UWB relay with physical layer security enhancement

Motivated by the demand for alternative ways to enhance wireless security, this paper proposes a technique to enhance wireless security at physical layer. It takes advantage of rich multipath environment to focus radio energy on some desired spatial spot, while achieving nulling at other spots. This can be viewed as a physical-layer security enhancement. What makes spatial focusing and nulling possible is transmit waveform optimization based on propagation environments. The concept and principle are studied in an ultra-wideband (UWB) relay network scenario. Using a matched filter receiver at test points, the optimization problem is actually the Quadratically Constrained Quadratic Program (QCQP) which is known as an NP-hard problem, thus the Semidefinite Program (SDP) as a relaxation technique is used to obtain suboptimal results. The feasibility is numerically validated using the UWB outdoor channel model. It is found that, by choosing a proper transmit waveform, the worst-case information leakage at unintended locations can be significantly minimized, with acceptable attenuation at the intended location.