ADSL-like transmission****
Java applets simulate here ADSL-like transmissions ... Basically ADSL uses
multiple carrier modulation : multiple QAM (Quadrature Amplitude modulation).
QAM (Quadrature Amplitude modulation) ?
Basically QAM is an amplitude modulated sine wave at the same time as an
amplitude modulated cosine wave. Mathematics say that a sine + cosine at
the same frequency gives a cosine with a phase: this is why QAM can be
considered as an amplitude and phase modulation.
Also, if for instance, 2^3=8 amplitude levels can be be recognised at the
receiver, a triplet of bits can be assigned to each one : 000 1st level,
001 2nd etc.
Thus, sine + cosine combined with 8 amplitude levels => 2*3 = 6 bits
transmitted during the unit time ("symbol time"). A conventional modem
works like this. Experiment this with menu <one channel> and
button <transmit> . Receiver is opened with ...<receiver>
!
You can add noise in the transmission (click and drag in the middle box
within green rectangle), and see the perturbation on the signal and on
the receiver side.
ADSL (Asymmetric Digital Subscriber Line ) ?
An ADSL modem is n usual modems working at the same time on n
frequency subbands...
Transmitter: For each channel (frequency sub-band) one cosine
carrier + one sine are amplitude modulated by a part of the message bits.
This is done in paralel for all sub-bands (channels), i.e. this is a Fourier
synthesis (inverse Fourier transform). The transmitted (ADSL) signal is
built as the sum for all sub-bands k of : x(k)*cos(k*2*Pi*fo*t) + y(k)*sin(k*2*Pi*fo*t)
where x(k) and y(k) are numbers repesenting the carried bits i.e. the information(<transmit>
to
build the signal).
Receiver: The transmitted signal is now Fourier expanded : coefficients
are just the message bits.
You can see the bits restitution (and the message) when opening the receiver
(spectrum) window: signal+noise power spectrum : full coloured rectangle,
cosine components in blue, sine components in yellow.
The number of bits/channel depends on the sub-band noise: you can choose
it on this applet by drawing (with mouse) in the left box. Also, you can
add noise wherever you want by drawing in the middle box. Wrong bits are
in red. You can correct by reducing the number of bits transmitted in the
sub-band.<transmit> advances in the messages bits.
About ADSL, see also this
introductionand this one
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