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Re: [Discuss-gnuradio] Improvements made to GMSK demod

From: Jeff Brower
Subject: Re: [Discuss-gnuradio] Improvements made to GMSK demod
Date: Mon, 15 Oct 2007 16:27:52 -0500


> I've made some improvements to the built-in GMSK demodulator, based mostly on 
> Simon
> & Wang's "Differential Detection of Gaussian MSK in a Mobile Radio 
> Environment".
> Also see "Differential Detection of GMSK Using Decision Feedback", Yongacoglu,
> A.    Makrakis, D.    Feher, K.
> The intermediate steps I took were as follows:
> -Place a low-pass filter in front of the clock recovery block. Optimal 
> bandwidth is
> a function of both Eb/N0 and xmitter BT product.
> -Remove the quadrature demod (which was looking at change in phase over 1 
> sample).
> Replace with blocks to look at change in phase over 1 SYMBOL. (this is a true 
> 1 bit
> differential detector)
> -Instead of 1 bit differential detector, look at change in phase over 2 
> symbols (2
> bit differential detector). (requires diff encode on xmit side). This 
> produces an
> asymmetric eye diagram, so you have to bias it a little bit prior to clock
> recovery.
> Initial Bit Error Rate results (just to get an idea of relative performance):
> (xmit BT=0.25, 8 samp/sym, same input to all demods, at a certain Eb/N0)
> Original, unchanged GMSK demod:
> BER=0.38
> Original + pre CR filter:
> BER=0.0090
> True 1-bit diff detect + pre CR filter:
> BER=0.0056
> True 2-bit diff detect + pre CR filter:
> BER=0.00076
> Is this something that people would like to see checked in? If so, how's the 
> best
> way to do so...via a developer branch?

The BER improvement looks outstanding.  I would think this should be an 


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