. Physical layer security (PLS) which protects data secrecy based on information-theoretic techniques, has lately received significant academic interest. The growth of wireless communications to 5G and beyond presents new difficulties for physical layer security research. As the existing Radio-Frequ
Hybrid Radio Frequency (RF) and Free Space Optics (FSO) communication systems represent a promising solution for modern communication challenges, offering the robustness of RF and the high data rates of FSO. As these systems become increasingly important in applications ranging from last-mile connectivity to satellite communication, ensuring their security, particularly at the physical layer, has emerged as a critical concern. This survey explores the state of the art in physical layer security (PLS) for mixed RF-FSO systems, highlighting key techniques, challenges, and solutions. We also outline future research directions to enhance the security and performance of these hybrid communication systems. By integrating insights from the latest research, this paper aims to provide a comprehensive understanding of the current landscape and future potential of PLS in mixed RF-FSO systems.
The open nature of radio propagation enables ubiquitous wireless communication. This allows for seamless data transmission. However, unauthorized users may pose a threat to the security of the data being transmitted to authorized users. This gives rise to network vulnerabilities such as hacking, eavesdropping, and jamming of the transmitted information. Physical layer security (PLS) has been identified as one of the promising security approaches to safeguard the transmission from eavesdroppers in a wireless network. It is an alternative to the computationally demanding and complex cryptographic algorithms and techniques. PLS has continually received exponential research interest owing to the possibility of exploiting the characteristics of the wireless channel. One of the main characteristics includes the random nature of the transmission channel. The aforesaid nature makes it possible for confidential and authentic signal transmission between the sender and the receiver in the physical layer. We start by introducing the basic theories of PLS, including the wiretap channel, information-theoretic security, and a brief discussion of the cryptography security technique. Furthermore, an overview of multiple-input multiple-output (MIMO) communication is provided. The main focus of our review is based on the existing key-less PLS optimization techniques, their limitations, and challenges. The paper also looks into the promising key research areas in addressing these shortfalls. Lastly, a comprehensive overview of some of the recent PLS research in 5G and 6G technologies of wireless communication networks is provided.
Physical layer security (PLS) has emerged as a new concept and powerful alternative that can complement and may even replace encryption-based approaches, which entail many hurdles and practical problems for future wireless systems. The basic idea of PLS is to exploit the characteristics of the wireless channel and its impairments including noise, fading, interference, dispersion, diversity, etc. in order to ensure the ability of the intended user to successfully perform data decoding while preventing eavesdroppers from doing so. Thus, the main design goal of PLS is to increase the performance difference between the link of the legitimate receiver and that of the eavesdropper by using well-designed transmission schemes. In this survey, we propose a conceptual, generic, and expandable framework for classifying the existing PLS techniques against wireless passive eavesdropping. In this flexible framework, the security techniques that we comprehensively review in this treatise are divided into two primary approaches: signal-to-interference-plus-noise ratio-based approach and complexity-based approach. The first approach is classified into three major categories: first, secrecy channel codes-based schemes; second, security techniques based on channel adaptation; third, schemes based on injecting interfering artificial (noise/jamming) signals along with the transmitted information signals. The second approach (complexity-based), which is associated with the mechanisms of extracting secret sequences from the shared channel, is classified into two main categories based on which layer the secret sequence obtained by channel quantization is applied on. The techniques belonging to each one of these categories are divided and classified into three main signal domains: time, frequency and space. For each one of these domains, several examples are given and illustrated along with the review of the state-of-the-art security advances in each domain. Moreover, the advantages and disadvantages of each approach alongside the lessons learned from existing research works are stated and discussed. The recent applications of PLS techniques to different emerging communication systems such as visible light communication, body area network, power line communication, Internet of Things, smart grid, mm-Wave, cognitive radio, vehicular ad-hoc network, unmanned aerial vehicle, ultra-wideband, device-to-device, radio-frequency identification, index modulation, and 5G non-orthogonal multiple access based-systems, are also reviewed and discussed. The paper is concluded with recommendations and future research directions for designing robust, efficient and strong security methods for current and future wireless systems.
Physical layer security (PLS) has emerged as a promising approach to enhance the security of wireless communication systems. This paper provides a comprehensive review of PLS, including its basic concepts, key techniques, and recent developments. We begin by introducing the basic principles of PLS, including the concept of secrecy capacity and the key assumptions underlying PLS. We then discuss various techniques that have been proposed to enhance PLS, such as artificial noise, cooperative jamming, and beamforming. Next, we review recent research on PLS in different scenarios, including multi-user systems, cognitive radio networks, and wireless sensor networks. Finally, we conclude by discussing future research directions and open challenges in PLS.
Due to the dramatic increase in high data rate services and in order to meet the demands of the fifth-generation (5G) networks, researchers from both academia and industry are exploring advanced transmission techniques, new network architectures and new frequency spectrum such as the visible light and the millimeter wave (mmWave) spectra. Visible light communication (VLC) particularly is an emerging technology that has been introduced as a promising solution for 5G and beyond, owing to the large unexploited spectrum, which translates to significantly high data rates. Although VLC systems are more immune against interference and less susceptible to security vulnerabilities since light does not penetrate through walls, security issues arise naturally in VLC channels due to their open and broadcasting nature, compared to fiber-optic systems. In addition, since VLC is considered to be an enabling technology for 5G, and security is one of the 5G fundamental requirements, security issues should be carefully addressed and resolved in the VLC context. On the other hand, due to the success of physical layer security (PLS) in improving the security of radio-frequency (RF) wireless networks, extending such PLS techniques to VLC systems has been of great interest. Only two survey papers on security in VLC have been published in the literature. However, a comparative and unified survey on PLS for VLC from information theoretic and signal processing point of views is still missing. This paper covers almost all aspects of PLS for VLC, including different channel models, input distributions, network configurations, precoding/signaling strategies, and secrecy capacity and information rates. Furthermore, we propose a number of timely and open research directions for PLS-VLC systems, including the application of measurement-based indoor and outdoor channel models, incorporating user mobility and device orientation into the channel model, and combining VLC and RF systems to realize the potential of such technologies.
5G network is expected to support massive user connections and exponentially increasing wireless services, which makes network security unprecedentedly important. Unlike traditional security-guaranteeing techniques which rely heavily on cryptographic approaches at upper layers of the protocol stack, physical-layer security (PLS) solutions fully take advantages of the characteristics of wireless channels to degrade the received signal qualities at the malicious users, and realize keyless secure transmission via signal design and signal processing techniques. PLS avoids the difficulties in the distribution and management of secret keys, and provides flexible security levels through adaptive transmission protocol design. Moreover, PLS techniques match the features of 5G networks well. Therefore, the application of PLS to 5G networks is a promising solution to address the security threats. This article presents a comprehensive review of the state-of-the-art PLS techniques, and discusses their applications in 5G networks. We first summarize the principle and advantages of PLS techniques, and point out the reasons why PLS is suitable for 5G networks. Then, we review the existing PLS methods in literature, and highlight several PLS solutions that are expected to be applied in 5G networks. Finally, we conclude this article and figure out some further research directions.