However, 5G already has some tools to alleviate such pressure in terms of capacity enhancement, such as millimeter wave (mmWave) communications, network densification, and massive multiple-input multiple-output (MIMO) technology, to name a few. Since 5G is a reality, studies on the sixth generation of mobile communication networks (6G) have already started and moved quite forward [4; 5; 6; 7]. Similar to the sharp advancement from the legacy networks to 5G, 6G is expected to be a paradigm shift in terms of network approaches, as mentioned in a recent report in [8]. More specifically, 6G is envisioned to shift the focus from conventional requirements including delay, data rate, reliability to global coverage, CO\({}_{2}\) emission, spectral efficiency, etc. [8]. However, this does not mean that 6G will not improve the data rate, latency, and reliability of the existing networks, because it is expected that 6G will enhance the aforementioned conventional requirements significantly [4], which indeed makes it more challenging for 6G networks. That is why, for instance, the terahertz (THz) spectrum is considered for 6G in order to open up more bandwidth for capacity enhancement and increased data rate purposes [4; 5; 6; 7]. The picture that we have been trying to frame so far is that there is and will be a need for capacity enhancement in the current and upcoming generations of mobile communication networks. As mentioned earlier in this section, although each generation involves various concepts to make them immune to the rise in capacity demand, there is always a need for higher capacity provision. Furthermore, during scenarios involving unusual gathering of people, such as music concerts, sports competitions, fairs, etc., which only take place for short durations over long periods of time, a crowd cluster around a hotspot (i.e., the location of the event), increasing the level of challenge for the existing cellular network infrastructures, which are typically not designed for such rare events. If not for special occasions, in a usual scenario, there are spatio-temporal changes in the traffic loads of base stations (BSs), so that, for a particular BS, the traffic load changes dynamically over the time of a day, and days of a week [9; 10; 11; 12]. This poses an interesting phenomenon that BSs do not always operate at full load; rather, the peak times of a day, for example, are limited to a certain period. In this regard, a pop-up solution would be required for such kinds of scenarios, and unmanned aerial vehicle (UAV)-assisted wireless networking has been introduced as a viable option for pop-up networking [13]. The idea here is that small cell (SC) BSs are deployed on UAVs (called UAV-BSs), and they are sent to the hotspots for capacity enhancement purposes as long as there is a need, providing pop-up connectivity. With that, not only the users are satisfied as they secure connectivity, but also the mobile network operators are relaxed as no fixed infrastructure deployment is necessary. In addition to the capacity enhancement purposes, UAV-BSs can also be used in emergency scenarios, including earthquakes, floods, etc., where the fixed infrastructure is either fully or partially damaged and down [14]. Since the end-to-end connectivity cannot be provided in such scenarios, UAV-BSs come as helping hands, such that they are sent to affected regions and operate either as standalone BSs in place of the damaged ones or as a relay BS that relays the data to the nearest healthy node of the communication system. This is an important vertical of UAV-assisted wireless networking, as in such scenarios, the transmitted message would be critical and can save the lives of people.