In this paper, a frequency diverse array-based directional modulation with artificial noise synthesis scheme is proposed to enhance the physical layer security of wireless communications. We aim to optimize the secrecy performance by jointly optimizing the frequency offsets, the beamforming vector, and the artificial-noise projection matrix (ANPM). Specifically, we address the physical layer security problems for known locations of proximal eavesdropper (Eve) and legitimate user (LU). The beamforming vector and frequency offsets are designed to preserve the signal power at LU. The ANPM is calculated to minimize the effect of AN on LU. Furthermore, we extend our approach to the case of multi-LUs with unknown Eve locations. Being different from the case of a single LU, the frequency offsets across array antennas are optimized to equally allocate transmitted power to each LU. The numerical results show that the proposed method can provide a higher secrecy performance than conventional DM methods. In the case of multi-LUs with unknown Eve locations, the proposed method can provide a high secrecy capacity while achieving almost equal achievable capacity to each LU.

In this paper, we aim to address the physical layer security problem for proximal legitimate user (LU) and eavesdropper (Eve) in the case of close-located LU and Eve, where conventional directional modulation (DM) methods may fail to provide efficient secure performance. In order to handle this problem, an optimal frequency offsets of frequency diverse array-based DM with artificial noise (AN) scheme is proposed. We maximize the secrecy capacity by optimizing the frequency offsets and designing AN projection matrix. The optimization problem of the frequency offsets is solved by a block successive upper-bound minimization method to iteratively obtain stationary convergence solutions. By elaborately calculating frequency offsets across array antennas, we can decouple the angle-range correlation and maximize recreational secrecy capacity, resulting in the improvement of the security performance of wireless communications. Numerical results show that the proposed method can provide a higher secrecy capacity than conventional DM schemes with the changes of bandwidth, power allocation factor, and power for the case of proximal LU and Eve. In addition, the proposed scheme is suitable for the case of multi-Eves.

In this paper, a novel multi-beam transmission scheme is proposed to enhance physical layer (PHY) security. This scheme utilizes index modulation (IM) based on frequency diverse array (FDA) beamforming with cooperative legitimate users (LUs). By leveraging IM based on FDA, the indices associated with multiple LUs are directly used as additional sources to transmit information. Using this scheme, the confidential messages can be retrieved only if all the index information and the corresponding symbols are obtained. This adds an extra level of PHY security hence reduces the possibility of information interception by eavesdroppers (Eves). Here instead of maximizing the secrecy rate, we propose a minimum message transmission power criterion to design the FDA beamforming vector, subject to prescribed minimum received power at each of the LUs, in addition to the orthogonality constraints of the remaining LUs. This provides a reliable and accurate transmission assurance for the LUs to obtain the corresponding messages under minimum message transmission power. To further deteriorate the quality of the received signals at Eves, we also add artificial noise (AN). Moreover, by elaborating the time-variant characteristics, we provide a new and more accurate perspective for the range-time dimension secure transmission. Numerical simulations confirm the effectiveness of the proposed scheme.

Directional modulation and artificial noise (AN)-based methods have been widely employed to achieve physical-layer security (PLS). However, these approaches can only achieve angle-dependent secure transmission. This paper presents an AN-aided decomposed and distributed directional modulation (D3M) scheme for secure wireless communications, which takes advantage of the spatial signatures to achieve an extra range-dimension security apart from the angles. Leveraging decomposed and distributed structure, each of modulated signal is represented by mutually orthogonal in-phase and quadrature branches, which are transmitted by two distributed transmitters to enhance PLS. In particular, we first aim to minimize transmit message power by integrated design of the transmit beamformers, subject to prescribed received signal-to-noise ratio (SNR) for the legitimate user (LU) and no inter-branch interference. This guarantees reliable and accurate transmission for the LU with the minimum transmit message power. Considering the leakage power on the sidelobes, AN is superimposed on the messages to try to mask the confidential information transmission. Simulation results demonstrate the security enhancement of our proposed D3M system.

In this paper, we consider the physical layer (PHY) security problem for hybrid wiretapping wireless systems in millimeter wave transmission, where active eavesdroppers (AEs) and passive eavesdroppers (PEs) coexist to intercept the confidential messages and emit jamming signals. To achieve secure and reliable transmission, we propose an artificial noise (AN)-aided robust multi-beam array transceiver scheme. Leveraging beamforming, we aim to minimize transmit power by jointly designing the information and AN beamforming, while satisfying valid reception for legitimate users (LUs), per-antenna power constraints for transmitter, as well as all interception power constraints for eavesdroppers (Eves). In particular, the interception power formulation is taken into account for protecting the information against hybrid Eves with imperfect AE channel state information (CSI) and no PE CSI. In light of the intractability of the problem, we reformulate the considered problem by replacing non-convex constraints with tractable forms. Afterwards, a two-stage algorithm is developed to obtain the optimal solution. Additionally, we design the received beamforming weights by means of minimum variance distortionless response, such that the jamming caused by AEs can be effectively suppressed. Simulation results demonstrate the superiority of our proposed scheme in terms of energy efficiency and security.

Frequency diverse array- (FDA-) based directional modulation (DM) is a promising technique for physical layer security, due to its angle-range dependent transmit beampattern. However, the existing schemes are not suitable for the broadcasting scenario, where there are multiple legitimate users (LUs) to receive the confidential message. In this paper, we propose a novel random frequency diverse array- (RFDA-) based DM scheme to realize the point to multi-point broadcasting secure transmission in both angle and range dimension. In the first stage, the beamforming vector is designed to maximize the artificial noise (AN) power, while satisfying the power requirement of LUs for transmitting the confidential message simultaneously. In the second stage, the AN projection matrix is obtained by maximizing signal-to-interference-plus-noise ratio (SINR) at the LUs. The proposed scheme only broadcasts the confidential message to the locations of LUs while the other regions are covered by AN, which promotes the security of the wireless broadcasting system. Moreover, it is energy efficient since the power of each LU is under accurate control. Numerical simulations are presented to validate the performance of the proposed scheme.