School of Science and Engineering
AL AKHAWAYN UNIVERSITY
LAB REPORT
CSC 3371L
Student’s Name: Mohamed Chaouni
Date: Fall 2021
Supervised by: Dr. Mohammed Abbou
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1. INTRODUCTION
The purpose of the lab experiments ..
 Write the objectives using your own words.
Understanding the mechanisms involved in message transmission through 4
procedures by putting various analogue and digital modulation and demodulation
techniques to use in actual simulations.
 Briefly describe any special equipment, systems, software that you have used to
execute the lab procedure.
We mostly used a software to execute the lab procedures (simulated models), which
is MATLAB and the Simulink integrated package.
Intended objectives:
To determine an AM signal's modulation index.
To define an AM signal's frequency spectrum.
To explain the technology of QPSK digital modulation.
To create a graphic of the QPSK constellation
We aimed to distinguish a message signal, a carrier signal, and AM
modulated/demodulated waveforms using the program.
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2. THEORETICAL BACKGROUND
The technique of superimposing a low-frequency signal over a high-frequency carrier
signal is known as modulation or variation of the RF carrier wave in response to the
information in a low-frequency signal.
During our Labs for analog transmissions, we looked at amplitude modulation, and for
digital signals, we looked at QPSK.
Modulation alters the structure of a carrier wave in order to encapsulate the data that
we want to send. Modulation is akin to encoding data in the carrier wave.
Time domain representation of the waves
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Modulating signal:
Carrier signal:
Where,
Am and Ac are the amplitude of the modulating signal and the carrier signal
respectively.
fm and fc are the frequency of the modulating signal and the carrier signal respectively.
Then, the equation of Amplitude Modulated wave will be
Modulation Index:
When a carrier wave is modulated and the modulated level is computed, the attempt is
referred to as Modulation Index. It specifies the amount of modulation a carrier wave
receives.
By rearranging the s(t) equation we obtain:
Where, μ is Modulation index and it is equal to the ratio of Am and Ac. Mathematically,
we can write it as
The AM spectrum:
The relative quantities of distinct frequency components of any signal are represented
by a spectrum.
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QPSK modulation:
Data is communicated by shifting the phase of a constant frequency carrier signal in
phase shift keying, a digital modulation procedure. PSK assigns a unique pattern of
binary digits to each phase based on the number of phases selected.
Quadrature Phase Shift Keying (QPSK) is a technique that employs four phases to
represent four binary digits. This allows twice as much data to be delivered without
requiring more bandwidth than standard PSK.
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3. EXPERIMENTAL PROCEDURES
We simulated amplitude modulation with 3 variations of the modulation index using
MathLab:
Procedure 1:
M-File:
fs: sampling frequency
d: duration of the signal
n: total nb of samples
t: time vector
sinusoidal signal with a frequency of fm= 50 Hz
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Carrier with fc =1000 Hz
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amplitude modulated waveform:
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three results in one page
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AM1 AM2 AM3:
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Procedure 2:
Simulink Model Simulating AM
Spectrum of the signal (Using Fourier transformation to get the spectrum)
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Results:
Information Signal:
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Carrier Signal:
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Frequency Spectrum:
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Procedure 3:
This program seeks to implement Envelope Detection, a basic demodulation technique.
This necessitates a modulation index of no more than one.
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The recovered signal here is from the modulated signal that was from the original one
below. So, this signal is not perfect since it didn’t go through the cascaded filters,
furthermore, the more we increase the number of the filters we get a smoother signal
graph. The more cascaded filters we use the modulated signal becomes less distorted.
So the larger the signal the better we use as many filters needed since the first
demodulated signal can become distorted from all the noise
After 3 cascaded filters:
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Original Signal:
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Recovered Signal (after 1st filter):
After 2nd cascaded filter:
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After 3 cascaded filters
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Scope 6:
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Procedure 4:
Quadrature Phase-Shift Keying modulation and demodulation:
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4. ANALYSIS AND DISCUSSION
Amplitude Modulation analysis (Procedure 1)
Modulation Index of 0.5:
When Am = 0.5 V, the index u = Am/Ac = 50%, it is obvious that we have an
undermodulation of the waveform This is evident from the fact that
Amax = Ac + Am = 1.5V, with Psb = (u2)/8 * (Ac)2 = 31.25mW for the sidebands.
