Downlink Transmission in RIS-Enhanced Cell-Free Network: Analysis and Optimization

This letter examines the performance of RIS-enhanced cell-free network (RIS-CFN) with multiple user groups under different transmission modes, subject to fronthaul capacity constraints. It derives the SINR under both coherent and non-coherent transmission modes. Unlike coherent transmission, non-coherent transmission is unaffected by asynchronous reception. An adequate number of access points (APs) can meet the data rate requirements of user equipment (UE). Based on these insights, an iterative joint power allocation and passive beamforming algorithm (JPAPB) is developed for non-coherent transmission, considering selecting partial APs for UE and fronthaul capacity limit. Simulations show that non-coherent transmission with JPAPB outperforms coherent transmission under limited fronthaul capacity. Experimental results also validate JPAPB’s effectiveness and the performance gains from RIS.