# 3D Path Planning Algorithms in UAV-Enabled Communications Systems: A Mapping Study
## 4 Results
### Implementation Aspects of Path Planning  
One of the aspects that researchers take into account for path planning is the number of UAVs. Table 6 shows two groups of UAVs that can be observed, the simple ones, with only one UAV used (17 papers) and multiple UAVs where more than one UAV is used for the mission.  
Another important aspect is the obstacles that the UAVs must avoid. Table 7 shows that most of the contributions do not consider obstacles or only focus on known obstacles such as buildings or trees, so 78.38% (29 articles) of the contributions conducted their research with static non-technical obstacles, and only 5.41% of the contributions researchers consider technical obstacles. For instance, paper A-12 developed path-planning techniques for light shows with UAVs. They required powerful computing units and onboard sensors, which are extremely expensive for a large-scale group of UAVs, becoming a technical hurdle for this research.  
One of the most critical aspects that few researchers have considered is the environment, such as wind, snow, or rain, which can affect the flight of the UAVs, causing them to change their planned route and even causing the mission to fail. In contribution A-14, the researchers carry out 3D path planning based on radio maps for UAVs connected to cell phones. Here, the UAVs are connected to the BS to update their flight information and replan their route through algorithms based on predictive control. Figure 13 shows that the researchers have considered a hostile and uncertain environment only in 14% (A-02, A-14, A-15, A-26, and A-29) of the contributions.  
Figure 12: Optimization objective functions vs. analyzed papers.  
Figure 13: Static or dynamic environments that have been considered in path planning.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**ID** & **Aptitude** & **Collisions** & **Cost** & **Data** & **Energy** & \begin{tabular}{c} **Environmental** \\ **Model** \\ \end{tabular} & \
\begin{tabular}{c} **Turning** \\ **Radius** \\ \end{tabular} & **Restrictions** &
\begin{tabular}{c} **Flight** \\ **Hazards** \\ \end{tabular} & **Time** & **Vigilance** & **Multiple** \\ \hline A-01 & & x & & & & & & & \\ \hline A-02 & & & x & & & & & & \\ \hline A-0  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**ID** & **Aptitude** & **Collisions** & **Cost** & **Data** & **Energy** & \begin{tabular}{c} **Environmental** \\ **Model** \\ \end{tabular} & \begin{tabular}{c} **Turning** \\ **Radius** \\ \end{tabular} & \begin{tabular}{c} **Restrictions** \\ **Hazards** \\ \end{tabular} & \
\begin{tabular}{c} **Time** \\ **Vigilance** \\ \end{tabular} &
\begin{tabular}{c} **Multiple** \\ **Multiple** \\ \end{tabular} \\ \hline A-25 & & & & x & & & x & & x \\ \hline A-26 & & & & & & & x & & x \\ \hline A-27 & x & & & & x & & & x & x \\ \hline A-28 & & & & x & & & x & & x & x \\ \hline A-29 & & & & x & & x & & & x \\ \hline A-30 & & & & x & x & & & & x \\ \hline A-31 & & & x & x & & & & x \\ \hline A-32 & & & x & x & & & & x \\ \hline A-33 & & x & & & & & x & x \\ \hline A-34 & x & & & & & & & & \\ \hline A-35 & & x & & & & & & & \\ \hline A-36 & & x & & & & & & & x \\ \hline A-37 & x & & & x & & x & & x & x \\ \hline \hline \end{tabular}
\end{table}
Table 5: _Cont._