Spectral Efficiency of Cell-Free Massive MIMO with Superimposed Pilots Over Spatially Correlated Rician Fading Channels

Abstract

This paper investigates the spectral efficiency (SE) of superimposed pilots (SP) in cell-free massive multiple-input multiple-output systems under spatially correlated Rician fading channels. By introducing the SP, both the data and pilot symbols can be transmitted simultaneously in the same time/frequency units, effectively reducing the pilot contamination. The channel estimation is realized by a linear minimum mean squared error estimator. We further provide the closed-form SE expression based on maximal ratio combining, considering both perfect and imperfect pilot removal. Thereafter, the large-scale fading decoding coefficient vector is derived. Numerical results validate the theoretical analyses and show that SP outperforms regular pilots when serving a large number of users.