Date: Mon, 22 Jan 2001 23:14:18 -0000 (GMT) From: Duncan Barclay <dmlb@dmlb.org> To: David Scheidt <dscheidt@enteract.com> Cc: freebsd-chat@FreeBSD.ORG, Brad Knowles <brad.knowles@skynet.be>, Michael C.Wu <keichii@iteration.net>, Greg Lehey <grog@lemis.com> Subject: Re: Mobile phone coverage (was: VCD (was Re: cvs commit: src/sys Message-ID: <XFMail.010122231418.dmlb@computer.my.domain> In-Reply-To: <Pine.BSF.4.21.0101212221400.55216-100000@shell-2.enteract.com>
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On 22-Jan-01 David Scheidt wrote: > On Mon, 22 Jan 2001, Greg Lehey wrote: > >:On Sunday, 21 January 2001 at 21:10:08 -0600, Michael C . Wu wrote: >:> On Mon, Jan 22, 2001 at 03:42:45AM +0100, Brad Knowles scribbled: >:>> At 12:35 PM +1030 2001/1/22, Greg Lehey wrote: >:>> >:>>> That's not the issue here. BTW, for the USA you need a three-band >:>>> phone. That is, incidentally, the only kind of phone which will work >:>>> just about anywhere (I'm not sure about Israel and Korea). >:>> >:>> Uhh, I think you need more than that. Let's count: >:>> >:>> AMPS/NAMPS >:>> US TDMA (1900Mhz? 900Mhz?) >:>> US CDMA (1900Mhz? 900Mhz?) >:>> US GSM (1900Mhz) >:> >:> US 1900 and 900 > > The US is 1900 and 800, not 900. The 900 Mhz freqs used by GSM in the rest > of the world are the property of the US military. To be honest, I'm not > sure there are any AMPS band GSM carriers in the US. Maybe Bell South DCS? > >:I've been looking at web pages, and it doesn't sound to me that 3G is >:a new transmission technology, just something that climbs on the back >:of one. Is there any reason why it should be tied to CDMA and not to >:GSM? > > Yes. CDMA has much better use of bandwidth. All users of a CDMA channel > use it simultanously, not time multiplexed. The data travelling across the > air interface are encoded with a walsh code, which allows the receiver to > pick out each data stream, because some mathmattecian was bloody clever. > I'm unable to find a good web reference right at the moment, but CDMA is > clearly a better choice than any of the TDMA solutions. Are you basing your "clearly a better choice" on the fact the CDMA uses Walsh codes? A few things - CDMA needs a greater bandwidth to transmit a channel than TDMA. This is because of the Walsh codes that are convolved with the data a 3.84Mb/s when the data is only 9.6kb/s. Having users transmit on the same frequency increases the noise power in each users receiver and from Shannon this places a limit on the capacity. Transmitted power must be adjusted much more carefully than in TDMA to ensure that the basestation isn't overloaded by a user close it to (near-far problem). Here CDMA as an RF technology wins is in multipath resiliance. Each data bit is spread over a wider bandwidth by the Walsh code making it resiliant to frequency dependent fading. Becuase Walsh codes are orthogonal, easy to correlate against them. With mutil-path effects that cause bits to run into each other (inter-symbol interferance) the Walsh code allows the receiver to very accurately equalise the delays out (Rake receiver and Turbo-coding). Duncan > David Duncan --- ________________________________________________________________________ Duncan Barclay | God smiles upon the little children, dmlb@dmlb.org | the alcoholics, and the permanently stoned. dmlb@freebsd.org| Steven King To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-chat" in the body of the message
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