why uplink frequency of sattelite is grater than downlink?

Answer 1

None of the above answers really understand the question except for tlbs101. In most Duplex communication links, there is an uplink (UL) and downlink (DL). This is only for FDD while TDD mode uses time slots for the different directions. So the questions is, why are satellite links assigned with UL at the higher band and DL at the lower band?

Power efficiency is important for satellite as power is limited in the bird. The higher the frequency, the less efficient the power amps are at transmission and also a slightly higher free-space-loss. UL is from ground to bird and DL is from bird to ground. Thus the bird should Tx at the lower frequency (DL at lower band) while the ground which is not limited by power can Tx at the higher frequency and also have a higher gain antenna. It is not much, but every bit counts for the bird.

In terrestrial links, they are reverse for cellular operation, i.e. UL is on the lower frequency side. The same reason hold as the handset Tx to base-station is the UL, while the DL is base to handset. Thus to give better performance to the handset the UL is in the lower band, thus the reverse of terrestrial links versus assignments for satellite links.

Answer 2

Radio frequency energy loss (propagation loss) is a function of frequency. The higher in frequency you go, the more the loss there is through the atmosphere. This is just a fact of physics. So, for a given band (i.e. the Ka band 19-22 GHz), since the satellite transmitter in space has to cover a wide physical area, and the signal will already be 'diluted' by a semi-directional antenna it makes sense to transmit at the lower frequencies in the band to maximize the amount of power available at your 'dish'.

The ground transmitter antenna, on the other hand, may be highly directional (as there is only one "point" in the sky where the satellite is geosynchronously 'parked'). For the same power level, the antenna can have much higher gain, so they can afford to 'waste' some power by transmitting at the highest frequencies in the band.

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Answer 3

It could be either greater or less - it should just be different in order to reduce interference between the 2, permitting simultaneous transmission and reception.

Answer 4

2 way radio communication needs 2 frequencies if it is to be full duplex i.e. transmit and receive simultaneously.

Answer 5

the satellite broadband uses your telephone line to transmit and your satellite dish to receive. the bandwidth of the satellite dish allows much faster information transfer than the bandwidth on the telephone line, so you receive information much faster than you transmit it.