Wednesday, November 5, 2014

Transponder Downlink dBW Contours

The forward channel is physically transmitted from the satellite transponder to the remote terminal.

The remote terminal receive figure of merit (G/T in dB/K) plays a role in the link budget, particularly if it varies as a function of steering angle.

The SFD that arrives upon the receive terminal aperture is proportional to the satellite transponder downlink power, which is a result of the inherent gain, amplification, and back-off settings in the satellite itself in combination with the signal received from the teleport.

The SFD is strongest at the "beam peak" and weakest at the "beam edge".

Generally a network is designed so that beam edge is 4-7 dB down from beam peak.

In combination, this means the PSD presented to the modem varies about 4-7 dB across a given transponder beam.  This is true in both wide beam and spot beam.

Spot beams are smaller than wide beams, hence a moving remote terminal would fly across the contours more quickly in a spot beam than in a wide beam.  This also leads to frequent spot-spot handoffs.

The limit in PSD or SFD is established in the beam peak, all other areas would be lesser.

I model Ku wide beam transponders varying from +50 dBW at beam peak down to +45 dBW at beam edge.

For Ku or Ka spot beams, I assume beam peak could range between +56 and +61 dBW, and between +51 and +56 dBW at beam edge.

Average performance in either a spot beam or a wide beam is a function of which contour you are operating.

It is pointless to argue whether on average spot beams or wide beams yield closest to beam peak, so I always look at any given scenario across multiple contours from beam peak to beam edge.

Peter Lemme

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