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Satellite communication systems serve as an indispensable component of wireless heterogeneous networks in 5G era for providing various critical civil and military applications. However, due to the broadcast nature and full accessibility of wireless medium, serious security threats exist from such systems. As an effort to address this issue, this paper, for the first time, investigates the secure communication in a non-geostationary orbit (NGSO) satellite system from a physical layer security perspective. Specifically, we focus on the downlink of an NGSO satellite which provides services to a fixed earth station and is wiretapped by a fixed eavesdropper. We first apply three types of orbiting models to characterize the movement state of the satellite. Based on the orbiting models, we then provide theoretical analysis for the secure communication performance of such a system. The expressions of two fundamental performance metrics, secrecy capacity and secrecy outage probability, are derived in a closed form for any system time. Finally, we conduct extensive simulations to validate our theoretical performance analysis and illustrate the security performance in a practical NGSO satellite communication system.

Satellite communication system is expected to play an important role in wireless networks because of its appealing contributions to ubiquitous coverage, content multicast and caching, reducing user expenditure, and so on. However, due to the inherent broadcasting nature and serious channel conditions, satellite communication system is highly vulnerable to eavesdropping attacks. As an initial step towards this end, this paper focuses on the physical layer security technique and explores the secrecy performance of a multibeam satellite system, where multiple legitimate users are served and each user is exposed to an eavesdropper located in the same beam. With perfect channel state information at the satellite and adopting the complete zero-forcing approach for signal processing, we first derive the optimal beamforming vectors to maximize the achievable secrecy rate. Based on this, we further calculate the secrecy outage probabilities of an individual user and the whole system, respectively. Finally, simulation and numerical results are provided to show the secrecy performance of the multibeam satellite system.

This paper studies the secure transmission over Rayleigh fading channels, where the communication between a multi-antenna(N-antenna) transmitter and a desired single-antenna receiver is overheard by a multi-antenna(Mantenna) eavesdropper. We consider a practical scenario where the transmitter can only obtain the average channel knowledge of the eavesdropper and the imperfect instantaneous channel knowledge of the legitimate receiver through the use of pilot training and digital feedback. Two outage events occur in the system — a connection outage event to ensure decode correctly and a secrecy outage event to ensure secure transmission, closed-form expressions for dual outage probabilities are derived. Before transmission, the transmitter selects the wiretap coding parameters to achieve secure transmission based on the instantaneous realization of the main channel, and the closed-form expression for secrecy throughput is also derived. By this, we further determine the optimal secrecy rate and the optimal power allocation between information signals and artificial noise signals to maximize the secrecy throughput. Moreover, we explicitly examine the impacts of N and M on the secrecy throughput and optimal power allocation.