In systems that allow device-to-device (D2D) communications, user pairs in close proximity communicate directly without using an access point (AP) as an intermediary. D2D communications leads to improved throughput, reduced power consumption and interference, and more flexible resource allocation. We show that the D2D paradigm also provides significantly improved security at the physical layer, by reducing exposure of the information to eavesdroppers from two relatively high-power transmissions to a single low-power hop. We derive the secrecy outage probability (SOP) for the D2D and cellular systems, and compare performance for D2D scenarios in the presence of a multi-antenna eavesdropper. The cellular approach is only seen to have an advantage in certain cases when the AP has a large number of antennas and perfect channel state information.

We study the optimality of linear precoding for the two-receiver multiple-input multiple-output (MIMO) Gaussian broadcast channel (BC) with confidential messages. Secret dirty-paper coding (S-DPC) is optimal under an input covariance constraint, but there is no computable secrecy capacity expression for the general MIMO case under an average power constraint. In principle, for this case, the secrecy capacity region could be found through an exhaustive search over the set of all possible matrix power constraints. Clearly, this search, coupled with the complexity of dirty-paper encoding and decoding, motivates the consideration of low complexity linear precoding as an alternative. We prove that for a two-user MIMO Gaussian BC under an input covariance constraint, linear precoding is optimal and achieves the same secrecy rate region as S-DPC if the input covariance constraint satisfies a specific condition, and we characterize the corresponding optimal linear precoders. We then use this result to derive a closed-form sub-optimal algorithm based on linear precoding for an average power constraint. Numerical results indicate that the secrecy rate region achieved by this algorithm is close to that obtained by the optimal S-DPC approach with a search over all suitable input covariance matrices.

This paper considers a multiple-input multiple-output (MIMO) Gaussian wiretap channel model, where there exists a transmitter, a legitimate receiver and an eavesdropper, each equipped with multiple antennas. Perfect secrecy is achieved when the transmitter and the legitimate receiver can communicate at some positive rate, while ensuring that the eavesdropper gets zero bits of information. In this paper, the perfect secrecy rate of the multiple antenna MIMO wiretap channel is maximized for arbitrary numbers of antennas under the assumption that the transmitter performs beamforming based on the generalized singular value decomposition (GSVD). More precisely, the optimal allocation of power for the GSVD-based precoder that maximizes the achievable secrecy rate is derived. Numerical results are presented to illustrate that the achievable secrecy rate of the GSVD-based precoding approach is nearly identical to that of the optimal scheme.

This paper studies linear beamforming based on the generalized singular value decomposition (GSVD) for the two-receiver multiple-input multiple-output (MIMO) Gaussian broadcast channel with confidential messages. The transmitter has two independent messages, each of which is intended for one of the receivers but needs to be kept as secret as possible from the other. Recently, it has been proved that, under an input power-covariance constraint, the secret dirty paper coding (S-DPC) scheme is optimal, but under the average power constraint, there is not a computable secrecy capacity expression for the general MIMO case. In fact, for this case, the secrecy capacity region must in general be found through an exhaustive search over the set of all possible matrix power constraints. Clearly, this exhaustive search, as well as the complexity of dirty-paper encoding and decoding, motivates us to consider low complexity linear beamforming technique whose performance is close to the optimal S-DPC scheme. In this paper, we propose a GSVD-based beamforming scheme for the general MIMO broadcast channel with confidential messages. Moreover, an optimal power allocation is obtained to maximize the sum-secrecy rates for the GSVD-based beamforming technique, under the average power constraint. Numerical results are presented to illustrate that the secrecy rate region of the linear precoding approach is nearly identical to that of the optimal S-DPC scheme.

We study the achievable rate regions of the multiple-input multiple-output (MIMO) interference channel with confidential messages sent to two receivers, assuming the transmitters use linear precoding. Each receiver is assumed to be an eavesdropper for the other link, and the transmitters employ various techniques to increase the secrecy rate of their own link or that of the network. We describe both cooperative and noncooperative transmission schemes for Gaussian interference channels, and derive their achievable secrecy rate regions. Cooperation is made possible if the transmitters share a portion of their channel state information, allowing them to appropriately adjust their precoders and improve the overall secrecy performance of the network. A game-theoretic formulation of the problem is adopted to allow the transmitters to find an operating point that balances network performance and fairness. The benefit of cooperation is demonstrated via several numerical examples.