Joint Power Allocation and Beamforming for Active IRS-aided Directional Modulation Network

To boost the secrecy rate (SR) of the conventional directional modulation (DM) network and overcome the double fading effect of the cascaded channels of passive intelligent reflecting surface (IRS), a novel active IRS-assisted DM system is investigated in this paper. Aiming to maximize the SR, two power allocation (PA) strategies, called maximizing SR based on fractional programming (FP) (Max-SR-FP) and maximizing SR based on derivative operation (DO) (Max-SR-DO), are proposed by jointly designing the PA factors, beamforming vector, and phase shift matrix of IRS, subject to the power constraint at IRS. The former with higher performance employs the FP and successive convex approximation (SCA) algorithms to design the confidential message PA factor and the total PA factor at the base station, and the SCA algorithm is also utilized to design the beamforming vector and the phase shift matrix of the IRS. The latter with lower complexity adopts the DO, and equal amplitude reflecting (EAR) and general power iterative (GPI) methods to solve them, respectively. The simulation results show that compared with the benchmark PA schemes, both the proposed PA schemes achieve a significant SR performance improvement. Moreover, the SR gap between two proposed schemes decreases gradually with the increases of the number of IRS phase shift element.