## 2 Related Works  
Most of the research studies concerning the optimization of optical transport networks in centralized RANs is related to the problem of radio processing functions (BBU/DU/CU) placement. The authors of [16; 17] formulated MILP models for the BBU placement problem in WDM optical networks with the goals to minimize the number of BBU sites and the amount of fiber connections. In [24], an MILP formulation was proposed for the dimensioning of BBU resources and optical transponders in active WDM optical networks with direct connections and without traffic aggregation between remote sites and the BBUs. Heuristic approaches for planning survivable fiber connections in WDM optical transport networks based on ring topologies were proposed in [25]. The DU/CU placement problem in a ring WDM optical aggregation network was modeled as an MILP optimization problem in [26]. The authors of [27; 28] proposed reinforcement learning algorithms for the DU/CU placement and optical path (lightpath) provisioning problem in optical xHaul networks, where the goal was to minimize the number of DU/CU sites and the bandwidth usage. The flexible splitting of the BBU functions and placing them in a RAN connected by means of an optical transport network was studied in [29]. The dynamic allocation of transmission resources for the transport of xHaul traffic in a passive optical network (PON) was studied in [30; 31]. Other related works have focused on the problem of connection provisioning with the optimization of transmission resources [32; 33], network survivability [34; 35], and energy efficiency [36; 37] in optical fronthaul networks.  
In the above discussed works, the main focus was on planning lightpath connections in generic scenarios in which either the underlying WDM optical network was pre-deployed, usually assuming the use of active WDM devices, which are expensive, or the network design process did not account for transmission constraints in the optical layer. Additionally, point-to-point lightpath/fiber connections were considered without traffic aggregation at intermediate nodes. Eventually, the optimization objectives were related to the usage of radio processing and transmission resources, whereas the optimization of network cost was neglected.  
In our recent works [19; 20], in which we analyzed the application of a passive WDM solution in a 5G packet-optical xHaul network, we took into account the impact of a mean ingful physical-layer constraint, related to the optical power budget, on the transmission distance during the xHaul network planning process. In these studies, we considered the use of a MUX device, also for the realization of OADMs, of specific capacity corresponding to the WDM system capacity. This assumption simplified the problem modeling, but it did not allow for the optimized deployment of MUX and OADM devices on designed TPs. Therefore, to fill this research gap, in this work, we address the problem of optimized OADM placement with proper MUX selection, taking into account the availability of different types of MUX/OADM devices, with the optimization goal related to the deployment cost of the optical transport network. To the best of our knowledge, both the problem studied and the MILP optimization model proposed have not been considered in the literature so far.