The document describes a simulation project for a communication link using AM and PSK modulation. Students are asked to design and simulate a communication link using AM modulation to transmit an audio signal, investigating the effects of different message signal frequencies and modulation indices. They also simulate communication links using BPSK and QPSK modulation schemes, comparing the performance of each in terms of bandwidth efficiency and required signal power. The project uses Matlab and Simulink to generate signals, design modulators and demodulators, and simulate the overall communication links.

1. Communication Link Simulation
Author: University of Hertfordshire
Date created:
Date revised: 2009
Abstract
The following resources come from the 2009/10 BEng (Hons) in Digital Communications & Electronics
(course number 2ELE0064) from the University of Hertfordshire. All the mini projects are designed as level
two modules of the undergraduate programmes.
The objective of this module is to have built communication links using existing AM modulation, PSK
modulation and demodulation blocks, constructed AM modulators and constructed PSK modulators using
operational function blocks based on their mathematical expressions, and conducted simulations of the links
and modulators, all in Simulink®.
Use Matlab®/ Simulink® to design a communication link for AM audio broadcasting. The message signal is a
mono audio signal although you may not be able to transmit the full audio frequency range that is normally
required for high quality sound.
Contents
Communication Link Simulation........................................................................................................................1
Day 1. Design and simulation of a communication link using AM....................................................................2
Day 2. Design and simulation of communication links using PSK....................................................................3
Matlab® and Simulink® Assignment.................................................................................................................4
Time and frequency domains of a square wave............................................................................................4
Sampling.......................................................................................................................................................4
Spectrum of a Sampled Signal......................................................................................................................5
Digital filters...................................................................................................................................................5
Credits..............................................................................................................................................................7
In addition to the resources found below there are supporting documents which should be used in
combination with this resource. Please see:
Mini Projects - Introductory presentation.
Mini Projects - E-Log.
Mini Projects - Staff & Student Guide.
Mini Projects - Standard Grading Criteria.
Mini Projects - Reflection.
You will also need the ‘Mini Project- Communication Link Simulation’ presentations containing the ‘Channels
Signal and Noise’, and the ‘Digital Modulation’ lectures..
© University of Hertfordshire 2009. This work is licensed under a Creative Commons Attribution 2.0 License.

2. Mini Project- Communication Link Simulation
Day 1. Design and simulation of a communication link using AM
Expected Outcomes for the day:
To have built communication links using existing AM modulation and demodulation blocks, constructed AM
modulators using operational function blocks based on their mathematical expressions, and conducted
simulations of the links and modulators, all in Simulink®.
Assessment Criteria:
Diagrams of communication links and modulators, simulated/calculated results and performances such as
spectra (frequency domain), waveforms (time domain), bandwidth, power and SNR, analysis and
discussions of results
Detailed Requirements:
Use Matlab®/ Simulink® to design a communication link for AM audio broadcasting. The message signal is a
mono audio signal although you may not be able to transmit the full audio frequency range that is normally
required for high quality sound.
The specification for the link is as follows:
Required signal to noise ratio (SNR) at the demodulated audio output of the receiver: 40 dB for a 1 kHz
message signal at 50% modulation (m = 0.5).
*Carrier frequency: 1.35 MHz
*Maximum RF bandwidth available 9 kHz
*Channel loss = 120 dB
*Channel noise power spectral density = -150dBm/Hz
Find out the following:
What is the highest frequency of the message signal that can be transmitted without exceeding the specified
RF bandwidth? For this message frequency, save a time domain plot and a frequency domain plot showing
the modulated RF output from the transmitter.
How much carrier power is required in order to achieve the required SNR? For this carrier power, how much
power is there in each sideband for the m = 0.5?
What is the SNR at the demodulated output if the frequency of the message signal is changed to the
following frequencies:
• 100 Hz
• The highest frequency that can be transmitted without exceeding the specified RF bandwidth
What is the SNR at the demodulated output if the modulation index m is increased to 1?
What happens if m > 1, e.g. if m = 1.1? Compare the demodulated output from the receiver in the time
domain and in the frequency domain for m = 1 and m = 1.1 and explain why a modulation index greater than
1 must be avoided in an AM link.
Prompts:
In order to complete the work required in the above, you will need to
• Generate baseband and carrier sinewave signals and AWGN noise
• Construct a channel model with constant loss and AWGN noise
• Construct an AM modulator with operational function blocks based on time-domain AM expression
• Construct a communications link using the built AM modulator, built channel model, and exiting AM
demodulator block in Simulink®.
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3. Mini Project- Communication Link Simulation
Day 2. Design and simulation of communication links using PSK
Expected Outcomes for the day:
To have built communication links using existing PSK modulation and demodulation blocks, constructed PSK
modulators using operational function blocks based on their mathematical expressions, and conducted
simulations of both links and modulators, all in Simulink®.
Assessment criteria:
Signals generated, link and modulator diagrams, simulation results including waveforms, constellations, BER
and SNR (or Eb/No), evaluation of results, contrasting between BPSK and QPSK.
Key Tasks:
• Generate baseband binary signals and carrier sinewave signals and AWGN noise
• Simulate and evaluate a communications link using BPSK with existing mod and demod blocks
• Simulate and evaluate a communications link using QPSK with existing mod and demod blocks
• Construct a BPSK modulator with operational function blocks based on the time-domain BPSK
expression, and simulate and evaluate the BPSK modulator.
