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 -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Steven Clark Sent: Thursday, June 05, 2008 5:53 PM To: Gregory Maxwell Cc: gnuradio mailing list Subject: Re: [Discuss-gnuradio] Posted: Enhanced GMSK demodulator On Thu, Jun 5, 2008 at 5:28 PM, Gregory Maxwell <[EMAIL PROTECTED]> wrote: > On Thu, Jun 5, 2008 at 3:25 PM, Steven Clark <[EMAIL PROTECTED]> wrote: > [snip] >> It should be a >> drop-in replacement for the existing demodulator in designs, except >> that the transmitted data needs to be differentially encoded (you will >> NOT, however, need a differential decoder on the receive side). > > Why? > > All the cases I can think of would be most logically implimented with > the same interface (encoded or non-encoded) on both the encoder and > decoders. > > If it's just an arbitrary decision then perhaps it should be changed > before it enters the tree and people build other radios expecting that > particular interface. > It is not an arbitrary decision -- it is required. I do agree that it seems weird, but it is just a quirk of the way the math works out for a 2-bit differential detector that it removes a differential encoding in the process. The Simon & Wang paper explains it better than I can: "The dashed lines around the differential encoder indicate that it is present for a two-bit differential detector but absent for a one-bit differential detector at the receiver. As discussed in [16], this differential encoding operation is required for the two-bit detector in order that hard decisions made on the detector output reflect decisions on the true input data sequence and not a differentially decoded version of it as would be the case without the differential encoder at the transmitter input." If you can't get your hands on the Simon & Wang paper, let me know and I can send you a copy of it. -Steven _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
