# Planning of Optical Connections in 5G Packet-Optical xHaul Access Network
## 1 Introduction
Optical fiber technologies are a first choice for deployment of such links as they satisfy both requirements and provide some additional capabilities, such as wavelength division multiplexing (WDM). WDM increases significantly the capacity of an optical fiber thanks to multiplexing of signals transmitted on different wavelengths using either passive or active WDM equipment installed at the link ends [12]. Passive WDM reduces about 4-6 times the cost of active WDM since it does not use signal amplification and dispersion compensation components, in addition to the fact that it utilizes less expensive WDM equipment [13]. Therefore, passive WDM solutions are often preferred in limited-distance applications (up to 20 km), such as 5G access networks. WDM enables aggregation of traffic from remote sites by means of OADM devices, which combine selected wavelengths at particular intermediate sites into an optical transmission path going from an end remote site through intermediate sites to a hub site, as shown in Figure 1. The application of an OADM at an intermediate site increases utilization of the WDM link, and, at the same time, it releases from the need to establish a dedicated transmission path between the hub and the site. It leads to the reduction of the number of fiber connections and installed WDM equipment, which translates into lower network deployment costs [14; 15; 16]. Recently, a commercial solution available under the trade name flexiHaul has been proposed for a 5G packet-optical xHaul access network. This solution consists of a TSN Ethernet switch [17] and a passive WDM system xWave 400G [16], which allows for the aggregation of wavelengths on optical paths by means of OADMs. In a 5G access transport network based on a transmission system with the above discussed features, a basic network design problem concerns planning of optical connections between remote sites and a hub site. The problem consists of the selection of routes for optical transmission paths over the network, together with the selection of intermediate nodes at which OADMs are located and traffic is aggregated. These decisions are constrained by the optical power budget of the system, which determines the maximum length of optical paths (i.e., transmission reach/distance). Moreover, the use of OADMs introduces losses of optical power, which results in a shortened transmission reach. As a consequence, there is a certain tradeoff that limits the use of many OADMs on longer paths, although such paths might pass through and allow for gathering traffic from a larger number of remote sites. As aggregation of traffic is desirable for decreasing network cost; hence, proper routing and OADM placement decisions are required when planning optical connections in the network. The length of routing paths and resulting propagation delays may be limited additionally due to low-latency requirements of specific 5G services. In this work, we focus on modeling and optimal solving the discussed connection planning problem in a 5G packet-optical xHaul access network. The main contributions of this work are the following:  
1. development and application of a physical-layer transmission model, based on optical power budget calculations and assuming the properties of a passive WDM system, for estimating the reach of transmission paths,
2. formulation of an ILP optimization problem for generation of optimal solutions to the connection planning problem in the 5G packet-optical xHaul access network considered, and
3. assessment of the impact of wavelength aggregation (by means of OADMs) on network performance in different scenarios assuming realistic transmission system parameters. To the best of our knowledge, the optimization problem addressed and the ILP formulation proposed have not been considered in the literature yet. In addition, we are not aware of a similar work in which a physical-layer transmission model was included to optimization of a 5G packet-optical xHaul access network. The remainder of this article is organized as follows.