Why in satellite communication uplink frequency is greater than downlink frequency?

Answer 1

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.