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Re: P3D information



As someone who works in the cellular industry, I can say with confidence
that the passing of analog for digital was not what it was cracked up to be.
Digital at best gives you approximately a 3:1 capacity improvement over
analog (OK perhaps 6:1 in some cases, but normally it's about 3:1).  At
Motorola, we had developed the NAMPS narrow band analog system which
provided the same 3:1 capacity improvements.  But it never really caught on
because of the "digital" revolution.

Digital phones are great when they work.  However, due to the extremely low
sampling rates that are used in order to get the necessary narrow bandwidth,
voice quality is sometimes quite poor.  Now this is partially due to the
fact that most PCS providers think they can build out a 1.9 GHz network with
the same cell site spacing that an 800 MHz network has.  Or at least the
marketing people think this way.  The other issue is that background noise,
and so forth is not digitized well.

CDMA in particular is horrible from my experience.  I had always thought
this was going to be the best and winner over any of the other modes.  In
fact, at Motorola, we had scrapped our domestic TDMA development because of
the difficulties Ericsson had when rolling out their TDMA systems.  We went
with CDMA  and GSM only (with the exception of our PDC system for Japan).
Now, after having used CDMA, I think TDMA is a better choice.  Personally, I
use TDMA with Nextel's system and find it's voice quality to be better than
anything else out there (we have no GSM really here in Chicago yet).
Nextel's TDMA system using Motorola's iDEN technology is really the best I
have heard.  The voice quality is nearly as good as analog.  It beats CDMA
hands down.

The other problem with digital networks as someone else pointed out is that
you either get S9 reception or S0.  There is no in between.  This is good in
that you get better signal reception with low received signal levels.
However, when you hit the point where you can't decode the bits any more it
just falls off like a brick.  With an analog system you can still make sense
out of the static.

My biggest concern is that all of the "3G" cellular systems are based on
CDMA technology which I think is the wrong way to go based on the quality I
have seen from CDMA so far.  We could end up with yucky cellular networks
very soon.

Now, to bring satellites into play here as that is what this is about....

Given the fact that satellites are sometimes weak signal devices, how well
will digital forms of modulation work?  I know P3D will be far better than
AO-10, but there are times on AO-10 when I've worked stations that I don't
think you could work digitally due to low signal levels.  Yes, I know that
things like PSK-31 can copy signals below the noise floor, but that data
rate certainly isn't high enough to support voice.  Of course if P3D is as
good as it is supposed to be, perhaps this is all moot and we'll have plenty
of signal level to play with.

73,

Jon
NA9D

----- Original Message -----
From: "Hamish Moffatt" <hamish@cloud.net.au>
To: <amsat-bb@AMSAT.Org>
Sent: Monday, October 09, 2000 8:18 AM
Subject: Re: [amsat-bb] P3D information


> On Mon, Oct 09, 2000 at 09:24:48AM +1000, Tony Langdon wrote:
> > phones go.  The GSM system had obvious limitations, due to its TDMA
design
> > and the distances we Aussies want to span with mobile comms.
Limitations
> > that don't apply to analog systems.  More recently, CDMA was implemented
as
> > the successor tom analog, and touted as being equal.  Guess what?  it
didn't
> > prove to be the case, and people are lamenting the passing of analog.
>
> Well, I don't think it's really been given a fair go. It's not very long
> ago that Telstra announced CDMA as the replacement for the analogue
> system; I don't see how they've had time to replace the entire network
> since then. Nor much incentive for them to do so, due to lack of
> competition.
>
>
> Hamish
> --
> Hamish Moffatt VK3SB <hamish@debian.org> <hamish@cloud.net.au>
> ----
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