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Re: [Discuss-gnuradio] Putting a correlator in the FPGA

From: Dan Halperin
Subject: Re: [Discuss-gnuradio] Putting a correlator in the FPGA
Date: Fri, 02 Mar 2007 15:33:40 -0800
User-agent: Thunderbird (Windows/20070221)

Eric Blossom wrote:
I think you're going to want the CORDIC and CIC in place.  Otherwise

What do you mean by "in place"? I was planning on leaving them there, and inserting the correlator between them. Do you mean I don't want to jump in the middle?

you're looking at the full IF passband of the daughterboard (I'm not
sure how wide the unmodified RFX-2400 IF is.)  In any event,
decimating by two is going to make your job easier and you won't lose
any useful info.  Maybe  you just want a fixed decimate by 2 CIC (it
would take up fewer resources than the general one that's currently in
the FPGA).

Are you perhaps proposing

64Msps complex w/11MHz info -> fixed CIC decim x2 -> fixed Barker Correlator/Decim x 8 -> 4Msps complex w/1MHz info?

When I said that I might push some of the decimation to the FIR, I was thinking

64Msps complex w/11MHz info -> fixed Barker Correlator/Decim x{2,4?} -> (current, variable) CIC decim x{8,4} -> 4Msps complex w/1MHz info.

There are no hardware multipliers in the Cyclone.  However, I think
you're falling victim to the "I used MATLAB and then some other piece
of magic code generator software" syndrome.  This can probably all be
made to fit.  Why do you think you need 16-bit coefficients?  How
about 8-bit, or even 1-bit (I'm not kidding).  You'll also probably want to

This is clearly a better idea. I just made the coefficients match the input width, but there's no real reason to do this. With using only 8- or 6-bit coefficients the space more than halves. And yes, I'm looking into throwing some of the data into RAM. There's not a lot of that on the Cyclone though.

Before diving in, you may want to spend a bit of time searching the
IEEE, etc pubs for 802.11 implementation articles.

Definitely. The one thing I've learned so far is that there's no standard pulse shaping filter. Brilliant.


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