氏学位名

廖 雪宁名

博士（システム情報科学）

学 位 記 番 号

第３９号

学位授与年月日

平成３０年９月２０日

学位論文題目

Physical Layer Security Performance Study for Two-Hop Wireless
Networks with Buffer-Aided Relay Selection

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

As wireless communication technologies continue to evolve rapidly, an
unprecedented amount of sensitive information, such as financial data, physical healthdetails and personal profile data, are transmitted through various wireless networks.
However, the broadcast nature of wireless medium makes it difficult to shield thesesensitive information from unauthorized users (eavesdroppers), and thus securing
wireless communication is becoming an increasingly urgent demand. Physical layer(PHY) security has been proposed as one promising technology to provide security
guarantee for wireless communications, owing to its unique advantages over traditional
cryptography-based mechanisms, like an everlasting security guarantee and no need forcostly secret key distribution/management and complex encryption algorithms. This
thesis therefore focuses on the PHY security performance study for tow-hop wirelessnetworks with buffer-aided relay selection (a typical PHY security technique), where
relay buffers will be adopted to help the transmission of the message.
We first investigate the security-delay trade-off of the buffer-aided relay selection
scheme in a two-hop wireless network with multiple randomize-and-forward (RF) relays.
To evaluate the security and delay performances of the system, we derive analyticalexpressions for the end-to-end (E2E) secure transmission probability (STP) and the
expected E2E delay under both perfect and partial eavesdropper channel state
information (CSI) cases. These analytical expressions help us to explore the inherent

trade-off between the security and delay performances of the concerned system. In
particular, the results in this paper indicate that: 1) the maximum E2E STP increases
as the constraint on the expected E2E delay becomes less strict, and such trend is moresensitive to the variation of the number of relays than that of the relay buffer size; 2) on
the other hand, the minimum expected E2E delay tends to decrease when a less strict
constraint on E2E STP is imposed, and this trend is more sensitive to the variation ofthe relay buffer size than that of the number of relays.
We then investigate the PHY security performances of two-hop wireless networks
with multiple DE relays, for which we extend the buffer-aided relay selection with RF
relays and propose a new buffer-aided relay selection scheme to resist theeavesdropper’s combining decoding in two-hop wireless networks with DF relays. To
validate the efficiency of the new scheme, a theoretical framework is developed toanalyze the E2E delivery process of a packet. Based the theoretical framework, wederive the closed form of the security and delay performances in terms of the E2E STP
and the expected E2E delay. Then, extensive numerical results are conducted tovalidate the efficiency of the new buffer-aided relay selection scheme, and thesecurity-delay trade-off issue is also studied to explore the achievable E2E STP(expected E2E delay) region under a given expected E2E delay (E2E STP) constraint.
Finally, comparisons are made between the new buffer-aided relay selection scheme andthe Max-Ratio scheme, results show that our new scheme outperforms the Max-Raitobuffer-aided relay selection scheme in terms of the E2E STP.

審査結果の要旨
This thesis focuses on the fundamental physical layer (PHY) security performance study for
two-hop wireless networks with the security technique of buffer-aided relay selection. Morespecifically, this thesis conducts the studies on: 1) the PHY security performance of buffer-aided
relay selection scheme for two-hop wireless networks with randomize-and-forward (RF) relays,
where different codebooks are adopted at the source and relay nodes respectively, and 2) the PHY
security performance of buffer-aided relay selection scheme for two-hop wireless networks withdecode-and-forward (DF) relays, where the same codebook is adopted at the source and the relay
nodes. In the first study, this thesis presents an attempt on the analysis of security-delay trade-off
issue for two-hop wireless networks with RF relays. In the second study, this thesis aims to propose
new buffer-aided relay selection scheme for two-hop wireless networks DF relays and provides

analysis on the E2E secure transmission probability (STP) and the expected E2E delay of theproposed scheme.
・論文の構成Chapter 1 Introduction
Chapter 2 Related Works
Chapter 3 Physical Layer Security Performance Study of Buffer-Aided Relay Selection Scheme for
Two-hop Wireless Networks with RF Relays
Chapter 4 Physical Layer Security Performance Study of Buffer-Aided Relay Selection Scheme for
Two-hop Wireless Networks with DF Relays
Chapter 5 Conclusions
Appendices A, B・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
This thesis studies the fundamental security performances of wireless networks with physicallayer (PHY) security technology. More specifically, this thesis focuses on the PHY security
technique of buffer-aided relay selection and conducts the studies of: 1) the PHY security
performance

ofbuffer-aided

relay

selection

fortwo-hop

wireless

networks

withrandomize-and-forward (RF) relays, 2) the PHY security performance of buffer-aided relay selection
for two-hop wireless networks with decode-and-forward (DF) relays.
In the first work, we consider the security-delay issue of the buffer-aided relay selection
scheme in a two-hop wireless network with RF relays. Available works on this topic suffer from two
main limitations: they usually consider the security of a single link to validate the security
performance, which fails to capture end-to-end (E2E) performance of the network; and they rarely
consider the packet delay issue. To address these limitations, we consider in this work the E2E
security and delay performances of the concerned network. To validate the security and delay
performances of the network, a theoretical framework is first developed based on the Markov chain
theory to depict the E2E delivery process of a tagged packet. With the help of the framework and thequeuing theory, analytical expressions of the E2E secure transmission probability (STP) and theexpected E2E delay are then derived. Finally, extensive numerical results are conducted to examine
the effect of network parameters on the network performances, and explore the security-delay
trade-off in terms of the E2E STP and the expected E2E delay.
In the second work, we study the PHY security performance of two-hop wireless networks
with DF relays. Despite much work on this topic, the PHY security performances study with theconsideration of buffer-aided relay selection technique for wireless network with DF relays remains

an open problem. We consider a two-hop wireless network with DF relays and propose a newbuffer-aided relay selection scheme to resist the combining decoding of the signals by eavesdroppers.
With help of the Markov chain theory, queuing theory and the probability theory, closed form
expressions of the E2E STP and the expected E2E delay are first derived. Then, extensive numericalresults are conducted to validate the efficiency of the new buffer-aided relay selection scheme overavailable ones.
・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. The studies conducted in this thesis will be helpful for the design of secure wireless networks.
The results in the E2E security-delay study illustrate the tradeoffs between security and delay of
a wireless network, which can serve as a guideline for the design of wireless networks withvarious performance requirements.
2. The results in the buffer-aided relay selection scheme study for networks with DF relays show
the impact of eavesdropper’s decoding strategy on network security, and thus shed light on thedesign of new PHY security techniques to effectively counteract such more hazardous
eavesdropping attacks.
3. The results in this thesis help us to understand the potentials of buffer-aided relay selection
technique in counteracting the eavesdropping attacks. In particular, the proposed buffer-aided
relay selection scheme could open a new approach for the design of more powerful buffer-aided
relay selection schemes by fully considering not only the link quality but also other inherent
properties of wireless networks.