The outcome is a waveform that is acceptable.
A Modulation Index of 1:
We attain perfect modulation with Am = 1V, when the waveform increases.
The voltage rises to 2V and then falls to 0V. Psb is equal to 125mW.
The outcome is a waveform that is acceptable.
A Modulation Index of 1.5:
Results in an u = 150 percent when Am is set to 1.5V. After decreasing to 0V, the
waveform becomes distorted with a phase reversal. Amin = 2.5V, Amax = 2.5V
Psb= -0.5V = 281.25mW.
Unless the phase reversal is corrected, this results in an unsatisfactory waveform.
To conclude when the index exceeds 1, choosing a modulation index becomes a job of
maximizing AM's low power efficiency and phase reversal.
Amplitude Modulation with simulink analysis (Procedure 2)
The findings of the modulated signals acquired with simulink from the spectrum
analyzer are comparable to the Modulated Signals with Varying Modulation Indexes,
with the peaks representing the sidebands and the carrier.
Am = 1
The proportion of power in the sidebands is calculated as follows:
277.565 mW total power
30.771 mW power in both sidebands
Percentage of power in sidebands: 11%
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Am = 0.5
The proportion of power in the sidebands is calculated as follows:
369.874 mW total power
123.081 mW power in both sidebands
Percentage of power in sidebands: 33.28%
Am = 1.5
The proportion of power in the sidebands is calculated as follows:
523.727 mW total power
276.934 mW power in both sidebands
Percentage of power in sidebands: 52.88%
The % power of the sidebands is consistent with the conceptual viewpoint presented in
the theoretical background.
To conclude, due to a constant carrier power, the power imbalance between the
sidebands and the carrier moves in favor of the sidebands as the modulation index
grows. This has the negative consequence of causing phase reversals in the output
signal, which must be filtered out.
Envelope Detection analysis (Procedure 3)
-The envelope detector removes the high carrier frequencies from the modulated signal
while maintaining the information stream. It takes numerous passes to smooth out the
signal.
-Adding a low-medium frequency noise signal changes the information signal and causes
data loss.
-Adding a high-frequency noise signal has minimal effect since the filters remove the
high frequencies from the signal.
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QPSK analysis (Procedure 4)
When we compare QPSK to modulation schemes that transmit one bit per symbol, we
notice that QPSK is advantageous in term of bandwidth efficiency since the QPSK can
use both baseband signal and the frequency at the same time, and it transmits two bits
during each symbol, and the modulation schemes can transmit one bit per signal.
Therefore, the bandwidth efficiency of QPSK is higher by a factor of 2.
5. CONCLUSION
To conclude we completed the objectives outlined in the introduction by going
through the four procedures, and we learned that modulation and demodulation are
critical in the field of computer science. We created and simulated an Amplitude
Modulation Setup (the process of combining the signal and carrier messages to obtain
the modulated signal) using MATLAB and SIMULINK signal and commutation toolboxes
and monitored the effects of changing the modulation index on the power balance
between the carrier and sidebands, as well as on the Modulated Signal. We observed a
virtual version of the theoretical equation used to plot the demodulation and
modulation graphs in procedure 2. The 3rd procedure featured Envelope Detection, a
low-cost and simple-to-implement demodulation technology in which we monitored the
effects noise has on the Envelope Detection method. Finally, the 4th procedure where
we moved on to digital modulation techniques through QPSK. We developed a model to
modulate a bit stream into four varying values by utilizing phase shifts. These values
were later mapped onto a constellation chart.
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6. REFERENCES
https://byjus.com/physics/modulation-and-demodulation/
https://www.tutorialspoint.com/analog_communication/analog_communication_ampli
tude_modulation.htm
https://man.fas.org/dod-101/navy/docs/es310/AM.htm
https://www.allaboutcircuits.com/technical-articles/quadrature-phase-shift-keyingqpsk-modulation/
https://www.maximintegrated.com/en/glossary/definitions.mvp/term/QPSK/gpk/244#:
~:text=Quadrature%20Phase%20Shift%20Keying%20(QPSK,PSK%20using%20the%20sa
me%20bandwidth.
https://www.edumedia-sciences.com/en/media/311-amplitude-modulation-am
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