## 4 Protocol Stack Splitting for the 5G Radio Node  
Protocol stack splitting between a remote and one central processing unit has been proposed for LTE small cells. The minimum fronthaul CPRI rate to support two antennas LTE small cell with 20 MHz is about 1.23 Gbit/s [18]. The adoption of MAC/PHY layer splitting allows to drastically reduce the required rate and permits to telecom operators to consider low cost options for fronthauling [19]. The possibility for the radio node to adapt its radio transmission/reception capabilities to the available fronthaul transport capacity is another interesting option we are going to discuss in the following. Radio node flexibility can be achieved by introducing a programmable PHY/RF protocol layer. The parameters and types of waveforms to be implemented (at runtime) in the node could be selected so that the overall required transport capacity to the central processing unit does not exceed the available fronthaul capacity. Thus, the parameters of the radio waveforms (e.g., bandwidth, frequency, power, etc.) could be selected in order to fit the available fronthaul bit rate. For example, if the traffic originating from one 20 MHz band LTE interface cannot match the transport fronthaul capacity, the LTE band could be reduced down to 10 MHz or even to 5 MHz. Focusing on FttC transport capacity at the node tends to reduce with the distance of the distributor from the Cabinet. Thus, even in these cases it could be necessary to adapt waveform parameters in the node to the available fronthaul transport capacity. The availability of a programmable PHY/RF layer can be helpful to solve several problems related to 5G deployment due to the still uncertain characteristics of the 5G radio interfaces and to the integration of different radio access technologies. The bandwidth to be managed by the node could be selected on the basis of the PHY baseband computational capacity and then on the characteristics of the processors, FPGA, ASIC, GPU, etc. practically realizing the board of the radio access node. Assuming the bandwidth managed by the small cell is fixed the main functional requirements of the programmable PHY/RF layer are listed in the following points. * Implement different baseband (BB) waveforms (e.g., modulation/demodulation, synchronization, coding, etc.) from one or more radio standards as well as non-standard/proprietary waveforms used for example by sensors or, in general, by any IoT device. The main constraint is that the sum of the bands of the waveforms and the transmitted power do not exceed the band managed by the node (e.g., 20 MHz) and the maximum transmit power, respectively. * Support carrier frequency agility allowing to transmit each BB waveform at any carrier frequency inside the spectrum of interest, i.e., from some tens of MHz up to 6 GHz. Thanks to carrier agility the node can support more standard/non-standard narrowband/wideband radio interfaces, simultaneously. * Allow communications with the remote unit in accordance with CPRI interface in the case of PHY (BB)/RF splitting or with FAPI interface in the case of MAC/PHY splitting, as depicted in Figure 2.  
In the case fronthaul capacity provided by the FttC is high enough to accommodate CPRI-based transmissions in one or both directions, the node can be configured to provide the central unit with the samples of signals. In general, the transport capacity provided by the fronthaul in the DS direction can be high enough to support CPRI while the US can be the limiting case. Thus, the node could be configured to support CPRI transmissions on the DS direction and FAPI transmissions on the US direction. In this case, the node could operate in a mixed CPRI/FAPI mode. More in general, it could be possible to divide the bandwidth assigned to the node into sub-bands.