Hybrid Directional Modulation and Beamforming for Physical Layer Security Improvement Through 4-D Antenna Arrays

The physical layer security techniques have made great progress in the past few years. The essential idea of physical layer transmission is to maximize the randomness of undesired channels. The 4-D antenna arrays formed by introducing time as the fourth controlling degree of freedom can be used to regulate the radiated fields in space, time, and frequency domains simultaneously. Thus, 4-D antenna arrays are an effective carrier for achieving physical layer secure transmission. However, the traditional secure transmission approach based on 4-D antenna arrays always inevitably leads to high sidelobe levels (SLLs) of radiated power or less channel randomness. Aimed at this problem, a novel physical layer secure transmission approach based on 4-D antenna arrays is proposed, which combines the advantages of traditional phased arrays and 4-D arrays. The approach can be used to reduce the SLL of radiated power and improve the channel randomness by optimizing the time sequences and static amplitude weighting. Moreover, the signal distortion caused by time modulation can be compensated in the desired direction by premodulating the transmitted signals. Both the numerical and experimental results demonstrate the effectiveness of the proposed approach.