ELEC 470

Prof. Rich Kozick

Spring, 1998

**Design Project 2 **

Recovery of Message Signals from a
Frequency Division Multiplexed Mystery Signal

**Date Assigned:** Wednesday, March 4, 1998

**Date Due:** Monday, March 9 and Wednesday, March 11, 1998

### The Problem:

Because of your expertise in analog communication systems,
you have been hired by a special agent to recover some
messages from a sampled signal.
Frequency division multiplexing (FDM) with some form
of analog modulation (AM, DSB-SC, SSB, VSB, FM) was used to
encode the messages to produce the signal that is available
to you.
Your job is to analyze this signal, determine how many
messages are present, determine the carrier frequencies,
determine the type of modulation used at each carrier frequency,
and finally recover each of the individual messages
through an appropriate demodulation process.
### Some Things That Are Known:

- More than one message signal is present.
- The message signals have zero DC (or average) level.
- The bandwidth of each message signal is less than 200 Hz.
- Each carrier frequency is a multiple of 1000 Hz.
- A different type of modulation may be used at each
carrier frequency.

### Some Hints:

- The sampled signal that you are to analyze can be loaded
into MATLAB by first saving the file
`data`
in your account with the filename `data`, and then
typing at the MATLAB prompt:
`load data -ascii
`

x = data;

clear data

The variable `x` contains the signal to be processed.
- The spacing between samples in this file is 50 microseconds,
which corresponds to a sampling rate of 20,000 samples per second.
- The FFT is a useful tool for determining the
carrier frequencies and the modulation type.
- The example MATLAB program
`mod_ex.m`
shows how to use the FFT, perform various types of demodulation,
and perform low-pass and high-pass filtering.
Note that the sampling rate in `mod_ex.m`
is *different* from the sampling rate in this project.
- If you need a coherent detector, then the phase of
the local oscillator will be important.
Think of some (simple?) ways to determine the proper phase.
- Using matlab5 on the Sun computers is recommended.
The programs may work fine on other platforms and versions,
but I have not tested them.

### What to Submit and When:

- For Monday, March 9, please come to class with
questions that you have, and I will help you.
You should have a very good idea by Monday about which
types of modulation are present, but you may have questions
about how to do certain processing in MATLAB.
In other words, by Monday you should have analyzed the
available signal and developed a system-level description
of the processing that is needed to recover the messages.
I will help you to implement your system in MATLAB.
- Submit your results on Wednesday, March 11.
Your results should include a discussion of your approach,
including how you determined the modulation types,
how you performed the processing, the MATLAB program
that you used, and plots of the demodulated message
signals in the time and frequency domains.
- Your report should be mostly explanation, and not
a MATLAB program with some plots attached!

Thank you and have fun!