From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp85.iad3a.emailsrvr.com (smtp85.iad3a.emailsrvr.com [173.203.187.85]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by lists.bufferbloat.net (Postfix) with ESMTPS id 8B7453CB39 for ; Thu, 2 Sep 2021 13:36:34 -0400 (EDT) Received: from app7.wa-webapps.iad3a (relay-webapps.rsapps.net [172.27.255.140]) by smtp35.relay.iad3a.emailsrvr.com (SMTP Server) with ESMTP id 89514435D; Thu, 2 Sep 2021 13:36:33 -0400 (EDT) Received: from deepplum.com (localhost.localdomain [127.0.0.1]) by app7.wa-webapps.iad3a (Postfix) with ESMTP id 74ED4600C5; Thu, 2 Sep 2021 13:36:33 -0400 (EDT) Received: by apps.rackspace.com (Authenticated sender: dpreed@deepplum.com, from: dpreed@deepplum.com) with HTTP; Thu, 2 Sep 2021 13:36:33 -0400 (EDT) X-Auth-ID: dpreed@deepplum.com Date: Thu, 2 Sep 2021 13:36:33 -0400 (EDT) From: "David P. Reed" To: dickroy@alum.mit.edu Cc: "'Bob McMahon'" , starlink@lists.bufferbloat.net, "'Make-Wifi-fast'" , "'Cake List'" , "'codel'" , "'cerowrt-devel'" , "'bloat'" , "'Rodney W. Grimes'" MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_20210902133633000000_50854" Importance: Normal X-Priority: 3 (Normal) X-Type: html In-Reply-To: <03CA2CDA3EC5415DA229F835BE039994@SRA6> References: <202108101410.17AEAR4w075939@gndrsh.dnsmgr.net> <5AF5551E2A7041168E7071FDA0F6B8EC@SRA6> <03CA2CDA3EC5415DA229F835BE039994@SRA6> X-Client-IP: 209.6.168.128 Message-ID: <1630604193.476312238@apps.rackspace.com> X-Mailer: webmail/19.0.11-RC X-Classification-ID: 7be18fdb-24bb-4bd5-9d11-d79055ea61af-1-1 Subject: Re: [Cerowrt-devel] [Cake] [Starlink] [Make-wifi-fast] Due Aug 2: Internet Quality workshop CFP for the internet architecture board X-BeenThere: cerowrt-devel@lists.bufferbloat.net X-Mailman-Version: 2.1.20 Precedence: list List-Id: Development issues regarding the cerowrt test router project List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 02 Sep 2021 17:36:34 -0000 ------=_20210902133633000000_50854 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable =0AI just want to thank Dick Roy for backing up the arguments I've been mak= ing about physical RF communications for many years, and clarifying termino= logy here. I'm not the expert - Dick is an expert with real practical and t= heoretical 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.=0A =0AIt 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 a= nd cronies from asserting these things as absolute truths (backed by actual= professors, especially professors of Economics like Coase, but also CS pro= fessors, network protocol experts, etc. who aren't physicists or practicing= RF engineers).=0A =0AThe 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 c= auses even engineers to be cocky. Or as the saying goes, a Clear View doesn= 't mean a Short Distance.=0A =0AIf there are 40 satellites serving 10,000 g= round terminals simultaneously, exactly what is the propagation environment= like? I can tell you one thing: if the phased array is digitized at some s= ample rate and some equalization and some quantization, the propagation REA= LLY matters in serving those 10,000 ground terminals scattered randomly on = terrain that is not optically flat and not fully absorbent.=0A =0ASo how wi= ll Starlink scale? I think we literally don't know. And the modeling matter= s.=0A =0ARecently a real propagation expert (Ted Rapaport and his students)= did a study of how well 70 GHz RF signals propagate in an urban environmen= t - Brooklyn. The standard model would say that coverage would be terrible= ! Why? Because supposedly 70 GHz is like visible light - line of sight is r= equired or nothing works.=0A =0ABut in fact, Ted, whom I've known from bein= g on the FCC Technological Advisory Committee (TAC) together when it was ac= tually populated with engineers and scientists, not lobbyists, discovered t= hat scattering and diffraction at 70 GHz in an urban environment significan= tly expands coverage of a single transmitter. Remarkably so. Enough that "c= ellular architecture" doesn't make sense in that propagation environment.= =0A =0ASo all the professional experts are starting from the wrong place, a= nd amateurs perhaps even more so.=0A =0AI hope Starlink views itself as a "= research project". I'm afraid it doesn't - partly driven by Musk, but equal= ly 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). T= his is a problem with today's regulation of spectrum - experiments are barr= ed, both by law, and by competitors who can claim your system will destroy = theirs and not work.