Active Intelligent Reflecting Surface Aided RSMA for Millimeter-Wave Hybrid Antenna Array

The active intelligent reflecting surface (IRS) can amplify the incident signal besides reflecting, thus alleviating the double-fading effect of the cascaded link. However, if the interference caused by the cascaded link cannot be effectively cancelled, the alleviated effect may result in increased interference among users, thus limiting the performance gain brought by the active IRS. To this end, an active IRS aided rate-splitting multiple access (RSMA) scheme is proposed in this paper for the millimeter-wave hybrid antenna array. In the proposed scheme, a user grouping method is designed to circumvent the narrow beamwidth constraint, and the 2-layer rate-splitting (RS) strategy applicable to the user grouping result and the corresponding successive interference cancellation are designed to cancel interference. To evaluate the max-min fairness (MMF) performance of the proposed scheme, an MMF optimization problem (OP) is formulated, where the digital precoder, the RS vector, and the reflecting vector are jointly optimized. An alternating optimization (AO) algorithm with a novel AO strategy is proposed to solve the MMF OP, where the RS vector is optimized in two subproblems. An initialization method is designed specially to start the AO algorithm. Simulation results illustrate that the proposed scheme can more fully exploit the active IRS, and although the active IRS consumes additional power, it can bring energy efficiency gain to system.