  Previous works on cell-free massive MIMO mostly consider physical-layer and
fronthaul transport aspects. How to deploy cell-free massive MIMO functionality
in a practical wireless system is an open problem. This paper proposes a new
cell-free architecture that can be implemented on top of a virtualized cloud
radio access network (V-CRAN). We aim to minimize the end-to-end power
consumption by jointly considering the radio, optical fronthaul, virtualized
cloud processing resources, and spectral efficiency requirements of the user
equipments. The considered optimization problem is cast in a mixed binary
second-order cone programming form and, thus, the global optimum can be found
using a branch-and-bound algorithm. The optimal power-efficient solution of our
proposed cell-free system is compared with conventional small-cell implemented
using V-CRAN, to determine the benefits of cell-free networking. The numerical
results demonstrate that cell-free massive MIMO increases the maximum rate
substantially, which can be provided with almost the same energy per bit. We
show that it is more power-efficient to activate cell-free massive MIMO already
at low spectral efficiencies (above 1 bit/s/Hz).