=0A =0ABut it is also a problem when "fans" start setti= ng expectations way too high. Like claiming that Starlink will eliminate an= y need for fiber. We don't know that at all!=0A =0A =0A =0A =0A =0A =0A =0A= On Tuesday, August 10, 2021 2:11pm, "Dick Roy" said:= =0A=0A=0A=0A=0ATo add a bit more, as is easily seen below, the amplitudes o= f each of the transfer functions between the three transmit and three recei= ve antennas are extremely similar. This is to be expected, of course, sinc= e the =E2=80=9Caperture=E2=80=9D of each array is very small compared to th= e distance between them. What is much more interesting and revealing is th= e relative phases. Obviously this requires coherent receivers, and ultimat= ely 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:^)))=0A =0AAlso, 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 receiv= ers. What this means is the CALIBRATION is CRUCIAL! Those who do not cali= brate, are doomed to fail!!!! I suspect that it is in calibration where t= he major difference in performance between vendors=E2=80=99=E2=80=99 produc= ts can be found :^))))=0A =0AIt=E2=80=99s complicated =E2=80=A6 =0A =0A=0A= =0AFrom: Bob McMahon [mailto:bob.mcmahon@broadcom.com] =0ASent: Tuesday, Au= gust 10, 2021 10:07 AM=0ATo: dickroy@alum.mit.edu=0ACc: Rodney W. Grimes; C= ake List; Make-Wifi-fast; starlink@lists.bufferbloat.net; codel; cerowrt-de= vel; bloat=0ASubject: Re: [Starlink] [Cake] [Make-wifi-fast] [Cerowrt-devel= ] Due Aug 2: Internet Quality workshop CFP for the internet architecture bo= ard=0A =0A=0AThe slides show that for WiFi every transmission produces a co= mplex frequency response, aka the h-matrix. This is valid for that one tran= smission only. The slides show an amplitude plot for a 3 radio device henc= e the 9 elements per the h-matrix. It's assumed that the WiFi STA/AP is sta= tionary 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 f= requency response. Each RX produces the h-matrix ahead of the MAC. These ma= y not be symmetric in the real world but that's ok as transmission and rece= ption 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 rea= l 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.=0A=0ABob=0A =0A=0A=0AOn Tue, Aug 10, 2021 at 9:13 AM Dick = Roy <[ dickroy@alum.mit.edu ]( mailto:dickroy@alum.mit.edu )> wrote:=0AWell= , I hesitate to drag this out, however Maxwell's equations and the=0A invar= iance of the laws of physics ensure that all path loss matrices are=0A reci= procal. What that means is that at any for any given set of fixed=0A bound= ary conditions (nothing moving/changing!), the propagation loss between=0A = any two points in the domain is the same in both directions. The=0A "multip= athing" in one direction is the same in the other because the=0A two-parame= ter (angle1,angle2) scattering cross sections of all objects=0A (remember t= hey are fixed here) are independent of the ordering of the=0A angles. =0A= =0A Very importantly, path loss is NOT the same as the link loss (aka link= =0A budget) which involves tx power and rx noise figure (and in the case of= =0A smart antennas, there is a link per spatial stream and how those links = are=0A managed/controlled really matters, but let's just keep it simple for= this=0A discussion) and these generally are different on both ends of a li= nk for a=0A variety of reasons. The other very important issue is that of t= he=0A ""measurement plane", or "where tx power and rx noise figure are bein= g=0A measured/referenced to and how well the interface at that plane is=0A = "matched". We generally assume that the matching is perfect, however it=0A= never is. All of these effects contribute to the link loss which determine= s=0A the strength of the signal coming out of the receiver (not the receive= =0A antenna, the receiver) for a given signal strength coming out of the=0A= transmitter (not the transmit antenna, the tx output port). =0A=0A In th= e real world, things change. Sources and sinks move as do many of the=0A o= bjects around them. This creates a time-varying RF environment, and now=0A= the path loss matrix is a function of time and a few others things, so it= =0A matters WHEN something is transmitted, and WHEN it is received, and the= two=0A WHEN's are generally separated by "the speed of light" which is a f= t/ns=0A roughly. As important is the fact that it's no longer really a path= loss=0A matrix containing a single scalar because among other things, the = time=0A varying environment induces change in the transmitted waveform on i= ts way to=0A the receiver most commonly referred to as the Doppler effect w= hich means=0A there is a