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 Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
