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

[Dan Halperin]

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.

-Dan