I just want to thank Dick Roy for backing up the arguments I've been making
about physical RF communications for many years, and clarifying terminology
here. I'm not the expert - Dick is an expert with real practical and
theoretical experience - but what I've found over the years is that many who
consider themselves "experts" say things that are actually nonsense about radio
systems.
It seems to me that Starlink is based on a propagation model that is quite
simplistic, and probably far enough from correct that what seems "obvious" will
turn out not to be true. That doesn't stop Musk and cronies from asserting
these things as absolute truths (backed by actual professors, especially
professors of Economics like Coase, but also CS professors, network protocol
experts, etc. who aren't physicists or practicing RF engineers).
The fact is that we don't really know how to build a scalable LEO system.
Models can be useful, but a model can be a trap that causes even engineers to
be cocky. Or as the saying goes, a Clear View doesn't mean a Short Distance.
If there are 40 satellites serving 10,000 ground terminals simultaneously,
exactly what is the propagation environment like? I can tell you one thing: if
the phased array is digitized at some sample rate and some equalization and
some quantization, the propagation REALLY matters in serving those 10,000
ground terminals scattered randomly on terrain that is not optically flat and
not fully absorbent.
So how will Starlink scale? I think we literally don't know. And the modeling
matters.
Recently a real propagation expert (Ted Rapaport and his students) did a study
of how well 70 GHz RF signals propagate in an urban environment - Brooklyn.
The standard model would say that coverage would be terrible! Why? Because
supposedly 70 GHz is like visible light - line of sight is required or nothing
works.
But in fact, Ted, whom I've known from being on the FCC Technological Advisory
Committee (TAC) together when it was actually populated with engineers and
scientists, not lobbyists, discovered that scattering and diffraction at 70 GHz
in an urban environment significantly expands coverage of a single transmitter.
Remarkably so. Enough that "cellular architecture" doesn't make sense in that
propagation environment.
So all the professional experts are starting from the wrong place, and amateurs
perhaps even more so.
I hope Starlink views itself as a "research project". I'm afraid it doesn't -
partly driven by Musk, but equally driven by the FCC itself, which demands that
before a system is deployed that the entire plan be shown to work (which would
require a "model" that is actually unknowable because something like this has
never been tried). This is a problem with today's regulation of spectrum -
experiments are barred, both by law, and by competitors who can claim your
system will destroy theirs and not work.
But it is also a problem when "fans" start setting expectations way too high.
Like claiming that Starlink will eliminate any need for fiber. We don't know
that at all!
On Tuesday, August 10, 2021 2:11pm, "Dick Roy" <dick...@alum.mit.edu> said:
To add a bit more, as is easily seen below, the amplitudes of each of the
transfer functions between the three transmit and three receive antennas are
extremely similar. This is to be expected, of course, since the “aperture” of
each array is very small compared to the distance between them. What is much
more interesting and revealing is the relative phases. Obviously this requires
coherent receivers, and ultimately if you want to control the spatial
distribution of power (aka SDMA (or MIMO in some circles) coherent
transmitters. It turns out that just knowing the amplitude of the transfer
functions is not really all that useful for anything other than detecting a
broken solder joint:^)))
Also, do not forget that depending how these experiments were conducted, the
estimates are either of the RF channel itself (aka path loss),or of the RF
channel in combination with the transfer functions of the transmitters and//or
receivers. What this means is the CALIBRATION is CRUCIAL! Those who do not
calibrate, are doomed to fail!!!! I suspect that it is in calibration where
the major difference in performance between vendors’’ products can be found
:^))))
It’s complicated …
From: Bob McMahon [mailto:bob.mcma...@broadcom.com]
Sent: Tuesday, August 10, 2021 10:07 AM
To: dick...@alum.mit.edu
Cc: Rodney W. Grimes; Cake List; Make-Wifi-fast;
starl...@lists.bufferbloat.net; codel; cerowrt-devel; bloat
Subject: Re: [Starlink] [Cake] [Make-wifi-fast] [Cerowrt-devel] Due Aug 2:
Internet Quality workshop CFP for the internet architecture board
The slides show that for WiFi every transmission produces a complex frequency
response, aka the h-matrix. This is valid for that one transmission only. The
slides show an amplitude plot for a 3 radio device hence the 9 elements per the
h-matrix. It's assumed that the WiFi STA/AP is stationary such that doppler
effects aren't a consideration. WiFi isn't a car trying to connect to a cell
tower. The plot doesn't show the phase effects but they are included as the
output of the channel estimate is a complex frequency response. Each RX
produces the h-matrix ahead of the MAC. These may not be symmetric in the real
world but that's ok as transmission and reception is one way only, i.e. the
treating them as repcripocol and the matrix as hollows symmetric isn't going to
be a "test blocker" as the goal is to be able to use software and programmable
devices to change them in near real time. The current approach used by many
using butler matrices to produce off-diagonal effects is woefully inadequate.
