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 with a power adjusting strategy between transmitter and active IRS is proposed in this paper. Then, a joint optimization of maximizing the SR is cast by alternately optimizing the power allocation (PA) factors, transmit beamforming at the BS, and reflect beamforming at the active IRS, subject to the power constraint at IRS. To tackle the formulated non-convex optimization problem, a high-performance scheme of maximizing SR based on fractional programming (FP) and successive convex approximation (SCA) (Max-SR-FS) is proposed, where the FP and SCA methods are employed to optimize the PA factor of confidential message and the PA factor of power allocated to the BS, and the SCA algorithm is also utilized to design the transmit beamforming and phase shift matrix of the IRS. To reduce the high complexity, a low-complexity scheme, named maximizing SR based on derivative operation (DO) and general power iterative (GPI) (Max-SR-DG), is developed, where the DO and methods of the equal amplitude reflecting (EAR) and GPI are adopted to derive the PA factors and IRS phase shift matrix, respectively. Simulation results show that with the same power constraint, both the proposed schemes harvest about 12 percent and 70 percent rate gains over the equal PA and passive IRS schemes, respectively.