It is possible to use analog modulation or digital modulation for satellite communication. Earlier the analog modulation was being used. Frequency modulation technique was the obvious choice for its low power requirement and better noise immunity. If analog modulation is being used then the FDMA technique is used.
The total available bandwidth of a frequency band is 500 mhz. It is divided into 12 frequency slots are allotted to different earth stations. If a particular station is not operating at a given instant of time then its slot remains idle. Here different earth station can use their allotted slot for all the 24 hours a day. No synchronization is required.
All the earth stations are allowed to use the satellite all the time. The guard bands are provided in order to avoid the interference between the adjacent channels.
The earth stations will transmit at uplink frequencies and the satellite will convert them and retransmit on down link frequencies.