Detailed Requirements for bullet link tasks 2 and 3:
1. You must measure BER against SNR or Eb/No and plot the performance curves according to the
data obtained.
2. For the same noise level, in order to achieve a BER of 10-4, what is the signal power ratio of the
BPSK and QPSK links?
3. Therefore, comment on BPSK and QPSK in terms of bandwidth efficiency and signal power
required.
4. Show waveforms at different points of the link with different SNR (or Eb/No)
5. Show the constellations of the modulators
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4. Mini Project- Communication Link Simulation
Matlab® and Simulink® Assignment
Time and frequency domains of a square wave
A square wave signal with unit frequency can be expressed as a summation of sinusoidal signals as
shown by the equation
1 1 1 1
square wave = sin(t ) + sin(3t ) + sin(5t ) + sin(7t ) + sin(9t ) + ....
3 5 7 9
Demonstrate the above principle using the Simulink®. Use five sinusoidal signals with the required
frequencies and amplitudes to produce an approximation for a square wave signal with 1 rad/sec frequency
and amplitude of 1. Plot the resultant signal. Save your work for future reference. To add the five sinusoidal
signals, use the sum in the math object.
Use the Simulink® to generate a square wave signal with 1 amplitude and 1 rad/sec frequency. Use the
power spectral density block to plot the spectrum of the square wave.
Sampling
Sampling of a signal can be achieved by multiplying the signal by a square wave signal, which has two
possible values 0 and 1. The principle of sampling can be illustrated using the Simulink® as shown in the
following example.
Example: Construct the system shown below. The pulse generator is used to produce the square wave
signal. The integrator works as a low pass filter.
sampling
signal sampled
signal
filtered
original
signal
signal
a) Set the frequency of the sinusoidal signal to 1 rad/sec. Set the frequency of the square wave signal
to 10 rad/sec. Note the sampled and filtered signals.
b) Reduce the frequency of the square wave signal to 6 rad/sec and record what you noticed.
c) Reduce the frequency of the square wave signal to 4 rad/sec and record what you noticed.
d) Reduce the frequency of the square wave signal to 3 rad/sec and record what you noticed.
e) Reduce the frequency of the square wave signal to 2 rad/sec and record what you noticed.
f) Reduce the frequency of the square wave signal to 1 rad/sec and record what you noticed.
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5. Mini Project- Communication Link Simulation
From this example what you can conclude.
Q. The minimum frequency of the square wave in order to sample the sinusoidal signal in the above example
correctly is ________ rad/sec.
Spectrum of a Sampled Signal
Connect the diagram shown below.
sampling sampled
signal signal
original filtered
signal signal
a) Set the frequency of the sinusoidal signal to 1 rad/sec. Set the frequency of the square wave signal
to 10 rad/sec. Notice spectrum of the original signal, the sampled and filtered signals.
b) Reduce the frequency of the square wave signal to 6 rad/sec. Notice the spectrum of the original
signal, the sampled and filtered signals.
c) Reduce the frequency of the square wave signal to 4 rad/sec. Notice the spectrum of the original
signal, the sampled and filtered signals.
d) Reduce the frequency of the square wave signal to 3 rad/sec. Notice the spectrum of the original
signal, the sampled and filtered signals.
e) Reduce the frequency of the square wave signal to 2 rad/sec. Notice the spectrum of the original
signal, the sampled and filtered signals.
f) Reduce the frequency of the square wave signal to 1 rad/sec. Notice the spectrum of the original
signal, the sampled and filtered signals.
From this example what you can conclude?
The minimum frequency of the square wave in order to sample the sinusoidal signal in the above example
correctly is ________ rad/sec.
Digital filters
Digital filters are made up of three basic components: adders, multipliers and delays. The figure
below shows a sample averager. Construct this using Matlab®/Simulink® and plot the result. Add a
spectral analyser and plot the output.
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6. Mini Project- Communication Link Simulation
Increase the number of samples averaged as shown below. Plot the results using a scope and then a
spectral analyser. What do you conclude?
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7. Mini Project- Communication Link Simulation
Credits
This resource was created by the University of Hertfordshire and released as an open educational resource
through the Open Engineering Resources project of the HE Academy Engineering Subject Centre. The Open
Engineering Resources project was funded by HEFCE and part of the JISC/HE Academy UKOER
programme.
© University of Hertfordshire 2009
This work is licensed under a Creative Commons Attribution 2.0 License.
The name of the University of Hertfordshire, UH and the UH logo are the name and registered marks of the
University of Hertfordshire. To the fullest extent permitted by law the University of Hertfordshire reserves all
its rights in its name and marks which may not be used except with its written permission.
The JISC logo is licensed under the terms of the Creative Commons Attribution-Non-Commercial-No
Derivative Works 2.0 UK: England & Wales Licence. All reproductions must comply with the terms of that
licence.
The HEA logo is owned by the Higher Education Academy Limited may be freely distributed and copied for
educational purposes only, provided that appropriate acknowledgement is given to the Higher Education
Academy as the copyright holder and original publisher.
Screen shots taken from Matlab® and/or Simulink ®, both of which are trade marks of The MathWorks, Inc.
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