RE: [Discuss-gnuradio] Posted: Enhanced GMSK demodulator

[Long, Jeffrey P.]

Steve-

You know what, you might be right here. I am starting to remember that
it could be BT dependent. My application called out for a BT of .5
which is not too severe(we didn't really have ACI issues) so in reality
the threshold might be good enough right thru the center. With
something more severe .35 or less maybe it makes sense to move the
threshold up. It might also be dependent on how much quantization and
oversampling you have at that point. With not much resolution in time
or amplitude it might not make much of a difference.

-Jeff

-----Original Message-----
From: Steven Clark [mailto:address@hidden 
Sent: Friday, June 06, 2008 12:27 PM
To: Long, Jeffrey P.
Cc: gnuradio mailing list
Subject: Re: [Discuss-gnuradio] Posted: Enhanced GMSK demodulator

On Fri, Jun 6, 2008 at 9:30 AM, Long, Jeffrey P. <address@hidden>
wrote:
> Steven-
>
> Did you actually find that the decision threshold needs to be biased?
I
> actually implemented the 2 bit differential detector on a custom asic
> that was targeted at streaming audio(in 2004) and during the
> simulations I found that moving the bias point did very little for
> performance. Maybe in a floating point environment it makes a little
> difference? I agree that it has superior performance over other
> techniques, due to that asymmetric increase in eye opening. Did you
> happen to notice that the math is actually performing a dot(or cross
I
> forget which) product between two vectors separated by 2 bit times? I
> think I learned that from Lindsey's book. I think he calls these
> techniques differently coherent. Its the best bang for your buck if
you
> want a robust demod without worry about carrier recovery.
Unfortunately
> the startup company where I did the design (Aura Communications) is
> gone but the chip lives on in the acquiring company so it wasn't a
> complete waste of time. :)
>
> -Jeff

Jeff-
That's interesting about the threshold. Do you remember what BT you
were using? The lower the BT, the more asymmetric the eye and
therefore the more you need to bump up the threshold to have it still
be near the center of the eye. The optimum threshold also depends on
SNR...increasing slightly as Eb/N0 increases. I didn't muck around too
much with the threshold, I just eyeballed the charts at the end of the
Simon&Wang paper and picked a value (0.1) that seemed close to the
most common cases. It should be somewhere between 0.0 and 0.2 for
certain.

-Steven