frequency translation/shift for each (multi-)path = of which there=0A are in general an uncountably infinite number because thi= s is a continuous=0A world in which we live (the space quantization experim= ent being conducted in=0A the central US aside:^)). As a consequence of the= se physical laws, the=0A entries in the path loss matrix become complex fun= ctions of a number of=0A variables including time. These functions are quit= e often characterized in=0A terms of Doppler and delay-spread, terms used t= o describe in just a few=0A parameters the amount of "distortion" a complex= function causes. =0A=0A Hope this helps ... probably a bit more than you r= eally wanted to know as=0A queuing theorists, but ...=0A=0A -----Original M= essage-----=0A From: Starlink [mailto:[ starlink-bounces@lists.bufferbloat.= net ]( mailto:starlink-bounces@lists.bufferbloat.net )] On Behalf Of=0A Rod= ney W. Grimes=0A Sent: Tuesday, August 10, 2021 7:10 AM=0A To: Bob McMahon= =0A Cc: Cake List; Make-Wifi-fast; [ starlink@lists.bufferbloat.net ]( mail= to:starlink@lists.bufferbloat.net );=0A[ codel@lists.bufferbloat.net ]( mai= lto:codel@lists.bufferbloat.net ); cerowrt-devel; bloat=0A Subject: Re: [St= arlink] [Cake] [Make-wifi-fast] [Cerowrt-devel] Due Aug 2:=0A Internet Qual= ity workshop CFP for the internet architecture board=0A=0A > The distance m= atrix defines signal attenuations/loss between pairs. It's=0A > straightfo= rward to create a distance matrix that has hidden nodes because=0A > all "s= ignal loss" between pairs is defined. Let's say a 120dB=0A attenuation=0A= > path will cause a node to be hidden as an example.=0A > =0A > A = B C D=0A > A - 35 120 65=0A > B - 65 65=0A > = C - 65=0A > D -=0A > =0A > So i= n the above, AC are hidden from each other but nobody else is. It does=0A >= assume symmetry between pairs but that's typically true.=0A=0A That is not= correct, symmetry in the RF world, especially wifi, is rare=0A due to topo= logy issues. A high transmitter, A, and a low receiver, B,=0A has a good = path A - > B, but a very weak path B -> A. Multipathing=0A is another maj= or issue that causes assymtry.=0A=0A > =0A > The RF device takes these dist= ance matrices as settings and calculates the=0A > five branch tree values (= as demonstrated in the video). There are=0A > limitations to solutions thou= gh but I've found those not to be an issue to=0A > date. I've been able to = produce hidden nodes quite readily. Add the phase=0A > shifters and spatial= stream powers can also be affected, but this isn't=0A > shown in this simp= le example.=0A > =0A > Bob=0A > =0A > On Mon, Aug 2, 2021 at 8:12 PM David = Lang <[ david@lang.hm ]( mailto:david@lang.hm )> wrote:=0A > =0A > > I gues= s it depends on what you are intending to test. If you are not=0A going=0A = > > to=0A > > tinker with any of the over-the-air settings (including the n= umber of=0A > > packets=0A > > transmitted in one aggregate), the details o= f what happen over the air=0A > > don't=0A > > matter much.=0A > >=0A > > B= ut if you are going to be doing any tinkering with what is getting=0A sent,= =0A > > and=0A > > you ignore the hidden transmitter type problems, you wil= l create a=0A > > solution that=0A > > seems to work really well in the lab= and falls on it's face out in the=0A > > wild=0A > > where spectrum overlo= ad and hidden transmitters are the norm (at least=0A in=0A > > urban=0A > >= areas), not rare corner cases.=0A > >=0A > > you don't need to include the= m in every test, but you need to have a way=0A > > to=0A > > configure your= lab to include them before you consider any=0A > > settings/algorithm=0A >= > ready to try in the wild.=0A > >=0A > > David Lang=0A > >=0A > > On Mon,= 2 Aug 2021, Bob McMahon wrote:=0A > >=0A > > > We find four nodes, a prima= ry BSS and an adjunct one quite good for=0A lots=0A > > of=0A > > > testing= . The six nodes allows for a primary BSS and two adjacent=0A ones.=0A > > = We=0A > > > want to minimize complexity to necessary and sufficient.=0A > >= >=0A > > > The challenge we find is having variability (e.g. montecarlos) = that's=0A > > > reproducible and has relevant information. Basically, the d= istance=0A > > matrices=0A > > > have h-matrices as their elements. Our chi= ps can provide these=0A > > h-matrices.=0A > > >=0A > > > The parts for sol= id state programmable attenuators and phase shifters=0A > > > aren't very e= xpensive. A device that supports a five branch tree and=0A 2x2=0A > > > MIM= O seems a very good starting point.