Frequency agile array based directional modulation

Directional modulation (DM) based on phased array (PA) can realize angle-dependent secure transmission. In this paper, frequency agile array (FAA) based DM technique is proposed to achieve range-angle-dependent secure transmission. Different from the conventional frequency diverse array (FDA), whose frequency offsets applied to the array is fixed, FAA can achieve a distortionless constellation at the target location and randomly distorted constellations at other locations by changing the frequency offsets at the symbol rate. Two frequency offset selection schemes are presented. The first scheme randomly selects the frequency offset applied to each element from a given set, and the received signal is randomly distorted both in amplitude and phase except the target location. The second scheme selects the optimal frequency offsets with lower sidelobe based on ant colony algorithm (ACO). Further, the sidelobe level is relaxed appropriately to seek multiple near optimal solutions on the basis of the optimal frequency offsets. The simulation results show that the proposed method generates higher bit error rate (BER) at non-target locations and narrower information beamwidth near the target location, which provides better secure transmission performance compared with the conventional FDA.