. The multiple-input multiple-output (MIMO) system has been generally considered as one of the key technologies for the next generation wireless communication systems. MIMO systems which utilize multiple antennas in both the transmit side and the receive side can overcome the abovementioned challenges since they are able to increase the channel capacity and the spectrum usage efficiency without the need for additional channel bandwidth. The detection algorithm is a big bottleneck in MIMO systems. Generally, it is expected to fulfill two main goals simultaneously: low computational complexity and good error rate performance. However, the existing detection algorithms are either too complicated or suffering from very bad error-rate performance. The purpose of this thesis is to comprehensively investigate the detection algorithms of MIMO systems, and based on that, to develop new methods which can reduce the computational complexity while retain good system performance. Firstly, the background and the principle of MIMO systems and the previous work on the MIMO decoding algorithms conducted by other researchers are thoroughly reviewed. Secondly, the geometrical analysis of the signal detection is investigated, and a geometric decoding algorithm which can offer the optimum BLER performance is proposed. Thirdly, the semidefinite relaxation (SDR) detection algorithms are extended to high-order modulation MIMO systems, and a novel SDR detector for 256-QAM constellations is proposed. The theoretical analysis on the tightness and the complexity are conducted. It demonstrates that the proposed SDR detector can offer better BLER performance, while its complexity is in between those of its two counterparts. Fourthly, we combine the SDR detection algorithms with the sphere decoding. This is helpful for reducing the computational complexity of the traditional sphere decoding since shorter initial radius of the hyper sphere can be obtained. Finally, the novel lattice-reduction-aided SDR detectors are proposed. They can provide near-optimum error rate performance and achieve the full diversity gain with very little computational complexity added compared with the stand-alone SDR detectors. / published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

The best MPhil thesis in the Faculties of Dentistry, Engineering, Medicine and Science (University of Hong Kong), Li Ka Shing Prize,2010-11. / published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

It has been envisioned that in the near future, wireless ad hoc networks would populate various application fields, ranging from disaster relief, environmental monitoring, surveillance, to medical applications, the observation of chemical and biological processes and community mesh networks. The decentralized and self-organizing nature of wireless ad hoc networks makes distributed algorithms fit very well in these networks, which however pose great challenges to the algorithm designers as they try to achieve optimal efficiency in communications. In this thesis, I develop a set of distributed algorithms addressing these challenges and solving some fundamental communication problems in wireless ad hoc networks. Communications in wireless ad hoc networks happen on a shared medium, and consequently are subject to interference. The first part of the thesis focuses on disseminating information on multiple-access channels while avoiding collisions. For both single-channel and multi-channel networks, the complexity of information dissemination is investigated, and nearly optimal distributed algorithms are proposed. The second part of the thesis focuses on designing efficient distributed algorithms for some fundamental problems under the physical Signal-to-Interference-plus-Noise-Ratio (SINR) interference model. The SINR model defines global fading interference with which the success of a signal reception depends on all simultaneous transmissions. Compared with graph based models, the SINR model reflects the fading and cumulative nature of radio signals. Hence, the SINR model represents the physical reality more precisely. However, the global nature of the SINR model makes the analysis of distributed algorithms much more challenging. Two types of fundamental problems are addressed in this part. The first type is closely related to communication coordination, including the wireless link scheduling problem and the node coloring problem. The second type of problems are about basic communication primitives, including the local broadcasting problem and the multiple-message broadcast problem. I investigate the complexity of these fundamental problems under the SINR interference model, and present efficient or optimal distributed algorithms. In the third part of the thesis, I propose a general interference model that can include commonly adopted interference models as special cases, and study whether efficient distributed algorithms can still be designed and analyzed in such a general model. Specifically, the affectance model is proposed in this part, which depicts the relative interference (affectance) on communication links caused by transmitting nodes. Both graph based models and the SINR model can be transformed into the affectance model. Under this general model, distributed algorithms with worst-case guarantees for the local broadcasting problem are presented. I also show how to make use of the developed techniques to get nearly optimal algorithms under the graph based model and the SINR model. / published_or_final_version / Computer Science / Doctoral / Doctor of Philosophy

The thesis report presents the designs of compact planar ultra-wideband (UWB) antennas for wireless devices applications. Three main designs of UWB antennas are studied, namely, single UWB antennas, UWB multiple-input-multiple-out(MIMO)antennas, and transparent UWB antennas on the screens of mobile phones. For single UWB antennas, the designs of two compact planar monopole antennas with compact sizes of 26×28 mm2and 30×39.3mm2are presented. The UWB operations of the antennas are achieved using a ground slot under the feed line, offsetting the feed line and the radiator from the middle of the ground plane and smoothly transforming the feed line. Simulation and measurement show that the two antennas can achieve an ultra-wide bandwidth with approximately omnidirectional patterns. A deep notch-band in5.1-5.85 GHz is created in one of the UWB antennas by employing two pairs of meander lines (MLs), one pair being close to the feed line and the other pair along the upper edge of the ground plane. At the notch frequency, the simulated efficiency is only 4%. Three compact UWB-MIMO antennas with very compact sizes of 26×40 〖mm〗^2, 21×38 〖mm〗^2, and 22×36 〖mm〗^2 are designed. Each of them is designed using two UWB antenna elements perpendicularly or symmetrically placed. Different techniques such as using ground stubs besides the radiators, cutting inclined slots on the ground, and adding a T-shaped protruding from ground are proposed to lower mutual coupling between the two antenna elements. One of the antennas is designed to generate a notched band in 5.15-5.85 GHz using two ground strips. Simulation and measurement results show that these antennas can cover the entire UWB of 3.1-10.6GHz with mutual coupling of less than -15 dB, and envelope correlation coefficient of less than 0.1. An UWB antenna is designed using a transparent conductive film for applications on mobile phone screens. The effects of a finger touching the screen are studied. Results show that, with the radiator on the bottom side of the screen and a thin film with a thickness of 0.05 mm on the top side to separate the finger and the antenna, the effects of the finger can be minimized. In measurement of monopole antennas with small ground planes, due to the feeding cable used, there are always discrepancies between the simulated and measured results in radiation patterns, efficiencies, and gains at lower frequencies. To verify that the discrepancies in the results of these studies are indeed due to the feeding cable used in measurement, the models of the feeding cables are developed and used for simulation. Results show that, by using the cable model, the simulated and measured results in radiation patterns, efficiencies, and gains agree very well. / published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy