  Employing massively distributed antennas brings radio access points (RAPs)
closer to users, thus enables aggressive spectrum reuse that can bridge gaps
between the scarce spectrum resource and extremely high connection densities in
future wireless systems. Examples include cloud radio access network (C-RAN),
ultra-dense network (UDN), and cell-free massive multiple-input multiple-output
(MIMO) systems. These systems are usually designed in the form of
fiber-wireless communications (FWC), where distributed antennas or RAPs are
connected to a central unit (CU) through optical front-hauls. A large number of
densely deployed antennas or RAPs requires an extensive infrastructure of
optical front-hauls. Consequently, the cost, complexity, and power consumption
of the network of optical front-hauls may dominate the performance of the
entire system. This article provides an overview and outlook on the
architecture, modeling, design, and performance of massively distributed
antenna systems with non-ideal optical front-hauls. Complex interactions
between optical front-hauls and wireless access links require optimum designs
across the optical and wireless domains by jointly exploiting their unique
characteristics. It is demonstrated that systems with analog
radio-frequency-over-fiber (RFoF) links outperform their baseband-over-fiber
(BBoF) or intermediate-frequency-over-fiber (IFoF) counterparts for systems
with shorter fiber length and more RAPs, which are all desired properties for
future wireless communication systems.
