From owner-freebsd-wireless@FreeBSD.ORG Wed Aug 27 23:13:35 2014 Return-Path: Delivered-To: freebsd-wireless@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [8.8.178.115]) (using TLSv1 with cipher ADH-AES256-SHA (256/256 bits)) (No client certificate requested) by hub.freebsd.org (Postfix) with ESMTPS id 7EB45E59 for ; Wed, 27 Aug 2014 23:13:35 +0000 (UTC) Received: from mail-qa0-x235.google.com (mail-qa0-x235.google.com [IPv6:2607:f8b0:400d:c00::235]) (using TLSv1 with cipher ECDHE-RSA-RC4-SHA (128/128 bits)) (Client CN "smtp.gmail.com", Issuer "Google Internet Authority G2" (verified OK)) by mx1.freebsd.org (Postfix) with ESMTPS id 3F4E43990 for ; Wed, 27 Aug 2014 23:13:35 +0000 (UTC) Received: by mail-qa0-f53.google.com with SMTP id w8so216502qac.26 for ; Wed, 27 Aug 2014 16:13:34 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:sender:in-reply-to:references:date:message-id:subject :from:to:cc:content-type; bh=tc9Cd361lB6xhoiMoTEWCl9hDYK/lAz5J8ZCjkbdmkg=; b=V1I+yPb9KhyzEoIt8c3KSgIGhHKI1J83n1S321Xzr5B38znEDMCnhJzkhQKdNDJP1M Zf3xlWGfXFU8pRQVRxunfmguojmFtONso5dMkzWR039OfTJVJTJExZdmxKkDM8P+vHHp 94ZzBww1mfWWhUWUmFUidIxcz2LgA3bcYCTvcToHw3OaQPtLpCMKxvzerRLvqVZu/XML 7emnlUnNmOdvLhZ8uBqQTrMy2wCfLPUElpgy4iZNeA+sPFh93fshz+UA+JaN0feHwB1S LICl2tQ4n4SuyMa6uIivMY3GMycLK/2hLMJTVfgfBd/JALLr9t8OQblkBUxcvKrN8Z62 t5Kw== MIME-Version: 1.0 X-Received: by 10.224.36.4 with SMTP id r4mr151153qad.69.1409181214400; Wed, 27 Aug 2014 16:13:34 -0700 (PDT) Sender: adrian.chadd@gmail.com Received: by 10.224.39.139 with HTTP; Wed, 27 Aug 2014 16:13:34 -0700 (PDT) In-Reply-To: <53FE6513.8040107@bartk.us> References: <53FE5CF4.1000901@bartk.us> <53FE6513.8040107@bartk.us> Date: Wed, 27 Aug 2014 16:13:34 -0700 X-Google-Sender-Auth: ufVZM5_zcwszsWE7ogKc9Xi-BFE Message-ID: Subject: Re: FreeBSD TDMA: Legalizing 440MHz 802.11 modems From: Adrian Chadd To: Bart Kus Content-Type: text/plain; charset=UTF-8 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 23:13:35 -0000 On 27 August 2014 16:09, Bart Kus wrote: > 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? It isn't a single clock variable. The chip is a huge state machine and lots of timers, so all of the timing related things that are counts that reflect time (eg number of ticks for 802.11 things to occur) need to be recalculated. > 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. :) Well, there's an FFT going on, and you still will have 52 subcarriers, so divide your 200KHz up into 64 bins (52 subcarriers + guard bits + sync tones.) normally for 20MHz they're 31.25KHz wide. For 200KHz that's 0.3125KHz wide, or 312.5Hz wide. There's not a lot of gap between each carrier and we aren't over-sampling. I don't think the chip was ever really designed for that. Also yes, there's AGC and other bits that have likely only been characterised for a max hold time of hm, 12mS? Whatever the 1MHz CCK * maximum packet length is. -a