  The recent development of high-altitude platforms (HAPs) has attracted
increasing attention since they can serve as a promising communication method
to assist satellite-terrestrial networks. In this paper, we consider an
integrated three-layer satellite-HAP-terrestrial network where the HAP support
dual-band connectivity. Specifically, the HAP can not only communicate with
terrestrial users over C-band directly, but also provide backhaul services to
terrestrial user terminals over Ka-band. We formulate a sum-rate maximization
problem and then propose a fractional programming based algorithm to solve the
problem by optimizing the bandwidth and power allocation iteratively. The
closed-form optimal solutions for bandwidth allocation and power allocation in
each iteration are also derived. Simulation results show the capacity
enhancement brought by the dual-band connectivity of the HAP. The influence of
the power of the HAP and the power of the satellite is also discussed.
