Active Reconfigurable Intelligent Surface-Assisted Cell-Free Massive MIMO.

This work analyzes the spectral efficiency (SE) of the active Reconfigurable intelligent surfaces (RIS)-assisted cell-free (CF) massive multiple-input multiple-output (mMIMO) under Rayleigh fading channels. Closed-form expression is derived for the achievable SE which is shown to be exact when compared with the Monte Carlo simulations. The active RIS significantly improves the performance of the CF mMIMO. Also, the active RIS shows substantially higher sum SE performance with respect to the passive RIS case under the same power budget.