Superimposed Pilots are Beneficial for Mitigating Pilot Contamination in Cell-Free Massive MIMO

In this letter, we present the first study for uplink cell-free massive multi-input multi-output (MIMO) systems with superimposed pilots (SPs). Closed-form linear minimum mean square error (LMMSE) channel estimation and achievable spectral efficiency (SE) expressions with maximal ratio combining (MRC) receiver filters are derived. The tractability of the closed-form results facilitates analyzing the effects of key design system parameters, such as the number of mobile terminals (MTs), the power balance factor between pilot and data symbols, and the length of coherence interval. Several significant insights are attained by assuming that the introduced pilot interference can or cannot be perfectly canceled from received data symbols. All the theoretical results are corroborated by simulation experiments.