Secure directional modulation for frequency diverse arrays via multiparameter weighted fractional Fourier transform

Physical layer security in directional modulation systems can be enhanced by frequency diverse arrays (FDAs), whose range-angle-dependent beampatterns provide additional spatial selectivity beyond conventional phased arrays. However, conventional artificial noise-aided FDA schemes often suffer from reduced power efficiency due to the allocation of transmit power for interference generation. This paper proposes a novel directional modulation framework for FDAs based on the multiparameter weighted fractional Fourier transform (MPWFRFT). In the proposed scheme, modulated symbol blocks are first processed by MPWFRFT and then transmitted via the FDA, jointly exploiting spatial selectivity and signal-domain secure transmission. We formulate the MPWFRFT structure in a multistage form and derive received signal models for both the legitimate receiver and potential eavesdroppers. While the legitimate receiver can reliably recover symbols using a matched inverse MPWFRFT, eavesdroppers generally experience degraded recovery performance due to spatial and transform-parameter mismatches under the considered passive-eavesdropper model. An achievable secrecy rate is obtained, revealing the effectiveness of this two-layer security mechanism. Simulation results demonstrate that the proposed scheme improves the secrecy rate and reduces the secrecy outage probability compared with conventional artificial-noise-aided FDA designs.