An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> I. INTRODUCTION <s> Security is one of the most important issues in communications. Security issues arising in communication networks include confidentiality, integrity, authentication and non-repudiation. Attacks on the security of communication networks can be divided into two basic types: passive attacks and active attacks. An active attack corresponds to the situation in which a malicious actor intentionally disrupts the system. A passive attack corresponds to the situation in which a malicious actor attempts to interpret source information without injecting any information or trying to modify the information; i.e., passive attackers listen to the transmission without modifying it. Information Theoretic Security focuses on confidentiality issues, in which passive attacks are of primary concern. The information theoretic approach to achieving secure communication opens a promising new direction toward solving wireless networking security problems. Compared to contemporary cryptosystems, information theoretic approaches offer advantages such as eliminating the key management issue; are less vulnerable to the man-in-the-middle and achieve provable security that is robust to powerful eavesdroppers possessing unlimited computational resources, knowledge of the communication strategy employed including coding and decoding algorithms, and access to communication systems either through perfect or noisy channels. Information Theoretic Security surveys the research dating back to the 1970s which forms the basis of applying this technique in modern systems. It proceeds to provide an overview of how information theoretic approaches are developed to achieve secrecy for a basic wire-tap channel model as well as for its extensions to multiuser networks. It is an invaluable resource for students and researchers working in network security, information theory and communications. <s> BIB001 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> I. INTRODUCTION <s> This paper provides a comprehensive review of the domain of physical layer security in multiuser wireless networks. The essential premise of physical layer security is to enable the exchange of confidential messages over a wireless medium in the presence of unauthorized eavesdroppers, without relying on higher-layer encryption. This can be achieved primarily in two ways: without the need for a secret key by intelligently designing transmit coding strategies, or by exploiting the wireless communication medium to develop secret keys over public channels. The survey begins with an overview of the foundations dating back to the pioneering work of Shannon and Wyner on information-theoretic security. We then describe the evolution of secure transmission strategies from point-to-point channels to multiple-antenna systems, followed by generalizations to multiuser broadcast, multiple-access, interference, and relay networks. Secret-key generation and establishment protocols based on physical layer mechanisms are subsequently covered. Approaches for secrecy based on channel coding design are then examined, along with a description of inter-disciplinary approaches based on game theory and stochastic geometry. The associated problem of physical layer message authentication is also briefly introduced. The survey concludes with observations on potential research directions in this area. <s> BIB002 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> I. INTRODUCTION <s> Wireless networking plays an extremely important role in civil and military applications. However, security of information transfer via wireless networks remains a challenging issue. It is critical to ensure that confidential data are accessible only to the intended users rather than intruders. Jamming and eavesdropping are two primary attacks at the physical layer of a wireless network. This article offers a tutorial on several prevalent methods to enhance security at the physical layer in wireless networks. We classify these methods based on their characteristic features into five categories, each of which is discussed in terms of two metrics. First, we compare their secret channel capacities, and then we show their computational complexities in exhaustive key search. Finally, we illustrate their security requirements via some examples with respect to these two metrics. <s> BIB003 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> I. INTRODUCTION <s> There has recently been significant interest in applying the principles of information-theoretical security and signal processing to secure physical layer systems. Although the community has made progress in understanding how the physical layer can support confidentiality and authentication, it is important to realize that there are many important issues that must be addressed if physical layer security is ever to be adopted by real and practical security systems. In this article, I briefly review several different flavors of physical layer security (at least for wireless systems), and then identify aspects (a.k.a. weaknesses) where the foundation for physical layer security needs to be strengthened. I then highlight that the opportunities for applying physical layer security to real systems will be quite rich if the community can overcome these challenges. In the course of the article, I note new directions for the community to investigate, with the objective of keeping physical layer security research targeted at having a practical impact on real systems. <s> BIB004 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> I. INTRODUCTION <s> The concept of physical layer security builds on the pivotal idea of turning the channel’s imperfections, such as noise and fading, into a source of security. This is established through appropriately designed coding techniques and signal processing strategies. In this vein, it has been shown that fading channels can enhance the transmission of confidential information and that a secure communication can be achieved even when the channel to the eavesdropper is better than the main channel. However, to fully benefit from what fading has to offer, the knowledge of the channel state information at the transmitter (CSIT) is of primordial importance. In practical wireless communication systems, CSIT is usually obtained, prior to data transmission, through CSI feedback sent by the receivers. The channel links over which this feedback information is sent can be either noisy, rate-limited, or delayed, leading to CSIT uncertainty. In this paper, we present a comprehensive review of recent and ongoing research works on physical layer security with CSIT uncertainty. We focus on both information theoretic and signal processing approaches to the topic when the uncertainty concerns the channel to the wiretapper or the channel to the legitimate receiver. Moreover, we present a classification of the research works based on the considered channel uncertainty. Mainly, we distinguish between the cases when the uncertainty comes from an estimation error of the CSIT, from a CSI feedback link with limited capacity, or from an outdated CSI. <s> BIB005 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> I. INTRODUCTION <s> As a complement to high-layer encryption techniques, physical layer security has been widely recognized as a promising way to enhance wireless security by exploiting the characteristics of wireless channels, including