  To enhance coverage and improve service continuity, satellite-terrestrial
integrated radio access network (STIRAN) has been seen as an essential trend in
the development of 6G. However, there is still a lack of theoretical analysis
on its coverage performance. To fill this gap, we first establish a system
model to characterize a typical scenario where low-earth-orbit (LEO) satellites
and terrestrial base stations are both deployed. Then, stochastic geometry is
utilized to analyze the downlink coverage probability under the setting of
shared frequency and distinct frequencies. Specifically, we derive mathematical
expressions for the distances distribution from the serving station to the
typical user and the associated probability based on the maximum bias power
selection strategy (Max-BPR). Taking into account real-world satellite antenna
beamforming patterns in two system scenarios, we derive the downlink coverage
probabilities in terms of parameters such as base station density and orbital
inclination. Finally, the correctness of the theoretical derivations is
verified through experimental simulations, and the influence of network design
parameters on the downlink coverage probability is analyzed.
