What is the RF design of a high altitude platform station for providing cellular coverage from the stratosphere?

The RF design of a high altitude platform station (HAPS) for providing cellular coverage from the stratosphere operates an airborne base station at 18-22 km altitude (in the stratosphere, above weather and commercial aircraft) to provide cellular-like coverage over a ground area of 50-200 km diameter. The RF design addresses: link budget (the HAPS-to-ground path loss at 2 GHz over 20 km is approximately 120 dB (much less than the 200+ dB for a GEO satellite); this favorable path loss allows: use of standard cellular frequency bands (700 MHz, 1.8 GHz, 2.1 GHz, mmW), standard mobile phone transmit power (23 dBm) to achieve adequate uplink SNR, and data rates comparable to terrestrial macro cells (10-100 Mbps per user)), antenna system (the HAPS carries a large multi-beam antenna array pointing downward; it creates 100-1000+ spot beams covering the ground service area; each beam serves as a cell sector; the antenna provides 20-35 dBi gain per beam; antenna type: active phased array or multibeam reflector antenna), frequency reuse (the spot beams enable frequency reuse similar to terrestrial cellular: 3-7 color reuse patterns across the beams, multiplying the system capacity), and interference management (the HAPS must not interfere with terrestrial cellular networks operating on the same frequencies; 3GPP and ITU define: power flux density limits at the ground to protect terrestrial systems, coordination zones where the HAPS must reduce power or avoid certain frequencies, and beam shaping to minimize sidelobe radiation toward terrestrial base stations)).