In this dissertation, channel modeling for Terahertz (THz) channels and designs of nano devices for THz communications are studied. THz communication becomes more and more important for future wireless communication systems that require an ultra high data rate, which motivates us to propose new nano device designs based on graphene and new system models for the THz channel. Besides, the multiple-input multiple-output (MIMO) antenna technique is well known to increase the spectral efficiency of a wireless communications system. Considering THz channels' particular characteristics, MIMO systems with reconfigurable antennas and distributed antennas are proposed. We compare the differences between MIMO systems in the GHz and THz bands, and highlight the benefits of using multi antennas in the THz band. The work on nano device designs provides two antenna designs with single walled carbon nanotubes (SWCNTs) and graphene nano ribbon (GNR). First, we analyse the spectral efficiency of an SWCNT bundled dipole antenna based MIMO system in the Terahertz band. Two scenarios are considered: the large scale MIMO and the conventional scale MIMO. It is found that, in order to get the maximum spectral efficiency, the CNT bundle size should be optimized to obtain a tradeoff between the antenna efficiency and the number of antennas for a given area. We also discuss the random fluctuation in the bundle size during the CNT bundled antenna fabrication which reduces the system spectral efficiency. Then, we propose reconfigurable directional antennas for THz communications. The beamwidth and direction can be controlled by the states of each graphene patch in the antenna, and the states can be easily configured by changing the electrostatic bias voltage on each element. The work on reconfigurable MIMO system proposes a new antenna array design for MIMO in the THz band. First, the path loss and reflection models of the THz channel are discussed. Then, we combine the graphene-based antenna and the THz channel model and propose a new MIMO antenna design. The radiation directions of the transmit antennas can be programmed dynamically, leading to different channel state matrices. Finally, the path loss and the channel capacity are numerically calculated and compared with those of the GHz channel. The results show that for short range communications, the proposed MIMO antenna design can enlarge the channel capacity by both increasing the number of antennas and choosing the best channel state matrices. The work on MIMO channels proposes a statistical model for the MIMO channel with rough reflection surfaces in the THz Band. First, our analysis of scattering from a rough surface indicates that the reflection from a single surface can be a cluster of rays. Secondly, a new MIMO model for THz communications is proposed. In this model, the number of multipaths is highly dependent on the roughness of the reflecting surfaces. When the surface is ideally smooth, the MIMO channel is sparse and as a result, the capacity is sub-linear with the MIMO scale. On the other hand, when the surface is rough, more degrees of freedom are provided by the scattered rays. Finally, channel capacities with different surface roughness are numerically calculated and compared between different MIMO scales. The results show that in contrast to the GHz range, large scale THz multiple antennas may not provide as much multiplexing gain. Therefore, it is necessary to determine the antenna scale according to the actual propagation environment. The work on distributed antenna systems (DAS) proposes a new DAS model in the THz band. First, the model of DAS in the THz frequency is discussed, which has fewer multipaths than that in the GHz band. Then, we analyze the characteristics of the DAS model and point out that the channel is very sparse if the number of antennas on the base station (BS) is very large. Besides, we provide reasons for the fact that DAS can have a large number of degrees of freedom. We compare the capacities of MIMO systems with DAS and without DAS. The results show that for THz channels, increasing the number of antenna units (AUs) is much more important than increasing the number of antennas in one AU. Finally, we propose an antenna selection and precoding scheme which has very low complexity. / Graduate

Recently, a number of empirical models have been introduced in the literature for the behavior of direct path used in the design of algorithms for RF based indoor geolocation. Frequent absence of direct path has been a major burden on the performance of these algorithms directing researchers to discover algorithms using multipath diversity. However, there is no reliable model for the behavior of multipath components pertinent to precise indoor geolocation. In this dissertation, we first examine the absence of direct path by statistical analysis of empirical data. Then we show how the concept of path persistency can be exploited to obtain accurate ranging using multipath diversity. We analyze the effects of building architecture on the multipath structure by demonstrating the effects of wall length and wall density on the path persistency. Finally, we introduce a comprehensive model for the spatial behavior of multipath components. We use statistical analysis of empirical data obtained by a measurement calibrated ray-tracing tool to model the time-of- arrival, angle-of-arrival and path gains. The relationship between the transmitter-receiver separation and the number of paths are also incorporated in our model. In addition, principles of ray optics are applied to explain the spatial evolution of path gains, time-of-arrival and angle-of-arrival of individual multipath components as a mobile terminal moves inside a typical indoor environment. We also use statistical modeling for the persistency and birth/death rate of the paths.

