Le 25/06/2009 10:27, Invisible nous fit lire :
> Darren New wrote:
>> Invisible wrote:
>>> Really? That's interesting... I got to the CDMA article *from* the
>>> GSM article. o_O
>>
>> Sure. The old analog phones used FDM (Frequency Division
>> Multiplexing), and GSM uses (I think) TDMA - time division multiple
>> access.  There's also phase-division multiple access, as well as
>> various combinations thereof.
> 
> Hmm, OK. Apparently,
> 
> "The modulation used in GSM is Gaussian minimum-shift keying (GMSK), a
> kind of continuous-phase frequency shift keying. In GMSK, the signal to
> be modulated onto the carrier is first smoothed with a Gaussian low-pass
> filter prior to being fed to a frequency modulator, which greatly
> reduces the interference to neighboring channels (adjacent channel
> interference)."
> 
> So not CDMA at all. Weird...

Not at all... but 3G (successor of GSM, in term of end-user technology/availabilty) is
using CDMA.
European use of UMTS (3G) is based on W-CDMA (W- : wideband), but not only.

GSM is 2G (quite old), playing with sub-channels (124 of 200kHz each, with 8 time
slot) in
TX & RX separated bands. Payload per timeslot is about 24.7 kbit/s (and no link
aggregation).
Of the 124 sub-frequencies, for a given point in space, they must be split between, at
least and optimaly, 3 ground-antenna (think plane-coloring... with some antenna having
different forms of cover, and some covers being influenced by the actual topology and
buildings)

GSM transmission is coded in the phase of a constant amplitude signal, allowing to
elect
the best antenna rather easily for the receiver (pick the strongest received
amplitude);
Phase modulation being one of the best modulation for issue with moving apparatus and
echo
of signal on buildings.

But it does not allow to share timeslots.

Between GSM and UMTS, GPRS and EDGE are extending the GSM system.
GPRS allow link aggregation (upto 8 timeslots, if the ground antenna is idle enough to
allow that), and then EDGE use a higher-density coding in the phase modulation to
extend
the bandwidth of GPRS (with limitation to the range... there must be a trade-off).

It's all like the old modem technology, giving new legs to an old horse.
(9600/28k/56k...)

CDMA (UMTS/...) is to GSM what ADSL is to modem. A totally different beast. The end
user
still surf the internet (resp. make phone calls), the technology in the middle is just
not
the same. And yet, it's only an electrical (resp. a radio) signal on a telephonic wire
(resp. on the air).

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