Why is uplink fregency greater than the downlink fregency ?

Question

submit with mathematical evidance ??

Answer 1

No reason I know of. They can't be the same, so one has to be higher than the other.

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Answer 2

Carrier power on the earth uplink is greater than carrier power at the satellite downlink.  Carrier power on the satellite is limited by the power budget, dictated by the size and weight (for launch) of the solar panels.  Carrier power on the earth uplink transmitter is not limited by those constraints on the satellite and can be many times greater than that of the satellite downlink transmitter.

The baseband carrier signal-to-noise ratio is proportional to:

-10*log(Fs*K*Tr*B)

where:

Fs is the receiver noise figure

K is Boltzmanns constant

T is the receiver temperature

B is the bandwidth

So, as the bandwidth increases, or as the upper limit of the band increases, the signal-to-noise ratio drops.

The frequency of the carrier versus the free-space path loss (from satellite to earth) may be plotted as a linear function on a log-log scale.  The higher the frequency, the larger the path loss.

Since carrier power is limited in the satellite, it would not make sense to put it at a frequency that would incur more loss.  It makes sense to put it at the lowest possible frequency (within the required band), to minimize the signal-to-noise ratio at the receiver.

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