=0A > > >=0A > > > Bob=0A > > >=0A > > >= On Mon, Aug 2, 2021 at 4:55 PM Ben Greear <[ greearb@candelatech.com ]( ma= ilto:greearb@candelatech.com )>=0A > > wrote:=0A > > >=0A > > >> On 8/2/21 = 4:16 PM, David Lang wrote:=0A > > >>> If you are going to setup a test envi= ronment for wifi, you need to=0A > > >> include the ability to make a fe ca= ses that only happen with RF, not=0A > > with=0A > > >> wired networks and= =0A > > >>> are commonly overlooked=0A > > >>>=0A > > >>> 1. station A can = hear station B and C but they cannot hear each=0A other=0A > > >>> 2. stati= on A can hear station B but station B cannot hear station A=0A 3.=0A > > >>= station A can hear that station B is transmitting, but not with a=0A stron= g=0A > > >> enough signal to=0A > > >>> decode the signal (yes in theory yo= u can work around interference,=0A but=0A > > >> in practice interference i= s still a real thing)=0A > > >>>=0A > > >>> David Lang=0A > > >>>=0A > > >>= =0A > > >> To add to this, I think you need lots of different station devic= es,=0A > > >> different capabilities (/n, /ac, /ax, etc)=0A > > >> differen= t numbers of spatial streams, and different distances from=0A the=0A > > >>= AP. From download queueing perspective, changing=0A > > >> the capabiliti= es may be sufficient while keeping all stations at same=0A > > >> distance.= This assumes you are not=0A > > >> actually testing the wifi rate-ctrl al= g. itself, so different=0A throughput=0A > > >> levels for different statio= ns would be enough.=0A > > >>=0A > > >> So, a good station emulator setup (= and/or pile of real stations) and=0A a=0A > > few=0A > > >> RF chambers and= =0A > > >> programmable attenuators and you can test that setup...=0A > > >= >=0A > > >> From upload perspective, I guess same setup would do the job.= =0A > > >> Queuing/fairness might depend a bit more on the=0A > > >> statio= n devices, emulated or otherwise, but I guess a clever AP could=0A > > >> e= nforce fairness in upstream direction=0A > > >> too by implementing per-sta= queues.=0A > > >>=0A > > >> Thanks,=0A > > >> Ben=0A > > >>=0A > > >> --= =0A > > >> Ben Greear <[ greearb@candelatech.com ]( mailto:greearb@candelat= ech.com )>=0A > > >> Candela Technologies Inc [ http://www.candelatech.com= ]( http://www.candelatech.com )=0A > > >>=0A > > >=0A > > >=0A > >=0A > = =0A > -- =0A > This electronic communication and the information and any fi= les=0A transmitted =0A > with it, or attached to it, are confidential and a= re intended solely for =0A > the use of the individual or entity to whom it= is addressed and may=0A contain =0A > information that is confidential, le= gally privileged, protected by privacy=0A=0A > laws, or otherwise restricte= d from disclosure to anyone else. If you are =0A > not the intended recipie= nt or the person responsible for delivering the =0A > e-mail to the intende= d recipient, you are hereby notified that any use, =0A > copying, distribut= ing, dissemination, forwarding, printing, or copying of =0A > this e-mail i= s strictly prohibited. If you received this e-mail in error, =0A > please r= eturn the e-mail to the sender, delete it from your computer, and =0A > des= troy any printed copy of it.=0A=0A [ Charset UTF-8 unsupported, converting.= .. ]=0A > _______________________________________________=0A > Starlink mai= ling list=0A > [ Starlink@lists.bufferbloat.net ]( mailto:Starlink@lists.bu= fferbloat.net )=0A > [ https://lists.bufferbloat.net/listinfo/starlink ]( h= ttps://lists.bufferbloat.net/listinfo/starlink )=0A > =0A _________________= ______________________________=0A Starlink mailing list=0A[ Starlink@lists.= bufferbloat.net ]( mailto:Starlink@lists.bufferbloat.net )=0A[ https://list= s.bufferbloat.net/listinfo/starlink ]( https://lists.bufferbloat.net/listin= fo/starlink )=0A=0AThis electronic communication and the information and an= y files transmitted with it, or attached to it, are confidential and are in= tended solely for the use of the individual or entity to whom it is address= ed and may contain information that is confidential, legally privileged, pr= otected by privacy laws, or otherwise restricted from disclosure to anyone = else. If you are not the intended recipient or the person responsible for d= elivering the e-mail to the intended recipient, you are hereby notified tha= t any use, copying, distributing, dissemination, forwarding, printing, or c= opying of this e-mail is strictly prohibited. If you received this e-mail i= n error, please return the e-mail to the sender, delete it from your comput= er, and destroy any printed copy of it. ------=_20210902133633000000_50854 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable

