on different wavelengths in a fiber link by means of an optical MUX device. Passive WDM solutions, which are cost effective, as they do not require signal amplification and dispersion compensation, are especially appropriate for limited-distance 5G xHaul networks [9, 11]. Concurrently, Ethernet can improve the utilization of optical connections by means of statistical multiplexing of the packets carrying the radio data of the flows aggregated at a remote site [12] and encapsulated using such protocols as the enhanced common public radio interface (eCPRI) [13] and radio over Ethernet (RoE) [14]. For the prioritized transmission of latency-sensitive Ethernet frames, e.g., related to fronthaul flows, time-sensitive networking (TSN) features are defined in [15]. The WDM and TSN Ethernet technologies reduce the demand for direct fiber connections between the remote sites and the hub, which decreases the xHaul network deployment cost [16, 17, 18]. The further reduction in the demand for optical fiber resources can be achieved by the application of passive OADM devices. The OADMs allow to aggregate the wavelengths from a number of remote sites in a single optical TP [19, 20], as shown in Figure 1, instead of using several direct fiber connections between each of the remote sites and the hub. The flexiHaul system [21], which includes a passive WDM system called xWave 400G [18] and a TSN Ethernet switch [22], is an example of a commercial solution supporting the above-discussed functionalities. The application of OADMs increases the complexity of the xHaul network planning problem since the decisions concerning the routing of TPs between the remote sites and the hub should be handled jointly with a proper choice of intermediate remote nodes at which the OADMs are placed. An important factor constraining the routing decisions is the maximum transmission distance, which, in a passive optical system such as flexiHaul, depends on the optical power budget, particularly on the attenuation of optical components belonging to a TP. The attenuation as well as the cost of OADMs increases with the number of channels (wavelengths) added to (dropped from) the aggregated WDM optical signal [23]. Therefore, a reasonable approach is to apply the OADMs in a flexible way, namely, according to the wavelength demands of particular remote sites to optimize the path length and reduce the network cost. The main goal of this work is to investigate the cost savings achieved by the flexible (optimized) application of OADMs for wavelength aggregation at remote sites in a packet-optical xHaul network. To this end, we formulate a novel MILP optimization model that accounts for different types of OADMs, applied flexibly according to the demands, and incorporates the power-budget calculations into the model. The model focuses on the optimization of the network cost and embraces the trade-off between the use of OADMs, which involves some costs, and the savings due to the reduced demand for optical fibers. The optimization model is applicable for xHaul access network deployment scenarios, in which the WDM optical transport network makes use of passive optical elements and the  
Figure 1: Example of optical TP from remote site 1 to the hub with wavelength aggregation by means of OADMs at intermediate remote sites 2 and 3 in packet-optical xHaul access network supporting 4G, 5G, and private network traffic. aggregation of traffic (wavelengths) is realized in the optical domain by means of OADMs. The major contributions of this study are as follows:  
1. Development of an MILP model for physical layer-aware and cost-oriented optimization of OADM placement in packet-optical xHaul networks realized using passive WDM devices. 2. Evaluation of cost saving due to the optimized application of OADMs for network scenarios with realistic cost and transmission system parameters. 3. Assessment of the complexity of solving the MILP model by means of numerical experiments performed in different network scenarios.