  We study the important and challenging problem of virtualized radio access
network (vRAN) design in its most general form. We develop an optimization
framework that decides the number and deployment locations of central/cloud
units (CUs); which distributed units (DUs) each of them will serve; the
functional split that each BS will implement; and the network paths for routing
the traffic to CUs and the network core. Our design criterion is to minimize
the operator's expenditures while serving the expected traffic. To this end, we
combine a linearization technique with a cutting-planes method in order to
expedite the exact solution of the formulated problem. We evaluate our
framework using real operational networks and system measurements, and follow
an exhaustive parameter-sensitivity analysis. We find that the benefits when
departing from single-CU deployments can be as high as 30% for our networks,
but these gains diminish with the further addition of CUs. Our work sheds light
on the vRAN design from a new angle, highlights the importance of deploying
multiple CUs, and offers a rigorous framework for optimizing the costs of
Multi-CUs vRAN.
