| Summary: | Traditional methods to secure communications rely on algorithms at protocol stack level, which are based on mathematically derived cryptographic protocols, to encrypt transmitted signals. With the steep rise in the data traffic and storage each year, and the strong increase in computational power available for unauthorized decryption, traditional mathematical cryptographic measures might be compromised in the upcoming years. To protect communications at the lowest level, physical layer security (PLS) methods have been proposed recently. These techniques aim to ensure secure communication links by exploiting the known unique characteristics of the physical medium between communicating nodes in order to distort a transmitted signal in such a way that only the intended receivers are unaffected by the distortion, effectively reducing the eavesdropping capabilities of any unintended party. In this way, it is the channel itself which is secured. These techniques come at the expense of several trade-offs between transmission resources and the degree of secrecy achieved. Of our particular interest is a class of PLS methods which can be referred to as `security enhancing techniques', in which minimal priors about eavesdroppers are assumed. In this dissertation we introduce innovative, wave scattering-based concepts and algorithms for the successful protection of both wireless and wired communication channels at the physical layer. This new class of approaches for physical layer (PHY) secure communications involve analog, controllable and reconfigurable scattering media (termed "micromedia'") positioned near the communicating nodes. This implementation is achieved via truly physical, analog means by exploiting the use of unilateral pilot signals for channel estimations. Prior research in this nascent field has addressed the important case of wireless channels as well as the more manageable wired communication channel case, the latter in connection with applications to power line communications (PLC) which make use of the available smart grid and other existing infrastructure. On the other hand, within the wireless regime, most existing work has focused on the canonical free-space case scenario, or -~within mathematical, statistical information theoretic frameworks~- on the classical random matrix models such as those for fading and other channels. Little attention has been given to the important case of unknown, complex, fixed or possibly slow-fading (block-fading) wireless channels, where little prior information is available other than that facilitated by the parties through explicit, cooperative communication. In addition, within the wired channel scenario, attention has been given only to techniques already developed for the wireless case. Furthermore, in both cases, many of the proposed algorithms are implemented mathematically rather than physically or analogically. In contrast, our proposed scattering approaches involve analog reconfigurable media and are applicable not only to free space but also to rather complex and unknown media. Our particular interest are applications to communications in indoor channels, tunnels, caves, and similar media, including underwater channels. Within this wireless regime, we develop novel secure communication methodologies applicable to single-input-single-output (SISO), multiple-input-single-output (MISO) and multiple-input-multiple-output (MIMO) systems. The developed methods for the SISO case are subsequently extrapolated also to the transmission line wired channel. The presented results and computer simulations demonstrate the feasibility for achieving secure communication links by means of the proposed techniques as well as the relevant trade-offs between communication resources and secrecy. Importantly, the proposed techniques provide security for the SISO case which had so far not been covered as a security enhancing technique. In the wired case, we address the transmission-line-like channels and more complex circuit networks. Motivation is provided by applications to secure communication in PLC networks, including the smart grid, as well as near field communication systems, intra-chip communication, and even novel modems for secure communication via conventional wired RF and optical channels. In this work, particular attention is given to single-phase, one-dimensional transmission line systems and planar circuit-like networks. The ideas, as presented for those particular classes of systems, can, of course, be readily extended to more general wired systems, and thus have relevance also for potential application to PLC systems where one deals in general with multi-phase transmission lines, electric machines, and other components that can be modeled via suitable lumped or distributed system parameters--Author's abstract
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