[Starlink] Starlink "beam spread"

[Ulrich Speidel]

See below.

On 1/09/2022 2:30 am, Mike Puchol via Starlink wrote:
> A lot of detail on the RF side, and you raise some valid points! A few 
> clarifications:
>
>   * Ka band is used exclusively for gateway links, and both satellite
>     and gateway use parabollic antennas, where sidelobes etc. are
>     greatly reduced compared to an ESA.
>
Note I worked on the basis of parabolics, so that bit would be covered.
>
>   * Ku band is used exclusively for service links to terminals, and
>     from FCC filings, we know that given Nco = 1, the constellation
>     will not project two overlapping co-frequency beams. How much they
>     extend this overlap “safety zone” away from the 3dB contour is not
>     known, but could be calculated given enough information about the
>     terminal.
>
Indeed. I used Ka band instead of Ku for my calculations even for 
downlink to end user because the narrower beams in that case would work 
in Starlink's favour. Entirely hypothetical, of course.
>
> As per some specific points:
>
>     But that's just the width at which the beam drops to half its
>     EIRP, not the width at which it can no longer interfere. For that,
>     you need the 38 dB width - or thereabouts - if you can get it, and
>     this will be significantly more than the 1.2 degrees or so of 3dB
>     beam width.
>
>
> You are correct in that the interference will come from an extended 
> footprint, but how much the extended frequency affects the terminal is 
> also a function of receive antenna selectivity, angle of arrival, 
> receiver gain, etc. The Starlink terminal is not an omnidirectional 
> antenna in receive, it is also selective by forming a receive beam 
> with significantly more gain in a specific direction, thus increasing 
> the SNR of the wanted signal. It would be interesting to dig into this 
> one deeper, and see the effect on frequency re-use, that’s for sure!
>
>     That's orders of magnitude more than the re-use spatial separation
>     you can achieve in ground-based cellular networks
>
>
> You are comparing an infrastructure that has evenly distributed 
> “towers”, to a cellular network that can adjust density of towers by 
> reducing output power and placing more of them closer together, 
> forming smaller and smaller cells - which comes at a cost. I believe 
> it’s unfair to compare any satellite constellation to a cellular 
> network in these terms.
Indeed, it's grossly unfair. But such is life ;-)
>
>     We really don't know the beam patterns that we get from the birds
>     and from the Dishys, and without these it's difficult to say how
>     much angular separation a ground station needs between two
>     satellites using the same frequency in order to receive one but
>     not be interfered with by the other. 
>
>
> Oh but we do know the beam patterns - they are in the GXT files that 
> accompany the Schedule S in FCC filings. I took them and created this 
> view:
>
>
>
> The three colors are 2dB, interpolated 3dB, and 4dB contours. I use 
> these to calculate beam spread in the capacity simulation.

That actually underlines my point nicely. If even the 4 dB contour in 
E-W direction is already in the order of 50 km across, then how wide 
will the interference contour be? Way more than for a mobile network's 
nano cells.

>     Basically, there are just too many variables in this for me to be
>     overly optimistic that re-use by two different sources within a
>     Starlink cell is possible.
>
>
> We know from gRPC data from the terminal itself that there is a 
> primary beam, and a backup beam, and we know they come from different 
> satellites. Re-use with the same frequency is not possible, as they 
> would be violating Nco = 1, so that point is moot.
Good. We're on the same song sheet then.
>
> Best,
>
> Mike
-- 

****************************************************************
Dr. Ulrich Speidel

School of Computer Science

Room 303S.594 (City Campus)

The University of Auckland
u.speidel at auckland.ac.nz  
http://www.cs.auckland.ac.nz/~ulrich/
****************************************************************


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