# SDR Proof-of-Concept of Full-Duplex Jamming for Enhanced Physical Layer Security
## 1 Introduction  
Secrecy against eavesdroppers has always been a concern in communication systems. This threat is particularly relevant in wireless communications where, due to their broadcast nature, it is difficult to restrict the communication to an intended receiver ([PERSON]) while guaranteeing secrecy against an illegitimate one ([PERSON]) [1]. 5G and internet-of-things (IoT), with \"everything\" connected, have just aggravated these concerns [2]. [PERSON] was the first to address the secrecy problem [3] paving the way for the study of information theoretically secure coding schemes. However, practical schemes able to guarantee information theoretic security are still unknown for real world channels [4]. Instead, security and secrecy of today's communications mainly rely on modern cryptographic schemes that settle on the premise of the computational infeasibility of breaking the cryptosystem [5]. However, the definition of infeasible changes over time as technology matures. Besides, new vulnerabilities are always being found in these schemes compromising the security; for instance, the \"Heartbleed\" bug [6] was a serious vulnerability in OpenSSL cryptographic software library compromising all communications using SecureSocket Layer (SSL)/Transport Layer Security (TLS) over the Internet. In addition, the traditional encryption-based algorithms and standards are not always perfectly appropriate for resource-limited IoT scenarios because of the computational cost and energy consumption [7]. Physical layer security (PLS) has found renewed interest recently as an additional means of defence to realise wireless secrecy in communications [8; 9]. PLS exploits the random characteristics of the medium, such as noise, fading, and interference, along with the several degrees of freedom granted on the design of a communication system such as coding design, MIMO capability, beamforming, relaying, cooperative communications, jamming, etc. [9; 10]. PLS can prove useful as a complement to modern cryptography to further secure wireless communications without relying on the computational intractability of certain functions. Also, it should be emphasized that this level of security is placed as a complement and not a replacement to modern cryptography. The origin of PLS traces back, in fact, to [PERSON] [3] and [PERSON] [11], although with a surge of interest in latter years. [PERSON] in his seminal work [11] defined the concept of the wiretap channel that considers a three-party communication setup, composed of a legitimate transmitter and receiver ([PERSON] and [PERSON]) and an eavesdropper ([PERSON]), and in which the wiretap channel between [PERSON] and [PERSON] is degraded with respect to the main channel between [PERSON] and [PERSON]. Also, under this setup, [PERSON] showed that it is possible to build wiretap codes to achieve reliable communication to [PERSON] and secrecy against [PERSON]. Motivated by this result several practical coding schemes have been proposed [8; 12; 13]. However, most of these schemes rely on [PERSON]'s wiretap channel assumption of [PERSON] having a disadvantage with respect to [PERSON], which is difficult to assure at all times. This led to the development of friendly/cooperative jamming schemes where there is one helper node causing interference to degrade [PERSON]'s channel [14; 15; 16]. However, these type of schemes have drawbacks such as the need for synchronization and willingness to cooperate by the helper node. This work addresses these drawbacks through a solution in which [PERSON] (rather than a third party) acts himself as a jammer as shown in Figure 1, while transmitting in the same frequency in which he is receiving (i.e., full-duplex (FD)), therefore suffering from self-interference (SI) upon reception [17].