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Monday, January 5, 2015

Return Channel Link Budget



The return channel performance operates under a combination of limiting factors

  • Native Performance
    • EIRP available
    • Symbol rate (committed spectrum)
    • G/T of uplink satellite receiver
  • PSD Limits (co and cross polarization) along and normal to the GSO
    • Different limits, Ku-band and Ka-band


Uplink signal strength of a spot beam is not impacted by interference (by its very nature), unlink the forward channel downlink signal strength.

Ultimately, energy per bit is related to EIRP, in that it is largely independent of symbol rate or modulation.

The maximum data rate is scaled by the achievable EIRP of a given remote terminal.

Return channel spectral efficiency is scaled by both EIRP and symbol rate.

Spot beams naturally bring enhanced G/T compared to wide beams, as a result of their antenna design.  

From looking at FCC filings, I conclude the following G/T ranges:
  • wide beam (Ku band) uplink G/T from -2 dB/K beam edge up to +6 dB/K beam peak.
  • spot beam (Ku band) uplink G/T from +10 dB/K at beam edge to +16 dB/K at beam peak.
  • spot beam (Ka band) uplink G/T from (+8, +12, +18) dB/K at beam edge to (+13, +16, +22) dB/K at beam peak

The increase in uplink G/T scales the signal strength, yielding a substantial benefit in spectral efficiency and achievable data rates.

A regulated PSD emissions mask (as a function of antenna gain) as it overlays the GSO, and across the GSO, and in both co-polarization and cross-polarization measurements.  

Antenna patterns have unique characteristics that cannot be generically modeled. 

The aviation remote terminal boresite beamwidth is generally the limiting factor when accounting for spectral mask compliance.  Boresite beamwidth is scaled by the extent of the aperture along the the GSO and across the GSO.   

Skew angle describes the rotation of an aviation remote terminal where zero degrees aligns the most discriminating axis along the GSO and 90 degrees which aligns the least discriminating axis along the GSO.   

The allowable PSD (as per the spectral emissions mask) is a function of skew angle, where the greatest PSD is at zero degrees skew angle and the least PSD is at 90 degrees skew angle.