On Superimposed Pilots Transmission for RIS-Assisted Cell-Free Massive MIMO

This article studies the reconfigurable intelligent surface (RIS)-assisted cell-free massive multiple-input-multiple-output (MIMO) systems under superimposed pilots (SPs) transmission. We derive a closed-form expression for the linear minimum mean square error channel estimation and the spectral efficiency (SE) under a maximally-ratio-combined receiver filter. Vital factors including the number of reflecting elements and the channel coherence time are investigated. Improved particle swarm optimization (IPSO) algorithm is utilized to optimize the power scaling factor for maximizing the sum-SE. Results show that the sum-SE grows with more RIS units, and SP performs better than regular pilot (RP) transmission. Moreover, optimizing the power scaling factor by IPSO achieves around 17% improvement in sum-SE.