PHY Security of Wiretap Channels: From Point-to-Point to Multiple Access
Information secrecy is crucial to a wireless communication system due to the broadcast nature of radio-frequency transmissions. Traditionally, secrecy has been provided by designing upper-layer sophisticated protocols. More specifically, the problem of securing the information from malicious eavesdropping nodes has been tackled by relying on the conventional encryption mechanism. To improve the system security, physical-layer (PHY) security has been recently recognized as a valuable tool to guarantee information-theoretic confidentiality of messages transmitted over the wireless medium. In this dissertation, we propose new efficient schemes to secure the wireless links of various wireless communications networks. We investigate both the frequency non-selective and selective channels for different relaying networks. For the frequency-selective channels, we use the orthogonal-frequency division-multiplexing (OFDM) transmission scheme to convert the channel into a set of flat-faded sub-channels where each OFDM block begins with a cyclic-prefix sequence to eliminate the inter-block interference. For frequency-selective and non-selective channels, we propose secure schemes and evaluate their performance. Our proposed artificial noise aided schemes are based on the spatial and temporal degrees of freedom provided by the multiple antennas at the legitimate nodes and the cyclic prefix of the OFDM systems, respectively. We provide in-depth analysis for the PHY security of many wireless systems architectures including single-input single-output (SISO) systems, relay-aided SISO systems, multiple-input multiple-output (MIMO) systems, and relay-aided MIMO systems. In addition, we investigate the impact of data buffers at communicating nodes as well as the presence of friendly jamming nodes on the achievable secrecy rates. The duplexity capabilities of the relay nodes are also investigated and their impacts on the achievable secrecy rates are quantified. To make the communication systems more robust against eavesdropping, we also investigate securing wireless networks in the presence of a set of untrusted relay nodes where the relay nodes are assumed to be honest but curious as in, e.g., the aggregating units in the smart grids, and the servers in governmental intelligence systems. As a case study on the impact of passive and active security attacks, we quantified the gain of our proposed encoding and secure schemes in terms of improving the demand-side management's security and reducing the monetary loss to the utility.