Energy-Efficient DU/CU Placement in O-RAN Using Graph-Based Site Minimization
We study the problem of Distributed Unit (DU) & Centralized Unit (CU) placement in Open RAN for reducing the energy footprint of the network, under explicit distance & bandwidth constraints on real-world Radio Unit (RU) topologies. We show that the DU / CU placement can be formulated as a minimum dominating set (MDS) problem on graphs derived from latency & bandwidth constraints, enabling exact solutions that minimize the number of deployed nodes. To further refine the placement while preserving this minimum deployment cardinality, we propose a sequential distance-weighted MDS approach that selects, among all minimum-cardinality solutions, the one reducing the load transport cost. We evaluate the proposed method on a real-world node topology using a detailed energy model capturing both computational and transport costs. The results show that the MDS formulation significantly reduces infrastructure footprint compared to a clustering-based baseline, leading to a global RAN energy gain of around 14%, while the sequential refinement provides additional gains reducing latency and transport energy cost.