And we're paying about $2.5K per each butler.
Bob
On Tue, Aug 10, 2021 at 9:13 AM Dick Roy <[ dick...@alum.mit.edu ](
mailto:dick...@alum.mit.edu )> wrote:
Well, I hesitate to drag this out, however Maxwell's equations and the
invariance of the laws of physics ensure that all path loss matrices are
reciprocal. What that means is that at any for any given set of fixed
boundary conditions (nothing moving/changing!), the propagation loss between
any two points in the domain is the same in both directions. The
"multipathing" in one direction is the same in the other because the
two-parameter (angle1,angle2) scattering cross sections of all objects
(remember they are fixed here) are independent of the ordering of the
angles.
Very importantly, path loss is NOT the same as the link loss (aka link
budget) which involves tx power and rx noise figure (and in the case of
smart antennas, there is a link per spatial stream and how those links are
managed/controlled really matters, but let's just keep it simple for this
discussion) and these generally are different on both ends of a link for a
variety of reasons. The other very important issue is that of the
""measurement plane", or "where tx power and rx noise figure are being
measured/referenced to and how well the interface at that plane is
"matched". We generally assume that the matching is perfect, however it
never is. All of these effects contribute to the link loss which determines
the strength of the signal coming out of the receiver (not the receive
antenna, the receiver) for a given signal strength coming out of the
transmitter (not the transmit antenna, the tx output port).
In the real world, things change. Sources and sinks move as do many of the
objects around them. This creates a time-varying RF environment, and now
the path loss matrix is a function of time and a few others things, so it
matters WHEN something is transmitted, and WHEN it is received, and the two
WHEN's are generally separated by "the speed of light" which is a ft/ns
roughly. As important is the fact that it's no longer really a path loss
matrix containing a single scalar because among other things, the time
varying environment induces change in the transmitted waveform on its way to
the receiver most commonly referred to as the Doppler effect which means
there is a frequency translation/shift for each (multi-)path of which there
are in general an uncountably infinite number because this is a continuous
world in which we live (the space quantization experiment being conducted in
the central US aside:^)). As a consequence of these physical laws, the
entries in the path loss matrix become complex functions of a number of
variables including time. These functions are quite often characterized in
terms of Doppler and delay-spread, terms used to describe in just a few
parameters the amount of "distortion" a complex function causes.
Hope this helps ... probably a bit more than you really wanted to know as
queuing theorists, but ...
-----Original Message-----
From: Starlink [mailto:[ starlink-boun...@lists.bufferbloat.net ](
mailto:starlink-boun...@lists.bufferbloat.net )] On Behalf Of
Rodney W. Grimes
Sent: Tuesday, August 10, 2021 7:10 AM
To: Bob McMahon
Cc: Cake List; Make-Wifi-fast; [ starl...@lists.bufferbloat.net ](
mailto:starl...@lists.bufferbloat.net );
[ co...@lists.bufferbloat.net ]( mailto:co...@lists.bufferbloat.net );
cerowrt-devel; bloat
Subject: Re: [Starlink] [Cake] [Make-wifi-fast] [Cerowrt-devel] Due Aug 2:
Internet Quality workshop CFP for the internet architecture board
> The distance matrix defines signal attenuations/loss between pairs. It's
> straightforward to create a distance matrix that has hidden nodes because
> all "signal loss" between pairs is defined. Let's say a 120dB
attenuation
> path will cause a node to be hidden as an example.
>
> A B C D
> A - 35 120 65
> B - 65 65
> C - 65
> D -
>
> So in the above, AC are hidden from each other but nobody else is. It does
> assume symmetry between pairs but that's typically true.
