  The fifth-generation of mobile radio technologies is expected to be agile,
flexible, and scalable while provisioning ultra-reliable and low-latency
communication (uRLLC), enhanced mobile broadband (eMBB), and massive machine
type communication (mMTC) applications. An efficient way of implementing these
is by adopting cloudification, network function virtualization, and network
slicing techniques with open-radio access network (O-RAN) architecture where
the base-band processing functions are disaggregated into virtualized radio
unit (RU), distributed unit (DU), and centralized unit (CU) over front/mid-haul
interfaces. However, cost-efficient solutions are required for designing
front/mid-haul interfaces and time-wavelength division multiplexed (TWDM)
passive optical network (PON) appears as a potential candidate. Therefore, in
this paper, we propose a framework for the optimal placement of RUs based on
long-term network statistics and connecting them to open access-edge servers
for hosting the corresponding DUs and CUs over front/mid-haul interfaces while
satisfying the diverse QoS requirements of uRLLC, eMBB, and mMTC slices. In
turn, we formulate a two-stage integer programming problem and time-efficient
heuristics for users to RU association and flexible deployment of the
corresponding DUs and CUs. We evaluate the O-RAN deployment cost and latency
requirements with our TWDM-PON-based framework against urban, rural, and
industrial areas and show its efficiency over the optical transport network
(OTN)-based framework.
