FREQUENCY HOPPING SPREAD SPECTRUM

FREQUENCY- HOP SPREAD SPECTRUM

AIM

To simulate the frequency-hop 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.

FREQUENCY HOP SPREAD SPECTRUM

In a frequency-hop spread spectrum technique, the spectrum of a data modulated carrier is widened by changing the carrier frequency in a pseudo random manner. In this the carrier hops randomly from one frequency to another. There are two frequency hopping.

1. Slow-frequency hopping

In this the symbol rate R_s is an integer multiple of the hop rate R_h. ie. Several

Symbols are transmitted on each frequency hop.

R_s = nR_h

2. Fast-frequency hopping

In this the hop rate R_h is an integer multiple of the symbol rate R_s.ie. Carrier frequency will change or hop several times during the transmission of one symbol.

R_h = nR_s

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.

FREQUENCY HOPPING SPREAD SPECTRUM

clc;

clear all;

close all;

% Generation of bit pattern

s=round(rand(1,25));%generating 20 bits

signal=[];

carrier=[];

t=[0:2*pi/119:2*pi];

for k=1:25

if s(1,k)==0

sig=-ones(1,120);%120 minus ones for bit0

else

        sig=ones(1,120);

    end

    c=cos(t);

    carrier = [carrier c];

    signal = [signal sig];

end

subplot(4,1,1);

plot(signal);

axis([-100 3100 -1.5 1.5]);

title('|bf|it original bit sequence');

% BPSK modulation of the signal

bpsk_sig=signal.*carrier; %modulating the signal

subplot(4,1,2);

plot(bpsk_sig);

axis([-100 3100 -1.5 1.5]);

title('|bf|itbpsk modulated signal');

% Preparation of 6 new carrier frequencies

t1=[0:2*pi/9:2*pi];

t2=[0:2*pi/19:2*pi];

t3=[0:2*pi/29:2*pi];

t4=[0:2*pi/39:2*pi];

t5=[0:2*pi/59:2*pi];

t6=[0:2*pi/119:2*pi];

c1=cos(t1);

c1=[c1 c1 c1 c1 c1 c1 c1 c1 c1 c1 c1 c1];

c2=cos(t2);

c2=[c2 c2 c2 c2 c2 c2];

c3=cos(t3);

c3=[c3 c3 c3 c3];

c4=cos(t4);

c4=[c4 c4 c4];

c5=cos(t5);

c5=[c5 c5];

c6=cos(t6);

% Random frequency hops to form a spread_signal

spread_signal=[];

for n=1:25

    c=randint(1,1,[1,6]);

switch(c);

case(1)

spread_signal = [spread_signal c1];

case(2)

spread_signal = [spread_signal c2];

case(3)

spread_signal = [spread_signal c3];

case(4)

spread_signal = [spread_signal c4];

case(5)

spread_signal = [spread_signal c5];

case(6)

spread_signal = [spread_signal c6];

end

end

subplot(4,1,3);

plot(spread_signal);

axis([-100 3100 -1.5 1.5]);

title('|bf}if spread signal with 6 frequencies');

% Spreading BPSK signal into wideband with total of 12 frequencies

freq_hopped_sig = bpsk_sig.*spread_signal;

subplot(4,1,4);

plot(freq_hopped_sig);

axis([-100 3100 -1.5 1.5]);

title('|bf|if frequency hopped spread spectrum signal');

% Expressing the FFT's

figure,subplot(2,1,1);

plot([1:3000],freq_hopped_sig);

axis([-100 3100 -1.5 1.5]);

title('|bf|if frequency hopped spread spectrum signal or in fft');

subplot(2,1,2);

plot(abs(fft(freq_hopped_sig)));

################################################################################

RESULT

Thus the MATLAB programs for frequency hopping spread spectrum modulation techniques were executed and the waveforms were obtained.