[Discuss-gnuradio] Re: [Commit-gnuradio] gnuradio-core/src/lib/general gr_correlate_acce...

['Eric Blossom']

[Moved to list, following my own recommendation ;)]

Regarding: searching for normal and conjugated match in 
gr_correlate_access_code.

On Wed, Jun 28, 2006 at 05:03:09PM -0400, Tom Rondeau wrote:
> 
> > 
> > This is OK, but there's a much simpler, and faster way to handle the
> > problem.
> > 
> > There's no need to check for the conjugate directly, or to run
> > multiple shift registerrs.  Just look at the hamming distance.  E.g.,
> > a perfect match of the conjugate will have the maximum hamming
> > distance, whereas a perfect match of the normal access will be 0.
> 
> But if we have a threshold value for the conjugate, (max-threshold) would
> trigger on a number of wrong solutions, wouldn't it? We would pass anything
> up that has between 52 and 64 bit errors in the access code.

If you're doing simultaneous detection of the normal & conjugate by any
method, you've got the same problem.  Note that this is a good reason
*not* to enable both normal and conjugate when using DPSK.


Say the access code was 10 long, and the threshold was 2.

When detecting both, the map goes like this:

  hamming dist   result
  ------------   -----
      0           Normal match (perfect)
      1           normal match (one off)
      2           normal match (two off)
      3           --
      4           --
      5           --
      6           --
      7           --
      8           conj match (two off)
      9           conj match (one off)
     10           conj match (perfect)


Eric