In satellite communications physical layer security is rather challenging to achieve due to the broadcasting nature of satellite downlinks. We investigate how the advances in multiuser multiple-input multiple-output satellite communications can protect downlinks from eavesdropping. The additional degrees of freedom in multi-antenna systems can not only be utilized for higher spectral efficiency but also to improve the security of the links. Our analysis shows that the area where a hostile interceptor is able to capture the signal is small due to the interference of other user signals. Moreover, the area can be tuned by applying a modified precoding matrix.
In satellite communications physical layer security is rather challenging to achieve due to the broadcasting nature of satellite downlinks. We investigate how the advances in multiuser multiple-input multiple-output satellite communications can protect downlinks from eavesdropping. The additional degrees of freedom in multi-antenna systems can not only be utilized for higher spectral efficiency but also to improve the security of the links. Our analysis shows that the area where a hostile interceptor is able to capture the signal is small due to the interference of other user signals. Moreover, the area can be tuned by applying a modified precoding matrix.
In satellite communication downlinks, physical layer security is challenging to achieve due to their broadcasting nature and Line-of-Sight channel characteristics. This paper provides a precoding algorithm to secure the downlinks of multiple users against multiple eavesdroppers with optimization of the minimum secrecy capacity. By the use of artificial noise, a positive secrecy capacity is achievable even if the number of eavesdroppers is higher than the number of beams. We demonstrate that a multiple-reflector antenna design provides a significantly higher secrecy and throughput performance when compared to a single-reflector design due to additional degrees of freedom, exhibited by the signal phases. The total vulnerability region is introduced as a new figure of merit with respect to unidentified eavesdroppers.
In multiuser multiple-input multiple-output (MUMIMO) systems for beyond-5G, many user groups are served by the same transmitter in a time-based scheduling. Considering all unscheduled users as eavesdroppers puts a high complexity on algorithms to achieve physical layer security (PLS). Moreover, due to the Line-of-Sight channel properties, PLS is even more challenging to achieve in satellite communication (SATCOM) downlinks. In multibeam MU-MIMO satellite systems, which will be part of beyond-5G networks, the spatial degrees of freedom can be used for both throughput and secrecy improvements. We extend a user-fairness zero-forcing algorithm with artificial noise to achieve secure precoding even if the channel state information of the eavesdroppers is unknown. We define the secrecy outage probability for Line-of-Sight MU-MIMO SATCOM channels and analyze how it can be reduced by the aforementioned algorithm. Moreover, we introduce a trade-off between a high system throughput and a low secrecy outage probability for the cost-effectiveness considerations of PLS.
Wireless broadcast allows widespread and easy information transfer. However, it may expose the information to unintended receivers, which could include eavesdroppers. As a solution, cryptography at the higher network levels has been used to encrypt and protect data. Cryptography relies on the fact that the computational power of the adversary is not enough to break the encryption. However, due to increasing computing power, the adversary power also increases. To further strengthen the security and complement the encryption, the concept of physical layer security has been introduced and surged an enormous amount of research. Widely speaking, the research in physical layer security can be divided into two directions: the information-theoretic and signal processing paradigms. This thesis starts with an overview of the physical layer security literature and continues with the contributions which are divided into the two following parts. In the first part, we investigate the information-theoretic secrecy rate. In the first scenario, we study the confidentiality of a bidirectional satellite network consisting of two mobile users who exchange two messages via a multibeam satellite using the XOR network coding protocol. We maximize the sum secrecy rate by designing the optimal beamforming vector along with optimizing the return and forward link time allocation. In the second scenario, we study the effect of interference on the secrecy rate. We investigate the secrecy rate in a two-user interference network where one of the users, namely user 1, requires to establish a confidential connection. User 1 wants to prevent an unintended user of the network to decode its transmission. User 1 has to adjust its transmission power such that its secrecy rate is maximized while the quality of service at the destination of the other user, user 2, is satisfied. We obtain closed-form solutions for optimal joint power control. In the third scenario, we study secrecy rate over power ratio, namely ``secrecy energy efficiency''. We design the optimal beamformer for a multiple-input single-output system with and without considering the minimum required secrecy rate at the destination. In the second part, we follow the signal processing paradigm to improve the security. We employ the directional modulation concept to enhance the security of a multi-user multiple-input multiple-output communication system in the presence of a multi-antenna eavesdropper. Enhancing the security is accomplished by increasing the symbol error rate at the eavesdropper without the eavesdropper's CSI. We show that when the eavesdropper has less antennas than the users, regardless of the received signal SNR, it cannot recover any useful information; in addition, it has to go through extra noise enhancing processes to estimate the symbols when it has more antennas than the users. Finally, we summarize the conclusions and discuss the promising research directions in the physical layer security.
Physical layer security has drawn significant attention in recent years because of its advancement in preventing confidential information leakage to eavesdroppers. In this paper, we focus on secure beamformer designs in multiuser multi-input multi-output (MU-MIMO) systems with a multi-antenna eavesdropper attempting to overhear the confidential information of a particular user. Unlike the existing work, we aim to exploit the inherent multi-user interference to disrupt the reception of the eavesdropper and enhance the security of wireless communications. Particularly, utilizing practical signal-to-interference-noise metrics, the objective of the secure beamformer design is to minimize the eavesdropper's SINR, while providing legitimate users with required prespecified SINRs to guarantee the qualify of service. We propose secure beamformer designs for both downlink and uplink transmissions, under the conditions of known and unknown eavesdropper's channels, respectively. With the proposed secure beamformers, the multiuser signal behaves like the artificial noise or distributed friendly jamming, which can further degrade the reception of the eavesdropper. Extensive simulation studies confirm our analytical performance predictions and illustrate the advantages of proposed beamformer designs for secure MU-MIMO transmissions.
This letter generalizes the precoding-aided spatial modulation (PSM) to a multiuser (MU) downlink scenario. With precoding vectors elaborately designed, a proposed MU-PSM scheme has the capability to resist a multiple antenna eavesdropper. Specifically, signal precoding matrices (SPMs) are designed to cancel the MU interference and modulate partial information bits on the indices of receive antennas. To improve physical layer security, scrambling on SPMs is conceived to construct a fast-varying precoding matrix, which contributes to the degradation at Eve's blind estimation and detection. Moreover, issues of detection algorithms and a lower bound of secrecy rate are addressed to verify security enhancement in the proposed scheme.
In this paper we investigate the physical layer security for downlink MU-MIMO systems where the transmitter and the eavesdropper are equipped with multiple antennas, while the legitimate users have single antenna. This is a more challenging topic because the eavesdropper appears to be more powerful than the legitimate users. We propose a transmission scheme able to enhance secure transmission for legitimate users, by intentionally applying different beamforming matrices to pilot signals and data signals. The beamforming matrix for data signals is constructed in a way that legitimate users can derive the channel matrix experienced by data signals based on pilot signals, whereas the eavesdropper fails to obtain the channel matrix experienced by data signals even with the aid of pilot signals. Therefore coherent detection at the eavesdropper is impossible and the signal-plus-interference-to-noise (SINR) of the eavesdropper is significantly degraded. In addition, the proposed beamforming design is formulated as an optimization problem to maximize the minimum SINR. We theoretically prove that with this formulation, the resultant optimal SINR is not affected by the proposed format of beamforming matrix for data signals. Numerical results show that, the proposed beamforming design outperforms the conventional beamforming design in terms of ergodic secrecy sum rate. Furthermore, with suitable beamforming designs, the ergodic secrecy sum rate achieved will not be noticeably affected by the number of antennas at the eavesdropper, although it is significantly degraded with conventional beamforming design.