Theoretical Upper Limit Analysis of Uplink Spectral Efficiency Based on Superimposed Pilots in Cell-Free Massive MIMO Systems

Cell-Free massive MIMO breaks through the concept of traditional cells and is considered to be a promising technology in future mobile communication. The technology can obtain sufficient macro diversity gain by deploying a large number of distributed Access Points (AP) to improve the quality of user service. The orthogonal pilot resources available for user allocation are very limited due to the restriction of channel coherence time, some users will have to reuse the same pilot sequences in the conventional multiplexed pilot (MP) configuration mode, which may cause serious pilot contamination problem. Therefore, utilizing the superimposed pilot (SP) configuration mode and based on the linear minimum mean square error (LMMSE) criterion and the maximum ratio combining (MRC) technique, the channel estimation of Cell-Free massive MIMO system was investigated, and the upper bound closed-form expression of the uplink spectrum efficiency (SE) of the system was derived in this paper. The simulation results show that the SP configuration mode can effectively reduce the impact of pilot contamination, thereby significantly improve the SE of the system.