Physical-layer security (PLS) is a new paradigm for secure communication without requiring secret key exchange and management. Moreover, PLS with frequency diverse subarray (FDSA) can better control information leakage in the angle-range domain, which mitigates the security weakness of the phased array caused by its lack of range resolution. In this paper, we propose a three-dimensional (3D) directional modulation (DM) using randomized radiation with FDSA for enhanced PLS, employing a planar array. In addition, nonuniform frequency offsets (FOs) are considered as FO configurations (FOCs) for FDSA to concentrate on the mainlobe and suppress the undesired sidelobes in 3D space, where logarithmically increasing FOC (L-FOC), Hamming window-based FOC (H-FOC), and piecewise trigonometric FOC (P-FOC) are introduced. Characterizing the process of selecting the random subsets for randomized radiation, we provide the exact analysis of the secrecy rate of the proposed scheme. Moreover, FOs applied to FDSA and the number of random subsets are optimized with a genetic algorithm (GA)-based optimization strategy. We evaluate the proposed schemes in terms of secrecy rate and vulnerable volume, where the simulation results verify our analysis and show that nonuniform FOCs are a more favorable choice for FDSA compared to uniform FOC (U-FOC).

Frequency diverse subarrays (FDSA) with linear frequency offsets (Lin-FO) can generate mainlobe to be confined around the desired receiver on an angle-range plane, which mitigates vulnerabilities in conventional randomized radiation techniques implemented with a phased array. However, the angle-range-coupled beampattern of subarrays imposes limitations on improving the secrecy rate. In this paper, we propose randomized radiation using FDSA with nonlinear FO designs for enhanced physical-layer security (PLS) in mmWave communications. Non-linear FO designs, including logarithmically increasing FO (Log-FO), Hamming window-based FO (HW-FO), and piecewise trigonometric FO (PT-FO), enable the formation of angle-range-decoupled beampatterns focused on the intended receiver. The exact secrecy rate of the proposed scheme is derived and validated through simulations. The evaluation of various FO designs reveals reduced vulnerable areas and highlights the impact of transmit power on their performance, offering valuable insights for FO design selection.

To achieve a global coverage and enhance throughput, low Earth orbit (LEO) satellites have been adopted in B5G and 6G communications, because they can provide lower latency and higher service density compared to those with higher orbital altitude. However, in the LEO satellite networks, ground users may observe significant Doppler, which should be estimated and compensated for reliable communications. Thus, in this paper, we provide a comprehensive review of the existing studies on Doppler characterization and compensation.