Communication via satellite as unguided scheme

Satellite communication is an unguided (wireless) transmission scheme: microwave signals are sent through free space from an earth station to a satellite, which relays them to another earth station.

Design

 Source -> Modulator -> Up-converter -> HPA -> Uplink dish
                                                ~~~ 6 GHz uplink ~~~>
                                  Satellite transponder:
                      LNA -> Frequency translator -> TWT/power amp
                                                ~~~ 4 GHz downlink ~~~>
 Downlink dish -> LNB -> Down-converter -> Demodulator -> Destination

1. Orbit: geostationary (GEO, 35,786 km) for fixed coverage, LEO/MEO for low delay.
2. Frequency bands: C band (6 GHz up / 4 GHz down), Ku band (14/12 GHz), Ka band (30/20 GHz). The uplink uses a higher frequency than the downlink to reduce satellite power.
3. Earth station: modulator (QPSK/PSK), up-converter, high power amplifier and parabolic dish antenna; receive side with LNA/LNB, down-converter and demodulator.
4. Satellite transponder: receives, amplifies, shifts the frequency (to avoid interference) and retransmits.
5. Link budget: free-space path loss FSPL = 20 log d + 20 log f + 92.45 dB (d in km, f in GHz). For GEO at 4 GHz: 91.1 + 12.0 + 92.45 = 195.6 dB, so high-gain antennas and a low-noise receiver are needed.
6. Multiple access: FDMA, TDMA or CDMA to share the transponder.
7. Delay: one hop (up and down) is 2 x 119 ms = about 238 ms (round trip about 0.5 s), which must be considered for protocols.
8. Rain fading margins, error control coding and power supply (solar panels) complete the design.