Role of altitude when exploring optimal placement of UAV access points

UAVs (Unmanned Aerial Vehicles) are candidates for serving as wireless access points in disaster or mass event scenarios. Here we elaborate on the question where to optimally place the available UAVs in space for a Poisson user distribution on the ground. Since the number of possible placement combinations is too high for a systematic search, we approach the optimal solution by a simple heuristic bio-inspired procedure. It turns out that changes in altitude contribute almost as much to increasing the average spectral efficiency as changes in horizontal directions. However, increasing the altitude requires much more energy than an equal movement in a horizontal direction. We show that restricting altitude movements by an attenuation factor can result in similar results as a procedure without such altitude restrictions. Even more intriguingly, keeping all UAVs at one favorable fixed altitude clearly outperforms the procedure with altitude flexibility. This has implications for the time UAVs can stay aloft with their on-board energy.