Doctor of Philosophy, The Ohio State University, 2018, Electrical and Computer Engineering

Wireless communication networks have become ubiquitous in recent years. Current wireless applications are possible thanks to small WiFi cells that provide high-speed indoor coverage and outdoor macro-cells that support user mobility. Next generation wireless networks will use similar architectures to enable new applications such as augmented and virtual reality, the internet of things, ultra-high definition video streaming, and massive data transmission and storage. However, these applications require unprecedented high-speed data transfer capabilities enabled by large frequency bandwidths. Motivated by spectrum scarcity in bands below 6 GHz, previously unused millimeter-wave (mmWave) bands, where large bandwidths are available, are now considered for future wireless networks. The necessity for efficient communication techniques for such large bandwidths and mmWave frequencies is the main motivation for this dissertation, with a focus on the complex radiowave propagation conditions found in indoor environments. Propagation mechanisms such as multiple reflections, diffractions, and transmissions through walls are commonly found in indoor wireless communications, which cause variations in the received signal along its bandwidth (wideband or frequency-selective channels). Traditionally, antenna arrays have been used together with beamforming (linear processing) techniques to improve the system's performance. However, those techniques were designed for narrowband systems (e.g., zero-forcing or matched filtering) and their application to wideband systems requires additional processing that increases system's complexity. In the first part of this dissertation, we tackle the problem of beamforming in frequency-selective channels with two approaches: \emph{i}) we use the electromagnetic time-reversal (TR) effect to directly design novel wideband beamformers, and \emph{ii}) we generalize the block-diagonalization (BD) procedure used in narrowband channels to the freque (open full item for complete abstract)

Committee: Fernando Teixeira (Advisor) Subjects: Electrical Engineering

PhD, University of Cincinnati, 2006, Engineering : Computer Science and Engineering

One of the main challenges in next generation wireless networks is to integrate heterogeneous wireless technologies to provide seamless connectivity, with guaranteed Quality of Service (QoS), to mobile users “anytime, anywhere and with any device”. In this dissertation, we investigate the problem of integrating cellular networks and Wireless Local Area Networks (WLANs) with the multi-hop communication paradigm used in Mobile Ad hoc Networks (MANETs) to exploit all the connectivity alternatives available to different types of Mobile Stations (MSs). We propose an integrated architecture based on three basic functionalities, namely, topology discovery, gateway discovery, and link quality estimation. We combine these three functionalities into an integrated routing mechanism that exploits all connectivity alternatives available in a generic heterogeneous scenario. Then, we provide a simulation-based analysis of our architecture and integrated routing mechanism in different heterogeneous networking scenarios. Our results show improvements in network's capacity and coverage achieved by our architecture as compared to isolated networks. The results also highlight the importance of the link quality estimation in providing QoS to users, as well as indicate that multi-hop links can be exploited in a controlled network configuration, but the QoS in multi-hop routes cannot be always guaranteed. Furthermore, we address the problem of selecting the best connectivity opportunity for a given service type based on the applications' QoS requirements, as well as on the network condition and user mobility profile. We propose the Connectivity opportunity Selection Algorithm (CSA) that allows MSs to select the connectivity opportunity most appropriate for a given type of service and mobility profile. Furthermore, we describe how our proposed selection algorithm can be introduced into the IEEE 802.21 standard for Media Independent Handover services.

Committee: Dr. Dharma Agrawal (Advisor) Subjects: Computer Science

Master of Science (MS), Ohio University, 2009, Electrical Engineering (Engineering and Technology)

This thesis describes a comparison of ray tracing results obtained using a commercial ray tracing software tool (Wireless Insite) to measured channel impulse response results for 5 GHz wireless band channels. Four sets of measurements were taken at an open parking lot by varying the transmitter and receiver positions with respect to the building. The channel impulse response, mean excess delay and root mean square delay spread were obtained from the measurements. We approximated the same environment in the software and obtained the channel impulse response and root-mean-square (RMS) delay spread (DS). The ray tracing and measured power delay profiles (PDPs) were first compared qualitatively, then the measured and ray tracing RMS-DS values were compared quantitatively. An accuracy analysis was done by varying the objects used in the ray tracing program and assessing the effects of this on results. A perturbation analysis was also conducted by varying by small amounts the transmitter distance, receiver distance, the number of reflections, and combinations of these to assess potential errors in our measured distance and hypothesized numbers of significant reflections. The results showed that the PDPs obtained using ray tracing are comparable to the measured results qualitatively and the perturbation analysis helped in modeling the inaccuracies involved during the measurements.

