Optimal AP Deployment in Cell-Free Massive MIMO Systems with LoS/NLoS Transmissions: A Stochastic Geometry Approach

Cell-free massive multiple -input multipleoutput (MIMO) is a promising technology for future wireless communications, where a large number of distributed access points (APs) simultaneously serve all users over the same time-frequency resources. Since users and APs may locate close to each other, the line-of-sight (LoS) transmission occurs more frequently in cell-free massive MIMO systems. Hence, in this paper, we investigate the cell-free massive MIMO system with LoS and non-line-of-sight (NLoS) transmissions, where APs and users are both distributed according to Poisson point process. Using tools from stochastic geometry, we derive a tight lower bound for the user downlink achievable rate and we further obtain the energy efficiency (EE) by considering the power consumption on downlink payload transmissions and circuitry dissipation. Based on the analysis, the optimal AP density and AP antenna number that maximize the EE are obtained. It is found that compared with the previous work that only considers NLoS transmissions, the actual optimal AP density should be much smaller, and the maximized EE is actually much higher.