Chapter 6: Modulation and Multiplexing Techniques

Here is the work-script for chapter 6

18 November 2011
We 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.
We 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.
We plotted the spectrum of the baseband signal and the AM carrier.

21 November 2011
For the 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 needed.
Click here for a demonstration of AM and FM.

We  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.

23 November 2011
We looked at the improvement factors. Then we turned to multiplexing.


25 November 2011
We looked at digital baseband signals and digital modulation and the bandwidth of a digitally modulated signal. The

28 November 2011
We 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.

30 November 2011
We learned about TDM and why raised cosine filters are often used in communication networks.