## 1 Introduction  
The 5G network can be seen as the natural evolution of LTE and LTE Advanced (LTE-A) radio technologies. LTE-A introduces several arrangements to improve performance, such as carrier aggregation, improved MIMO, Coordinated Multipoint (CoMP), Relay Stations and Heterogeneous Networks are for enhancing coverage and link capacity. Nevertheless, differently from 4G mobile networks, 5G connectivity is mainly oriented to application. This means that 5G does not only provide wireless connectivity, but the 5G network should organize itself to guarantee tight latency and high bandwidth requirements of each application/use case and to accommodate a huge number of devices. For this reason requirements of future 5G networks are commonly specified in terms of use cases, instead of radio interface key performance indicators (KPI) as in previous 2G up to 4.5G systems. The 5G networks should be designed to support a wide range of new applications and use cases, including vehicular ones with traffic safety control, smart homes, critical infrastructures, industry processes, very-high-speed media delivery, Internet of Things (IoT), and so on [1; 2]. The overall aim of 5G is to provide ubiquitous connectivity for any kind of devices and applications that may benefit from being connected [3; 4]. The 5G access network will not be based on one specific radio-access technology. Rather, several vendors imagine 5G as flexible, adaptive portfolio/platform of access and connectivity solutions capable to address the demands and requirements of mobile communications as soon as possible after year 2020. Main differences between present LTE-A and 5G can be listed as follows [5; 6]:at the time of this writing, LTE/LTE-A networks are going through a rapid deployment/consolidation, while 5G networks mostly comprise of research papers and pilot projects. Widespread deployment of 5G networks is targeted not earlier than 2020;
* LTE mostly focused on the availability of raw bandwidth; 5G aims at providing pervasive connectivity to users/applications wherever they are, even independent of their moving speed;
* although LTE standard has been modified to incorporate a variant called machine type communications (MTC) for the IoT traffic, 5G technologies are being designed from grounds up to support MTC-like devices;
* the 5G networks are not going to be a monolithic network entity and will be built around a combination of existing radio access technologies: 2G, 3G, LTE, LTE-A, Wi-Fi, etc. The capability for future 5G terminals to support multi-connectivity is an important requirement. Finally, the 5G access will experiment with a new access network architecture based on dense/ultra-dense deployment of a myriad of access points in the service area. This is a distinctive feature of future 5G network with respect to the existing 4G. In principle, this network will be governed using Cloud-RAN (Radio Access Network) functionalities [7] and cognitive radio techniques. Self-organizing features will allow the dense/ultra-dense infrastructure to automatically decide about the type of channel to be offered to the user, differentiate between mobile and fixed objects, and will be able to adapt to conditions at a given time. The dense/ultra-dense network concept originates from the possibility of splitting the classical base station functionalities into RF and Baseband and to provide their implementation into two distinct units. In particular, the samples of signals captured by the antennas at the remote RF radio unit (RRU) (mounted for example on a pole) are sent to the baseband unit (BBU) where they are processed at PHY to recover data. The BBU may also include the higher protocol layer typically L2 and/or L3. The RRU and the BBU communicate using an optical link. In a more general setting, one BBU can process signals from more RRUs. This has led to the concept of centralized BBU allowing agility, faster service delivery, cost savings, and, most important, improved coordination of radio capabilities across a set of RRUs.