From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp78.iad3a.emailsrvr.com (smtp78.iad3a.emailsrvr.com [173.203.187.78]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by lists.bufferbloat.net (Postfix) with ESMTPS id C83C43B29E for ; Tue, 28 Nov 2017 15:04:32 -0500 (EST) Received: from smtp10.relay.iad3a.emailsrvr.com (localhost [127.0.0.1]) by smtp10.relay.iad3a.emailsrvr.com (SMTP Server) with ESMTP id 8AE2E684E; Tue, 28 Nov 2017 15:04:32 -0500 (EST) Received: from app59.wa-webapps.iad3a (relay-webapps.rsapps.net [172.27.255.140]) by smtp10.relay.iad3a.emailsrvr.com (SMTP Server) with ESMTP id 7569F652B; Tue, 28 Nov 2017 15:04:32 -0500 (EST) X-Sender-Id: dpreed@reed.com Received: from app59.wa-webapps.iad3a (relay-webapps.rsapps.net [172.27.255.140]) by 0.0.0.0:25 (trex/5.7.12); Tue, 28 Nov 2017 15:04:32 -0500 Received: from reed.com (localhost.localdomain [127.0.0.1]) by app59.wa-webapps.iad3a (Postfix) with ESMTP id 663B440069; Tue, 28 Nov 2017 15:04:32 -0500 (EST) Received: by apps.rackspace.com (Authenticated sender: dpreed@reed.com, from: dpreed@reed.com) with HTTP; Tue, 28 Nov 2017 15:04:32 -0500 (EST) X-Auth-ID: dpreed@reed.com Date: Tue, 28 Nov 2017 15:04:32 -0500 (EST) From: dpreed@reed.com To: "=?utf-8?Q?Toke_H=C3=B8iland-J=C3=B8rgensen?=" Cc: make-wifi-fast@lists.bufferbloat.net MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_20171128150432000000_17641" Importance: Normal X-Priority: 3 (Normal) X-Type: html In-Reply-To: <0982A196-2DA3-46E6-B18C-DADDAE252F6E@toke.dk> References: <0982A196-2DA3-46E6-B18C-DADDAE252F6E@toke.dk> Message-ID: <1511899472.416610020@apps.rackspace.com> X-Mailer: webmail/12.9.9-RC Subject: Re: [Make-wifi-fast] Tinfoil hats for WiFi routers X-BeenThere: make-wifi-fast@lists.bufferbloat.net X-Mailman-Version: 2.1.20 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 28 Nov 2017 20:04:32 -0000 ------=_20171128150432000000_17641 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable =0AThough it is said to have "worked" with 802.11ac, I wonder what you do t= o preserve diversity in the 4 antennas, esp. if not in a straight line.=0A = =0AIn principle, diffusive scattering actually helps MIMO performance, but = the scattering has to be "random" to be most effective. For example, if yo= u put up a flat mirror behind the 4 antennas, and you picture the coverage = as a "directed beam" (which is not an ideal way to think about 4x4 MIMO at = all, but it's easier to picture), you'll end up with a reflected version of= the backside of the beam that goes off in a different direction from the f= irst. That would "split" the energy so the backlobe gets wasted because it= does not reach the intended receiver.=0A =0AOFDM has an advantage in refle= ctive environments, because the symbol duration is quite large compared to = the value c/f where c is lightspeed and f is the central frequency of the b= and. This means that phase mixing of the subcarriers due to reflections/sc= attering has much less impact on OFDM.=0A =0ASo, if you can concentrate the= transmitted energy from the four antennas of the source onto the four ante= nnas of the receiver, even with phase shifts, you can do better.=0A =0ABut = 4x4 MIMO is already doing a lot of concentration of energy, so I would thin= k that no matter what the device is, it won't help 802.11ac as much as it w= ould help non-MIMO systems.=0A =0AAnd 802.11b (DSSS) would suffer from symb= ol distortion due to phase mixing. But that depends on the physical dimensi= ons of the device that is wrapped. If small compared to the symbol duration= * c, then it won't matter that much.=0A =0A =0A =0A =0A =0A=0A=0AOn Tuesda= y, November 28, 2017 12:57pm, "Toke H=C3=B8iland-J=C3=B8rgensen" said:=0A=0A=0A=0A> Supposed to be 3D-printed to point the signal in th= e right direction. Can't find=0A> the paper, just the press release:=0A> ht= tps://www.eurekalert.org/pub_releases/2017-11/dc-aw110717.php=0A> =0A> -Tok= e=0A> _______________________________________________=0A> Make-wifi-fast ma= iling list=0A> Make-wifi-fast@lists.bufferbloat.net=0A> https://lists.buffe= rbloat.net/listinfo/make-wifi-fast ------=_20171128150432000000_17641 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable

Though it is said = to have "worked" with 802.11ac, I wonder what you do to preserve diversity = in the 4 antennas, esp. if not in a straight line.

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In principle, diffusive scattering= actually helps MIMO performance, but the scattering has to be "random" to = be most effective.  For example, if you put up a flat mirror behind th= e 4 antennas, and you picture the coverage as a "directed beam" (which is n= ot an ideal way to think about 4x4 MIMO at all, but it's easier to picture)= , you'll end up with a reflected version of the backside of the beam that g= oes off in a different direction from the first.  That would "split" t= he energy so the backlobe gets wasted because it does not reach the intende= d receiver.

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So, if yo= u can concentrate the transmitted energy from the four antennas of the sour= ce onto the four antennas of the receiver, even with phase shifts, you can = do better.

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But 4x4 MIMO is already doing a lot of concentration of energy, so I would= think that no matter what the device is, it won't help 802.11ac as much as= it would help non-MIMO systems.

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And 802.11b (DSSS) would suffer from symbol distorti= on due to phase mixing. But that depends on the physical dimensions of the = device that is wrapped. If small compared to the symbol duration * c, then = it won't matter that much.

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On Tuesday, November 28, 2017 12:57pm, "Toke H=C3=B8iland-J= =C3=B8rgensen" <toke@toke.dk> said:

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> Supposed to be 3D-printed = to point the signal in the right direction. Can't find
> the paper,= just the press release:
> https://www.eurekalert.org/pub_releases/= 2017-11/dc-aw110717.php
>
> -Toke
> ______________= _________________________________
> Make-wifi-fast mailing list
> Make-wifi-fast@lists.bufferbloat.net
> https://lists.bufferb= loat.net/listinfo/make-wifi-fast

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