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

[Eric Blossom]

On Fri, Mar 02, 2007 at 10:27:44AM -0800, Dan Halperin wrote:
> Hi,
> 
> I'm trying to put the Barker correlator in the FPGA. This should greatly 
> improve the SNR of the BBN 802.11 code.
> 
> The plan is:
> 
> 64Msps complex w/11MHz info -> Barker Correlator -> Decim by 16 -> 4Msps 
> complex with 1MHz info
> 
> instead of
> 
> 64Ms complex w/11MHz info -> Decim by 8 -> 8Msps complex with 11MHz info 
> -> Barker Correlator
> 
> .
> 
> I resampled the 11-bit Barker code to 64 taps (is this even the right 
> thing to do? MATLAB still gave me all real numbers) and used the Altera 
> IP FIR compiler to build a multiplier-less FIR. However, this FIR, when 
> I set everything to bit-width 16 (dropping all but the most significant 
> 16 of the output), uses ~1/3 of the LEs of the FPGA. Is this expected or 
> did I do something grossly wrong?
> 
> In any case, I'm going to need at least 2 of them, one for TX and one 
> for RX, and quite possibly 4 if I have to stick one each on the input 
> and output I and Q paths. I definitely don't have that much room, and 
> I've already removed the secondary RX and TX paths in the core.
> 
> If I let the FIR do some of the decimation, I can make it smaller, but I 
> don't understand the DSP enough to know that that's ok.
> 
> Also, I'm currently planning on inserting the FIR after the CORDIC and 
> before the CIC decimation filter. Is that the right place? If I put it 
> before the CORDIC then maybe I would only have to use 1 FIR on the RX path?

Disclaimer: I haven't actually looked at how to implement what you're
trying to do ;)

I think you're going to want the CORDIC and CIC in place.  Otherwise
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).

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
take a look at the Altera App Note on techniques for synthesizing
multipliers.  There are lots of options, particularly if you have
cycles or RAM available.

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

Eric