I just want to thank D= ick Roy for backing up the arguments I've been making about physical RF com= munications for many years, and clarifying terminology here. I'm not the ex= pert - Dick is an expert with real practical and theoretical experience - b= ut what I've found over the years is that many who consider themselves "exp= erts" say things that are actually nonsense about radio systems.

=0A

 

=0A

It seems to me that Star= link 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 abs= olute truths (backed by actual professors, especially professors of Economi= cs like Coase, but also CS professors, network protocol experts, etc. who a= ren't physicists or practicing RF engineers).

=0A

&n= bsp;

=0A

The fact is that we don't really know how t= o build a scalable LEO system. Models can be useful, but a model can be a t= rap that causes even engineers to be cocky. Or as the saying goes, a Clear = View doesn't mean a Short Distance.

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= =0A

If there are 40 satellites serving 10,000 ground te= rminals simultaneously, exactly what is the propagation environment like? I= can tell you one thing: if the phased array is digitized at some sample ra= te and some equalization and some quantization, the propagation REALLY matt= ers in serving those 10,000 ground terminals scattered randomly on terrain = that is not optically flat and not fully absorbent.

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=0A

So how will Starlink scale? I think w= e literally don't know. And the modeling matters.

=0A

Recently a real propagation expert (Ted= Rapaport and his students) did a study of how well 70 GHz RF signals propa= gate in an urban environment - Brooklyn.  The standard model would say= that coverage would be terrible! Why? Because supposedly 70 GHz is like vi= sible light - line of sight is required or nothing works.

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=0A

But in fact, Ted, whom I've kno= wn from being on the FCC Technological Advisory Committee (TAC) together wh= en it was actually populated with engineers and scientists, not lobbyists, = discovered that scattering and diffraction at 70 GHz in an urban environmen= t significantly expands coverage of a single transmitter. Remarkably so. En= ough that "cellular architecture" doesn't make sense in that propagation en= vironment.

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So = all the professional experts are starting from the wrong place, and amateur= s perhaps even more so.

