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

Master of Science, University of Akron, 2013, Electrical Engineering

Cooperative relaying is an effective approach to enhance the reliability and data-rate of wireless channels, where network nodes assist each other by relaying transmissions. Among different cooperative relaying protocols, the amplify-and-forward (AF) scheme is popular due to its performance and the ease of implementation. This thesis investigates the cooperative AF half-duplex multi-relay systems from an information-theoretic point of view where multiple relays use channel state information (CSI) to cooperate with the source and destination. The first part of the thesis is dedicated to the orthogonal AF (OAF) protocol where the source and relays using orthogonal channels for transmission. For a given input covariance matrix at the source, an optimal power allocation scheme among the relays is first developed via the optimal instantaneous power amplification coefficients to maximize the end-to-end achievable rate. The optimization problem is concave and is solved by Karush-Kuhn-Tucker conditions. The ergodic channel capacity is then derived by jointly optimizing the input covariance matrix at the source and the power sharing scheme among the relays. This is a bi-level non-convex optimization problem and can be solved using Tammer decomposition method. Furthermore, the capacity-achieving input covariance matrix is analyzed in high and low signal-to-noise ratio (SNR) regimes. At high SNRs, it is demonstrated that the transmit power should be distributed equally to all broadcasting phases at the source. On the other hand, at low SNRs, the optimal way is to use only a relay having the strongest cascaded source-relay and relay-destination channels and the source spends all the power in the broadcasting phase corresponding to that relay. The second-part of the thesis extends the results and analysis to the non-orthogonal AF (NAF) protocol, which is considered as a general description of AF relaying. Given the nature of broadcasting and receiving collisions in NAF (open full item for complete abstract)

Committee: Nghi Tran Dr. (Advisor); Hamid Bahrami Dr. (Committee Member); S.I. Hariharan Dr. (Committee Member) Subjects: Electrical Engineering

Doctor of Philosophy, The Ohio State University, 2006, Electrical Engineering

We propose novel cooperative protocols for various coherent flat-fading channels composed of half-duplex nodes. We consider relay, cooperative broadcast (CB), multiple-access relay (MAR) and cooperative multiple-access (CMA) channels and devise efficient protocols for them. We evaluate the proposed protocols using the diversity-multiplexing tradeoff (DMT). We also consider ARQ cooperative channels where users are provided with ACK/ NACK signals indicating success or failure of destination in decoding their messages. We show that utilization of ARQ techniques not only improves the tradeoff achieved by non-ARQ protocols, but also provides novel opportunities for cooperation that are otherwise unavailable. A distinguishing feature of the protocols proposed in this dissertation is that they do not rely on orthogonal subspaces, allowing for a more efficient use of resources. In fact, based on our results one can argue that the sub-optimality of previously proposed protocols stems from their use of orthogonal subspaces rather than the half-duplex constraint. We also provide a better understanding of the asymptotic relationship between the probability of error, transmission rate, and signal-to-noise ratio, as compared to what DMT offers. In particular, we identify the limitation imposed by the multiplexing gain notion and provide a new formulation, called the throughput-reliability tradeoff (TRT), that avoids this limitation. The new characterization is then used to elucidate the asymptotic trends exhibited by the outage probability curves of MIMO channels.

Committee: Hesham El Gamal (Advisor) Subjects: