  Cell-Free (CF) Massive multiple-input multiple-output(MIMO) is a distributed
antenna system, wherein a large number of back-haul linked access points
randomly distributed over a coverage area serve simultaneously a smaller number
of users. CF Massive MIMO inherits favorable propagation of Massive MIMO
systems. However, the level of favorable propagation which highly depends on
the network topology and environment may be hindered by user' spatial
correlation. In this paper, we investigate the impact of the network
configuration on the level of favorable propagation for a CF Massive MIMO
network. We formulate a user grouping and scheduling optimization problem that
leverages users' spatial diversity. The formulated design optimization problem
is proved to be NP-hard in general. To circumvent the prohibitively high
computational cost, we adopt the semidefinite relaxation method to find a
sub-optimal solution. The effectiveness of the proposed strategies is then
verified through numerical results which demonstrate a non-negligible
improvement in the performance of the studied scenario.
