To generate the waveform of Phase Shift Keying (PSK) signal using MATLAB

Objective:

To generate the waveform of Phase Shift Keying (PSK) signal using MATLAB

Software required:

1. MATLAB
2. Computer installed with Windows XP or higher Version

Theory

Generation of PSK: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 2πfct

S₂(t) =√2Eb/Tb (2πfct+π) = - √ 2Eb/Tb Cos 2πfct where 0 ≤ t< T _b and

E_b=Transmitted signed energy for bit

The carrier frequency f _c= n/T _b for some fixed integer n.

Antipodal Signal:The pair of sinusoidal waves that differ only in a relative phase shift of 180° are called antipodal signals.

BPSK Transmitter: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 is a BSPK signal as shown in figure 7.1.

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.

Program:

%ASK Modulation

clc;

clear all;

close all;

%Generate Carrier Signal

Tb=1;

t=0:Tb/100:Tb;

fc=2;

c=sqrt(2/Tb)*sin(2*pi*fc*t);

%generate message signal

N=8;

m=rand(1,N);

t1=0;t2=Tb

for i=1:N

t=[t1:.01:t2]

if m(i)>0.5

m_s=zeros(1,length(t));

m(i)=1;

m_s=ones(1,length(t));

else

m(i)=0;

m_s=-1*ones(1,length(t));

end

message(i,:)=m_s;

%product of carrier and message signal bpsk

sig(i,:)=c.*m_s;

%Plot the message and BPSK modulated signal

subplot(5,1,2);

axis([0 N -2 2]);

plot(t,message(i,:),'r');

title('message signal(POLAR form)');

xlabel('t--->');

ylabel('m(t)');

grid on;

hold on;

subplot(5,1,4);

plot(t,bpsk_sig(i,:));

title('BPSK signal');

xlabel('t--->);

title('BPSK signal');

ylabel('s(t)');

grid on;

hold on;

t1=t1+1.01;

t2=t2+1.01;

end

hold off

%plot the input binary data and carrier signal

subplot(5,1,1);

stem(m);

title('binary data bits');

xlabel('n--->');

ylabel('b(n)');

grid on;

subplot(5,1,3);

plot(t,c);

title('carrier signal');

xlabel('t--->');

ylabel('c(t)');

grid on;

Observation:The desired BPSK waveforms i.e. binary data, message signal, carrier signal and output waveforms are shown in figure 6.1.

Conclusion:The program for binary PSK modulation has been simulated in MATLAB and observed the desired waveforms.

Questions:

1. Write down the drawback of DPSK compared to BPSK?
2. Why do we make 180 degree phase shift in PSK and why not 90 or 270? Comment on this.
3. Compare FSK and PSK.