  To relieve the stress on channel estimation and decoding complexity in
cell-free massive multiple-input multiple-output (MIMO) systems, user grouping
problem is investigated in this paper, where access points (APs) based on
time-division duplex (TDD) are considered to serve users on different time
resources and the same frequency resource. In addition, when quality of service
(QoS) requirements are considered, widely-used max-min power control is no
longer applicable. We derive the minimum power constraints under diverse QoS
requirements considering user grouping. Based on the analysis, we formulate the
joint power and user grouping problem under QoS constraints, aiming at
minimizing the total transmit power. A generalized benders decomposition (GBD)
based algorithm is proposed, where the primal problem and master problem are
solved iteratively to approach the optimal solution. Simulation results
demonstrate that by user grouping, the number of users served in cell-free MIMO
systems can be as much as the number of APs without increasing the complexity
of channel estimation and decoding. Furthermore, with the proposed user
grouping strategy, the power consumption can be reduced by 2-3 dB compared with
the reference user grouping strategy{, and by 7 dB compared with the total
transmit power without grouping.
