Hi,
>
>On Wednesday, 7 November 2001 11:46 PM Marc Printz wrote:
>
> >> Now: that is interesting. How does the call setup work? Tedious?
> >> Or could you in principle with the press of one link get a voice
> >> connection to an IVR system up and at the same time an i-mode
> >> connection? What about i-appli, is it possible to have concurrent
> >> i-appli running? THIS would be excellent.
>
>
>Without having exact details of FOMA's internal handset and base station
>architecture (which I am guessing they keep secret) one cant say exactly
>how it is implimented (anyone that knows specifics please sing out and
>feel free to correct the figures that I guessed)
You're actually describing GPRS, not WCDMA. :-)
WCDMA is a lot harder to explain than a TDMA system like GPRS. I'll give it
a try anyway.
WCDMA is a standard using CDMA. CDMA stands for code-division multiple
access. It means that a radio resource is divided among users based, not on
timeslots (as in TDMA) or on different frequencies (as in FDMA) but, on
codes. Each user is assigned a specific "code" which is used to encode the
message. All users then transmit simultaneously using the same frequencies.
One normally uses the so-called Cocktail Party Analogy to illustrate this.
Assume that you understand English but not French or Russian. You're at a
cocktail party and in your vicinity there are three simulatenous
conversations, in English, French and Russian, respectively. Since you only
understand English, you will capture the information in the English
conversation, but the French and Russian ones will be just like noise to
you. Similar for a monoglot Frenchman or Russian. On the other hand, a
person fluent in both French and English will -- at least in theory -- be
able to follow both conversations. Voila, there's your multimode keitai!
It's "just" a matter of signal processing to decode several concurrent
messages in realtime.
That's the thing with CDMA. You can have 10 or 100 channels using the same
frequency band, but your receiver, decoding with _your_ code will only hear
the message intended for you. The other channels will be just background
noise.
I don't want to go into too many technical details about the encoding, but
it's actually quite simple. One thing it does, is that it increases the
bandwidth of the message. Another word for the encoding is therefore
_spreading_ and CDMA is referred to as a "spread-spectrum radio technique".
Spread-spectrum techniques have been used by the military for decades. One
of the more astounding features is that the signal strength of the message
is allowed to be very weak, since you have to decode it with the proper
code. For instance, it's possible to send a message with a signal strength
that lies far below the background noise level! This means that your enemy
won't even know that you're transmitting something unless he's listening
with the right code. Spread-spectrum techniques are also very robust against
jamming.
Regards,
Joran
.
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Received on Sat Nov 10 18:45:26 2001