Re: [Discuss-gnuradio] fast dot product?

[Jens Elsner]

On Wed, Jul 26, 2006 at 11:08:25PM -0700, Eric Blossom wrote:
> On Thu, Jul 27, 2006 at 12:47:46AM +0200, Jens Elsner wrote:
> > Hello,
> > 
> > I'd like to calculate the complex dot product with GNU Radio.
> > 
> > Is there a way to do it faster that this? This correlation takes 
> > "ages"...
> 
> You can probably repurpose the existing gr_fir_ccc.h code.
> See gnuradio-core/src/lib/filter/gr_fir_ccc.h
> It doesn't apply the conjugate, so you'll need to work that into your
> b values.   If you use gr_fir_ccc.h, you'll get hand-coded SIMD
> assembler on x86 and x86-64, and generic C++ code on the rest.
Thanks, I'll have a look at it. 

> The set_taps operation is relatively expensive.  For our purposes, we
> expected it to be done infrequently.  This allowed us to amortize the
> cost of building multiple copies of the taps at different alignments.
> This was required to take advantage of the 128-bit loads with SSE.
I'm using guard interval cross correlation to synchronize the OFDM
frame and to estimate the frequency offset. The correlation code is
the bottle neck, and I guess anything will be faster than what I have
now.

Jens