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Path: sparky!uunet!lll-winken!telecom-request
From: das...@rpi.edu (Anindadeb Vijaykumar Dasgupta)
Newsgroups: comp.dcom.telecom
Subject: Digital Cellular Telephony
Message-ID: <telecom12.17.6@eecs.nwu.edu>
Date: 7 Jan 92 18:08:34 GMT
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X-Telecom-Digest: Volume 12, Issue 17, Message 6 of 8

I read somewhere that in most large cities cellular carriers are
switching to digital systems due to saturation of existing cells.  I
couldn't tell what the advantages of this would be:

With analog transmission, each equipment would need 4 KHz. while with
8 bit PCM, 64 Kbps would be needed, which would surely translate to a
higher frequency than 8 KHz.  This definitely does not free up any
bandwidth.  Are these carriers using source coding?  Won't that make
the cellular phones more expensive/bulky?

I am not a telecom person and maybe I am missing the point here.
Could someone explain the advantages of digital over analog (other
than improvement in voice quality, if any) in cellular systems?

Thanks in advance.


Aninda DasGupta    (ani...@networks.ecse.rpi.edu)

Path: sparky!uunet!wupost!uwm.edu!lll-winken!telecom-request
From: rdipp...@cancun.qualcomm.com (Ron Dippold)
Newsgroups: comp.dcom.telecom
Subject: Re: Digital Cellular Telephony
Message-ID: <telecom12.21.2@eecs.nwu.edu>
Date: 9 Jan 92 19:26:43 GMT
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X-Telecom-Digest: Volume 12, Issue 21, Message 2 of 12

das...@rpi.edu (Anindadeb Vijaykumar Dasgupta) writes:

> I read somewhere that in most large cities cellular carriers are
> switching to digital systems due to saturation of existing cells.

They're moving towards it, as fast as they can.

> I couldn't tell what the advantages of this would be:

> With analog transmission, each equipment would need 4 KHz. while with
> 8 bit PCM, 64 Kbps would be needed, which would surely translate to a
> higher frequency than 8 KHz.  This definitely does not free up any
> bandwidth.  Are these carriers using source coding?  Won't that make
> the cellular phones more expensive/bulky?

Okay, you're assuming that the analog phone is much more efficient in
bandwidth than it actually is.  In actuality, each phone completely
occupies a 30 KHz channel, and each phone is transmitting at thre
watts for marginal quality.  And it still sounds bad.

With digital encoding done correctly (so you can use Viterbi
decoding), you need much less power to get your data through (you're
just looking for on-off instead of a FM sound waveform).  In addition,
if you're using CDMA, CDMA doesn't require that you divide up your
bandwidth into bandwith wasting 30 KHz chunks.  Intead, each phone
uses exactly as much bandwidth as it needs.  More phones just mean
more "noise" (not noise heard by the user, but as in signal to
noise ...)

For example, if you had 50 users that needed computers, then the
analog analagous way to do it would be to buy two mainframes, and let
only one user at a time use each computer, even if all they needed to
do was simple word processing.  The CDMA way would be to buy one
computer that was 20 times more powerful and give each user a
terminal, so they only use as much of the computer power as they need,
and everyone can use it at once.

Even better, we use active power control (something we can do because
it's digital).  The mobile and cell channel elements transmit with
only as much power they need.  If you're close to the cell, you don't
need much power.  As you get farther away, it slowly boosts power.

Finally, with digital we can (and do) use a variable rate vocoder.  In
this way CDMA makes use of the Voice Activity Factor of conversation:
either of the parties involved is not saying something about 60% of
the time.  (Interestingly, even when we force the vocodoer to use a
maximum rate of half its top rate, it still sounds better than my
analog phone).

What it works out to is that while an analog phone transmits at three
watts, and the cell-site power requirements are truly horrible, CDMA
phones transmit in the milliwatt range, and the entire cell site
transmits with only as much power as a couple of the radios in the
analog cell.

Bottom line: We have an officially capacity tested (tests observed by
the major companies in the industry) CDMA system in the field that
gives a capacity improvement of 10 to 30 times (depending on
conditions) over an AMPS system, with better voice quality, better
handoffs, and less dropped calls.

Efficient use of resources is the key (plus a lot of geniuses in the
theory department, and then ignoring those who claimed we couldn't do
it).

One further advantage: The digital medium is a lot more flexible.
When an AMPS phone has to transmit control information, the voice
blanks out.  With CDMA, we can just vocode at half the normal rate and
send the control information in the other half of the frame.  Result:
undetectable loss of voice quality instead of complete loss of voice.

In addition, we can divide the channel between different data sources,
so you could send voice and data (from a modem, perhaps) on the same
channel.  What we can do is limited only by the messages we can think
up to send back and forth.

Another voice quality advantage: because it's a digital vocoder,
rather than sending the analog waveform out, we can more easily do a
lot more filtering on it.  For example, continuous background noises
(such as a car engine or the wind) can be severely reduced.

There are further advantages, but that should be enough.

Path: sparky!uunet!spool.mu.edu!telecom-request
From: ibbot...@rtsg.mot.com (Craig Ibbotson)
Newsgroups: comp.dcom.telecom
Subject: Re: Digital Cellular Telephony
Message-ID: <telecom12.23.1@eecs.nwu.edu>
Date: 9 Jan 92 21:23:47 GMT
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X-Telecom-Digest: Volume 12, Issue 23, Message 1 of 6

das...@rpi.edu (Anindadeb Vijaykumar Dasgupta) writes:

> I read somewhere that in most large cities cellular carriers are
> switching to digital systems due to saturation of existing cells.  I
> couldn't tell what the advantages of this would be:

> With analog transmission, each equipment would need 4 KHz. while with
> 8 bit PCM, 64 Kbps would be needed, which would surely translate to a
> higher frequency than 8 KHz.  This definitely does not free up any
> bandwidth.  Are these carriers using source coding?  Won't that make
> the cellular phones more expensive/bulky?

> I am not a telecom person and maybe I am missing the point here.
> Could someone explain the advantages of digital over analog (other
> than improvement in voice quality, if any) in cellular systems?

There are currently two digital cellular proposals.  One is TDMA (Time
Division Multiple Access) and the other is CDMA (Code Division
Multiple Access).  TDMA divides the existing 30kHz radio channels into
six time slots, yielding three equivalent voice channels.  TDMA
proponents say this will offer a 3.7 time capacity of existing analog
systems.  CDMA is a spread-spectrum technology which calls for a
channel bandwidth of 1.25 MHz.  The entire bandwidth is reused in
every cell site, and is shared by a number of customers with different
codes.  Proponents say it will offer a 20X capacity increase.  The
main advantages of digital cellular (regardless of technology) are
capacity increase and the introduction of data services.

It is going to be very interesting to see what happens in the digital
cellular market.  TDMA was the initial technology of choice, picked
over FDMA after lengthy trials here in the US.  After TDMA was chosen,
however, a company called Qualcomm met with some of the larger
cellular vendors and convinced them that CDMA could greatly increase
their capacity and provide some additional features.  Both
technologies are in market trials now, and the initial indications are
that both work.

The problem here is that different vendors are aligning themselves
with different technologies.  LA Cellular is promising a cellular
system using Ericsson TDMA technology sometime this year.  PACTEL and
one of the vendors in New York are heavily involved in CDMA.

A third technology has been introduced by Motorola, called N-AMPS.
N-AMPS stands for Narrowband Amps, and is a "digitally enhanced"
analog system.  It increases capacity by using 10kHz radio channels
instead of 30 kHz radio channels, using the existing analog
technology.  It also has a digital sub-audio channel which allows data
to be sent to the mobile.  This technology is intended as a bridge
between an analog and digital system.  I believe all Motorola mobile
and portable cellular phones now shipping are dual mode AMPS/NAMPS.

All of these technologies have their pluses and minuses.  TDMA is the
basis for GSM, which is in commercial service in Europe, so it is a
proven player.  NAMPS is in commercial service in Japan, and can be
used with Micro-TAC LITE sized phones (the other technologies cannot
be used with portables at this time).  CDMA promises the biggest
capacity increase and possibly higher quality handoffs.  The big
problem I see is that CDMA and TDMA are incompatible; the standards
currently call for all mobiles to be dual mode (digitial/analog), but
there is no thought at this time for a mobile to support both TDMA and
CDMA.  For example, if you are an LA Cellular TDMA user and attempt to
roam in a CDMA system, you will not be able to use the digital system
(you should always be able to go back to standard AMPS service,
however, no matter where you are).

This has also made it very difficult for manufacturers.  Ericsson and
Northern Telecom are firmly behind TDMA.  Motorola and AT&T are
developing CDMA, TDMA and NAMPS.  Hughes is pushing E-TDMA, a second
generation TDMA system which uses a half-rate vocoder to get an
eight-fold increase over today's analog capacities.  And Qualcomm is
pushing CDMA.

One thing should be noted about digital cellular.  Everyone is hot
about anything digital nowadays.  When cellular goes digital, however,
you will not see a quality improvement like that from LP records to
compact discs.  I quote the editor of Cellular Business:

"Cellular's digital technology is another beast altogether.  It might
sound better.  But chances are good that the customer won't even
notice a difference, except that some manufacturers phones might be
bigger.  And initially, they might be bulkier, too."  (Cellular
Business, "Moving Into Digital", supplement to October 1991 issue).


Craig Ibbotson, Motorola, Inc.  ...uunet!motcid!ibbotsonc
Cellular Infrastructure Division, Radio Telephone Systems Group

Path: sparky!uunet!sun-barr!ames!elroy.jpl.nasa.gov!usc!zaphod.mps.ohio-state.edu!unix.cis.pitt.edu!dsinc!telecom-request
From: r...@rtsg.mot.com (Bernard Rupe)
Newsgroups: comp.dcom.telecom
Subject: Re: Digital Cellular Telephony
Message-ID: <telecom12.29.3@eecs.nwu.edu>
Date: 10 Jan 92 15:10:04 GMT
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X-Telecom-Digest: Volume 12, Issue 29, Message 3 of 8

das...@rpi.edu (Anindadeb Vijaykumar Dasgupta) writes:

> I read somewhere that in most large cities cellular carriers are
> switching to digital systems due to saturation of existing cells.  I
> couldn't tell what the advantages of this would be:
 
No carriers in the US have a digital system up yet.  In fact, the
standard is still up in the air, with both TDMA (time division) and
CDMA (code division) being talked about.  A standard on TDMA is
essentially complete.  There is, however, a NAMPS (narrow band AMPS)
system in operation in Las Vegas.  NAMPS uses compression to squeeze
three voice channels (10KHz each) into the band one used to occupy
(30KHz).  This system was developed by Motorola to provide a bridge
between current technology and the digital technology of the future
(it also costs much less).

> With analog transmission, each equipment would need 4 KHz. while with
> 8 bit PCM, 64 Kbps would be needed, which would surely translate to a
> higher frequency than 8 KHz.  This definitely does not free up any
> bandwidth.  Are these carriers using source coding?  Won't that make
> the cellular phones more expensive/bulky?

In AMPS (current system) each channel uses FM and 30 KHz.  TDMA uses
digital compression so that three (or possibly more) channels fit into
the 30 KHz.  CDMA uses a spread spectrum technique to expand capacity
10 or even 20 times current.  Trials with both are going on as you
read this.  The price of the new phones may very well go up.  In the
case of NAMPS, the phones don't have to be more bulky.  Motorola's new
MicroTac Lite (7.7 oz) has the added NAMPS capability.  In the case of
the digital technologies, the answer is yes the phones need to be much
more bulky.  This is another reason why Motorola introduced NAMPS.

> I am not a telecom person and maybe I am missing the point here.
> Could someone explain the advantages of digital over analog (other
> than improvement in voice quality, if any) in cellular systems?

The voice quality should be about the same.  The main advantage is
that digital signals are much more resistant to noise, so greater
range can be obtained and interference can be minimized.


Bernie Rupe 		        1501 W. Shure Drive  Room 1315
Motorola, Inc.                  Arlington Heights, IL 60004
Cellular Infrastructure Group	+1 708 632 2814
		      r...@rtsg.mot.com

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