EEP316 Laboratory

Code Division Multiple Access (CDMA)

with

Direct-Sequence Spread Spectrum (DSSS)

30/1/13

Indra Bhushan 2010EE50548

Umang Gupta 2010EE50564

Vivek Mangal 2010EE50566

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

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('\bf\it 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('\bf\it 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('\bf\it DSSS Signal');

% Plotting the FFT of DSSS signal figure,plot([1:12120],abs(fft(dsss_sig)))

OBSERVATIONS:

DSBSC signal from the first multiplier.

Spread Sprectrum (DSSS) signal

Spectrum Analyser Output

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.