Frequency-hopping
spread spectrum
(Matlab
Implementation With An Example)
Hasan Mehmood ,Usama Zaka Khan
Department Of Electrical
Engineering, Institute Of Space Technology
Abstract: Frequency-hopping
spread spectrum (FHSS) is a method of transmitting radio signals by rapidly
switching a carrier among many frequency channels, using a pseudorandom
sequence known to both transmitter and receiver. It is utilized as a multiple
access method in the frequency-hopping code division multiple access (FH-CDMA)
scheme.[2]
This
phenomenon is implemented in matlab with help of an example to show how FHSS
can be achieved. Necessary graphs and matlab code are included in the report.
1.
Introduction: Spread spectrum is a
digital modulation technology and a technique based on principals of spreading
a signal among many frequencies to prevent interference and signal detection.
As the name shows it is a technique to spread the transmitted spectrum over a
wide range of frequencies. It started to be employed by military applications
because of its Low Probability of Intercept (LPI) or demodulation, interference
and anti-jamming (AJ) from enemy side. The idea of Spreading spectrum is to
spread a signal over a large frequency band to use greater bandwidth than the
Data bandwidth while the power remains the same. And as far as the spread
signal looks like the noise signal in the same frequency band it will be
difficult to recognize the signal which this feature of spreading provides
security to the transmission. Compared to a narrowband signal, spread
spectrum spreads the signal power over a wideband and the overall SNR is
improved because only a small part of spread spectrum signal will be affected
by interference. In a communication system in sender and receiver sides’ one
spreading generator has located which based on the spreading technique they synchronize
the received modulated spectrum.[1]
2.
Frequency Hopping spread spectrum: Frequency hopping
spread spectrum is a transmission technology used in wireless networks and a
technique to generate spread spectrum by hopping the carrier frequency. FHSS
uses narrow band signal which is less than 1 MHz, Inthis method data signal is
modulated with a narrowband carrier signal that "hops" in random and
hopping happens in pseudo-random "predictable" sequence in a regular
time from frequency to frequency which is synchronized at both ends. Using FHSS
technology improves privacy, it is a powerful solution to avoid interference
and multi path fading (distortion), it decreases narrowband interference, increases
signal capacity, improve the signal to
noise ratio, efficiency of bandwidth is high and difficult to intercept
also this transmission can share a frequency band with many types of
conventional transmissions with minimal interference. For frequency hopping a mechanism
must be defined to transmit data in a clear channel and to avoid the congested channels.
Frequency hopping is the periodic change of transmission frequency and hopping happens
over a frequency bandwidth which consists of numbers of channels. Channel which
is used as a hopped channel is instantaneous bandwidth while the hopping
spectrum is called total hopping bandwidth. Frequency hopping categorized into
slow hopping and fast hopping which by slow hopping more than one data symbol
is transmitted in same channel and by fast hopping frequency changes several times
during one symbol. Hopping sequence means which next channel to hop; there are two
types of hopping sequence: random hopping sequence and deterministic hopping
sequence. The focus of this work is on slow and deterministic frequency hopping
sequence. In a frequency hopping network, there can be different number of
receivers which one sender is designed as Base that is responsible to transmit
the synchronization data to the receivers.[2]
3. Pseudo code for Implementation: following
steps were undergone while making the code
1. Random
no. of bits were generated
2. NRZ
line coding was implemented
3. BPSK
modulation was done
4. Random
carrier frequencies were generated
5. A
spread signal was formed by randomly choosing the frequencies.
6. Frequency
hoped spread signal was formed.
NRZ was chosen to give
180 degree phase shift. And BPSK was used for simplicity. Choice of carrier
frequencies was random.
4.
Matlab Code’s Output: The code was implemented for random
25 bits message signal and following outputs were acquired from Matlab.
Figure:1
Different steps in producing a frequency hopped signal
are shown in figure 1.
For checking if our modulation in accurate we took
FFT f the system. For a correct modulation there must be Six peaks n the FFT as
the modulation is done on six different frequencies. The output acquired was as
below
Figure:2
As shown above there are six clusters in the FFT of
the FHSS modulated signal. This provides us the satisfaction that our design is
accurate.
5.
Matlab Code:
% Frequency Hopping Spread
Spectrum
%------------------------------------------------------------%
clc
clear
% Generation of bit pattern
s=round(rand(1,25)); % Generating 25 bits
signal=[];
carrier=[];
t=[0:2*pi/119:2*pi]; % Creating 120 samples for one
cosine
for k=1:25
if s(1,k)==0
sig=-ones(1,120); % 120 minus ones for bit 0
else
sig=ones(1,120); % 120 ones for bit 1
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\it BPSK
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 hopps 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([1:3000],spread_signal);
axis([-100 3100 -1.5 1.5]);
title('\bf Spread
Signal with 6 frequencies');
% Spreading BPSK Signal
into wider band with total of 5 frequencies
freq_hopped_sig=bpsk_sig.*spread_signal;
subplot(4,1,4)
plot([1:3000],freq_hopped_sig);
axis([-100 3100 -1.5 1.5]);
title('\bf Frequency
Hopped Spread Spectrum Signal');
% Expressing the FFTs
figure,subplot(2,1,1)
plot([1:3000],freq_hopped_sig);
axis([-100 3100 -1.5 1.5]);
title('\bf Frequency
Hopped Spread Spectrum signal and its FFT');
subplot(2,1,2);
plot([1:3000],abs(fft(freq_hopped_sig)));
6.
Conclusion: Through whole the exercise, the basic
concept of FHSS was gained. It’s implementation was learnt and the overall
grasp on the topic was made stronger.
7. Acknowledgement
The authors gratefully acknowledge Mrs. Salma Zainab
and Mr. Adnan Zafar for their valuable
support and guidance in this work.
8.
References:
[1] Frequency Hopping Spread Spectrum: An Effective
Way to Improve Wireless Communication Performance
[2].
www.wikipedia.com
https://en.wikipedia.org/wiki/Frequency-hopping_spread_spectrum
