UNIVERSITY OF GHANA
Computer Engineering Department
CPEN 308: Fundamentals of Information Transmission
Lab 4: Amplitude Modulation and Demodulation Implementation with
SIMULINK
Date:
Objective
To understand the theoretical foundations of Analog Communications as
well as of Double-Side-Band Amplitude Modulation and Demodulation
(DSB-AM)
To design the Simulink model of the DSB-AM to analyse each signal in
time and frequency domains using time scope and spectrum analyser
To examine the effects of the Additive Gaussian Channel (AWGN) in the
Simulink Model of DSB-AM
Theoretical Background
a. Review of Signals & Systems, Probability and Noise and Starters’
Guide
To understand the theory along with the experiments behind this course,
the review sections were prepared. It is highly recommended that the
Review and the Starters’ Guide are understood. Please see the instructor
for any further information.
As a reminder, Review of Signals & Systems, Probability and Noise is
valid for all experiments.
b. Fundamentals of Analog Communications
Analog Communication is an information transmitting mechanism, i.e.,
music, voice, and video using broadcast radio, walkie-talkies, or cellular
radio, and broadcast television. The significant invention made by
Marconi in 1895 was a radio. Later, the foundation of Trans-Atlantic
Communication Systems had been taken place. Although digital
communications systems are much more efficient, cost-saving, more
reliable, some communication systems are still analogue.
Analog communication techniques can be summarized as
Advantages of modulation:
o Size of the antenna reduces when a signal is modulated by a larger
frequency of a carrier.
Antenna size = L = λ = cfc,
where c = speed of the light =3×108m/s
o Using modulation to transmit the signal through space to long
distances. Therefore, Wireless Communication techniques has
raised our standards considerably.
o Modulation allows us to transmit multiple signals in the same
medium (i.e., Frequency Division Multiplexing, FDMA)
b. Amplitude Modulation and Demodulation
Let ωc = 2πfcωc = 2πfc be the carrier frequency in radians per second
where fc > Wfc > W. Then the amplitude modulated signal s(t) can be
expressed as
s ( t )=AC [ 1+ μm ( t ) ] cos ( 2 π f c t )
s ( t )=AC cos ( 2 π f c t ) + ACμm ( t ) cos ( 2 π f c t )
where μ is the modulation index defined in −1 < μ< 1
As an example, the following figure shows the Amplitude Modulation
with m ( t ) =sin(2 πt )❑,
AC=1, μ=0.9, and fc =10Hz
Recall: Modulation Property
⟺ 12[M(f−fc) +
m(t) is multiplied by cos(2πfct).
⟺
m(t)∙cos(2πfct) M(f+fc)], where f is the
frequency of m(t)m(t)∙cos(2πfct) 12[M(f−fc)+M(f+fc)],
In general, the AM modulation is summarized as:
In case of carrier, which could be used sine or cosine wave. Practically,
there is no difference except -90-degree phase shift.
Remark:
Any signal is summed by a constant value means that this signal is raised
by the same constant value with respect to the vertical axis in time
domain. In frequency domain, the constant value is represented by an
impulse at f= 0Hz
Demodulation
For AM demodulation, we will examine the Square-Law and Envelope
Detector techniques.
Demodulation by Squaring
s2(t)=((1+μm(t)) cos(2πfct))2, where cos2(wct)=12(1+cos(4πfct))
s2(t)=12(1+μm(t))2+12(1+μm(t))2cos(4πfct)
The high frequency is removed after filtering,
=12(1+μm(t))2, then=12(1+μm(t))2, then
M(t)=14(1+μm(t))
Synchronous Demodulator
The block diagram of synchronous demodulator is as shown
For the low pass to detect the information envelope, the frequency of the
carrier must be as high as possible. However, as you can imagine the
noise from the nature (i.e., white noise) cannot be filtered/removed
perfectly in such analogue transmissions (AM, or FM).
s(t)=sin(2πfct) +m(t)2sin(2πfct−2πfmt) −m(t)2sin(2πfct+2πfmt)
After the multiplication of s(t)×sin(2πfct)
= −m(t)2sin(2πfmt) −12sin(2πfct) −m(t)2(4πfct−2πfmt)
+m(t)4sin(2πfct+2πfmt)
Then, the low-pass filter removes the higher frequency components, so
we can recover m(t).
2. Building Simulink Model of Amplitude Modulator and Demodulator
Modulation
The Simulink design of an Amplitude modulator is in the following [2] (M.
Boulmalf, 2010)
Parameters:
Double click on the signal generator, and then set the frequency as
1 kHz with a waveform of sine
Adjust the carrier sine wave’s frequency as 20 kHz
Set the simulation time such as 0.01 to observe the signals clearly
Run your simulation
To observe the spectrum analyser, please increase the simulation
time to 1 or 2 seconds.
As it is clearly seen that the AM model is exactly based upon the
mathematical foundation provided in the theoretical section. The
message signal is multiplied by the modulation index, then it is added a
DC carrier, finally is multiplied with a sinusoidal carrier signal to
transmit the AM modulated signal.
Demodulation (Square-Law Demodulator)
Apply the similar procedure. You will have the demodulation structure as
shown in the following figure:
Specify the band edge frequency as 2*pi*X
Connect your modulation and demodulation models as shown.
Run your model, you will then observe the following
Simulation time is chosen to be 2 secs for the spectrum analysers.
Lab Tasks
1. [Synchronous Detector]
Build the model given below [3], and then set up the block parameters as
o m(t) with frequency of 1kHz and sample time:1/100kHz
o carrier: 10kHz, phase: π/2π/2 and sample time: 1/100kHz
o Local Oscillator (LO): same as carrier.
o Filter: Lowpass, Fs:100kHz, Fpass:6 and Fstop:12
o Set up the simulation time as 50k/100k
o Run your model
a. Observe the 3 spectrum analysers, then explain the
waveforms from the frequency point of view (Hint:
remember modulation property). Comment your result.
b. Change the simulation time to 500/100k (to clearly see the
sine wave). Compare signals in the time scope? Did you
recover m(t)? Is there any delay between two signals? If yes,
explain, why?
2. Build the Simulink model of AM modulator and demodulator
explained in this manual. You must determine the analogue filter’s
passband edge frequency. Then, explain the theoretical side of the
blocks. Use the notation as μ: modulation index, m(t), h(t) etc.
References
[1] H. Taub, D. L. (2008). Principles of Communication Systems (3rd ed.).
McGraw Hill.
[2] M. Boulmalf, Y. S. (2010). Teaching Digital and Analogue Modulation to
Undergraduate Information and Technology Students Using MATLAB and
Simulink. IEEE.
[3] Simulink model of Perfect Modulation and Demodulation, Software Defined
Radio using MATLAB & Simulink and the RTL-SDR, Strathclyde Academic
Media, 2015
[4] The MathWorks Inc ®, Envelope Detection,
