RE: [Discuss-gnuradio] Spread Specturm implementation in FPGA

[Tachwali, Yahia]

Hi Guys, 

Let me add my two cents here:

The chip synchronization can take care of the symbol synch since the chip 
correlation does not produce maximum values unless the symbol timing is 
correct. Normally, and what I mean by normally is typical cases to the best of 
my knowledge, the synchronization is done in two stages (fast and fine stage) 
where the fast stage perform synchronization with a step of half chip duration. 
Some design decision need to be made regarding the source of your timing 
extraction, will it be:
1- preamble (contains synch sequence at the beginning of each given number of 
data packets)
2- use separate synch channel (the way it is done in CDMA)
3- use blind synchronization or NDA (non-Data Aided) synch, which I doubt it is 
gonna be option for a "simple" transceiver.

I hope that can help a little!

Kind regards,
Yahia Tachwali
________________________________________
From: address@hidden address@hidden On Behalf Of Firas A. address@hidden
Sent: Saturday, January 10, 2009 11:02 PM
To: address@hidden
Subject: Re: [Discuss-gnuradio] Spread Specturm implementation in FPGA

Hi,


> Yong J. Chang" wrote:
> Hi All,
>
> I've worked on Embedded system and FPGA systems for several years and I
> joined the team working on GNU Radio > project. Because of my lack of
> knowledge about digital communication, I need some help of you all.
>
> The goal is simple PAM modulated communication with M-sequence spreading.
> So far as I know, this can be done in
> host PC level. However, our goal is that this is implemented in FPGA level
> in real-time because we have to determine > exact time of packet arriving.
>
> The question is following.
>
> If I implement the despreader ( is it called synchronizer? including
> detecting and tracking ) in FPGA, should I perform
> symbol timing recovery in host PC again? If we use the despreader and can
> detect the peak of correlation between
> received signal and spreading code, I don't think that I have to recover
> symbol timing again because the despreaded
> signal is already symbol-spaced signal captured at nice time. But I'm not
> sure that I'm right or wrong.
>
> And I hope that this can be done in Cyclone FPGA!!!!
>
> Thank you all!
>
> Y. J. Chang


I'm not sure how it is possible to do the spreading with PAM, but I think
doing spreading with PWM is much easier. Also to conserve space in USRP
FPGA, you have to use the image with 1 RX and 1 TX. This will free about 55%
of FPGA resources.

Best Regards,

Firas
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