In Time-of-Arrival (TOA) based indoor human tracking system, the human body mounted with the target sensor can cause non-line-of-sight (NLOS) scenario and result in significant ranging error. In this thesis, we measured the TOA ranging error in a typical indoor environment and analyzed sources of inaccuracy in TOAbased indoor localization system. To quantitatively describe the TOA ranging error caused by human body, we introduce a statistical TOA ranging error model for body mounted sensors based on the measurement results. This model separates the ranging error into multipath error and NLOS error caused by the on-body creeping wave phenomenon. Both multipath error and NLOS error are modeled as a Gaussian variable. The distribution of multipath error is only relative to the bandwidth of the system while the distribution of NLOS error is relative to the angle between human facing direction and the direction of Transmitter-Receiver, signal to noise ratio (SNR) and bandwidth of the system, which clearly shows the effects of human body on TOA ranging. An efficient way to fight against the TOA ranging error caused by human body is to employ site-specific channel models by using ray-tracing technology. However, existing ray-tracing softwares lack the propagation model that takes the effects of human body into account. To address that issue, this thesis presents a empirical model for near human body ultra-wideband (UWB) propagation channel that is valid for the frequency range from 3GHz to 8GHz. It is based on measurements conducted in a anechoic chamber which can be regarded as free space. The empirical model shows the joint propagation characteristics of the on body channel and the channel between body surface and external access point. It includes the loss of the first path, arrival time of the first path and the total pathloss. Models for all three aspects have been partitioned into two sections by a break point due to the geometrical property of human body and the creeping wave phenomenon. The investigation on first path behavior can be regarded as a theoretical basis of raytracing technique that takes the effects of human body into consideration.

ITC/USA 2012 Conference Proceedings / The Forty-Eighth Annual International Telemetering Conference and Technical Exhibition / October 22-25, 2012 / Town and Country Resort & Convention Center, San Diego, California / The extremely large electrical-size and complexity of terrain scene poses great challenge in channel modeling of aeronautic telemetry. It becomes even more difficult if severe multipath and fading present due to scattering and attenuation of ground, terrain objects and precipitation [Rice, 2004]. This is critical in more sophisticated test scenarios involving low flying unmanned air vehicles and helicopters tested over water at high sea states, in hilly terrain, or even over urban environment. Conventional ray tracing and simple Fresnel reflection are not sufficient to characterize such complex channels. Hence, the novel bidirectional analytic ray tracing and radiative transfer (RT²) is proposed for advanced telemetry channel modeling.

Modern society relies heavily on communication networks that in turn rely on both wired and wireless infrastructure. This work pertains to scenarios where a group of people or robots need to communicate in an environment where there is no preexisting communications infrastructure. These include sites of emergencies and disasters (e.g., inside burning buildings, search and rescue operations) and unexplored areas on Earth and other planets. Wireless ad hoc or mesh networks offer the ability to keep such entities connected, but they falter when any single entity wishes to leave the developed coverage area. Utilizing mobile repeater nodes can help, but is costly and complicated. By eliminating the need for repeater nodes to traverse the environment, their size and cost can be vastly reduced. This work explores the use of static "breadcrumb" repeater nodes to increase the reach of such a network. Determining when and where to place a static repeater node can be difficult in an environment where radio propagation characteristics are unknown. In this work, several algorithms for node placement are compared under the constraint that placement of a static repeater node should not dictate the entity's movement. The algorithms investigated range from calculating rolling averages to modeling channel parameters on-the-fly. The placement algorithms were configured to run in real-time on TP-Link MR-3040 portable WiFi routers and the approach is demonstrated in an outdoor uncharacterized environment.

