[Starlink] Starlink "beam spread"

Wed Aug 31 17:46:20 EDT 2022

I work on the assumption that Starlink satellites are, or at least will 
eventually be, processing IP packets. For inter-satellite routing it's 
more or less a must-have unless you have some other packet switching 
protocol layered in between.

On 1/09/2022 2:51 am, David Fernández via Starlink wrote:
> "DNS on Starlink satellites: Good idea, lightweight, and I'd suspect
> maybe already in operation?"
>
> Are the satellites processing IP packets? Are the ISLs even in
> operation? I have been told Starlink satellites are transparent.
>
>
> > Date: Thu, 1 Sep 2022 01:41:07 +1200
> > From: Ulrich Speidel <u.speidel at auckland.ac.nz>
> > To: David Lang <david at lang.hm>
> > Cc: Sebastian Moeller <moeller0 at gmx.de>, Ulrich Speidel via Starlink
> > <starlink at lists.bufferbloat.net>
> > Subject: Re: [Starlink] Starlink "beam spread"
> > Message-ID: <56e56b0f-07bd-fe0c-9434-2663ae9d4404 at auckland.ac.nz>
> > Content-Type: text/plain; charset=UTF-8; format=flowed
> >
> > Um, yes, but I think we're mixing a few things up here (trying to bundle
> > responses here, so that's not just to you, David).
> >
> > In lieu of a reliable Starlink link budget, I'm going by this one:
> >
> > 
> https://www.linkedin.com/pulse/quick-analysis-starlink-link-budget-potential-emf-david-witkowski/ 
> <https://www.linkedin.com/pulse/quick-analysis-starlink-link-budget-potential-emf-david-witkowski>
> >
> > Parameters here are a little outdated but the critical one is the EIRP
> > at the transmitter of up to ~97 dBm. Say we're looking at a 30 GHz Ka
> > band signal over a 600 km path, which is more reflective of the current
> > constellation. Then Friis propagation gives us a path loss of about 178
> > dB, and if we pretend for a moment that Dishy is actually a 60 cm
> > diameter parabolic dish, we're looking at around 45 dBi receive antenna
> > gain. Probably a little less as Dishy isn't actually a dish.
> >
> > Then that gives us 97 dBm - 178 dB + 45 dB = -36 dBm at the ground
> > receiver. Now I'm assuming here that this is for ALL user downlink beams
> > from the satellite combined. What we don't really know is how many
> > parallel signals a satellite multiplexes into these, but assuming at the
> > moment a receive frontend bandwidth of about 100 MHz, noise power at the
> > receiver should be around 38 pW or -74 dBm. That leaves Starlink around
> > 38 dB of SNR to play with. Shannon lets us send up to just over 1.25
> > Gb/s in that kind of channel, but then again that's just the Shannon
> > limit, and in practice, we'll be looking a a wee bit less.
> >
> > That SNR also gives us an indication as to the signal separation Dishy
> > needs to achieve from the beams from another satellite in order for that
> > other satellite to re-use the same frequency. Note that this is
> > significantly more than just the 3 dB that the 3 dB width of a beam
> > gives us. The 3 dB width is what is commonly quoted as "beam width", and
> > that's where you get those nice narrow angles. 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.
> >
> > But even if you worked with 1.2 degrees at a distance of 600 km and you
> > assumed that sort of beam width at the satellite, it still gives you an
> > >12 km radius on the ground within which you cannot reuse the downlink
> > frequency from the same satellite. That's orders of magnitude more than
> > the re-use spatial separation you can achieve in ground-based cellular
> > networks. Note that the 0.1 deg beam "precision" is irrelevant here -
> > that just tells me the increments in which they can point the beam, but
> > not how wide it is and how intensity falls off with angle, or how bad
> > the side lobes are.
> >
> > Whether you can re-use the same frequency from another satellite to the
> > same ground area is a good question. 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. 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. And
> > I haven't even looked at the numbers for Ku band here.
> >
> > CDNs & Co - are NOT just dumb economic optimisations to lower bit miles.
> > They actually improve performance, and significantly so. A lower RTT
> > between you and a server that you grab data from via TCP allows a much
> > faster opening of the congestion window. With initial TCP cwnd's being
> > typically 10 packets or around 15 kB of data, having a server within 10
> > ms of your client means that you've transferred 15 kB after 5 ms, 45 kB
> > after 10 ms, 105 kB after 15 ms, 225 kB after 20 ms, and 465 kB after 25
> > ms. Make your RTT 100 ms, and it takes half a second to get to your 465
> > kB. Having a CDN server in close topological proximity also generally
> > reduces the number of queues between you and the server at which packets
> > can die an untimely early death, and generally, by taking load off such
> > links, reduces the probability of this happening at a lot of queues.
> > Bottom line: Having a CDN keeps your users happier. Also, live streaming
> > and video conferencing aside, most video is not multicast or broadcast,
> > but unicast.
> >
> > DNS on Starlink satellites: Good idea, lightweight, and I'd suspect
> > maybe already in operation? It's low hanging fruit. CDNs on satellites:
> > In the day and age of SSDs, having capacity on the satellite shouldn't
> > really be an issue, although robustness may be. But heat in this sort of
> > storage gets generated mostly when data is written, so it's a function
> > of what percentage of your data that reaches the bird is going to end up
> > in cache. Generally, on a LEO satellite that'll have to cache baseball
> > videos while over the US, videos in a dozen different languages while
> > over Europe, Bollywood clips while over India, cooking shows while over
> > Australia and always the same old ads while over New Zealand, all the
> > while not getting a lot of cache hits for stuff it put into cache 15
> > minutes ago, would probably have to write a lot. Moreover, as you'd be
> > reliant on the content you want being on the satellite that you are
> > currently talking to, pretty much all satellites in the constellation
> > would need to cache all content. In other words: If I watch a cat video
> > now and thereby put it into the cache of the bird overhead, and then
> > send you an e-mail and you're in my neighbourhood and you watch it half
> > an hour later, my satellite would be on the other side of the world, and
> > you'd have to have it re-uploaded to the CDN on the bird that's flying
> > overhead our neighbourhood then. Not as efficient as a ground-based CDN
> > on our ground-based network that's fed via a satellite link.
> >
> > As long as Starlink is going to have in the order of hundreds of
> > thousands of direct users, that problem won't go away.
> >
> > On 31/08/2022 7:33 pm, David Lang wrote:
> >
> >> On Wed, 31 Aug 2022, Ulrich Speidel via Starlink wrote:
> >>
> >>> This combines with the uncomfortable truth that an RF "beam" from a
> >>> satellite isn't as selective as a laser beam, so the options for
> >>> frequency re-use from orbit aren't anywhere near as good as from a
> >>> mobile base station across the road: Any beam pointed at you can be
> >>> heard for many miles around and therefore no other user can re-use
> >>> that frequency (with the same burst slot etc.).
> >>
> >> not quite, you are forgetting that the antennas on the ground are also
> >> steerable arrays and so they can focus their 'receiving beam' at
> >> different satellites. This is less efficient than a transmitting beam
> >> as the satellites you aren't 'pointed' at will increase your noise
> >> floor, but it does allow the same frequency to be used for multiple
> >> satellites into the same area at the same time.
> >>
> >> David Lang
> >>
> > --
> > ****************************************************************
> > 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/ 
> <http://www.cs.auckland.ac.nz/~ulrich>
> > ****************************************************************
> >
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-- 
****************************************************************
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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