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On 4/06/2024 5:41 am, Mike Puchol wrote:<br>
<blockquote type="cite" cite="mid:f08df76c-f6ab-48ab-bcde-873554959ff1@Spark">
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<div dir="auto"><span style="font-family:Arial">While the
terminals use focused beams, Starlink does not operate in
isolation or using protected spectrum. They must comply with
EPFD limits, which are the reason why they just cannot land
two co-frequency beams that overlap, as they could be
causing harmful interference to other users of the same
spectrum.</span></div>
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</blockquote>
<p>Precisely.</p>
<p>Likewise, the famous "25 degrees over the horizon" isn't a nod to
"too many trees/houses around Dishy", but there to satisfy the
regulators that a transmitting beam from a satellite won't get
into (more or less horizontally pointing) terrestrial
communication systems (there's a softening of that requirement to
5 degrees minimum for beams to gateways at higher latitudes only).</p>
<p>It's also notable that this EPFD limit leads to a de-facto
standardisation of the downlink as such. In a "textbook" system,
you'd have some transmit power on the satellite, some antenna gain
up there, you'd then take into account the path loss from
spherical spreading of the signal on its way down, which gives you
a PFD at your receiver site that depends on the geometry of the
satellite-ground station arrangement. You'd then also look at your
receive antenna gain. That's determined by the antenna aperture,
which in the case of a phased array, is the collective area of all
antenna elements. And since you're steering your receive beam,
you'd be downrating the aperture by the cosine of your steering
angle. And then you'd end up with some signal to noise ratio at
the receiver which determines what modulation scheme you can use,
and that along with the bandwidth of the link determines your
capacity. But now you have a hard limit on your PFD imposed by the
regulatory EPFD limit. So what SpaceX do according to their FCC
filings is to reduce their EIRP at the satellite so the signal
stays under/at the EPFD limit on the Earth's surface, and they
adjust Dishy gain by using fewer of its elements the lower their
steering angle is. </p>
<p>Put in another way: If you could "see" Starlink satellite
signals, they would appear equally bright to you no matter where
in the sky they come from - you wouldn't be able to tell distance
from brightness. Similarly, you could think of Dishy as a pair of
sunglasses that are tinted most strongly in the middle but where
the tinting fades as you look sideways. Sounds complex but results
in SNR's that are a perfect fit for the modulation schemes (16QAM
for Gen 1, 64QAM / 256QAM for Gen 2), so saves on satellite and
Dishy having to negotiate modulation.</p>
<blockquote type="cite" cite="mid:f08df76c-f6ab-48ab-bcde-873554959ff1@Spark">
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Best,<br>
<br>
Mike</div>
<div name="messageReplySection">On Jun 3, 2024 at 09:43 -0700,
David Lang via Starlink <a class="moz-txt-link-rfc2396E" href="mailto:starlink@lists.bufferbloat.net"><starlink@lists.bufferbloat.net></a>,
wrote:<br>
<blockquote type="cite" style="border-left-color: grey; border-left-width: thin; border-left-style: solid; margin: 5px 5px;padding-left: 10px;">If
the ground stations were omnidirectional antennas, you would
be correct, but<br>
since they are phased array directional antennas, they can
steer the beam to<br>
receive one satellite even while a different one is
transmitting on the same<br>
frequency to the same cell.<br>
<br>
David Lang<br>
<br>
<br>
On Mon, 3 Jun 2024, Ulrich Speidel via Starlink wrote:<br>
<br>
<blockquote type="cite">Date: Mon, 3 Jun 2024 22:40:24 +1200<br>
From: Ulrich Speidel via Starlink
<a class="moz-txt-link-rfc2396E" href="mailto:starlink@lists.bufferbloat.net"><starlink@lists.bufferbloat.net></a><br>
Reply-To: Ulrich Speidel <a class="moz-txt-link-rfc2396E" href="mailto:u.speidel@auckland.ac.nz"><u.speidel@auckland.ac.nz></a><br>
To: <a class="moz-txt-link-abbreviated" href="mailto:starlink@lists.bufferbloat.net">starlink@lists.bufferbloat.net</a><br>
Subject: Re: [Starlink] musk: 28ms median latency on
starlink<br>
<br>
