Browsing by Author "Zhang, Yi"
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Item Physical Layer Security in MIMO Backscatter Wireless Systems(IEEE Transactions on Wireless Communications, 8/31/2016) Yang, Qian; Wang, Hui-Ming; Zhang, Yi; Han, ZhuBackscatter wireless communication is an emerging technique widely used in low-cost and low-power wireless systems, especially in passive radio frequency identification (RFID) systems. Recently, the requirement of high data rates, data reliability, and security drives the development of RFID systems, which motivates our investigation on the physical layer security of a multiple-input multiple-output (MIMO) RFID system. In this paper, we propose a noise-injection precoding strategy to safeguard the system security with the resource-constrained nature of the backscatter system taken into consideration. We first consider a multi-antenna RFID tag case and investigate the secrecy rate maximization (SRM) problem by jointly optimizing the energy supply power and the precoding matrix of the injected artificial noise at the RFID reader. We exploit the alternating optimization method and the sequential parametric convex approximation method, respectively, to tackle the non-convex SRM problem and show an interesting fact that the two methods are actually equivalent for our SRM problem with the convergence of a Karush-Kuhn-Tucker point. To facilitate the practical implementation for resource-constrained RFID devices, we propose a fast algorithm based on projected gradient. We also consider a single-antenna RFID tag case and develop a low-complexity algorithm, which yields the global optimal solution. Simulation results show the superiority of our proposed algorithms in terms of the secrecy rate and computational complexity.Item Subsurface Source Generation and Transport from Spills of Alcohol Blended Fuels(2014-05) Zhang, Yi; Rixey, William G.; Chellam, Shankar; Mohanty, Kishore K.; Vipulanandan, Cumaraswamy; Wang, Keh-HanThe goal of this research is to improve the current understanding of the physical-chemical impacts of alcohol in an alcohol-blended fuel on the distribution and transport characteristics of alcohol and non-aqueous phase liquid (NAPL) contaminants from fuel releases, which may result in new or enhanced groundwater quality issues. The principal aim is to build a framework for developing a predictive tool for guidance in field spill events associated with these blended fuels. The phase partitioning and plume transport characteristics of alcohol and hydrocarbons in the fuel mixtures were studied via 2-D visualization experiments and quantitative pore water analyses. NAPL source generation and distribution characteristics were observed and compared for spills of various alcohol blended fuels to a model aquifer. As alcohol content in the fuel mixture increased, a higher fraction of the NAPL in the fuel was mobilized due to both cosolvency and phase mobilization. Variations in the size, shape, and saturation of the residual NAPL source generated from different alcohol fuel blends impacted the dissolution rates of the hydrocarbon contaminants and the longevity of the NAPL source. Ethanol was the alcohol studied in this research. A multi-component, multi-phase flow simulator, UTCHEM, was used to quantitatively evaluate the impact of key physical-chemical properties that influence the transport behavior of alcohol as well as the distribution of the hydrocarbon contaminants in porous media. Numerical simulations compared well with physical model experiments for ethanol blended fuels to the aquifer in water-saturated conditions.