That is not correct, symmetry in the RF world, especially wifi, is rare
due to topology issues. A high transmitter, A, and a low receiver, B,
has a good path A - > B, but a very weak path B -> A. Multipathing
is another major issue that causes assymtry.
>
> The RF device takes these distance matrices as settings and calculates the
> five branch tree values (as demonstrated in the video). There are
> limitations to solutions though but I've found those not to be an issue to
> date. I've been able to produce hidden nodes quite readily. Add the phase
> shifters and spatial stream powers can also be affected, but this isn't
> shown in this simple example.
>
> Bob
>
> On Mon, Aug 2, 2021 at 8:12 PM David Lang <[ da...@lang.hm ](
> mailto:da...@lang.hm )> wrote:
>
> > I guess it depends on what you are intending to test. If you are not
going
> > to
> > tinker with any of the over-the-air settings (including the number of
> > packets
> > transmitted in one aggregate), the details of what happen over the air
> > don't
> > matter much.
> >
> > But if you are going to be doing any tinkering with what is getting
sent,
> > and
> > you ignore the hidden transmitter type problems, you will create a
> > solution that
> > seems to work really well in the lab and falls on it's face out in the
> > wild
> > where spectrum overload and hidden transmitters are the norm (at least
in
> > urban
> > areas), not rare corner cases.
> >
> > you don't need to include them in every test, but you need to have a way
> > to
> > configure your lab to include them before you consider any
> > settings/algorithm
> > ready to try in the wild.
> >
> > David Lang
> >
> > On Mon, 2 Aug 2021, Bob McMahon wrote:
> >
> > > We find four nodes, a primary BSS and an adjunct one quite good for
lots
> > of
> > > testing. The six nodes allows for a primary BSS and two adjacent
ones.
> > We
> > > want to minimize complexity to necessary and sufficient.
> > >
> > > The challenge we find is having variability (e.g. montecarlos) that's
> > > reproducible and has relevant information. Basically, the distance
> > matrices
> > > have h-matrices as their elements. Our chips can provide these
> > h-matrices.
> > >
> > > The parts for solid state programmable attenuators and phase shifters
> > > aren't very expensive. A device that supports a five branch tree and
2x2
> > > MIMO seems a very good starting point.
> > >
> > > Bob
> > >
> > > On Mon, Aug 2, 2021 at 4:55 PM Ben Greear <[ gree...@candelatech.com ](
> > > mailto:gree...@candelatech.com )>
> > wrote:
> > >
> > >> On 8/2/21 4:16 PM, David Lang wrote:
> > >>> If you are going to setup a test environment for wifi, you need to
> > >> include the ability to make a fe cases that only happen with RF, not
> > with
> > >> wired networks and
> > >>> are commonly overlooked
> > >>>
> > >>> 1. station A can hear station B and C but they cannot hear each
other
> > >>> 2. station A can hear station B but station B cannot hear station A
3.
> > >> station A can hear that station B is transmitting, but not with a
strong
> > >> enough signal to
> > >>> decode the signal (yes in theory you can work around interference,
but
> > >> in practice interference is still a real thing)
> > >>>
> > >>> David Lang
> > >>>
> > >>
> > >> To add to this, I think you need lots of different station devices,
> > >> different capabilities (/n, /ac, /ax, etc)
> > >> different numbers of spatial streams, and different distances from
the
> > >> AP. From download queueing perspective, changing
> > >> the capabilities may be sufficient while keeping all stations at same
> > >> distance. This assumes you are not
> > >> actually testing the wifi rate-ctrl alg. itself, so different
throughput
> > >> levels for different stations would be enough.
> > >>
> > >> So, a good station emulator setup (and/or pile of real stations) and
a
> > few
> > >> RF chambers and
> > >> programmable attenuators and you can test that setup...
> > >>
> > >> From upload perspective, I guess same setup would do the job.
> > >> Queuing/fairness might depend a bit more on the
> > >> station devices, emulated or otherwise, but I guess a clever AP could
> > >> enforce fairness in upstream direction
> > >> too by implementing per-sta queues.
> > >>
> > >> Thanks,
> > >> Ben
> > >>
> > >> --
> > >> Ben Greear <[ gree...@candelatech.com ]( mailto:gree...@candelatech.com
> > >> )>
> > >> Candela Technologies Inc [ http://www.candelatech.com ](
> > >> http://www.candelatech.com )
> > >>
> > >
> > >
> >
>
> --
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