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.
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:
Detector portion of demod should really be rewritten in C.
If sufficiently vetted, check into svn?
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.
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