**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.

**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}**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**

- 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.

**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.

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