Secure Satellite-Terrestrial Transmission via Hybrid Analog-Digital Beamforming

This paper deals with the physical layer security (PLS) of fifth generation (5G) cellular system operating at millimeter wave (mmWave) frequency in Satellite-Terrestrial network. By employing hybrid analog-digital precoder at both base station (BS) and satellite, we propose a realistic multicast Satellite-Terrestrial scheme, and study a constrained optimization problem to maximize the achievable secrecy rate (ASR) of the cellular user under the constraints of transmit power, constant modules and the signal-to-interference-plus-noise ratio (SINR) threshold of fixed satellite service. Due to the nonlinear coupling of the analog precoder and digital precoder, the problem is nonconvex and hard to solve. We reduce the optimization problem into a sequence of subproblems, each of which leads to a simple solution, by using the penalty dual decomposition (PDD) framework. Numerical results validate the effectiveness of the proposed algorithm.