johnlovestohate wrote:

> Hi all,
> Where can I find a practical block diagram of a Direct Sequence Spread
> Spectrum (DSSS) system? I looked for it everywhere but couldn't find any. I
> am aware of the general block diagrams but I am specifically interested in
> something that shows how the system looks in the real world. Currently, I
> am trying to implement a DSSS system on Gnuradio.
> 
> In another question if I am using RRC shaping in a BPSK based DSSS system
> how can I do a cross correlation between the received signal and the PN
> Sequence.

In order to receive DSSS, you've got to synchronize carrier frequency, 
symbol rate and the shift of PN sequence.

> I am using a real valued PN sequence whereas the received signal
> is a RRC filtered (to complete the raised cosine system) complex signal at
> baseband. The cross correlation using real valued pn sequence and the
> complex signal at baseband doesn't give me good results. 

Of course it won't.

> I am using FFTs to do this. 

Use FFT to do exactly what? Estimate the shift of PN sequence?

> The fact that the transmitted signal is RRC shaped might be a
> reason but I am not sure though.

Of course this is not a reason.

> Can someone who has experience in DSSS offer me a little help please? 

As FFT window is not synchronized to the full period of PN sequence, the 
FFT should cover 1/2 of the period. Before doing a correlation, you must 
get rid of the frequency offset. A common approach to cancel the offset 
is demodulate DSSS as DPSK.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

On Sep 17, 1:40�am, "johnlovestohate"
<aliatolemiss@n_o_s_p_a_m.gmail.com> wrote:
> Hi all,
> Where can I find a practical block diagram of a Direct Sequence Spread
> Spectrum (DSSS) system? I looked for it everywhere but couldn't find any. I
> am aware of the general block diagrams but I am specifically interested in
> something that shows how the system looks in the real world. Currently, I
> am trying to implement a DSSS system on Gnuradio.
>
> In another question if I am using RRC shaping in a BPSK based DSSS system
> how can I do a cross correlation between the received signal and the PN
> Sequence. I am using a real valued PN sequence whereas the received signal
> is a RRC filtered (to complete the raised cosine system) complex signal at
> baseband. The cross correlation using real valued pn sequence and the
> complex signal at baseband doesn't give me good results. I am using FFTs to
> do this. The fact that the transmitted signal is RRC shaped might be a
> reason but I am not sure though.
>
> Can someone who has experience in DSSS offer me a little help please?
>
> Thanks,
> John

Try "Spread Spectrum Communications", by
Marvin K. Simon
Jim K. Omura
Robert A. Scholtz
Barry K. Levitt

Maurice Givens

Hi all,
Where can I find a practical block diagram of a Direct Sequence Spread
Spectrum (DSSS) system? I looked for it everywhere but couldn't find any. I
am aware of the general block diagrams but I am specifically interested in
something that shows how the system looks in the real world. Currently, I
am trying to implement a DSSS system on Gnuradio.

In another question if I am using RRC shaping in a BPSK based DSSS system
how can I do a cross correlation between the received signal and the PN
Sequence. I am using a real valued PN sequence whereas the received signal
is a RRC filtered (to complete the raised cosine system) complex signal at
baseband. The cross correlation using real valued pn sequence and the
complex signal at baseband doesn't give me good results. I am using FFTs to
do this. The fact that the transmitted signal is RRC shaped might be a
reason but I am not sure though.

Can someone who has experience in DSSS offer me a little help please? 

Thanks,
John