Committee: Dr. David Matolak (Advisor); Dr. Jeffrey Dill (Committee Member); Dr. Roger Radcliff (Committee Member); Dr. Sergio Lopez (Committee Member) Subjects: Electrical Engineering

Master of Science in Electrical Engineering (MSEE), Wright State University, 2017, Electrical Engineering

As communication and radar technology continue to become increasingly sophisticated, the sophistication of technologies used by unintended parties to acquire transmitted information increases in direct proportion. As a result, entities such as the military and commercial communication industries require methods to protect transmitted information from undesired recipients. Furthermore, the frequency spectrum, a finite resource, is becoming increasingly congested due to inefficient utilization. This thesis presents a novel RF steganography concept that uses linear frequency modulated (LFM) radar signals capable of optimizing the use of the frequency spectrum and hiding digital communication within the LFM to covertly transmit to legitimate recipients. Finally, this work demonstrates that these joint radar/communication waveforms can be designed, transmitted, and received, using software defined radio.

Committee: Zhiqiang Wu Ph.D. (Advisor); Xiaodong Zhang Ph.D. (Committee Member); Yan Zhuang Ph.D. (Committee Member) Subjects: Communication; Engineering

Doctor of Philosophy, The Ohio State University, 2014, Electrical and Computer Engineering

The excessively growing demand on the wireless data services has raised major concerns of a potential degradation, if not a total collapse, of satisfactory mobile communications. On the other hand, the available spectrum for wireless communications has been reported to suffer from a daily underutilization problem that lasts from midnight to early morning hours. Such a discrepancy between the wireless traffic levels over the course of the day is essentially tied to the human activity patterns, whereby end users exhibit high demand characteristics during the day time creating the so-called peak hour load, and concurrently idle at the late night time yielding substantially low demand and the so-called off-peak hour load. Major research efforts have been exerted over the past few years to develop a radical remedy to such a problem threatening the future of high-quality wireless communications. However, almost all of the emerging solutions, including cognitive radio communications, time-dependent pricing, and WiFi offloading, rely on influencing the economical responsiveness of wireless users to delay their demand from the peak to the off-peak time. The resulting gains of these proposed solutions hinge on the tradeoff between the offered pricing incentives and the flexibility of the users to change their activity patterns. In this dissertation we bring to attention an unexploited degree of freedom in the realm of wireless resource allocation. That is, the human behavior is highly predictable. Motivated by the recent findings that affirm this observation, we propose the proactive resource allocation paradigm in wireless networks. In particular, we investigate the design of optimal proactive data download policies that service predictable peak hour demand during the off-peak time without affecting the activity patterns of end users, and study their potential gains. Starting with the idealistic scenario of fully predictable requests and essentially static data content (open full item for complete abstract)

Committee: Atilla Eryilmaz (Advisor); Ness Shroff (Committee Member); Eylem Ekici (Committee Member) Subjects: Electrical Engineering

Doctor of Philosophy, The Ohio State University, 2014, Electrical and Computer Engineering

A novel class of embroidered textile radio frequency (RF) circuit and antennas is presented. They are based on a new class of highly conductive metal-coated polymer fibers (e-fibers) and are used to realize a variety of wearable RF electronics woven into daily garments. The subject e-fibers are composed of high strength and flexible polymer core, typically 10um in thickness, and covered with a 2um thick metallic coating. Computerized embroidery machinery and high density stitching was employed to achieve high conductivity and precise fabrication of the e-fiber textiles woven into regular clothing. Using this process, prototype textile antennas and RF circuits were ''printed'' onto garments and evaluated. Their RF characteristics were measured in o-body (free-standing) and on-body configurations. It is demonstrated that the textile antennas exhibit excellent RF performances, nearly as good as conventional copper antennas. This is in addition to their excellent mechanical strength and flexibility. Importantly, the textile antennas can be inconspicuously woven into clothing, without affecting comfort, fashionality, and washability. Consequently, various textile-based conformal and wearable antennas and sensors were designed and measured for communications, medical sensing, and RF energy harvesting applications. Overall, the proposed conductive textile-based antennas and RF circuitry provide solutions to future RF functionalized and fashionable garments for wearable wideband antennas with omni-directional coverage as well as frequency and beam reconfiguration.

