From owner-freebsd-wireless@FreeBSD.ORG Wed Aug 27 23:09:10 2014 Return-Path: Delivered-To: freebsd-wireless@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) (using TLSv1 with cipher ADH-AES256-SHA (256/256 bits)) (No client certificate requested) by hub.freebsd.org (Postfix) with ESMTPS id 1749BDF8; Wed, 27 Aug 2014 23:09:10 +0000 (UTC) Received: from a.mx.bartk.us (173-10-122-205-BusName-Washington.hfc.comcastbusiness.net [173.10.122.205]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id ED19B38F4; Wed, 27 Aug 2014 23:09:09 +0000 (UTC) Received: from [192.168.45.6] (unknown [192.168.45.6]) by a.mx.bartk.us (Postfix) with ESMTP id 5076E3D004A; Wed, 27 Aug 2014 16:09:08 -0700 (PDT) Message-ID: <53FE6513.8040107@bartk.us> Date: Wed, 27 Aug 2014 16:09:07 -0700 From: Bart Kus User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.1.0 MIME-Version: 1.0 To: Adrian Chadd Subject: Re: FreeBSD TDMA: Legalizing 440MHz 802.11 modems References: <53FE5CF4.1000901@bartk.us> In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Cc: "freebsd-wireless@freebsd.org" X-BeenThere: freebsd-wireless@freebsd.org X-Mailman-Version: 2.1.18-1 Precedence: list List-Id: "Discussions of 802.11 stack, tools device driver development." List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 27 Aug 2014 23:09:10 -0000 Is the underclocking affecting the digital domain only? Or would there be some analog frequency response curves that would start falling off too? If it's a digital-only underclock I don't see why there would be any degradation (aside from the obvious speed decrease). Is this easily testable somehow, with a single clock variable? Yes, the subcarriers would get really narrow, but the sampling time would increase proportionately, so the FFT resolution would stay the same. I wonder if we'd be exceeding the hold time of the S&H circuit(s)... I don't really know anything about these chips, just making wild ass guesses based on generic modem architecture. :) --Bart On 08/27/2014 03:53 PM, Adrian Chadd wrote: > Hi! > > So, I'm not sure if we can underclock it _that_ far. The 5/10MHz > channels are implemented by underclocking various parts of the chip, > which results in everything being some fraction of 20MHz (or 2x > clocking it, resulting in 40MHz.) Bringing it all the way down to > 200KHz is a pretty tall ask. > > I know people have gone down to 2.5MHz on these chips, but I don't > know of anything lower than that. The OFDM subcarriers start being way > too narrow and I doubt you'd end up with anything sensible. > > It's likely that you'd want to create a custom DSP based solution to > do 200KHz side stuff at that symbol rate. > > > > -a > > > On 27 August 2014 15:34, Bart Kus wrote: >> Hello, >> >> I'm wondering if you can tell me if it's possible to modify the FBSD TDMA >> code to make this card: >> >> http://www.doodlelabs.com/products/radio-transceivers/sub-ghz-range/420-450-mhz-dl435-30/ >> >> legal to use in its intended spectrum. By default the card violates two >> Part 97 rules: >> >> 1) Emissions are limited to 200kHz bandwidth >> 2) Symbol rate is limited to 56kSym/s >> >> Is it possible to slow down the subcarrier symbol rate in that Atheros chip? >> Is it also possible to then space the subcarriers tighter together to >> respect the 200kHz emission bandwidth limit? They'd have to come closer >> together anyway to uphold OFDM subcarrier orthogonality. >> >> Thanks for any clues, >> >> --Bart >> >> PS: I'm asking in the interest of the HamWAN.org project. We're trying to >> find a more mobile solution, which penetrates through forests and some >> buildings. 440MHz seems to fit the bill, but hardware is really hard to >> come by. >> >> _______________________________________________ >> freebsd-wireless@freebsd.org mailing list >> http://lists.freebsd.org/mailman/listinfo/freebsd-wireless >> To unsubscribe, send any mail to "freebsd-wireless-unsubscribe@freebsd.org"