# An Overview of Key Technologies in Physical Layer Security
## 1 Introduction  
Wireless communication technology is a necessity for modern-day life because human beings depend on this technology for data transmission. In most cases, the said data contain confidential information such as banking transactions, military applications, and multimedia. The International telecommunication organization approximated that 53.6% of the world population, which amounts to about 4.1 billion people, were using the internet at the end of 2019 [1]. It is expected that this number will rise due to a rapid increase in active mobile subscribers as wireless networks continue to expand and new applications are developed. However, it is reported in [2] that an increasing number of mobile and wireless devices are affected by cyber-criminal activities. Cyber security ventures [2] predicted that cyber crime costs will increase to more than 6 trillion US dollars annually by 2021, from 3 trillion US dollars recorded in 2015. Therefore, it is highly important to improve wireless networks against cyber-criminal activities.  
Conventionally, the upper layers of the open system interconnect model are utilized to handle any discrepancies related to the attributes of authenticity, confidentiality, and privacy of data transmission. These attributes are mostly dependent on cryptographic algorithms which include secret-key distribution, public-key, and symmetric encryption. All these techniques function independently from the physical layer [3]. Based on the assumption that the eavesdropper has limited computing power ability, the above-mentioned techniques are considered to be secure. Moreover, they rely on underlying computational complexity for their robustness. Recent advances in quantum computing pose a serious threat to the currently used cryptographic schemes with their unlimited computational capacity [4]. Therefore, it is evident that the conventional methods in secure wireless communication are becoming less reliable.  
The open and superposition nature of wireless networks raises issues of confidentiality and security of the transmitted data when unintended users are present. Difficulties that prevent the transmitted signal from reaching the unintended users are a result of the broadcast nature. On the other hand, time variations and fluctuations in the wireless channel result in the arrival of multiple copies of the transmitted signal at the receiver. With that being said, security attacks in wireless networks may be categorized as active and passive attacks [5]. Passive attacks involve eavesdroppers who listen to the ongoing transmission silently and try to steal the transmitted information without interrupting legitimate transmission [6]. Under active attacks, the eavesdroppers use more aggressive and intrusive techniques that attempt to deteriorate the quality of the signal at the intended receiver. Common examples of these aggressive techniques are the denial of service, routing, and node malfunction attacks [5]. With exposure to so many attacks, wireless networks are required to have certain capabilities that will enable them to withstand and mitigate these attacks. The desired characteristics of a secure network include integrity, confidentiality and authentication, availability, and access control [5].