  In this letter, we study the on-demand UAV-BS placement problem for
arbitrarily distributed users. This UAV-BS placement problem is modeled as a
knapsack-like problem, which is NP-complete. We propose a density-aware
placement algorithm to maximize the number of covered users subject to the
constraint of the minimum required data rates per user. Simulations are
conducted to evaluate the performance of the proposed algorithm in a real
environment with different user densities. Our numerical results indicate that
for various user densities our proposed solution can service more users with
guaranteed data rates compared to the existing method, while reducing the
transmit power by 29%.
