  Cell-free massive multi-input multi-output (MIMO) has recently gained a lot
of attention due to its high potential in sixth-generation (6G) wireless
systems. The goal of this paper is to first present a unified modeling for
massive MIMO, encompassing both cellular and cell-free architectures with a
variable number of antennas per access point. We derive signal transmission
models and achievable spectral efficiency in both the downlink and uplink using
zero-forcing and maximal-ratio schemes. We also provide performance comparisons
in terms of per-user and sum spectral efficiency.
