What are uplink and downlink?
Every satellite communication link has two directions. The uplink is the radio signal transmitted from Earth (a user terminal, gateway, or ground station) to the satellite — it 'goes up' to orbit. The downlink is the signal transmitted from the satellite toward Earth — it 'comes down' to users. These two directions always operate on different frequencies to prevent the powerful uplink transmitter from desensitising or saturating the satellite's sensitive downlink receiver, which is located on the same spacecraft.
Frequency separation
The frequency gap between uplink and downlink is called the frequency duplex separation. In Ku-band VSAT, the uplink is at 14.0–14.5 GHz and the downlink at 11.7–12.75 GHz — a separation of about 2.5 GHz. In Ka-band HTS, the uplink (from user terminal to satellite) is at 27.5–31 GHz, and the downlink is at 17.3–20.2 GHz. In reverse, the gateway-to-satellite feeder uplink uses 27.5–31 GHz and the satellite-to-gateway feeder downlink uses 17.3–20.2 GHz. The satellite's payload uses diplexers and band filters to separate these frequencies at the antenna ports.
Asymmetric capacity
Most satellite broadband networks are asymmetric: the downlink (forward link, from hub to user) typically carries much more capacity than the uplink (return link, from user to hub). This matches internet usage patterns — users download far more than they upload. VSAT systems implement this asymmetry at the technology level: the forward link uses DVB-S2X with large bandwidth allocations, while the return link uses TDMA or MF-TDMA with shared time-slot access that efficiently multiplexes many low-duty-cycle users onto limited return capacity.