## 1 Introduction
Current optical fronthaul between the RRU and the BBU adopts Common Public Radio Interface (CPRI). The CPRI-based fronthaul requires tight latency and large transmission bandwidths. When the centralized BBU is moved into the Cloud, the Cloud-RAN governs the dense wireless access network. There is not a common agreement on the definition of dense/ultra-dense networks especially for outdoor environment. Several definitions can be found in the current literature [8; 9] and are given in terms of the achievable bit rate per km\({}^{2}\) or in terms of the number of nodes per km\({}^{2}\). For the purpose of this paper, we consider and extend the classification in [9] concerning the ultra-dense networks for outdoor:  
* number of nodes <5 nodes/km\({}^{2}\), macrocell network (in this case nodes are commonly indicated as base stations);
* number of nodes 5-10 nodes/km\({}^{2}\), microcell/small cell network;
* number of nodes 10-40 nodes/km\({}^{2}\), dense network;
* number of nodes >40 nodes/km\({}^{2}\), ultra-dense network. However, practical deployment of dense/ultra-dense network is severely limited by large costs of the fiber-based fronthaul infrastructure connecting RRUs to the BBU as well as to the backhaul. The costs of fiber to the home (FttH) to be used for fronthauling dense/ultra-dense wireless network have been analysed in [10; 11; 12]. It turns out that investments can be very risky for telecom operators due to the still uncertain revenue volumes from future 5G deployments. Furthermore, currently operators are still consolidating their 4G and forthcoming 4.5G networks providing the existing terminals with high radio link capacity. This uncertain economical scenario may significantly slow down the introduction of innovative 5G features brought by the dense/ultra-dense network deployment. Research on fronthaul alternatives to optical fiber is an important aspect for initial low cost, low investment risk and rapid deployment of 5G. Wireless fronthaul/backhaul is being analyzed in the current literature. Several solutions mostly based on millimeter waves have been considered. Due to their inherent flexibility, the adoption of radio technologies possibly integrated with optical fiber for 5G fronthauling and backhauling such as satellite, WiFi, or cellular systems are currently under study [13; 14]. In this paper we investigate on the possibility of using fiber to the cabinet (FttC)-based fronthaul. The adoption of DSL (Digital Subscriber Line)-based technologies for fronthauling has been mainly discussed with respect to the fiber to the distribution point (FttDp) access network topology using G.fast (ITU-T G9701) or the future X-G.fast [15]. CAPEX investments for an FttDp deployment can be compared with those based on optical fiber and the adoption of G.fast technology is still at initial stage and in some Countries (such as Italy) it is slowed down by regulatory issues related to the access market. To the best of authors' knowledge, the possible adoption of FttC networks based on Very High-speed Digital Subscriber Line type 2 (VDSL2) profile 35b [16] has not been thoroughly analyzed and investigated in the current literature and considerations have been restricted to the current FttC networks based on VDSL2 profile 17a. The introduction of the VDSL2 profile 35b is relatively recent and its potentials for fronthauling have not been adequately explored especially for supporting of 5G. Telecom operators have started to invest on VDSL2 35b and are currently updating their access network with this new technology. The enlarged bandwidth of 35b, the adoption of vectoring with the possibility of bonding more copper pairs and with phantom technology allowing combining pairs to electrically increase the overall number of pairs at the network termination (NT), allow to extend the VDSL coverage and to obtain significant bit rates that, as shown in this paper, can be used for fronthaul transport.