6: Modulation and Multiplexing Techniques
the work-script for chapter 6
18 November 2011
found the parameters for the quality of an analogue transmission and a
digital transmission, S/N and BER, respectively.
Each of the parameters depends on C/N and also on the modulation
technique. That gets us into analogue and
digital modulations. There are three types for each of the two kinds of
modulation. First we looked at analogue modulation.
plotted a block diagram with a carrier modulator having two inputs, the
carrier and the modulating signal which is
the baseband signal. The output is the modulated carrier. We assumed
for the carrier a cos wave and for the baseband signal also a cos wave,
however at a much lower frequency. We wrote down the equations for AM,
FM and PM.
We plotted the amplitude modulated carrier and imagined how it would
change if we changed the modulation index.
plotted the spectrum of the baseband signal and the AM carrier.
21 November 2011
general case of a continuum of a spectrum, we use the symbolic triangle
for the spectrum. We learned about upper side band (USB) and lower side
band (LSB). We also learned that AM does not provide an improvement of
the S/N in comparison to the C/N. Basically the highest S/N for
amplitude modulation cannot be larger than C/N. Then we turned to FM.
We plotted the the amplitude as a function of time for a simple cases
of a FM modulated carrier and looked at the
equation for a frequency modulated carrier. Bessel functions were
here for a demonstration of AM and FM.
looked at the FM waveform and thought how the spectrum of the FM
carrier would look. We realized that the theoretical bandwidth of an FM
waveform is infinite. However, Carson's rule gives a bandwidth where
most of the information is contained. We looked at examples. Then we
focussed on the S/N ratio for the FM case and learned that
it can indeed be larger than the C/N.
Although the theoretical bandwidth of an
FM waveform is infinite, Carson's rule gives a bandwidth where
most of the information is contained. We looked at examples.
We looked at the improvement
factors. Then we turned to multiplexing.
We looked at digital baseband signals and
digital modulation and the bandwidth of a digitally modulated signal.
saw the whole scheme of a transmitting and receiving end of a
typical FDM system.
We looked at an
example of how to compute the S/N ratio and saw that it indeed can be
much larger than the C/M ratio for FM. We looked at the bit error rate
and the improvement possibilities.
We learned about TDM and why
raised cosine filters are often used in communication networks.