fading, noise, and interference. In order to enhance the received signal power at legitimate receivers and impair the received signal quality at eavesdroppers simultaneously, multiple-antenna techniques have been proposed for physical layer security to improve secrecy performance via exploiting spatial degrees of freedom. This paper provides a comprehensive survey on various multiple-antenna techniques in physical layer security, with an emphasis on transmit beamforming designs for multiple-antenna nodes. Specifically, we provide a detailed investigation on multiple-antenna techniques for guaranteeing secure communications in point-to-point systems, dual-hop relaying systems, multiuser systems, and heterogeneous networks. Finally, future research directions and challenges are identified. <s> BIB006 </s> An Overview of Physical Layer Security With Finite-Alphabet Signaling <s> I. INTRODUCTION <s> Physical layer security (PHY-security) takes the advantages of channel randomness nature of transmission media to achieve communication confidentiality and authentication. Wiretap coding and signal processing technologies are expected to play vital roles in this new security mechanism. PHY-security has attracted a lot of attention due to its unique features and the fact that our daily life relies heavily on wireless communications for sensitive and private information transmissions. Compared to conventional cryptography that works to ensure all involved entities to load proper and authenticated cryptographic information, PHY-security technologies perform security functions without considering about how those security protocols are executed. In other words, it does not require to implement any extra security schemes or algorithms on other layers above the physical layer. This survey introduces the fundamental theories of PHY-security, covering confidentiality and authentication, and provides an overview on the state-of-the-art works on PHY-security technologies that can provide secure communications in wireless systems, along with the discussions on challenges and their proposed solutions. Furthermore, at the end of this paper, the open issues are identified as our future research directions. <s> BIB007
W IRELESS communication technologies have become an indispensable part of our everyday lives. As more and more data is being transmitted over wireless links, along with the reliability of the transmission, ensuring security of information transfer has become a challenging issue. Maintaining confidentiality of the transmitted data, preventing Manuscript corruption of the transmitted information and verifying authenticity of communication are the most important security requirements in a wireless network. Traditionally, protecting confidentiality of data transmission is addressed via computation-based mechanisms such as encryption. On the other hand, the security provided by these methods mainly relies on the conjecture that the encryption function is difficult to invert, which is not provable from a mathematical point of view. With the ever-increasing computing power, encryption may no longer prevent information leakage to sophisticated adversaries. Moreover, implementation of key-based secure communications requires complicated protocols for key distribution and management, which is highly challenging to implement in the cases of decentralized wireless networks. Securing transmission at the physical layer is an alternative or a complement to the cryptographic solutions. The basic idea is to exploit the inherent randomness of the channel for achieving secrecy. Dissimilar to encryption based methods, in the schemes employing physical layer security, no constraint is placed on the computational capability of the eavesdropper. Furthermore, no key distribution/management is required in its implementation. During the past decade, there has been an extensive growth of interest in studying the capabilities of physical layer for securing communication. Several surveys and overview papers are available describing the state-of-the-art on this topic. For instance, fundamentals of physical layer security are comprehensively explained in BIB001 - BIB003 . A review of recent results ranging from point-to-point communications to multiuser scenarios is given in BIB002 . Hyadi et al. BIB005 present a survey on physical layer security under channel state information (CSI) uncertainty at different nodes. A comprehensive overview on various multiple antenna techniques in physical layer security is provided in BIB006 . The potentials of physical layer security for safeguarding data confidentiality in the fifth generation (5G) networks is studied in  and two updated summaries of the recent research results on physical layer security techniques for 5G and next generation wireless networks are given in  and BIB007 . The existing literature reviews provide a comprehensive understanding of the fundamentals of physical layer security. However, less is said about the practical aspects. Adoption of physical layer security techniques in the next generation networks requires a number of important practical issues to be addressed. With this motivation, a high-level overview of these issues is provided in a recent IEEE Communications Magazine paper BIB004 . While BIB004 provides the big picture, our objective in this survey is to shed light on two important (and highly related) practical aspects, namely, secure transmission with discrete signaling and practical code construction. We present a detailed review of the related recent results, underline the important behavioral differences between Gaussian inputs and discrete signaling, and study the transmit signal design problem for securing communications in a variety of scenarios. We also provide a review of recent results on code design for secrecy, and make connections with the information theoretic results with finite inputs. The remainder of the paper is organized as follows. In Section II, we provide a description of the fundamental concepts in physical layer security including the wiretap channel model and different secrecy metrics. In Section III, we discuss the differences between the secrecy performance of constellation-constrained and Gaussian inputs over Gaussian wiretap channels. An overview of secure multi-antenna transmission schemes with finite-alphabet inputs is provided in Section IV. Sections V, VI, and VII discuss the scenarios with multi-carrier transmission, spread spectrum communications and multiuser channels, respectively. Section VIII focuses on applications of physical layer security techniques in the next generation wireless networks including 5G. An overview of practical code design approaches for secrecy is presented in Section IX, followed by concluding remarks and future research directions in Section X. The organizational structure of the paper is given in Fig. 1 . Notation: Vectors and matrices are denoted with lowercase and uppercase bold letters. Non-bold letters are used to denote scalar values and calligraphic letters denote sets. The expectation and probability mass (or density) function of a random variable W is denoted by E W {.} and P W (.), respectively. Pr(.) is used to denote the probability of an event. A n stands for a sequence of length n, i.e., A n = {A(1), A(2), . . . , A(n)}. The notation CN (m, R) denotes a circularly symmetric complex Gaussian random vector with mean vector m and covariance matrix R.