# An Overview of Key Technologies in Physical Layer Security
## 1 Introduction
### Related Works  
In this section, we provide a brief overview of some of the related works on the review of key technologies in PLS. It should be noted that this paper focuses only on key-less PLS technologies. The survey paper in [7] provides a comprehensive overview of security in the physical layer from both an information-theoretic point of view and optimization using multi-antenna techniques; however, it does not describe applications of PLS techniques in modern wireless communication networks. To the best of our knowledge, the reviews found in [8; 9] are the only ones that cover the application of conventional PLS techniques in 5G (and beyond) wireless communication systems. They give insight into some of the emerging technologies, including internet of things (IoT), multiple-input multiple-output (MIMO), energy harvesting, visible light communication (VLC), and UAV communication. However, there are a lot of papers available that focus on individual aspects of the technologies mentioned above.  
VLC: Recently, the authors in [10] provided a comprehensive survey of VLC applications in PLS. The paper also discusses VLC channel models, different network configurations, and also presents some of the precoding strategies. In this paper, however, we demonstrate, with some simulation plots, a VLC practical scenario model with realistic system design parameters and show how the optical power decay in VLC is beneficial in PLS.  
IRS: Another technology which has gained a lot of interest in PLS recently is intelligent reflecting surfaces, and there has been a lot of work published in this area. Some recent papers are [11; 12; 13]. The authors in [11] provide an overview of IRS technology, particularly its applications across the whole of wireless communication. They also outline the advantages IRS offers and some of the challenges in implementing IRS-aided systems. The optimization problems in the IRS-based PLS scheme are derived and solved in [12; 14]. In this paper we specifically demonstrate the application of IRS in PLS and evaluate the performance of an IRS-based scheme using the achievable secrecy rate metric.  
UAVs: There have been a couple of papers published in this area recently [15; 16; 17]. Current research is aimed at the integration of UAV communication networks with 5G technology (and beyond 5G) to safeguard the current and future wireless networks. We provide a brief overview and summary of some of the interesting works on the applications of UAVs in PLS.  
Satellite communication: This is another area that has become a trend in recent PLS research, and a couple of interesting works have been published in [18; 19; 20; 21]. [PERSON] et al. [18] provided an overview of precoding techniques in multibeam satellite communication systems. Another study [19] proposed the inter-satellite communication of small satellite systems. A comprehensive overview of PLS in space information networks is given in [21]. They proposed the integration of satellite and IoT to form a satellite-based IoT and also discussed current technologies dedicated to PLS in land mobile satellite communication networks. Our letter intends to summarize some of the promising technologies that are discussed in the literature.