## 2 FttC-Based Dense Network Architecture  
The principle architecture of 5G wireless dense/ultra-dense network adopting FttC-based fronthaul is depicted in Figure 1.  
In a typical FttC deployment, we have a number of Cabinets connected by optical fiber to the central office (CO) (i.e., the primary network). One or more cables, each one containing a few hundreds of copper pairs, depart from each Cabinet towards homes (i.e., the secondary network). Subsets of copper pairs are extracted from one single cable and are connected to a distributor, i.e., a remote small patch panel. Then, one single copper pair out of the panel is connected to the NT inside the user premises. Distributors are flexibility points, which ease operations and maintenance. Not all pairs assigned to one distributor are connected to the clients' NTs, i.e., a certain number of spare pairs exist in each distributor that can be used for maintenance, or assigned to a single user in addition to its original assigned pair, if higher transmission capacity is required. The availability of spare pairs at the distributors renders FttC an interesting option for supporting the fronthauling for 5G dense/ultra-dense network. We assume the \(m\)-th distributor is assigned \(M_{m}\) copper pairs and \(M=\sum_{m=1}^{n_{D}}M_{m}\) is the total number of copper pairs in the single cable from the Cabinet. The \(n_{D}\) is the number of distributors connected to the Cabinet by the same cable. Typically \(M=200\) or \(M=400\) and the copper cable can contain from 4 up to 8 binders each one with 25 or 50 copper pairs. Far-end crosstalk (FEXT) among copper pairs in the same binder is the most relevant and is the main responsible for performance limitations. We further assume the single 5G wireless node can be directly mounted at the Cabinet or can be connected to one distributor and linked to the DSLAM at the Cabinet using \(1\leq K_{i}<M_{i}\) copper pairs. The DSLAM contains all the functionalities to manage and control the wireless nodes at the distributors along the cable. A discussion on the protocol stack splitting for the 5G access nodes in Figure 1 is detailed in Section 4. As shown in Figure 1 the radio nodes can connect among them using a dedicated wireless link, then forming a local wireless network. The nodes connected to distributors on the same cable could optionally communicate among them using dedicated copper pairs (where available). Communicating nodes could even belong to different cables in the same or from different Cabinets. In the following, nodes are managed by different DSLAMs and then they should be coordinated at CO level, provided that both DSLAMs pertain to the same CO. The wireless network among nodes introduces one further degree of flexibility in the 5G dense network, as illustrated in Section 4. In order to reduce latency for mobile applications requiring computational resources in the cloud, the DSLAM could be equipped with one or more distributed cloud elements such as the mobile edge server (MES). The MES can process signals/data from local terminals captured by the dense network node. Optionally, the DSLAM could host the central processing unit in the case of node protocol stack splitting. Figure 1: Considered 5G dense network architecture with FttC-based fronthaul. As shown in Section 5, which provides results referring to the Italian network, the possibility of deploying 5G nodes at the distributors in addition to those located at the Cabinets allows increasing the radio node density to reach values compatible with those envisaged for the 5G ultra-dense network. Before concluding this Section, it is worth observing the backhaul bandwidth of the FttC network should be large enough to absorb the request for the additional fronthauling capacity from extra pairs. In fact, extra pairs used for maintenance purposes only, could have been excluded and/or only partially considered in the actual FttC backhaul planning.