Randomized Radiation Technique Exploiting Frequency Diverse Subarrays with Various Designs of Nonlinear Frequency Offsets

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. Nonlinear FO designs, including logarithmically increasing FO (LogFO), 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.