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

[Eric Blossom]

On Mon, Oct 15, 2007 at 04:28:01PM -0400, Steven Clark wrote:
> All-
> 
> 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)


How does it do with say, 2 samples/sym ?


> 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?
> 
> Tom Tsou- thanks for pointing the way to some of this stuff.
> 
> -Steven


Definitely interested in having this in the code base.
At least until the dust settles, we'd want it as an addition, not a
replacement of the existing code.

I'll send you the copyright assignment stuff in the next message.

Thanks!
Eric