## 5 Feature Challenges
### Beyond 5G  
In a 5G network, there are three categories of services whose requirements became the driving force for innovation, namely, massive machine type communications (mMTC), supporting millions of Internet of Things (IoT) devices with intermittent activity and transmissions of small data packets; ultra-reliable and low-latency communications (URLLC), allowing zero-latency communication with high reliability, such as in critical and emergency applications [96]; and enhanced mobile broadband (eMBB), accommodating traffic with high data rates, as well as cell-edge users' connectivity. Extended mMTC scenarios include hundreds of thousands of devices, automating smart city processes, such as waste disposal, asset tracking, intelligent transportation etc., demanding an Area Spectral Efficiency (ASE) exceeding 20 MB/s/m\({}^{2}\) that is well beyond 5G expectations [97, 98] and paves the way to its Long-Term Evolution (5G-LTE). Taking into consideration these facts, a promising idea to design the B5G network architecture appears under the name of Ultra-Dense Networking (UDN) [99, 100], implying a density of (even) thousands of nodes (drones or cars equipped with remote radio-head (RRH) or other 5G network modules) per square kilometer, while in a traditional mobile network, there are usually not more than twenty BSs in the same area. UDN is based on the deployment of static or moving, ground or flying Radio Access (RA) nodes offering improved coverage per user. More specifically, in UDNs, the transmission distance is reduced, resulting in improved link quality, while frequency reuse is maximized, increasing capacity. Under this paradigm, BSs and/or RRHs are closer to the user or are even following a user's trajectory, realizing the concept of moving access points [101]. For example, in a crowded event (Figure 8a), e.g., a music festival, a trade-fair and so on, additional connectivity is required, especially when they are hosted at less central locations where, usually, the available telco infrastructure cannot cope with the massive connectivity demands of event audiences (contrary to infrastructure-wise, well-provisioned environments like stadiums, where regular massive events are expected). A way to improve network capability for supporting such sporadic events would be the deployment of drones, positioned strategically for improved coverage and extra bandwidth to the event area, while at the same time, relaying over high-speed connections, the mobile data traffic between users and the core network. In addition, edge content caching can ease the stress on backhaul links and improve user experience. Figure 7: Autonomous vehicle in city driving. In the same context, many outdoor places of interest for groups of tourists are located slightly outside of the range of traditional telco infrastructure, including boat trips close to the coast/on a lake to see, group visits to historical sites, hiking/Segway trips to visit the local flora/fauna close to a city, etc. Connectivity can be provided to users moving around (Figure 8b), as a group, via drones, flying over the area where touristic activities are happening. The connectivity between the flying RA nodes and the core network would require some sort of medium-long-range transmission technology. A small number of drones will follow the tourists during their visits, lasting up to 80-100 minutes and cover short distances (\(<=1\)km), applying flight if required by drones. Finally, after big natural disasters, such as earthquakes or hurricanes, telecom networks may have gone down, leaving both people isolated and the emergency response teams without communication capabilities via infrastructure-based cellular networks. In this case, a mobile network should be rapidly deployed (Figure 8c), focused on the disaster area, allowing the emergency response teams and the people in the area to properly communicate and access data using either internet or any other physical means, such as voice, gestures, etc.