Secure Communication Optimization for MIMO NTN Cell-Free Networks

The nonterrestrial network (NTN) is a promising technology to achieve the ubiquitous, low latency, and high data rate next-generation communication system, however, since the broad application of the NTN, the security problem in the NTN is of paramount importance. In this regard, this article investigates a novel secure communication strategy within the NTN, where a legitimate user of the low-Earth orbit satellite-integrated autonomous vehicles network (ISAVN) is equipped with a high-gain antenna and attempts to cross-layer eavesdrop on the geostationary orbit (GEO) satellite network. In the considered NTN system, the cell-free massive multi-input-multi-output (CFmMIMO) network is utilized at the ISAVN to enhance the spectral efficiency of the terrestrial users. Furthermore, the ISAVN shares the millimeter-wave spectrum with the GEO satellite multicast downlink communication link. We first formulated a multiobjective optimization (MOO)-based joint beamforming design problem for the secure communication of the considered CFmMIMO NTN system to achieve a Pareto-optimal tradeoff between two conflicting and essential objectives: 1) minimizing total transmitted power and 2) maximizing the secret data rate of the satellite network. Thereafter, to address this mathematically complicated optimization problem, we utilized successive convex approximation methods to transform this intractable problem into an equivalent convex optimization problem. Then, the commercial optimization package is employed to achieve the optimal solution of the MOO problem, commencing from a feasible point that is identified by a semi-definite programming initialization algorithm. Ultimately, the numerical simulation results demonstrated that the Pareto-optimal tradeoffs for the formulated MOO problem were achieved using the proposed algorithm, and the effectiveness of our proposed algorithm was shown through comparisons with existing related methods.