Radio Frequency Informed Physical Layer Security - An Augmented Padlock to Wireless Transmission Secrecy

Electromagnetic induction, the earliest form of wireless energy transfer, was demonstrated by Michael Faraday in 1831, happening to be the infancy of radio communication. Since then, this technique in various scales was used for communications, eventually converging to a democratic name "Radio" in the early 1900s as we know it today. Radio frequency or RF communications are collectively and broadly referred to the communication of wireless nature using electromagnetic waves with a frequency of roughly a few kHz to several THz. RF communications are used for several applications ranging from leisure, meteorology, public broadcast, commercial, domestic and defense communications, and so forth. However, when information is communicated, often there is a need to distinguish among the intended receivers and eavesdroppers, for reasons of secrecy, confidentiality, and integrity, like for requirements from personal privacy, till national security. At present, wireless communications are mostly secured in the top level through mathematic encryption - the Advanced Encryption Standard (AES) and the Rivest-Shamir-Adleman (RSA) methods are some examples of this, and in smaller levels by techniques like range limiting, frequency diversity and similar. Anyhow the prominent mode of security is mathematic-based software encryption.

Now it is apparent that researchers (ethical or unethical) are working hard like the research community in software encryption to find techniques to crack into the information transmitted. Very often a scenario is encountered such that a transmitted spectrum is recorded and is then cracked over time using several reverse engineering and computational approaches. After all, software encryption is nothing but a complicated mathematical function that, in principle, is always solvable, provided enough time and computational effort is applied to the challenge. This is like eavesdropping an unknown language speaker, recording the conversation, and then transliterating it later with the help of a proficient translator.

The current advances in computing and artificial intelligence, like the advent of quantum computing makes this plot worse as computation power and competence is evolving day by day [1-4].

An added fortress to wireless communication in the shadow of such an impending vulnerability is hardware security, which is interchangeably called physical layer network security. This is synonymous to adding an extra padlock alongside a combination lock that extends the security over time and effort. Thus, physical layer wireless security is a set of hardware control approaches which makes sure that an effective reception of transmitted data constellation (often software encrypted) could be received in the right format only along pre-defined secure directions or locations. The transmission along all other directions or locations is either scrambled or prevented. This is a relevant concern and is backed by the fact that in a conventional antenna-based transmission, even though the directional gain of the antenna can be controlled, the constellation pattern of transmitted data is distributed in free space almost omnidirectionally around it. Thus, a receiver of adequate sensitivity would be able to record the transmitted data even though it is encrypted mathematically.

In this chapter, we broadly classify this concept, discuss several types of reported technologies for physical layer wireless security, and outline their comparisons. Finally, a future perspective of the same is drawn for readers who are interested in this promising research topic.