Efficient Secure Communication in 4-D Antenna Arrays Through Joint Space–Time Modulation

Traditional directional modulation techniques always sacrifice the effective radiated power in order to produce the effect of directional modulation, thus resulting in low radiation efficiency and leading to low channel capacity or short transmission distance. Generally, a signal is first modulated at the baseband and then modulated again at the antenna. The two modulations are always regarded as two separate steps. Therefore, these directional modulation techniques fail to maximize their modulation efficiency, resulting in possible performance degradation. Aimed at this problem, this article proposed a novel secure transmission approach named joint space–time modulation (JSTM) for improving the transmission efficiency of secure communication by using 4-D antenna arrays. By selecting the appropriate modulation strategy, the space–time degrees of freedom are integrated and the signal modulation can be completed with high efficiency, thus being able to reduce the attenuation in the signal modulation process. The proposed approach is able to improve the overall gain of the 4-D arrays close to that of the phased arrays while ensuring good security performance. Numerical and experimental results demonstrate the effectiveness of the proposed approach.