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[Discuss-gnuradio] Putting a correlator in the FPGA

From: Dan Halperin
Subject: [Discuss-gnuradio] Putting a correlator in the FPGA
Date: Fri, 02 Mar 2007 10:27:44 -0800
User-agent: Thunderbird (X11/20070105)


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?


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