  The coming extension of cellular technology to base-stations in low-earth
orbit (LEO) requires a fresh look at terrestrial 3GPP channel models. Relative
to such models, sky-to-ground cellular channels will exhibit less diffraction,
deeper shadowing, larger Doppler shifts, and possibly far stronger cross-cell
interference: consequences of high elevation angles and extreme "sectorization"
of LEO satellite transmissions into partially-overlapping spot beams. To permit
forecasting of expected signal-to-noise ratio (SNR), interference-to-noise
ratio (INR) and probability of outage, we characterize the powers of desired
and interference signals as received by ground users from such a LEO satellite.
In particular, building on the Shadowed Rician channel model, we observe that
co-cell and cross-cell sky-to-ground signals travel along similar paths,
whereas terrestrial co- and cross-cell signals travel along very different
paths. We characterize SNR, signal-to-interference ratio (SIR), and INR using
transmit beam profiles and linear relationships that we establish between
certain Shadowed Rician random variables. These tools allow us to simplify
certain density functions and moments, facilitating future analysis. Numerical
results yield insight into the key question of whether emerging LEO systems
should be viewed as interference- or noise-limited.
