[Discuss-gnuradio] Improvements made to GMSK demod

[Steven Clark]

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)

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