  Terrestrial communication networks have experienced significant development
in recent years by providing emerging services for ground users. However, one
critical challenge raised is to provide full coverage (especially in dense
high-rise urban environments) for ground users due to scarce network resources
and limited coverage. To meet this challenge, we propose a high altitude
platform (HAP)-reserved ground-air-space (GAS) transmission scheme, which
combines with the ground-to-space (G2S) transmission scheme to strengthen the
terrestrial communication and save the transmission power. To integrate the two
transmission schemes, we propose a transmission control strategy. Wherein, the
ground user decides its transmission scheme, i.e., switches between the GAS
link transmission and the G2S link transmission with a probability. We then
maximize the overall throughput and derive the optimal probability that a
ground user adopts the GAS transmission scheme. Numerical results demonstrate
the superiority of the proposed transmission control strategy.
