The benefits of cells in the sky

Cells in the sky can broadly be divided into high-altitude platforms (HAPs) and satellites. HAPs are based on flying platforms such as aircraft or balloons, operating at altitudes of up to about 60,000 feet, while satellites use a wide range of orbits from low-Earth-orbit (LEO) systems such as Iridium at 300–800 km above the Earth to geo-stationary (GEO) satellites such as those used for TV broadcasting at 36,000 km above the Earth's surface. Systems of each type have their own particular characteristics, which will be discussed in the following sections.

A cell in the sky can provide excellent outdoor coverage. Owing to its elevated position, obstacles such as mountains or buildings tend not to get in the way, allowing line-of-sight propagation from many locations. Large cells can readily be provided, enabling coverage of both urban and rural areas. Because much of the propagation is line-of-sight, higher frequencies, such as those above 3 GHz, can be used. These are inappropriate for cellular communications because the decrease in reflection and refraction at higher frequencies prevents good coverage, but this is generally not a problem for cells in the sky. At higher frequencies spectrum is both less expensive and more plentiful. A good example of this is TV broadcasting. Terrestrial TV broadcasting uses frequencies in the UHF range – about 500–800 MHz.