Physical Layer Security for Wireless and Quantum Communications

T his special issue is dedicated to security problems in wireless and quan⁃tum communications.Papers for this issue were invited,and after peer review,eight were selected for publication.The first part of this issue comprises four papers on recent advances in physical layer security for wireless networks.The second part comprises another four papers on quantum com⁃munications.

Wireless networks have become pervasive in order to guarantee global digital connectivity,and wireless devices have quickly evolved into multimedia smart⁃phones running applications that demand high-speed data connections.Multiuser multiple-input multiple-output(MIMO)wireless techniques meet this demand by achieving high spectral efficiency.Security is also regarded as critical in wireless multiuser networks because users rely on these networks to transmit sensitive data.Because of the broadcast nature of the physical medium,wirelessmultiuser commu⁃nication is very susceptible to eavesdropping,and it is essential to protect transmit⁃ted information.Wireless communications have traditionally been secured by net⁃work layer key-based cryptography.However,in large,dynamic wireless networks,classical cryptography might not be suitable.Classical cryptography tends to cause problems in terms of key distribution and management(for symmetric cryptosys⁃tems)and computational complexity(for asymmetric cryptosystems).Moreover,clas⁃sical cryptography is potentially vulnerable because it relies on the unproven as⁃sumption that certain mathematical functions are difficult to invert.Recently,meth⁃ods have been proposed to provide an additional level of protection and to achieve perfect secrecy without encryption keys.These methods,collectively referred to as physical layer security,exploit the randomness inherent in noisy channels.Physical layer security has been identified as the highest form of security and will be a criti⁃cal part of future communication networks.The core principle of physical layer se⁃curity is to restrict the amount of useful information that can be extracted at the sym⁃bol/signal level by an unauthorized receiver.Thisisachieved by carefully designing intelligent and appropriate coding and precoding techniques that exploit the wire⁃lessmedium's channel state information.As opposed to classic cryptography,phys⁃ical layer security is based on information-theoretic principles and does not rely on secret keys or the limited computational capacity of the eavesdropper.Over the past few years,the information-theoretic aspect of secrecy at the physical layer has at⁃tracted significant interest and promises to significantly affect both the theory and practical design of futurewirelessnetworks.

In“Location Verification Systems in Emerging Wireless Networks,”Yan and Malaney discuss location-based techniques and applications.They show that in re⁃cent years,there has been an explosion of activity related to location-verification techniques in wireless networks.This work has focused on intelligent transport sys⁃tem(ITS)becauseof themission-critical natureof vehiclelocation verification with⁃in ITS.The authors review recent research on wireless location verification related to the vehicular networks.In particular,they focus on location verification systems that rely on formal mathematical classification frameworks and show how many sys⁃temsare either partly or fully encompassed by such frameworks.

▶Jinhong Yuan

Jinhong Yuan received his BE and PhD degrees in electronics engineer⁃ing from Beijing Instituteof Technol⁃ogy in 1991 and 1997.From 1997 to 1999,he was a research fellow at the School of Electrical Engineer⁃ing,University of Sydney,Australia.In 2000,he joined the School of Electrical Engineering and Telecom⁃munications, University of New South Wales,Australia,and is currently a professor of tele⁃communications at that school.Dr.Yuan has authored two books,two book chapters,and more than 200 papers for telecommunications journals and conferences.He has also authored 40 industry reports.He is a co-inventor of one patent on MIMO systems and two patents on low-density parity-check(LDPC)codes.He has co-authored three pa⁃pers that have won Best Paper Awards or Best Poster Awards.His published work list is availableat http://www2.ee.unsw.edu.au/wcl/JYuan.html.Dr.Yuan is currently the NSW Chair of the joint Communications/Signal Proces⁃sions/Ocean Engineering Chapter of IEEE.He is also an associate editor for IEEE Transactions on Communica⁃tions.His research interests include error-control coding and information theory,communication theory,and wire⁃less communications.

▶Yixian Yang

Professor Yixian Yang is the Direc⁃tor of Information Security Center,Beijing University of Posts and Tele⁃communications.He is also chief of the National Engineering Laborato⁃ry for Disaster Backup and Recov⁃ery,National Key Laboratory for Network and Information Defense.He received his PhDdegree in elec⁃trical engineering and communica⁃tion systems from BUPTin 1988.His research interests in⁃clude network coding,coding theory,cryptography,infor⁃mation security,internet/intranet security,communication theory,graph theory,neural networks,signal processing,software radio,wavelet theory,discrete mathematics,and e-commerce.Dr.Yang has authored more than 500 re⁃search papers in academic journals such as IEEE Transac⁃tions on Communications,and has also authored textbooks and monographs.

▶Nanrun Zhou

Nanrun Zhou received his BSc de⁃gree in Physics Education and his MSc degree in Theoretical Physics from Jiangxi Normal University in 2000 and 2003.He received his PhD degree in Communications and Information Systems from Shanghai Jiaotong University in 2005.In 2006,he joined the School of Infor⁃mation Engineering,Nanchang Uni⁃versity,and is currently a professor and PhD supervisor at that school.From September 2011 to July 2012,he was a visiting scholar in the School of Computer Science,Beijing University of Posts and Telecommunications.Previously,he has been selected to the first and second ranks of the Ji⁃angxi Province Baiqianwan Talents for the New Century Programme,the Young Scientists of Jiangxi Province(Jing⁃gang Star),and the Ganpo Programme 555 for Outstanding Talent.His research interests include quantum communi⁃cation,quantum cryptography,optical image encryption,and wireless communication security.He has published more than 110 papers in refereed international journals and conferenceproceedings.

