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.

A novel sparse reconstruction method is developed for direction of arrival (DOA) estimation in the presence of unknown mutual coupling of uniform linear arrays (ULAs). In the proposed method, a sparse representation for single measurement vector (SMV) is first derived. Then, it is shown that the problem size can be reduced by a linear transformation to eliminate the redundant components in the SMV. Finally, by taking advantage of the banded symmetric Toeplitz structure of the mutual coupling matrix (MCM), a reweighted ℓ₁-norm minimization subject to an error-constrained ℓ₂-norm is introduced to determine the DOA estimates without mutual coupling compensation, further enhancing the sparsity and providing a robustness against the noise. Simulation results demonstrate the superiority of the proposed method over its state-of-the-art counterparts.

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.

In this letter, a 16 × 16 Ku-band circularly polarized phased array is proposed. The antenna element comprises a cross dipole, a metal cavity, and a threaded SMP connector. The metal cavity makes the gain of the antenna increased and the mutual coupling between the array elements is reduced, thus ensuring good impedance matching and axis ratio (AR) during the two-dimensional beam scanning. The mutual coupling of the center element at center frequency is -26.6 dB when the cavity is added. The proposed phased array achieves the impedance bandwidth (| S ₁₁ | ≤ - 10 dB) of 10.1% (13.1-14.5 GHz) when scanning ±40° in the xz plane and yz plane. AR of the main lobe is less than 2 dB in the total scanning range. The gain fluctuation is less than 3 dB during scanning. This letter presents the measurement and simulation results, verifying that the phased array has good performance.