## 4 Protocol Stack Splitting for the 5G Radio Node
Then, the samples of the signal in one sub-band (maybe a narrowband) could be transmitted using the CPRI interface at its basic bit rate (e.g., 614 Mbit/s) and the signals in the remaining sub-bands are first demodulated/decoded and then frame bits are sent to the central unit using the FAPI interface. The 5G dense/ultra-dense network adopting PHY programmable nodes provides other opportunities for telecom operators to create flexible and reconfigurable access networks. In fact, the telecom operator could adapt both the radio coverage of the single node as well as its access technology(ies) in accordance with the traffic load, the types of terminals and required services in the area. As an example, some nodes could be configured to provide access in accordance with the new 5G radio interfaces, while other could use LTE, HSPA and even GSM radio interfaces. In the case of increase of 5G traffic nodes could be re-programmed (even at runtime) to operate with 5G interfaces and the existing traffic on the other interfaces could be migrated to 5G or the remaining nodes implementing LTE or HSPA etc. The single node can be configured to capture traffic from sensors in the area even when they are using non-standard or proprietary radio interfaces. Finally, nodes could directly communicate among them using millimeter-wave technologies or by dynamically reserving a quota of the assigned band for this purpose. Thanks to relatively small coverage of every node, high line-of-sight transmission probability, and limited path transmission loss, ultra-dense wireless networks can make full use of high-frequency band and millimeter-wave transmission, whose spectrum resources are still sufficient. The creation of network among nodes opens to the possibility of implementing local coordination features that can be exploited for improving performance even in the case of MAC/PHY split. In fact, in this case recombination of received observables at PHY level at the central processing unit is not possible and then MIMO gain due to (natural) presence of more antennas capturing the signals of the same user is wasted. Thanks to local coordination receiving nodes could transfer the signal observables of one user to a reference local node, which is in charge to perform combination at PHY level and then to communicate data to the central unit by FAPI. An alternative to MIMO to improve link capacity for the user is offered by multi-connectivity. One terminal equipped with more transceivers can locally connect to more nodes simultaneously using the same or different radio interfaces. It is evident that, multi-connectivity can be used to manage handover among nodes. Finally, the availability of a PHY programmable layer in the nodes of a dense/ultra-dense network allows protecting the investments of telecom operators on 5G against the absence of standardization of 5G radio interfaces. The telecom operator can start to deploy programmable PHY nodes by adopting FttC fronthaul to start 5G dense/ultra-dense network at reduced costs and then risks. Figure 2: Protocol stack splitting alternatives: CPRI-based (**a**); FAPI-based (**b**).