  Unmanned aerial vehicles (UAV) have emerged as a practical solution that
provides on-demand services to users in areas where the terrestrial network is
non-existent or temporarily unavailable, e.g., due to natural disasters or
network congestion. In general, UAVs' user-serving capacity is typically
constrained by their limited battery life and the finite communication
resources that highly impact their performance. This work considers the
orthogonal frequency division multiple access (OFDMA) enabled multiple unmanned
aerial vehicles (multi-UAV) communication systems to provide on-demand
services. The main aim of this work is to derive an efficient technique for the
allocation of radio resources, $3$D placement of UAVs, and user association
matrices. To achieve the desired objectives, we decoupled the original joint
optimization problem into two sub-problems: (i) $3$D placement and user
association and (ii) sum-rate maximization for optimal radio resource
allocation, which are solved iteratively. The proposed iterative algorithm is
shown via numerical results to achieve fast convergence speed after fewer than
10 iterations. The benefits of the proposed design are demonstrated via
superior sum-rate performance compared to existing reference designs. Moreover,
results showed that the optimal power and sub-carrier allocation help to
mitigate the inter-cell interference that directly impacts the system's
performance.