Committee: John Volakis (Advisor); Fernando Teixeira (Committee Member); Christopher Baker (Committee Member) Subjects: Electrical Engineering; Electromagnetics

PhD, University of Cincinnati, 2005, Engineering : Electrical Engineering

A primary design objective in next generation wireless systems is to make efficient use of available bandwidth while keeping error rates low. Multiple Input, Multiple Output (MIMO) systems have shown promise toward fulfilling this objective. Space-time codes spread data symbols in time and space in order to create redundancy, which improves bit error rates (BER) but does not improve capacity. Spatial multiplexing techniques decompose the matrix channel into subchannels. This improves capacity but the resulting systems are not optimized with respect to BER. Linear Dispersion Codes (LDCs) are linear codes which spread data symbols using dispersion matrices. These codes guarantee high spectral efficiency and have been shown to exhibit good error performance. LDC designs based on frame theory of wavelets explicitly optimize both capacity and error performance. Conventional space-time codes such as Orthogonal Space-Time Block Codes (OSTBC) and spatial multiplexing systems assume that the transmitter has no knowledge of the channel. In some cases, however, feedback from receiver to transmitter can be established to convey channel state information (CSI). Such systems are called precoding systems. In this dissertation, we propose Precoded Linear Dispersion Codes (P-LDCs), a family of precoders which assume that the receiver has perfect channel knowledge while the transmitter has statistical information about the transmit and receive correlation matrices. P-LDCs can be viewed as a unifying design strategy which utilizes the LDC structure to implement an optimal capacity-achieving transmit covariance strategy. Utilizing partial CSI, P-LDCs form a linear space-time precoder using dispersion matrices derived according to capacity optimality criteria. P-LDC dispersion matrices are designed according to a two-fold objective: maximize spectral efficiency, then minimize pairwise codeword error probability for high signal to noise ratio (SNR). Finally, we analyze frame-based LDCs a (open full item for complete abstract)

Committee: Dr. James Caffery (Advisor) Subjects:

MS, University of Cincinnati, 2004, Engineering : Computer Science

Routing protocols in wireless ad hoc network are highly insecure and prone to various attacks owing to its inherent characteristics of open medium, dynamically changing topologies and distributed cooperation between the member nodes. Having a secure routing protocol in wireless ad hoc networks appears to be a problem that is not trivial to solve. We propose a scheme to enhance the fault-tolerance of cluster head's functionality in CBRP. CBRP with a single cluster head is single point of failure and unsuitable especially for functionalities like key distribution. By distributing the cluster head service to a group of cluster heads called Council nodes and utilizing the (k, n) secret sharing scheme, we can increase the fault tolerance of network manifolds against security attacks. Simulation results obtained demonstrates that our proposed algorithm enables simultaneous formation the Council based clusters, thereby making the scheme time efficient and comparable to CBRP. Results also show that since large size clusters are formed in Council based clusters, it is feasible to apply (k, n) secret sharing concepts. The scheme is more suitable for low mobility networks due to the less signaling overhead involved in during cluster reformations.

Committee: Dr. Dharma Agrawal (Advisor) Subjects: Computer Science

Doctor of Philosophy, The Ohio State University, 2007, Electrical Engineering

The central theme of this dissertation is the impact of feedback on the performance of wireless networks. Wireless channels offer a multitude of new challenges and opportunities that are uncharacteristic of wireline systems. We reveal the crucial role of feedback in exploiting the opportunities and in overcoming the challenges posed by the wireless medium. In particular, we consider three distinct scenarios and demonstrate the different ways in which feedback helps improve performance. We first consider cellular multicast channels and show that the availability of feedback allows for the cross-layer design of efficient multicast schedulers. Here we focus on two types of feedback scenarios: perfect channel state information (CSI) feedback and automatic repeat request (ARQ) feedback. We propose low-complexity multicast schedulers that achieve near-optimal asymptotic throughput-delay tradeoffs for both feedback scenarios. We further propose a cooperative multicast scheduler, requiring perfect CSI feedback, that achieves the optimal asymptotic scaling of both throughput and delay with the number of users. Next, we consider fading eavesdropper channels and reveal the importance of feedback in establishing secure communications. We characterize the secrecy capacity of such channels under the assumptions of full CSI and main (legitimate) channel CSI knowledge at the transmitter, and propose optimal rate and power allocation strategies. Interestingly, we show that the availability of CSI feedback enables one to exploit the time-varying nature of the wireless medium and achieve a perfectly secure non-zero rate even when the eavesdropper channel is more capable than the legitimate receiver channel on the average. We also propose a low-complexity on/off power allocation strategy and establish its asymptotic optimality. We then consider a minimal ARQ feedback scenario and propose transmission schemes that leverage the ARQ feedback to achieve non-zero perfect secrecy rates. Fina (open full item for complete abstract)