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=0A

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, w= hich 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 compet= itors who can claim your system will destroy theirs and not work.

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=0A

But it is also a proble= m when "fans" start setting expectations way too high. Like claiming that S= tarlink will eliminate any need for fiber. We don't know that at all!

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On Tuesday, August 10, 2021 2:11pm, "D= ick Roy" <dickroy@alum.mit.edu> said:

=0A=0A
=0A
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=0A

Also, do not forget that depending how these experiments were con= ducted, the estimates are either of the RF channel itself (aka path loss),o= r of the RF channel in combination with the transfer functions of the trans= mitters and//or receivers.  What this means is the CALIBRATION is CRUC= IAL!  Those who do not calibrate, are doomed to fail!!!!   I= suspect that it is in calibration where the major difference in performanc= e between vendors=E2=80=99=E2=80=99 products can be found :^))))

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It=E2=80=99s complicated =E2=80=A6

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From: Bob McMahon [mailto:bob.mcmahon@bro= adcom.com]
Sent: Tuesday, August 10, 2021 10:07 AM
To: dickroy@alum.mit.edu
Cc: Rodney W. Grimes; Cake List= ; Make-Wifi-fast; starlink@lists.bufferbloat.net; codel; cerowrt-devel; blo= at
Subject:= Re: [Starlink] [Cake] [Make-wifi-fast] [Cerowrt-devel] Due Aug 2: Internet= Quality workshop CFP for the internet architecture board

= =0A
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The slid= es show that for WiFi every transmission produces a complex frequency = response, aka the h-matrix. This is valid for that one transmission only.&n= bsp; The slides show an amplitude plot for a 3 radio device hence the 9 ele= ments per the h-matrix. It's assumed that the WiFi STA/AP is stationary suc= h that doppler effects aren't a consideration. WiFi isn't a car trying to c= onnect to a cell tower.  The plot doesn't show the phase effects but t= hey are included as the output of the channel estimate is a complex frequen= cy 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 recep= tion 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 b= e 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 o= ff-diagonal effects  is woefully inadequate. And we're paying abo= ut $2.5K per each butler.
3D""
Bob

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On Tue, Aug 10, 2021 = at 9:13 AM Dick Roy <dickroy@alu= m.mit.edu> wrote:

=0A
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=0A

Well= , I hesitate to drag this out, however Maxwell's equations and the
in= variance 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 fix= ed
boundary conditions (nothing moving/changing!), the propagation lo= ss between
any two points in the domain is the same in both direction= s. The
"multipathing" in one direction is the same in the other becau= se the
two-parameter (angle1,angle2) scattering cross sections of all= objects
(remember they are fixed here) are independent of the orderi= ng of the
angles. 

Very importantly, path loss is N= OT the same as the link loss (aka link
budget) which involves tx powe= r 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 re= ally 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/re= ferenced to and how well the interface at that plane is
"matched".&nb= sp; We generally assume that the matching is perfect, however it
neve= r 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).  &nbs= p;

In the real world, things change.  Sources and sinks mo= ve as do many of the
objects around them.  This creates a time-v= arying RF environment, and now
the path loss matrix is a function of = time and a few others things, so it
matters WHEN something is transmi= tted, and WHEN it is received, and the two
WHEN's are generally separ= ated by "the speed of light" which is a ft/ns
roughly. As important i= s the fact that it's no longer really a path loss
matrix containing a= single scalar because among other things, the time
varying environme= nt induces change in the transmitted waveform on its way to
the recei= ver 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 continuou= s
world in which we live (the space quantization experiment being con= ducted in
the central US aside:^)). As a consequence of these physica= l laws, the
entries in the path loss matrix become complex functions = of a number of
variables including time. These functions are quite of= ten characterized in
terms of Doppler and delay-spread, terms used to= describe in just a few
parameters the amount of "distortion" a compl= ex function causes.

Hope this helps ... probably a bit more th= an you really wanted to know as
queuing theorists, but ...

