Forward Channel Downlink Regulations

Ku band Downlink Regulations

47 CFR 25.134
Licensing provisions for Very Small Aperture Terminal (VSAT) and C-band Small Aperture Terminal (CSAT) networks.

(g) Applications for VSAT operation in the 12/14 GHz bands that meet the following requirements will be routinely processed:

(3) The maximum GSO FSS satellite EIRP spectral density of the digital modulated emission does not exceed 10 dB(W/4kHz) for all methods of modulation and accessing techniques.

Here is an FCC order reflecting on a ViaSat application for airborne use as being routinely approved as long as to not exceed 10 dBW/4 kHz.

It is possible to coordinate a higher downlink PSD with the neighboring satellites such that the FCC will approve on the basis of the letter of coordination.

Here is an example of Row 44 request for downlink PSD as high as 13 dBW/4 kHz.

I would normally assume a limit of 10 dBW/4 kHz on any forward channel downlink, which is achieved at the beam peak of the particular transponder.  
A common downlink PSD of 10 dBW/4 kHz presents the highest spectral efficiency regardless of committed spectrum or dBW, wide beam or spot beam.  

The downlink PSD scales with the transponder downlink EIRP contours, e.g., along a contour five dB down from beam center, in an optimal case, PSD would be 5 dBW/4 kHz.

Ka band Downlink Regulations

47 CFR 25.138

Blanket Licensing provisions of GSO FSS Earth Stations in the 18.3-18.8 GHz (space-to-Earth), 19.7-20.2 GHz (space-to-Earth), 28.35-28.6 GHz (Earth-to-space), and 29.25-30.0 GHz (Earth-to-space) bands.

Power flux-density (PFD) at the Earth's surface produced by emissions from a space station for all conditions, including clear sky, and for all methods of modulation shall not exceed a level of −118 dBW/m2/MHz, in addition to the limits specified in § 25.208 (d).

I have found examples of service providers using PFD max between -128 and -121 (dBW/MHz/m2).

Downlink Ku-band Limits Versus Ka-band Limits

PFD or spectral flux density (SFD) is one measure that can be shown to trend with spectral efficiency.

A Ku-band forward channel operating at the limit PFD as a Ka-band forward channel would operate about 10 dB higher PSD than normally permitted in Ku band.

Here are the three examples, using 30 MHz of spectrum (arbitrary), with EIRP adjusted to meet either the Ku-band PSD limit, the -121 dBW/MHz/m2, and the Ka-band SFD limit.

Operating at a maximum of -121 dBW/MHz/m2 PFD gives about a 7.5 dB advantage over equivalent Ku-band operating at 10 dBW/4 kHz PSD.

Peter Lemme
peter@satcom.guru

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