**AIM**

To
simulate the direct sequence spread spectrum modulation techniques
using MATLAB program and to calculate the bit error rate

**THEORY**

Spread
Spectrum is a means of transmission in which the data of interest
occupies a bandwidth in excess of the minimum bandwidth is necessary
to send the data.

The
primary advantage of a spread-spectrum communication system is its
ability to reject interference whether it is the unintentional
interference of another user simultaneously attempting to transmit
through the channel, or the intentional interference of a hostile
transmitter attempting to jam transmission.

Spread
Spectrum modulation was originally developed for military
applications where resistance to jamming is of major concern. Another
application is in multi access communication in which a number of
independent users are required to share a common channel without an
external synchronizing mechanism.

Principles
of spread spectrum modulation emphasis on direct sequence and
frequency hopping techniques. In a direct sequence spread spectrum
technique two stages of modulations are used. First, the incoming
data sequence is used to modulate a wide band code. This code
transforms the narrow band data sequence into a noise-like wide band
signal. The resulting wide band signal undergoes a second modulation
using a phase shift keying technique.

In
a frequency-hop spread spectrum technique, the spectrum of a data
modulator carrier is widened by changing the carrier frequency in a
pseudo random modulator manner. For their operation both of this
techniques rely on the ability of a noise like spreading code called
a pseudo random or pseudo noise sequence.

**PN SEQUENCE**

A
Pseudo Noise (PN) sequence is defined as a coded sequence of 0’s &
1’s with certain autocorrelation properties. The maximum length
sequence, a type of cyclic code is commonly used as a periodic PN
sequence.

In
case of spread spectrum a Periodic PN sequence is used with a period
of N = 2

^{m}-1, where m is the length of the shift register. Such sequences have long periods and require simple instrumentation in the form of a linear feedback shift register. PN sequence may also be a periodic. Such sequence is known as Barker sequence.**DIRECT SEQUENCE SPREAD SPECTRUM**

The
data sequence b (t) is a narrow band sequence whereas the PN sequence
c(t) is a wideband sequence. The product signal m (t) i.e.b (t)*c (t)
will have a spectrum that is nearly same as that of the PN sequence.

In
this the PN sequence performs the role of spreading the code by
multiplying the information bearing signal b (t) by the spreading
code c (t). Each information bit is chopped up into a number of small
time increments. This small time increment is called as chip.

**ALGORITHM**

- Generate the signal which is to be spread.
- Generate the PN sequence.
- Multiply the PN sequence with message signal.
- Display the pseudo sequence and frequency-hop spread sequence.

**OUTPUTS:**

**DIRECT SEQUENCE SPREAD SPECTRUM**

**ORIGINAL SEQUENCE**

**PSEUDO RANDOM BIT SEQUENCE**

**DSSS SIGNAL**

**FFT OF DSSS SIGNAL**

**RESULT**

Thus
the MATLAB program for direct sequence spread spectrum technique was
executed and the waveforms were obtained.

PROGRAM

clc;

clear all;

s=round(rand(1,20));

Pattern=[];

for i=1:20

if(s(1,i)==1)

SS=zeros(1,6);

else

SS=ones(1,6);

end

Pattern=[Pattern SS];

end

figure,plot(Pattern);

axis([1,120,-1,2]);

title('Original Signal');

Pr=round(rand(1,120));

figure,plot(Pr);

axis([1,120,-1,2]);

title('Pseudo Random Number');

Hopped_sig=xor(Pattern,Pr);

% Modulating Hopped Signal

DSSS_sig=[];

t=1:120;

fc=0.1;

f1=cos(2.*t.*pi.*fc);

f2=cos(2.*t.*pi.*fc+pi);

for j=1:120

if(Hopped_sig(j)==true)

DSSS_sig=[DSSS_sig f1];

else

DSSS_sig=[DSSS_sig f2];

end

end

figure,plot(DSSS_sig);

axis([1,1200,-1,2]);

title('DSSS Signal');

figure,plot(abs(fft(DSSS_sig)));

% axis([1,120,-1,2]);

title('FFT');

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