In satellite communication why the uplink frequency is greater than downlink frequency?

Answer 1

In satellite communications, the uplink frequency is of a higher value than the downlink mainly because of two reasons:

1. to mitigate the free space spreading losses, and the tropospheric losses (gases, clouds, rain), all of which are related to wavelength and therefore to frequency (wavelength=speed of light in vacuum/frequency). In a point to point scenario (fixed wireless links) this becomes irrelevant as the difference between the forward channel and the return channel is not very large. In this case, the assignment is a consequence of the coordination studies carried on in the framework of the ITU to minimize the impact one microwave service might have upon another (satellite on fixed terrestrial, WiMax on fixed satellite, fixed terrestrial on satellite, mobile telephony on satellite, etc).

2. Besides this transmission aspect, there is always the intra-system interference issue, which is the interference caused in your signals by other signals transmitted by your own system. in this case, to avoid interference from site A to Site B and viceversa, a frequency channel is assigned to Site A's transmissions and another to Site B's. in this case one of them is higher in frequency than the other.

I hope i am not confusing you more with this answer. In a nutshell,

a) one reason is interference, to avoid colliding as two cars might do if they travel in opposite sense on a highway

b) another is related to propagation of the signals, and to help the site which is more complicated to design and has more compromises of power/transmission power/heat etc (the satellite).

Regards,

Answer 2

Uplink freq is greater than the downlink frequency in satellite communication because the power requirement is less with higher freq than the low.And it is easier to fullfil higher power requirement at the earth station than at satellite.