-----Original Message-----
From: Starlink [mailto:starlink-bounces= @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; starlink@lists.bufferbloat.net;
codel@lists.bufferbloat.net;= cerowrt-devel; bloat
Subject: Re: [Starlink] [Cake] [Make-wifi-fast]= [Cerowrt-devel] Due Aug 2:
Internet Quality workshop CFP for the int= ernet architecture board

> The distance matrix defines signa= l attenuations/loss between pairs.  It's
> straightforward to= create a distance matrix that has hidden nodes because
> all "sig= nal  loss" between pairs is defined.  Let's say a 120dB
att= enuation
> path will cause a node to be hidden as an example.
>
>      A    B     = C    D
> A   -   35   120&= nbsp;  65
> B         -   =   65   65
> C          &n= bsp;    -       65
> D    =                      = ;-
>
> So in the above, AC are hidden from each other bu= t nobody else is. It does
> assume symmetry between pairs but that= 's typically true.

That is not correct, symmetry in the RF worl= d, especially wifi, is rare
due to topology issues.  A high tran= smitter, A,  and a low receiver, B,
has a good path A - > B, = but a very weak path B -> A.   Multipathing
is another m= ajor issue that causes assymtry.

>
> The RF device= takes these distance matrices as settings and calculates the
> fi= ve 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<= br /> > date. I've been able to produce hidden nodes quite readily. Add = the phase
> shifters and spatial stream powers can also be affecte= d, but this isn't
> shown in this simple example.
>
> Bob
>
> On Mon, Aug 2, 2021 at 8:12 PM David Lang= <david@lang.hm&g= t; wrote:
>
> > I guess it depends on what you are in= tending to test. If you are not
going
> > to
> &= gt; tinker with any of the over-the-air settings (including the number of > > packets
> > transmitted in one aggregate), the d= etails of what happen over the air
> > don't
> > ma= tter much.
> >
> > But if you are going to be doing= any tinkering with what is getting
sent,
> > and
&= gt; > 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 cas= es.
> >
> > you don't need to include them in every= test, but you need to have a way
> > to
> > config= ure your lab to include them before you consider any
> > settin= gs/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
> > > wa= nt to minimize complexity to necessary and sufficient.
> > >=
> > > The challenge we find is having variability (e.g. mon= tecarlos) that's
> > > reproducible and has relevant informa= tion. Basically, the distance
> > matrices
> > >= have h-matrices as their elements. Our chips can provide these
> = > h-matrices.
> > >
> > > The parts for so= lid 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 <greearb@candelatech.com>
>= ; > wrote:
> > >
> > >> On 8/2/21 4:16 = PM, David Lang wrote:
> > >>> If you are going to setu= p a test environment for wifi, you need to
> > >> include= the ability to make a fe cases that only happen with RF, not
> &g= t; with
> > >> wired networks and
> > >>= ;> are commonly overlooked
> > >>>
> > = >>> 1. station A can hear station B and C but they cannot hear eac= h
other
> > >>> 2. station A can hear station B = but station B cannot hear station A
3.
> > >> stati= on A can hear that station B is transmitting, but not with a
strong > > >> enough signal to
> > >>> decod= e 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)
> > >&= gt; different numbers of spatial streams, and different distances from
the
> > >> AP.  From download queueing perspective= , changing
> > >> the capabilities may be sufficient whil= e keeping all stations at same
> > >> distance.  Thi= s assumes you are not
> > >> actually testing the wifi ra= te-ctrl alg. itself, so different
throughput
> > >>= levels for different stations would be enough.
> > >> > > >> So, a good station emulator setup (and/or pile of re= al stations) and
a
> > few
> > >> RF ch= ambers and
> > >> programmable attenuators and you can te= st that setup...
> > >>
> > >>  Fr= om upload perspective, I guess same setup would do the job.
> >= >> Queuing/fairness might depend a bit more on the
> > &= gt;> station devices, emulated or otherwise, but I guess a clever AP cou= ld
> > >> enforce fairness in upstream direction
&g= t; > >> too by implementing per-sta queues.
> > >&g= t;
> > >> Thanks,
> > >> Ben
>= > >>
> > >> --
> > >> Ben Gre= ear <greear= b@candelatech.com>
> > >> Candela Technologies Inc=   http://www.= candelatech.com
> > >>
> > >
>= ; > >
> >
>
> --
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