Summary¶

The attached "gmskenhanced.py" file offers an enhanced GMSK demodulator that I believe to be superior to the GNURadio standard gmsk demodulator. 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). The enhanced demodulator should offer better bit error rate (BER) performance, especially for lower bandwidth-bittime (BT) products, such as the GSM standard BT=0.3, and in lower SNR conditions.

Details¶

The GNURadio standard gmsk demod is an attempt at a one-bit differential detector, and looks at the phase-change over one sample (should actually be over one symbol). The enhanced gmsk demod looks at the phase-change over two SYMBOLS and is therefore a two-bit-differential detector. The net effect is that inter-symbol-interference (ISI) is reduced: the eye openings in the resulting eye-diagrams are wider, and therefore more resistant to noise. A side-effect of a two-bit-differential detector is that the eye diagram is asymmetric, thus the decision threshold needs to biased. The constructor's default value (decision_threshold=0.1) should be fine for most cases.

Work based largely on:
Differential Detection of Gaussian MSK in a Mobile Radio Environment

Also see:
Differential Detection of GMSK Using Decision Feedback

And:
GMSK Modulation for Digital Mobile Radio Telephony

TODO¶

Code cleanup.

Detector portion of demod should really be rewritten in C.

If sufficiently vetted, check into svn?

Feedback¶

My BER testing shows a significant improvement, but I would like to hear from others. Please either post here or email me if you have a chance to try it out, even if it degrades performance or you can't make it work right.

Contact¶

[email protected] (please replace the mythical beast with "steven", sorry for the CAPTCHA...)