Re: [Discuss-gnuradio] Direct Sequence Spread Spectrum Question.

[Ahmed Zaheer]

Thanks a lot Albert:

I am multiplying my file source with GLFSR source and then giving the product to modulation blocks in transmitting flowgraph. In the receiving flowgraph after demodulating the signal I am passing the demodulated signal through PN Correlator block having same values of degree, mask and seed parameters and then save the file into sink source. I am not able to receive the same data(it only works when I use degree=1, mask and seed =0 for both GLFSR and PN Correlator i.e. making GLFSR a constant source of magnitude 1). Do I have to use the PN Correlator block in some other manner? Or I should use some other blocks after PN Correlator block to retrieve the original data? 

Regards.

Ahmed. 

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From: Albert Chun-Chieh Huang <address@hidden>
To: Ahmed Zaheer <address@hidden>
Cc: "address@hidden" <address@hidden>
Sent: Saturday, December 1, 2012 7:46 AM
Subject: Re: [Discuss-gnuradio] Direct Sequence Spread Spectrum Question.

Ahmed Zaheer <address@hidden> writes:

> I was trying to implement Direct Sequence Spread Spectrum in gnuradio. In the transmitting flowgraph I am using GLFSR source to multiply it with the file which I am transmitting. At the receiver end I am able to receive it by multiplying it with same GLFSR source with the same values. But there is no acquisition being done. I am seeing a block named PN Correlator. But I don't know what this block does and where should I use this block in the flowgraph. My question is does anybody know what the PN Correlator block does and is it performing the acquisition or not. 
>
> Any help/comments would be greatly appreciated. 
>
> Regards.

Ahmed,

After taking a look at digital_pn_correlator_cc.cc, I think it's a
serial code acquisition block as described in its header. The output
will be cross-correlation of PN sequence and input samples. Input sample
pointer is advanced by noutput_items samples for each call, and the
output value is derived from multiplying input samples by PN sequence,
both are length of whole PN sequence. If you think noutput_items to be
one, then it will be easier to understand this code. So, yes, it's
serial search code acquisition to my understanding. If you monitor the
output of digital_pn_correlator, you should see a spike when the
acquisition is achieved, and the interval between two spikes will be
roughly the length of PN sequence due to noise effect. I think its input
is time domain samples from your front-end.

This is my understanding of this block. Hope it helps.

Best Regards,

Albert

--
Albert Chun-Chieh Huang(黃俊傑)
Blog: Random Notes, http://alberthuang314.blogspot.com/