2. Spread Spectrum-DSSS & CDMA
Aim: Demonstration of some principle of direct sequence spread spectrum (DSSS)
system.
Theory:
Spread Spectrum- A modulation technique that spreads a signal’s power over a wide band
of frequencies. The main reasons for this technique is that the signal becomes much less
susceptible to electrical noise and interferes less with other radio-based systems.
A DSSS generator:
To generate a spread spectrum signal one requires:
1. A modulated signal somewhere in the RF spectrum
2. A PN sequence to spread it
A DSSS demodulator:
A demodulator for the DSSS of Figure 1 is shown in block form in Figure 3.
The input multiplier performs the de-spreading of the received signal, and the second
multiplier translates the modulated signal down to baseband. The filter output would
3. probably require further processing - not shown - to ‘clean up’ the waveform to binary
format.
The PN sequence at the receiver acts as a ‘key’ to the transmission. It must not only have the
same clock and bit pattern; it must be aligned properly with the sequence at the transmitter.
MATLAB code:
% Direct Sequence Spread Spectrum
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
clc
clear
% Generating the bit pattern with each bit 6 samples long
b=round(rand(1,20));
pattern=[];
for k=1:20
if b(1,k)==0
sig=zeros(1,6);
else
sig=ones(1,6);
end
pattern=[pattern sig];
end
plot(pattern);
axis([-1 130 -.5 1.5]);
title('bfit Original Bit Sequence');
% Generating the pseudo random bit pattern for spreading
spread_sig=round(rand(1,120));
figure,plot(spread_sig);
axis([-1 130 -.5 1.5]);
title('bfit Pseudorandom Bit Sequence');
% XORing the pattern with the spread signal
hopped_sig=xor(pattern,spread_sig);
% Modulating the hopped signal
dsss_sig=[];
t=[0:100];
fc=.1
c1=cos(2*pi*fc*t);
c2=cos(2*pi*fc*t+pi);
for k=1:120
if hopped_sig(1,k)==0
dsss_sig=[dsss_sig c1];
else
dsss_sig=[dsss_sig c2];
end
end
figure,plot([1:12120],dsss_sig);
axis([-1 12220 -1.5 1.5]);
title('bfit DSSS Signal');
4. % Plotting the FFT of DSSS signal
figure,plot([1:12120],abs(fft(dsss_sig)))
8. Demodulated Output
Comments:
The experiment demonstrates the DSSS transmission. In this we use more band-width than
required. The energy as shown is distributed across the whole band-width of transmitter.
Hence, signal can be transmitted as low SNR; infact the signal is transmitted using PRBS and
a knowledge of PRBS helps in demodulation on other side. However, it become very difficult
to intercept this signal as SNR is low and hence signal identification becomes difficult leave
alone demodulation. DSSS is used in various areas like CDMA, a similar method is freq.
hoping in which we sent signal at a random carrier frequency.
