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:[EMAIL PROTECTED] 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. <[EMAIL PROTECTED]> 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 _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
