The network slicing concept has gained much attention with the development of
software-defined network and network function virtualization technologies, enabling
logically isolated networks for different purposes in the same network infrastructure.
The virtualization of network functions enables functional split of radio access network
functions, which can fulfill different 5G radio access network requirements.
Functional split can be expressed as deciding the distribution of radio access network
functionalities between the radio unit placed at the edge, the distributed unit placed
close to the users, and the central unit placed centrally.
In this work, we considered the placement of virtualized radio access network functions
in a multi-tier 5G radio access network architecture and formulated the problem
as a Mixed-Integer Quadratically Constrained Programming, considering different
functional split options for each network slice separately. Furthermore, a heuristic
approach based on the best fit decreasing strategy, FlexBFDP, is proposed and compared
with a state-of-the-art solution existing in the literature. Our experiments with
different functional split options showed that flexible functional split utilizes physical
network resources and satisfies different network slice requirements better than fixed
functional split options. Whereas our experiments comparing the solutions of the
Gurobi solver, FlexBFDP, and the state-of-the-art algorithm showed that FlexBFDP
outperforms the state-of-the-art algorithm. Even though two out of three Gurobi solutions
outperform FlexBFDP in terms of optimization objective, FlexBFDP is preferable
in terms of computation time as it takes less time to find a solution.