Beamforming Orthogonality in Coupled Directional Modulation Arrays

This paper studies the orthogonality in a directional modulation (DM) transmitter equipped with an antenna array whose mutual coupling is considered. The antenna coupling will generate backward waves at the power amplifier (PA) outputs. This dynamic interaction will drive the PA load away from the system characteristic impedance thus introducing the loadpulling effect to the system which would result in the distortion to the DM system orthogonality. When the antenna coupling is considered, for a certain target user, the leakage radiation from the information beams designed for other users and from the generated artificial noise would become the interference to this user, and in one example given in this paper, the signal to interference ratios (SIR) of around 20 dB are observed. This compromise of the DM orthogonality would put a performance upper limit to a DM system considering a coupled antenna array.