1. The document describes an experiment demonstrating principles of direct sequence spread spectrum (DSSS) transmission. 2. In DSSS, a signal's power is spread over a wide bandwidth of frequencies, making it less susceptible to noise and interference. A DSSS transmitter uses a modulated signal and a pseudorandom number (PN) sequence to spread the signal. 3. At the receiver, the PN sequence acts as a key to de-spread the signal. It must match the transmitter in clock, bit pattern, and alignment with the transmitted sequence. MATLAB code is provided to generate and modulate a DSSS signal for demonstration purposes.

1. EEP316 Laboratory
Amplitude Shift Keying (ASK)
30/1/13
Indra Bhushan 2010EE50548
Umang Gupta 2010EE50564
Vivek Mangal 2010EE50566

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 SequenceSpread Spectrum
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
clc
clear
% Generating the bit pattern witheach bit 6 sampleslong
b=round(rand(1,20));
pattern=[];
fork=1:20
if b(1,k)==0
sig=zeros(1,6);
else
sig=ones(1,6);
end
pattern=[pattern sig];
end
plot(pattern);
axis([-1130 -.5 1.5]);
title('bfitOriginal Bit Sequence');
% Generating the pseudo randombitpattern forspreading
spread_sig=round(rand(1,120));
figure,plot(spread_sig);
axis([-1130 -.5 1.5]);
title('bfitPseudorandomBitSequence');
% XORing the pattern withthe spread signal
hopped_sig=xor(pattern,spread_sig);
% Modulating thehopped signal
dsss_sig=[];
t=[0:100];
fc=.1
c1=cos(2*pi*fc*t);
c2=cos(2*pi*fc*t+pi);
fork=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([-112220 -1.5 1.5]);
title('bfitDSSSSignal');

4. % Plotting the FFT of DSSSsignal
figure,plot([1:12120],abs(fft(dsss_sig)))

6. OBSERVATIONS:
DSBSC signal from the first multiplier.
Spread Sprectrum (DSSS) signal

7. Spectrum Analyser Output

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.