An antenna array system configured to offer directional dependent modulation has the capability to prevent eavesdroppers' attacks, thereby enhancing the security level of data transmission. In this paper, we propose artificial-noise-aided directional modulation transmitter utilizing a 4×4 Butler matrix with a four-element 2-D (i.e., range and angle) frequency diverse array (FDA) antenna to achieve secure transmissions, which outperforms the 1-D (i.e, angle) phased array scheme. The proposed scheme utilizes FDA Butler matrix excited by information data and injected artificial noise interference which radiates along all directions except the main information data direction. Thus, the radiation pattern during a particular transmission period will be range-angle dependent. The proposed scheme is evaluated by using constellation points in IQ space, bit error probability (BER), and secrecy capacity. Simulation results demonstrate that: 1) our scheme scrambles the constellation points along undesired direction(s) in both amplitude and phase, while preserving a clear constellation points along the pre-specified direction(s); 2) the scheme achieves better BER and secrecy capacity than that of the phased array based directional modulation scheme and other existing scheme; 3) the scheme significantly improve security performance especially in the range dimension.
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