DIRECT SEQUENCE SPREAD SPECTRUM

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

1. Generate the signal which is to be spread.
2. Generate the PN sequence.
3. Multiply the PN sequence with message signal.
4. 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');