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

From: Steven Clark
Subject: Re: [Discuss-gnuradio] Posted: Enhanced GMSK demodulator
Date: Fri, 6 Jun 2008 12:27:03 -0400

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

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


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