This paper is a survey of transmit antenna selection—a low-complexity, energy-efficient method for improving physical layer security in multiple-input multiple-output wiretap channels. With this method, a single antenna out of multiple antennas is selected at the transmitter. We review a general analytical framework for analyzing exact and asymptotic secrecy of transmit antenna selection with receive maximal ratio combining, selection combining, or generalized selection combining. The analytical results prove that secrecy is significantly improved when the number of transmit antennas increases.

This special issue is dedicated to security problems in wireless and quantum 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 communications.

Wireless networks have become pervasive in order to guarantee global digital connectivity, and wireless devices have quickly evolved into multimedia smartphones 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, wireless multiuser communication is very susceptible to eavesdropping, and it is essential to protect transmitted information. Wireless communications have traditionally been secured by network 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 cryptosystems) and computational complexity (for asymmetric cryptosystems). Moreover, classical cryptography is potentially vulnerable because it relies on the unproven assumption that certain mathematical functions are difficult to invert. Recently, methods 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 critical part of future communication networks. The core principle of physical layer security is to restrict the amount of useful information that can be extracted at the symbol/signal level by an unauthorized receiver. This is achieved by carefully designing intelligent and appropriate coding and precoding techniques that exploit the wireless medium’s channel state information. As opposed to classic cryptography, physical 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 attracted significant interest and promises to significantly affect both the theory and practical design of future wireless networks.

In“Location Verification Systems in Emerging Wireless Networks,”Yan and Malaney discuss location-based techniques and applications. They show that in recent years, there has been an explosion of activity related to location-verification techniques in wireless networks. This work has focused on intelligent transport system (ITS) because of the mission-critical nature of vehicle location verification within 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 systems are either partly or fully encompassed by such frameworks.

In“Wireless Physical Layer Security with Imperfect Channel State Information: A Survey,”Bao He et al. provide a comprehensive survey of physical layer security in wireless networks with imperfect channel state information (CSI) at communication nodes. The authors describe the main information-theoretic ways that secrecy is measured when CSI is imperfect. They also describe signal processing enhancements for secure transmission. These enhancements include secure on-off transmission, beamforming with artificial noise, and secure communication 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 problems and future research directions.

In“Methodologies of Secret-Key Agreement Using Wireless Channel Characteristics,”Ali and Sivaraman give an overview of current research on shared secret-key agreement between 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 secret-key agreement comprises four processes: sampling, quantization, information reconciliation, and privacy application. The authors also discuss existing challenges and future research directions.

In“An Introduction to Transmit Antenna Selection in MIMO Wiretap Channels,”Yang et al. propose transmit antenna selection as a low-complexity, enecgy-efficient way of improving 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 antenna selection. This framework includes receive maximal ratio combining, selection combining, or generalized selection combining. The results show that secrecy is significantly increased when the number of transmit antennas is increased.

Significant progress has been made in quantum communications as a result of increased support from governments and enterprises. There is a practical need for quantum communication, and it will significantly alter future communications. Quantum cryptography can benefit from the properties of quantum systems, e.g. entangled systems. Quantum entanglement lies at the heart of quantum information processing and communication. For a long time, entanglement was seen merely as a fancy feature that makes quantum mechanics counterintuitive. Quantum information theory has recently shown how quantum correlations are tremendously important to the formulation of new methods of information transfer and for algorithms based on quantum computers. Quantum correlation makes quantum information processing powerful and interesting. In a quantum many-particle system, classifying and quantifying correlations in a multipartite quantum state and determining how much knowledge about the quantum system can be acquired from subsystems are fundamental problems. The main task of quantum information processing and communication is the delivery of quantum states. The main focus of quantum information processing and communication is the delivery of quantum states. A quantum carrier or quantum channel can perform miracles compared with conventional signal processing and communication. In practice, it is very difficult to deliver entangled photons over long distances because of channel loss and detector noise. Quantum error correction coding is necessary for practical, reliable quantum information processing and can be performed in a noisy or real channel or in an imperfect processor.

In“Reducible Discord in Generic Three-Qubit Pure W States,”Zhihui Li et al. show that quantum correlation in generic three-qubit pure W states can be given by the two-qubit discord of these states. The authors show that reducing discord in the generalized three-qubit pure W state is complicated.

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 manner and is based on the five-qubit entangled Brown state. It allows Alice and Bob to simultaneously exchange arbitrary unknown states with the help of trusted Charlie. The authors show how to transfer arbitrary unknown states in a secure cooperative 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 quantum error-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 distances in quantum networks. The authors discuss the rate capacities 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 communication networks when the number of nodes in a multihop link tends towards infinity. The authors draw the concepts of classical ad hoc networks to optimize the rate capacity of one typical structure of a quantum repeater. The rate capacities of the recursive entanglement scheme (simultaneous entanglement scheme) and adjacent entanglement scheme are Ω(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 submitted papers. We hope the content of this issue is informative and helpful to all readers.