  This paper studies a cell-free massive multiple-input multiple-output (MIMO)
system where its access points (APs) and users are equipped with multiple
antennas. Two transmission protocols are considered. In the first transmission
protocol, there are no downlink pilots, while in the second transmission
protocol, downlink pilots are proposed in order to improve the system
performance. In both transmission protocols, the users use the minimum
mean-squared error-based successive interference cancellation (MMSE-SIC) scheme
to detect the desired signals. For the analysis, we first derive a general
spectral efficiency formula with arbitrary side information at the users. Then
analytical expressions for the spectral efficiency of different transmission
protocols are derived. To improve the spectral efficiency (SE) of the system,
max-min fairness power control (PC) is applied for the first protocol by using
the closed-form expression of its SE. Due to the computation complexity of
deriving the closed-form performance expression of SE for the second protocol,
we apply the optimal power coefficients of the first protocol to the second
protocol. Numerical results show that two protocols combining with
multi-antenna users are prerequisites to achieve the suboptimal SE regardless
of the number of user in the system.
