Active STAR-RIS Aided Cell-Free Massive MIMO: A Performance Study

This paper investigates the spectral efficiency (SE) of an active simultaneously transmitting and reflecting reconfigurable intelligent surface (aSTAR-RIS) aided cell-free massive multiple-input multiple-output (CF-mMIMO) system under correlated Rayleigh fading. In order to improve the system quality-of-service (QoS) and suppress the pilot contamination, fractional power control and an appropriate pilot assignment are adopted. Furthermore, a large-scale fading decoding (LSFD) method is introduced, based on which the analytical SE expression for the considered system is derived. Numerical and simulation results show that the adopted LSFD detection leads to higher SE than maximal ratio combining does, and as compared to an equivalent active RIS scheme, aSTAR-RIS significantly improves the SE of the CF-mMIMO system.