  Cell-free massive MIMO (CF-mMIMO) network and its user-centric (UC) version
are considered as promising techniques for the next generations of wireless
networks. However, fronthaul and backhaul assignments are challenging issues in
these networks. In this paper, energy efficiencies of uplink transmission for
the CF- and UC-mMIMO networks are studied, wherein access points (APs) are
connected to aggregation nodes (ANs) through radio frequency (RF) and/or
free-space optic (FSO) fronthauls, and the ANs are connected to a central
processing unit via fiber backhauls. The achievable data rates are derived by
taking into account the effects of hardware non-ideality at the APs and ANs,
FSO alignment and weather conditions. To have a robust and energy-efficient
network, especially in the presence of FSO misalignment and adverse weather
conditions, firstly, a cognitive RF--FSO fronthaul assignment algorithm is
proposed at the cost of sharing the available RF bandwidth between the access
and fronthaul links. Then, optimal power allocations at the users and APs are
investigated, and two analytical approaches are proposed to solve the
non-convex optimization problem. Through numerical results, we have discussed
how utilizing the cognitive RF--FSO fronthaul assignment achieves higher energy
efficiency compared to that of FSO-only, RF-only, or simultaneously using RF
and FSO fronthaul links, e.g., achieving up to $198\%$ higher energy efficiency
under unfavorable conditions. Moreover, the effects of FSO misalignment,
weather conditions, and power allocations on the performances of the CF- and
UC-mMIMO networks are discussed.
