Building a multi-user Direct Sequence Spread Spectrum model using SIMULINK

Thread Starter

Jyothi Naik

Joined Apr 23, 2014
I'm designing a multi user Direct sequence spread spectrum in SIMULINK as my project, with BPSK modulation, and I want to implement the technique of multi - user interference rejection using LMS equalizers. Basically I am connecting multiple users at the transmittter side, and picking one of them as my desired signal, trying to retrieve it back on the rceiver side, and calculating the error rate. I have used an AWGN channel. Below are the specifications I used.

Data generator : Bernoulii Sequence Generator, time period Tb = 0.6
Spreading code : PN sequence generator, time period Tc= 0.04

All filters used all Low pass, FIR filters, Rectangular window, Normalized frequency cut off = 0.95

LMS Linear equalizer : 16 taps, reference tap is 4

I used differenct generating polynomials for the PN sequence generators in each of the diffrent user to distinguish them, also the data generator, i changed its Probability of Zero each time to distinguish the data also.

The error rate refuses to go below 0.3 and the worst part is, with every increase in the user, the error rate decreases, which is wrong. I am unable to trace the error in the design. I have posted a picture of the model for better understanding.

To summarize it, I have the following doubts :
1) why does the error rate decrease as my no of users increase ?
2) The Lowpass FIR filter gives the least error at 0.95 cutoff - is this nearing to an ALL pass filter ?
3) Does the equalizer contribute to the error ?

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Joined Aug 21, 2008
The picture of the model appears to not be posted.

If you are lucky enough for somebody on this forum to be able to help you, you will have to post many more details than just a block diagram. You might want to post the details of the filters and PN sequence at a minimum.

Since you are simulating the whole system, at some point, the relative timing of the various interfering signals are synchronous. It might very well be that the demodulator is unable to spread the interfering signals to look like random noise because of this synchronization. Just a guess.
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