  The emerging cell-free massive multiple-input multiple-output (CF-mMIMO) is a
promising scheme to tackle the capacity crunch in wireless networks. Designing
the optimal fronthaul network in the CF-mMIMIO is of utmost importance to
deploy a cost- and energy-efficient network. In this paper, we present a
framework to optimally design the fronthaul network of CF-mMIMO utilizing
optical fiber and free space optical (FSO) technologies. We study an uplink
data transmission of the CF-mMIMO network wherein each of the distributed
access points (APs) is connected to a central processing unit (CPU) through a
capacity-limited fronthaul, which could be the optical fiber or FSO. Herein, we
have derived achievable rates and studied the network's energy efficiency in
the presence of power consumption models at the APs and fronthaul links.
Although an optical fiber link has a larger capacity, it consumes less power
and has a higher deployment cost than that of an FSO link. For a given total
number of APs, the optimal number of optical fiber and FSO links and the
optimal capacity coefficient for the optical fibers are derived to maximize the
system's performance. Finally, the network's performance is investigated
through numerical results to highlight the effects of different types of
optical fronthaul links.