A number of techniques for indoor and outdoor WiFi localization using received signal strength (RSS) signatures have been published. Little work has been performed to characterize the RSS signatures used by these WiFi localization techniques or to assess the accuracy of current channel models to represent the signatures. Without accurate characterization and models of the RSS signatures, a large amount of empirical data is needed to evaluate the performance of the WiFi localization techniques. The goal of this research is to characterize the RSS signatures, propose channel model improvements based on the characterization, and study the performance of channel models for use in WiFi localization simulations to eliminate the need for large amounts of empirical data measurements. In this thesis, we present our empirical database of RSS signatures measured on the Worcester Polytechnic Institute campus. We use the empirical database to characterize the RSS signatures used in WiFi localization, showing that they are composed of connective segments and influenced by the access point (AP) location within a building. From the characterization, we propose improving existing channel models by building partitioning the signal path-loss using site-specific information from Google Earth. We then evaluate the performance of the existing channel models and the building partitioned models against the empirical data. The results show that using site-specific information to building partition the signal path-loss a tighter fit to the empirical RSS signatures can be achieved.

Les avancées technologiques de ces dernières années, combinées au succès avéré et toujours croissant des communications sans fil, ont tout naturellement donné naissance à un nouveau type de réseaux sans fil, communément appelés Body Area networks. A terme, ces réseaux corporels sans fil doivent permettre à un ensemble de senseurs bio-médicaux répartis sur le corps humain de communiquer, soit pour échanger des informations en vue d'un traitement en temps réel du patient, soit pour enregistrer des données physiologiques en vue d'une analyse ultérieure. L’objectif de cette Thèse vise la réduction de la consommation énergétique au niveau des senseurs de sorte à leur garantir une autonomie de quelques mois, voire de quelques années. En réponse à cette contrainte énergétique, une association innovante de deux technologies émergentes est proposée, à savoir une combinaison des transmissions à ultra-large bande aux systèmes à multiples antennes. Une nouvelle architecture pour les réseaux corporels sans fil est donc envisagée pour laquelle les performances doivent être évaluées. Notre principale contribution à cet objectif consiste en la proposition d'une modélisation spatio-temporelle complète du canal de transmission dans le cadre de senseurs répartis autour du corps. Cette modélisation fait appel à la définition de nouveaux modèles, l'élaboration d'outils spécifiques d'extraction de paramètres et une compréhension fine des mécanismes de propagation liés à la proximité du corps humain. Ce manuscrit présente les résultats majeurs de nos recherches en cette matière.

Ray tracing based on geometric optics can be utilized for generating propagation models for arbitrary and complex environments. These methods can be employed to determine important wireless channel characteristics such as path gain and the channel impulse response which in turn can be used to deduce channel capacity. In this thesis, a fully vectorial 3-D ray-tracer is developed. The simulator is applied to study novel wireless systems such as ultra-wideband pulse propagation in complex railway tunnels and MIMO systems employing closely spaced low mutual coupling meta-material antennas. The computational complexity of the ray-tracing algorithm is reduced using optimizations and via the development of a novel hybrid method that combines the efficiency and accuracy of waveguide models with the flexibility of a ray-tracer. The resulting simulator is validated against measured results and demonstrated to show good agreement. Convergence of the solution using the ray-tracing method is also discussed.

Ray tracing based on geometric optics can be utilized for generating propagation models for arbitrary and complex environments. These methods can be employed to determine important wireless channel characteristics such as path gain and the channel impulse response which in turn can be used to deduce channel capacity. In this thesis, a fully vectorial 3-D ray-tracer is developed. The simulator is applied to study novel wireless systems such as ultra-wideband pulse propagation in complex railway tunnels and MIMO systems employing closely spaced low mutual coupling meta-material antennas. The computational complexity of the ray-tracing algorithm is reduced using optimizations and via the development of a novel hybrid method that combines the efficiency and accuracy of waveguide models with the flexibility of a ray-tracer. The resulting simulator is validated against measured results and demonstrated to show good agreement. Convergence of the solution using the ray-tracing method is also discussed.

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