Date: Wed, 27 Aug 2014 16:09:07 -0700 From: Bart Kus <me@bartk.us> To: Adrian Chadd <adrian@freebsd.org> Cc: "freebsd-wireless@freebsd.org" <freebsd-wireless@freebsd.org> Subject: Re: FreeBSD TDMA: Legalizing 440MHz 802.11 modems Message-ID: <53FE6513.8040107@bartk.us> In-Reply-To: <CAJ-VmokUyrvE-2bgkHDE9sCGJv_3EEffqWVWySvYf=7ra4PfUA@mail.gmail.com> References: <53FE5CF4.1000901@bartk.us> <CAJ-VmokUyrvE-2bgkHDE9sCGJv_3EEffqWVWySvYf=7ra4PfUA@mail.gmail.com>
next in thread | previous in thread | raw e-mail | index | archive | help
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 <me@bartk.us> 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"
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?53FE6513.8040107>