Experiment No: 12 Modulation Using Matlab Aim:: Advanced Communication Laboratory

Advanced Communication Laboratory
EXPERIMENT NO: 12
MODULATION USING MATLAB
AIM:
To perform digital modulation using MATLAB.
APPARATUS REQUIRED:
Pentium 4 processor, MATLAB Editor.
THEORY:
Amplitude Shift Keying (ASK) is the digital modulation technique. In amplitude shift keying,
the amplitude of the carrier signal is varied to create signal elements. Both frequency and
phase remain constant while the amplitude changes. In ASK, the amplitude of the carrier
assumes one of the two amplitudes dependent on the logic states of the input bit stream. This
modulated signal can be expressed as: Amplitude shift keying (ASK) in the context of digital
signal communications is a modulation process, which imparts to a sinusoid two or more
discrete amplitude levels. These are related to the number of levels adopted by the digital
message. For a binary message sequence there are two levels, one of which is typically zero.
Thus the modulated waveform consists of bursts of a sinusoid. There are sharp discontinuities
shown at the transition points. These result in the signal having an unnecessarily wide
bandwidth. Band limiting is generally introduced before transmission, in which case these
discontinuities would be rounded off. The band limiting may be applied to the digital
message, or the modulated signal itself. The data rate is often made a sub-multiple of the
carrier frequency.
Generation of PSK signal
PSK is a digital modulation scheme that conveys data by changing, or modulating, the phase
of a reference signal (the carrier wave). PSK uses a finite number of phases, each assigned a
unique pattern of binary digits. Usually, each phase encodes an equal number of bits. Each
pattern of bits forms the symbol that is represented by the particular phase. The demodulator,
which is designed specifically for the symbol-set used by the modulator, determines the phase
of the received signal and maps it back to the symbol it represents, thus recovering the
original data. In a coherent binary PSK system, the pair of signal S1(t) and S2 (t) used to
represent binary symbols 1 & 0 are defined by
S1 (t) = 2Eb/ Tb Cos 2fct
S2 (t) =2Eb/Tb (2fct+) = - 2Eb/Tb Cos 2fct where 0 t< Tb and
Eb = Transmitted signed energy for bit
The carrier frequency fc =n/Tb for some fixed integer n.
Antipodal Signal:
Department of Electronics & Communication Engineering
The pair of sinusoidal waves that differ only in a relative phase shift of 180 are called
antipodal
signals. The input binary symbols are represented in polar form with symbols 1 & 0
represented by constant amplitude levels Eb & -Eb. This binary wave is multiplied by a
sinusoidal carrier in a product modulator. The result in a BSPK signal.
Generation of FSK
Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information
is transmitted through discrete frequency changes of a carrier wave. The simplest FSK is
binary FSK (BFSK). BFSK uses a pair of discrete frequencies to transmit binary (0s and 1s)
information. With this scheme, the "1" is called the mark frequency and the "0" is called the
space frequency.In binary FSK system, symbol 1 & 0 are distinguished from each other by
transmitting one of the two sinusoidal waves that differ in frequency by a fixed amount.
Si (t) = 2E/Tb cos 2f1t 0 t Tb0 elsewhere
Where i=1, 2 & Eb=Transmitted energy/bit .Transmitted freq= i = (nc+i)/Tb, and n =
constant (integer), Tb = bit interval Symbol 1 is represented by S1 (t) ,Symbol 0 is
represented by S0 (t).
PROCEDURE:
ASK modulation
1. Generate carrier signal.
2. Start FOR loop
3. Generate binary data, message signal(on-off form)
4. Generate ASK modulated signal.
5. Plot message signal and ASK modulated signal.
6. End FOR loop.
7. Plot the binary data and carrier.
PSK modulation
1. Generate carrier signal.
2. Start FOR loop
3. Generate binary data, message signal in polar form
4. Generate PSK modulated signal.
5. Plot message signal and PSK modulated signal.
6. End FOR loop.

7. Plot the binary data and carrier.
FSK modulation
1. Generate two carriers signal.
2. Start FOR loop
3. Generate binary data, message signal and inverted message signal
4. Multiply carrier 1 with message signal and carrier 2 with inverted message signal
5. Perform addition to get the FSK modulated signal
6. Plot message signal and FSK modulated signal.
7. End FOR loop.
8. Plot the binary data and carriers.
MODEL GRAPH:
ASK modulation
PSK modulation
FSK modulation
RESULT: The program for ASK modulation, FSK modulation and PSK modulation has been
simulated in MATLAB and necessary graphs are plotted.

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Advanced Communication Laboratory

Advanced Communication Laboratory

Advanced Communication Laboratory

6. End FOR loop.

Department of Electronics & Communication Engineering

Advanced Communication Laboratory

Department of Electronics & Communication Engineering

Advanced Communication Laboratory

Department of Electronics & Communication Engineering

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