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

This paper studies the reconfigurable intelligent surface (RIS)-assisted cell-free massive MIMO Systems under superimposed pilots (SP) 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 the sum-SE grows with more RIS units, and SP perform better than regular pilot transmission. Moreover, optimizing the power scaling factor by IPSO achieves around 17% improvement in sum-SE.