## 10 Challenges and Open Research Problems
### Energy Harvesting Challenges  
It is well known that one of the biggest limitations to the widespread adoption of UAVs in cellular networking is their limited operating lifetime [310]. One of the most popular solutions for dealing with this problem is using energy harvesting technologies to enable UAVs to harvest energy from the environment to support the on-board battery. However, this technology is still in its infancy and fraught with many natural and technical challenges. For instance, solar-based energy harvesting still depends on climatic conditions and becomes unavailable when the sun is not shining [67; 304], especially in the winter (which can last several months in the Northern Hemisphere). Solar energy harvesting solutions are based on use of photovoltaic arrays, which are only suitable for fixed-wing UAVs [43]. Similarly, the availability of RF energy depends on the density of RF devices within the area. Most of the energy harvesting technologies in the literature have low energy transfer efficiency due to environmental (such as free space pathloss for RF energy harvesting) or device limitations (such as poor energy conversion ratio). Moreover, since UAVs are usually in motion, they keep losing LOS connection with the charging BS. As a matter of fact, the amount of energy harvested is still very small compared to the amount of power that can be stored in on-board batteries, especially when such energy comes from RF environment [349].  
The output of most energy harvesting techniques is still quite low due to the poor efficiency of the energy conversion and matching circuits [350]. The amount of energy captured from RF energy, for instance, depends on the area of the antenna elements [305]. Since most UAVs come in small form factors to limit their weight, their antennas are also small. Even where solar or other forms of energy are targeted for energy harvesting, the small form factor also implies that a limited area of the UAV is exposed to ambient energy,thereby limiting the amount of energy that can be harvested from such environments. Moreover, the distance between the UAVs and the charging (base) station as well as whether the UAV has an LOS connection to the base also affect the amount of energy received; the farther the UAV, the lower the energy received [311]. However, since UAVs are deployed to supplement the services provided by MBSs, it is desirable that they operate far from the BS in areas with poor coverage, which limits the power received from the macro cell.  
Another limitation associated with energy harvesting is the time it takes to recharge in-flight UAVs from harvested energy due to shortcomings in the charging rates. Finally, since energy harvesting is still in its infancy, there is a lack of a unified standard on it, which limits the widespread involvement of original equipment manufacturers (OEM) to develop wireless recharging devices.  
One way to improve the amount of energy harvested from the radio waves is to use directional antennas via beamforming [311; 351], which ensures that more of the transmitted RF energy is received at the UAV. There are other proposed solutions aimed at optimizing the charging time of UAVs by improving the energy transfer and conversion rates. Solutions related to the above two include optimal or near optimal trajectory planning to guarantee maximum exposure of the UAV to the RF field of the charging BS in cases where energy is harvested from the radio environment.