Committee: Hesham El Gamal (Advisor) Subjects:

Master of Science (MS), Ohio University, 2009, Electrical Engineering (Engineering and Technology)

The thesis provides statistical characterization and modeling of wireless channel transitions in both indoor and outdoor environments based on narrowband and wideband measurements. We characterize delay dispersion characteristics of the wireless channel as it transits from a line of sight (LOS) to a non-line of sight (NLOS) region. For the narrowband indoor channel transition measurements, received power levels versus frequency were measured in the 900 MHz unlicensed ISM frequency band. The narrow band channel results quantify some fading characteristics versus frequency and distance, and we also estimate values for the “power correlation” bandwidth. The power correlation values of 0.5 were observed for bandwidths of approximately 7 MHz. For the wideband channel transition measurements, power delay profiles (PDP) were measured in indoor and outdoor environments in the 5 GHz frequency band. Several delay spread parameters - root mean square delay spread, delay window, and channel impulse response X, dB (CIRX,dB) duration - were obtained for LOS and NLOS regions from the measured PDPs, and we quantify example changes in these parameters due to the transition. As expected, the delay spread parameters for NLOS regions are larger than those for LOS regions. Root mean-square delay spreads were found to increase from approximately 9 ns to 18 ns in going from LOS to NLOS regions, respectively. Wideband channel models were also developed for both regions for a bandwidth of 50 MHz. The channel models define tap amplitude fading distributions and parameters, tap energies, and Markov tap persistence parameters.

Committee: David W Matolak (Advisor); Jeffrey Dill (Committee Member); Trent Skidmore (Committee Member); Sergio Lopez (Committee Member) Subjects: Electrical Engineering; Engineering; Technology

Doctor of Engineering, Cleveland State University, 2011, Fenn College of Engineering

This research demonstrates the feasibility of utilizing high intensity laser power beaming (HILPB) systems as a conduit for robust free-space optical communications over large distances and in challenging atmospheric conditions. The uniqueness of vertical multi-junction (VMJ) photovoltaic cells used in HILPB systems in their ability to receive and to convert at high efficiency, very high intensity laser light of over 200 W/cm2, presents a unique opportunity for the development of the robust free space optical communication system by modulating information signals onto the transmitted high intensity photonic energy. Experiments were conducted to investigate and validate several optical communications concepts. A laser modulator was implemented to exhibit the excellent transient response of the VMJ technology at very high illumination intensities, and thus show its applicability to optical communications. In addition, beam polarization optic stages were employed to demonstrate a secure multi-channel communications scheme. The off-axis response of the receiver and the beam profile were characterized in order to evaluate the feasibility of developing acceptable pointing and tracking geometries. Finally, the impact of signal modulation on the total converted energy was evaluated and shown to have minimal effect on the overall power transmission efficiency. Other aspects of the proposed communication system are studied including: quantifying beamwidth and directivity, signal-to-noise-ratio, information bandwidth, privacy, modulation and detection schemes, transmission channel attenuation and disturbances (atmospheric turbulence, scintillation from index of refraction fluctuations, absorption and scattering from thermal and moisture variation) and beam acquisition tracking and pointing influence on the performance metrics of optical transmission technologies. The result of this research demonstrates the feasibility of, and serves as a comprehensive design guide for the i (open full item for complete abstract)

Committee: Taysir Nayfeh PhD (Advisor); Nigamanth Sridhar PhD (Committee Member); Petru Fodor PhD (Committee Member); John Turner PhD (Committee Member); Ana Stankovic PhD (Committee Member); Joseph Svestka PhD (Other) Subjects: Communication; Electrical Engineering; Engineering; Experiments; Optics