Joint Optimization of Transmit Power and Passive Beamforming for STAR-RIS-Assisted HetNets

The heterogeneous network (HetNet), as a promising deployment strategy, can significantly improve spectrum utilization by sharing the same spectrum resource between the macro base station (MBS) and multiple small base stations (SBSs). However, this leads to the severe interference to MBS users (MUEs) and SBS users (SUEs) inevitably. To further enhance the desired channels and suppress the interference links, simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RISs) are employed. In this paper, a novel STAR-RIS assisted HetNet is proposed. The objective is to maximize the weighted sum rate of all SUEs by jointly optimizing the transmit power and passive transmission and reflection beamforming under the constraint of the quality of service (QoS) for MUEs. To solve the formulated non-convex optimization problem, an effective iterative algorithm based on the alternating optimization method is designed. Specifically, with the given passive transmission and reflection beamforming, the transmit power is optimized by utilizing the successive convex approximation method. Then with the obtained transmit power, the passive transmission and reflection beamforming is solved by employing the Gaussian randomization method. Simulation results are provided to validate the convergence and evaluate the performance of the proposed algorithm for different parameters.