## 1 Introduction
In general, CPRI does not fit transport capacity of fronthaul based on FttC and VDSL2 technologies especially for upstream (US) transmissions. As an example, considering a radio node with two antennas providing access over a band of 20 MHz and using LTE technology will require up to 2.5 Gbit/s CPRI [17]. Compression is an option for reducing CPRI rate so to relax fronthaul requirements but bit rates remain relatively high for FttC and transfer delays increase due to the compression/decompression processing. As shown in this paper, achievable downstream (DS) bit rates can be compared to that of CPRI. However, this is achieved only in very specific situations i.e., vectoring is adopted, the radio node is close to the Cabinet and at least 6 or 8 bonded copper pairs (or 4 pairs with phantom) have been assigned to it to communicate with the DSLAM (DSL Access Multiplexer). To overcome the CPRI bit rate issues, which would prevent the adoption of FttC for fronthauling at least in the US direction, the splitting of the protocol stack of the radio node is an option to ease FttC fronthauling for initial 5G dense/ultra-dense network deployment, at least. The concept of protocol stack splitting has been introduced for 4G small cells and it is relatively recent. In 2014, the small cell forum (SCF) embarked on a study of small cell splitting and virtualization. The main focus of such research was on the impact that the transport has on the ability to decompose a small cell into a physical network function and a centralized/virtualized network function. Compared with traditional techniques requiring fiber, these studies have examined alternative protocol layer decompositions that can be supported over a range of transport options, enabling strict requirements on fronthaul latency and bandwidth to be reduced. Main results of these studies are presented in [18] and indicate the relaxation in fronthaul latency and bandwidth as the various functions are decomposed between the remote node and the central/virtualized entity. In the MAC/PHY split for LTE, all functionalities related to signal generation/detection are performed remotely at the remote unit while higher layer functionalities, e.g., scheduling, are performed at the central unit. The study concluded that the MAC/PHY split delivers most of the benefits of centralization, with only a small increase in transport performance, and is well aligned with the current small cell multi-vendor ecosystem approach based on the functional application platform interface (FAPI) and its evolution network FAPI (nFAPI). Specifically, it was agreed to move forward with the definition of a transportable interface based on a MAC/PHY decomposition. Furthermore, when coupled with HARQ (hybrid automatic repeat request) interleaving, the MAC/PHY approach could be supported over the packet switched backhaul service conventionally used to support small cell deployment. Some interesting results showing the benefits of MAC/PHY split on fronthaul requirements are reported in [18], Table 1. It can be observed that the MAC/PHY split can be supported using a fronthaul transport network providing 187.5 Mbit/s and 62.5 Mbit/s that are much lower than the CPRI bit rates required for PHY splitting of 1.075 Gbit/s for downlink and 922 Mbit/s for uplink. Referring again to the LTE case in [19], it has been observed that the fronthaul bit rate of the MAC/PHY split does not scale linearly with the number of antennas and that the rate follows the actual load of the cell and, most important, transport resources are not wasted in low load scenarios (The CPRI interface transfers signal samples at the same rate on the fronthaul even in the presence of only one low bit rate use). Furthermore, it was observed that splitting at layers higher than MAC provides a required fronthaul bit rate slightly lower compared to the MAC/PHY split since uplink measurements, user equipment (UE) reports, and MAC and RLC headers are used at the remote unit and thus are not transported to the central unit.