氏学位名高 嬋名

博士（システム情報科学）

学 位 記 番 号

第５６号

学位授与年月日

令和３年９月１６日

学位論文題目

Protocol Design and Performance Analysis for Covert Communications
in Relay-Assisted Wireless Systems

論文審査委員

主査 姜 暁鴻副査 稲村 浩副査 藤野 雄一
副査 和田 雅昭論

文 要 旨With the rapid evolution of wireless communication technologies and the wide
coverage of wireless hotspots, wireless systems are of paramount importance to provide
ubiquitous wireless connectivity for lots of critical applications in daily life. However,security and privacy are critical in existing and future wireless systems since a large
amount of confidential information (e.g., credit card information, physiologicalinformation for e-health) is transferred over the open wireless channels. How toguarantee information security has attracted increasing concerns from both academia
and industry recently. Covert communication is a potential technique to prevent
adversaries from detecting the existence of transmissions among both sides of thecommunication. Therefore, this thesis focuses on the protocol design and performance
analysis for covert communications in the fundamental relay-assisted wireless systems.
We first investigate the covert communication in a two-hop wireless communicationsystem with multiple relays, where a message is first transmitted from its source to aselected relay and then forwarded by the relay to its destination under the detection of apassive warden. We explore in detail the relay selection protocol design issue for thissystem. For evaluating the performance of covert communication, we develop atheoretical framework to analyze the transmission outage probability, the detection
error probability of warden, and covert capacity based on a relay selection protocol
proposed. We also explore covert capacity maximization through efficient numericalsearches under a given covertness requirement. Finally, extensive simulation andnumerical results are provided to illustrate our theoretical findings and theperformance of covert communication in two-hop wireless communication systems withmultiple relays.
We then introduce cooperative jamming technology into relay-assisted wirelesssystems to interfere with the warden. In order to determine the forward relay and thejammer, we illustrate a new relay/jammer selection protocol in such systems. In order toexplore the impact of cooperative jamming on the performance of covert communication,
we introduce a jam-generating threshold into the theoretical framework and we further
derive the expressions for three performance metrics, i.e., transmission outageprobability, the detection error probability of warden, and covert capacity. We also
explore covert capacity maximization through efficient numerical searches under given
covertness and outage requirements. Finally, we present extensive simulation andnumerical results to validate our theoretical results, as well as to demonstrate thatcooperative jamming can confuse the warden well to improve the performance of covert
communication.
We further extend our study to a scenario where the warden will actively attack thecommunication process as a jammer in relay-assisted wireless systems. We redefine thebehavior of the warden that it will perform detection and jamming throughout thecovert communication process. Notice that the transmitter has to increase thetransmission power to ensure successful decoding due to the jamming of the warden,
and thus increases the risk of being detected. To deeply understand such interactions,based on the related relay selection protocol designed previously, we develop a newtheoretical framework to analyze the transmission outage probability, the detection
error probability of warden, and covert capacity. Then, we optimize the covert capacity
through power control under given covertness and outage requirements. Finally,
extensive simulation and numerical results are provided to illustrate our theoreticalfindings and the performance of covert communication with an active warden in suchsystems.

審査結果の要旨
This dissertation first explores the relay selection protocol design for covert communication in
two-hop multi-relay wireless communication systems with a passive warden, and conducts
theoretical analysis to derive the corresponding achievable covert capacity performance. We thenintroduce cooperative jamming technology into such systems to interfere with the warden, anddesign related relay/jammer selection protocols and provide theoretical analysis to demonstrate theimpact of cooperative jamming on covert capacity. We further extend our study to the scenario
where the warden will actively attack the communication process as a jammer in such systems.
Finally, extensive simulations and numerical results are provided to demonstrate our theoreticalfindings and the performance of covert communication in such systems. The work in this thesissheds new insights on the future researches of covert communications in relay-assisted wirelesssystems.
・論文の構成Chapter 1 Introduction
Chapter 2 Related Works
Chapter 3 Covert Communication in Wireless Systems with Multiple Relays
Chapter 4 Cooperative Jamming based Covert Communication in Relay-Assisted Wireless Systems
Chapter 5 Cover Communications in Relay-Assisted Wireless Systems with an Active Warden
Chapter 6 Conclusion
・研究目的の妥当性，従来の手法との比較においての有意性，および理論・実験手法の新規性
We first investigate the covert communication in two-hop wireless systems with multiple relays,
where a message is first transmitted from its source to a selected relay and then forwarded by therelay to its destination under the detection of passive warden. We explore in detail the relay selection
issue and propose two different relay selection protocols for this system. For evaluating theperformance of covert communication, we develop a theoretical framework to analyze thetransmission outage probability, detection error probability and covert capacity for a given relay
selection protocol, and also explore the covert capacity maximization through efficient numericalsearches under the condition of covertness requirement.
We then introduce cooperative jamming technology into relay-assisted wireless systems tointerfere with the warden. In order to determine the forward relay and the jammer, we propose newrelay/jammer selection protocols in the system. In order to explore the impact of cooperative
jamming on the performance of covert communication, we introduce a jam-generating threshold into
the theoretical analysis and further derive the expressions for the three performance metrics.
Finally, we extend our study to the scenario where the warden will actively attack thecommunication process as a jammer in relay-assisted wireless systems. We redefine the behavior ofthe warden that it will perform detection and jamming throughout the covert communication process.

To deeply understand such interactions, based on the two relay selection protocols designed
previously, we develop a new theoretical framework to analyze the transmission outage probability,
covert capacity and detection error probability of warden. We also explore the covert capacity
optimization through power control under the covertness and outage requirements.
・得られた知見のシステム情報科学の分野における意義
The results of this thesis provide the following insights.
1. It is expected that the proposed relay selection protocols and related theoretical models will
provide an important guideline for the design of covert communication systems in practicalapplications.
2. The results obtained in this thesis can inspire subsequent theoretical researches on the study of thecovert communication in two-hop wireless systems with multiple relays.