  Backscatter 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 (KKT) 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.