Getting the satellite density up will help, but it will only
improve things<br>
so far.<br>
<br>
The problem on user downlink in particular is that there's a
limit on the<br>
maximum spectral power flux density that arrives from the
satellite in space<br>
on the ground. If you point all (mutually compatible) user
downlink beams<br>
from a single satellite at a single cell, you all but reach
that limit there.<br>
In fact, where SpaceX want to use two beams on the same
frequency but with<br>
opposite polarisations to the same cell, they must reduce
the transmit power<br>
on each beam by 3 dB (50%) in order to stay within the
limit. More satellites<br>
would give you more beams, but you can't point them at cells
that already<br>
have a beam on the same frequency in use from another
satellite (unless you<br>
de-rate on the power front, I guess). That seriously limits
what you can<br>
receive in terms of total capacity within a single cell to
what a single<br>
satellite's mutually compatible beams can deliver, which
appears to be about<br>
12 Gb/s on V1 and V1.5 birds, and 20 Gb/s on V2 (on Ku, if
you add in Ka-band<br>
and anticipate Dishys that can do Ka, then it's a lot more
for Ka). In<br>
practice, we know that a cell gets served by beams from
different satellites,<br>
but the overall constraint still applies - if you deploy
beam X from sat A<br>
and beam Y from sat B to the same cell, this makes the same
contribution to<br>
PFD as deploying both from the same satellite. Note that
Starlink sats do<br>
have multiple mutually incompatible beams that they can only
point at<br>
different cells, bringing Ku user downlink capacity up to 16
Gb/s on V1 and<br>
1.5, and 48 Gb/s on V2. But that only ups your chances of
getting a larger<br>
slice of those 12 or 20 Gb/s in your cell.<br>
<br>
Your best bet for continuing good service at the moment is
literally to tell<br>
your neighbours that Starlink is useless, so they don't sign
up and you can<br>
have your cake all to yourself ;-)<br>
<br>
On 3/06/2024 5:13 am, Dave Taht via Starlink wrote:<br>
<blockquote type="cite">Via elon musk:<br>
<br>
Starlink just achieved a new internal median latency
record of 28ms<br>
yesterday! Great work by the engineering and operations
teams.<br>
<br>
- <a class="moz-txt-link-freetext" href="https://twitter.com/elonmusk/status/1797282250574184587">https://twitter.com/elonmusk/status/1797282250574184587</a><br>
<br>
I of course, am very interested in y'all´s external
measurements of how<br>
well starlink is doing. For me, it is fantastic - 30Mbit
uploads nowadays,<br>
0<br>
latency on the upload (how?)<br>
<a class="moz-txt-link-freetext" href="https://www.waveform.com/tools/bufferbloat?test-id=2a1d139b-87cb-4ba4-a829-e2167801cffe">https://www.waveform.com/tools/bufferbloat?test-id=2a1d139b-87cb-4ba4-a829-e2167801cffe</a><br>
<br>
I also keep hoping that the rest of the ISP industry is
now paying<br>
attention and deploying stuff like fq_codel and cake and
libreqos, but, ah<br>
well - I will settle for starlink blowing past a lot of
dsl and cable and<br>
finding ways to get their density up.<br>
<br>
Anyone going to the Starship launch on the 6th?<br>
<br>
<br>
<br>
--<br>
<a class="moz-txt-link-freetext" href="https://www.youtube.com/watch?v=BVFWSyMp3xg&t=1098s">https://www.youtube.com/watch?v=BVFWSyMp3xg&t=1098s</a><br>
<a class="moz-txt-link-rfc2396E" href="https://www.youtube.com/watch?v=BVFWSyMp3xg&t=1098s"><https://www.youtube.com/watch?v=BVFWSyMp3xg&t=1098s></a> Waves
Podcast<br>
Dave Täht CSO, LibreQos<br>
<br>
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</blockquote>
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</blockquote>
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</blockquote>
<pre class="moz-signature" cols="72">--
****************************************************************
Dr. Ulrich Speidel
School of Computer Science
Room 303S.594 (City Campus)
The University of Auckland
<a class="moz-txt-link-abbreviated" href="mailto:u.speidel@auckland.ac.nz">u.speidel@auckland.ac.nz</a>
<a class="moz-txt-link-freetext" href="http://www.cs.auckland.ac.nz/~ulrich/">http://www.cs.auckland.ac.nz/~ulrich/</a>
****************************************************************
</pre>
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