Doctor of Philosophy, The Ohio State University, 2017, Electrical and Computer Engineering

Tremendous increase in throughput, reliability and security requirements in present and future wireless communication networks necessitates the migration towards the underutilized higher frequency bands. The premise of large scale multiple input multiple output (MIMO) technology deployment in these bands has the potential of fulfilling future network requirements. At the same time, large scale network deployment, or the so-called dense coverage (large number of small scale base stations), is another link level strategy that also has the potential of enhancing the overall network quality of service (QoS). Performance of MIMO communication systems is governed by the amount of channel state information (CSI) available at both transmitter and receiver especially when deployed in a dense coverage network which has the potential of high line of sight (LoS) opportunity. This thesis aims to address throughput, reliability and physical layer security aspects of MIMO communication systems deployed in a fading environment with a stable path between transmitter and receiver with limited CSI feedback. The research involves four major research directions: (1) Transmitter optimization for public messages with minimal form of CSI feedback, (2) Secrecy capacity and optimal transmission strategy for confidential messages under the same limited CSI feedback model with eavesdropper uncertainty, (3) Establishing fundamental limits of covert communication of MIMO AWGN channel and highlight the potential of having a dominant channel mode in establishing high covert rates, (4) Message source authentication over MIMO channel with dominant mode. We start by considering the MIMO channel with dominant LoS component where the only CSI available at the transmitter are the Rician factor and the physical direction of the receiver with respect to the transmitter antennas array. For this particular scenario, although the exact capacity still unknown in a closed form, we establish an upper bou (open full item for complete abstract)

Committee: Hesham El Gamal Professor (Advisor); Can Emre Koksal Professor (Advisor); Inder Gupta Professor (Committee Member); Sheila Morgan Professor (Committee Member) Subjects: Communication; Electrical Engineering; Information Science

Doctor of Philosophy, The Ohio State University, 2021, Electrical and Computer Engineering

Emerging technologies and infrastructure developments in information and communication systems have paved the way for the extraordinary exposure of information around the globe. Specifically, the ease and the reliable exchange of information have promoted cultural, social, and economic activities. Meanwhile, this exposure is being exploited against user privacy and data confidentiality. In response, there have been major activities in keeping information safe. These activities can be summarized under three main domains: 1) Authentication: granting only legitimate access to data at rest, 2) Confidentiality: protecting information from being leaked to unauthorized parties in transit, and 3) Privacy: concealing user identity and activities. Modern cryptography is a practical and standardized approach that provides a certain level of information security. Cryptosystems obfuscate data in a way that makes it almost impossible to recover the plaintext, even with significant computational resources, but they do not rule out brute force recovery of data. They are robust in the communication media, i.e., the attackers are ruled out to have access to the ciphertext without a problem. Another approach, which is based on the physical characteristics of the hardware and/or the location, has been emerged as a powerful technique that can achieve unconditional security, i.e., without any assumption on the computational resources of the attackers. These two approaches are complementary and future security approaches will likely utilize both. In this dissertation, we mainly focus on the physical layer approaches, in particular, hardware-aided approaches, and discuss ways on how they can be used to enhance encryption-based approaches. First, we study multiple-input multiple-output (MIMO)-aided covert communication (also referred to as communication with a low probability of detection): the session between two legitimate parties remains undetectable from an external eavesdropper. (open full item for complete abstract)

Committee: C. Emre Koksal (Advisor); Yingbin Liang (Committee Member); Daniel J. Gauthier (Committee Member) Subjects: Electrical Engineering; Information Science; Information Systems

Master of Science in Engineering, University of Akron, 2015, Electrical Engineering

Due to the broadcast nature of wireless channels, security and privacy are of utmost concern for future wireless technologies. However, securely transferring confidential information over a wireless network in the presence of adversaries still remains a challenging task. As one of the most important aspects of wireless communication security, Physical Layer (PHY) security has started gaining research attention in the past few years. In wireless PHY security, the breakthrough idea is to exploit the characteristics of wireless channels such as fading or noise to transmit a message from a source to an intended destination while trying to keep this message confidential from passive eavesdroppers. Unlike cryptographic methods, no computational constraints are placed on the eavesdroppers. Benefiting from information-theoretic studies in cooperative relaying communications, relaying strategies have also recently received considerable attention in the context of PHY security over wireless networks. Specifically, in wireless PHY security, relay nodes can be used as trusted nodes to support a secured transmission from a source to a destination in the presence of one or more eavesdroppers. This thesis studies a wireless relay network in which a source node wants to communicate securely to a destination node in the presence of an eavesdropper under the aid of an amplify-and-forward (AF) relay operating in full-duplex (FD) mode for further security enhancement. The focus is on the optimal power allocation (PA) schemes to maximize the secrecy rate in different wireless environments. The first part of the thesis considers the problem of optimizing the PA at the source node and the relay node to achieve the secrecy capacity for slowly varying fading channels. Under this consideration, the optimal PA problem is shown to be quasi-concave. As such, the globally optimal power allocation solution exists, and it is unique. A simple bisection method for root finding can then be used t (open full item for complete abstract)

Committee: Nghi Tran Dr. (Advisor); Shiva Sastry Dr. (Committee Member); Forrest Bao Dr. (Committee Member) Subjects: Electrical Engineering

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: