Subject: Roaming, CDMA, TDMA etc. (was Re: GSM Interference)
From: [email-masked] (Phil Price)
Date: 29 Nov 1993 23:37:19 GMT

In article <[email-masked]>, [email-masked] (Laurence Chiu)
wrote:

> I don't pretend to understand all this technical stuff about TDMA, GSM
> or CDMA but does this mean if various companies decided to implement
> different standards for digital cellular, then is roaming, which is
> pretty hard already, going to be that much harder as the poor user is
> going to have to know if his phone is compatible in the area he is
> roaming in?

Roaming problems are really unrelated to whether the phone uses AMPS,
TDMA, E-TDMA, GSM or CDMA (I'll explain these later). Roaming between
systems requires a supporting infrastructure and intersystem
protocols. These protocols have been standardised, but are still
evolving. The N.American standard is IS-41, and the European standard
is GSM (more precisely, the MAP protocols). These protocols define the
messaging and procedures for exchanging information between systems to
allow roaming. Most of the new phones will be dual-mode -- AMPS plus
one of the others, so you should be able to use it just about anywhere
(without having to know about the system that you are currently
using). Roaming is already available in Europe, and is rapidly being
implemented in N.America.

On a related issue, you should start seeing information about
'roaming' systems fairly soon. One company has just released a system
claiming coverage over the whole of N.America (MobiLink?), but I think
that is a PCN system. There are also several projects under way to
provide global (i.e.  world-wide) roaming -- the front runners in this
are Qualcomm's Globalstar, Motorolla's Iridium and TRW's Odessey
systems. These systems are all satellite-based but have fundamentally
different operating strategies. I know the most about Globalstar, and
I'm probably biased in my views, but I'll try to summarize them:

Globalstar - has 48 low earth orbiting satellites covering 70 degrees
north and south of the Equator. The satellites act as a communications
link to ground stations (called gateways) that connect the user to the
land network. the satellites have a very large coverage area on the
ground (roughly 2000 miles radius, I think) and so can be used as an
'extension' facility e.g. for rural or undeveloped areas. The user
terminals will generally be dual mode (though single-mode phones are
planned), where one mode is Globalstar and the other mode is that of
the local system that you choose (AMPS, CDMA, GSM etc.). The idea here
is that a user will use the local system when coverage is available,
or Globalstar coverage when it is not (i.e. when roaming) -- since the
satellite covers the temperate regions all around the globe, this
means that global roaming is potentially possible (when you get into
it, the problems become political or commercial in nature, rather than
technical).

Iridium -- has lots (76? whatever the element number of Iridium is) of
low-earth orbit satellites in polar orbits (I don't know what happens
if a few of them get close to each other at the poles!?). The funky
thing about Iridium is that the satellites do the switching in space
between themselves and then sends the signal down to earth at some
point close to the destination of the call i.e. it bypasses the
terrestrial systems (but apparently they are discussing an option to
interface to the terrestrial systems also). The advantages of this is
that it doesn't depend upon the terrestrial network (especially if
there isn't one!). The disadvantages are complexity (imagine all of
that hardware and software in space) and cost.

Odessey -- this is a TDMA-based system utilising Medium-earth orbit
satellites (i.e. you don't need as many satellites, but the user
terminals have to be much more powerful and there is more delay). I
don't know anything more about this system.

Maybe someone from Motorolla or TRW could clarify this a little?

> As an aside, are TDMA and CDMA implementations of a technology known
> generically as digital AMPS?

All of the cellular technologies (except N-AMPS) are basically digital
AMPS in that they are digital, use the AMPS call setup model and are
based upon the original AMPS model (the architecture anyway). Just
FYI, the main systems that are being pushed as successors to AMPS are:

N-AMPS -- This system is based upon FM waveform (as is AMPS), but
splits up the bands into sub-bands (three, I think) and also carries
signalling data overlayed on the voice signal. This provides a 3:1
capacity gain over AMPS, but still has all of the old fading and
error-correction problems of the old system (this is probably a biased
opinion, since I have worked on both E-TDMA and CDMA ;-)

TDMA - Known in the standards as IS-54, this uses a TDMA approach
where each frequency band is divided into six repeating timeslots
(frames). A user will be assigned two of these slots for conversation
(or data etc.) i.e.  there is a capacity gain of 3:1. Call setup is
achieved using AMPS and then 'handed-off' to TDMA.

E-TDMA -- Enhanced TDMA, developed by Hughes Network Systems, is an
extension of TDMA where a 'pool' of (TDMA) frequencies are used to
support many mobiles. The technology uses half rate vocoders (i.e.
doubles capacity) and digital speech interpolation (i.e. slots are
only assigned when needed).

GSM -- the European equivalent of TDMA, but with a much more strongly
defined architecture (on the network side) and a more integrated
system.  The system is fairly similar to TDMA, but doesn't rely on
AMPS for call setup and has lots of other features built into the
standard (data, FAX, messaging etc.).

CDMA -- Code Division Multiple Access -- the relevant standard is
IS-95. CDMA is a spread-spectrum technology i.e. instead of dividing
the data into timeslots, the data is 'spread' across a wide band of
frequencies using some pretty fancy coding tricks. All users can
transmit on the same frequencies at the same time -- the signals are
distinguished from each other by the coding schemes. Using this
scheme, the number of users is limited by the coding, rather than the
number of timeslots. I think the capacity gain is about 10:1 or more
(over AMPS), but there have been plenty of articles in the trade
magazines comparing CDMA to TDMA (and I'm no expert).

Hope this helps,

Phil