In“Wireless Physical Layer Security with Imperfect Chan⁃nel State Information:A Survey,”Bao He et al.provide a com⁃prehensive survey of physical layer security in wireless net⁃works with imperfect channel state information(CSI)at commu⁃nication nodes.Theauthorsdescribethemain information-the⁃oretic ways that secrecy is measured when CSI is imperfect.They also describe signal processing enhancements for secure transmission.Theseenhancementsinclude secureon-off trans⁃mission,beamforming with artificial noise,and secure commu⁃nication assisted by relay nodes or cognitive radio systems.The authors discuss the recent development of physical layer security in large,decentralized wireless networks as well as open problemsand futureresearch directions.

In“Methodologies of Secret-Key Agreement Using Wire⁃less Channel Characteristics,”Ali and Sivaraman give an over⁃view of current research on shared secret-key agreement be⁃tween two parties.This agreement is based on the wireless channel characteristics of the radio.The authors discuss the advantages of shared secret-key agreement over traditional cryptographic mechanisms and describe the theory behind this technique.They also describe the key agreement process,threat model,and typical performance metrics.A shared se⁃cret-key agreement comprises four processes:sampling,quan⁃tization,information reconciliation,and privacy application.The authors also discuss existing challenges and future re⁃search directions.

In“An Introduction to Transmit Antenna Selection in MI⁃MO Wiretap Channels,”Yang et al.propose transmit antenna selection as a low-complexity,enecgy-efficient way of improv⁃ing physical layer security in multiple-input multiple-output wiretap channels.The authors describe a general framework for analyzing the exact and asymptotic secrecy of transmit an⁃tenna selection.This framework includes receive maximal ra⁃tio combining,selection combining,or generalized selection combining.The results show that secrecy is significantly in⁃creased when thenumber of transmit antennasisincreased.

Significant progress has been made in quantum communica⁃tions as a result of increased support fromgovernmentsand en⁃terprises.There is a practical need for quantum communica⁃tion,and it will significantly alter future communications.Quantum cryptography can benefit fromthe propertiesof quan⁃tum systems,e.g.entangled systems.Quantum entanglement lies at the heart of quantum information processing and com⁃munication.For a long time,entanglement was seen merely as a fancy feature that makes quantum mechanics counterintui⁃tive.Quantum information theory has recently shown how quantum correlations are tremendously important to the formu⁃lation of new methods of information transfer and for algo⁃rithms based on quantum computers.Quantum correlation makes quantum information processing powerful and interest⁃ing.In a quantum many-particle system,classifying and quan⁃tifying correlations in a multipartite quantum state and deter⁃mining how much knowledge about the quantum system can be acquired from subsystems are fundamental problems.The main task of quantum information processing and communica⁃tion is the delivery of quantum states.The main focus of quan⁃tum information processing and communication is the delivery of quantum states.A quantum carrier or quantum channel can perform miracles compared with conventional signal process⁃ing and communication.In practice,it is very difficult to deliv⁃er entangled photons over long distances because of channel loss and detector noise.Quantum error correction coding is necessary for practical,reliable quantum information process⁃ingand can beperformed in anoisy or real channel or in an im⁃perfect processor.

In“Reducible Discord in Generic Three-Qubit Pure W States,”Zhihui Li et al.show that quantum correlation in ge⁃neric three-qubit pure W states can be given by the two-qubit discord of these states.The authors show that reducing discord in thegeneralized three-qubit pure Wstateiscomplicated.

In“Two-Way Cooperative Quantum Communication with Partial Entanglement Analysis,”Ying Guo et al.describe an improved cooperative two-way quantum communication scheme.This scheme works in a forward-and-backward man⁃ner and is based on the five-qubit entangled Brown state.It al⁃lows Alice and Bob to simultaneously exchange arbitrary un⁃known states with the help of trusted Charlie.The authors show how to transfer arbitrary unknown states in a securecoop⁃erative manner using encryption performed by trusted Charlie.

In“A Coding and Automatic Error-Correction Circuit Based on the Five-Particle Entangled State,”Xiaoqing Zhou et al.propose a quantum-coding and error-correction circuit for the five particle entangled state.This circuit can correct the bit-reversed or phase-flip error of one and two quantum states.The authors also simplify the design of a multiple quan⁃tumerror-correction circuit.

In“Optimal Rate for Constant-Fidelity Entanglement in Quantum Communication Networks,”Xutao Yu et al.describe how to achieve constant fidelity entanglement over long dis⁃tances in quantum networks.The authors discuss the rate ca⁃pacities of constant fidelity entanglement for both elementary and multihop links.In particular,the authors focus on the rate capacity of constant fidelity entanglement in quantum commu⁃nication networks when the number of nodesin a multihop link tends towards infinity.The authors draw the concepts of classi⁃cal ad hoc networks to optimize the rate capacity of one typical structure of a quantum repeater.The rate capacities of the re⁃cursive entanglement scheme (simultaneous entanglement scheme)and adjacent entanglement scheme areΩ(1/en)and Ω(1/n),respectively.

We thank all authors for their valuable contributions and all reviewers for their timely and constructive comments on sub⁃mitted papers.We hope the content of this issue is informative and helpful to all readers.