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From: | Nick Foster |
Subject: | RE: [Discuss-gnuradio] Why not use matched filter in GMSK demodulator |
Date: | Fri, 31 Jul 2009 17:36:07 +0000 |
Shizheng, For an alternative noncoherent GMSK demodulator that does a lot better than the stock one, see the GMSK receiver I put together for my AIS project: https://www.cgran.org/wiki/AIS It uses a modified decision-feedback equalizer to estimate the channel and remove ISI. It seems to do pretty well for my application. If you have a continuous signal (i.e., not in short bursts) you can even use the stock LMS DFE block. My application uses short packets, so I run them through the DFE multiple times to train it. The DFE avoids the colored-noise problem you get when you invert the channel with a matched filter. It's pretty simple, but it works. --n > Date: Thu, 30 Jul 2009 20:31:44 -0400 > From: address@hidden > To: address@hidden > Subject: Re: [Discuss-gnuradio] Why not use matched filter in GMSK demodulator > CC: address@hidden > > > > Shizheng Li wrote: > > Thank you for your reply! > > > > I think the generic receiver for CPM you mentioned is the optimal > > receiver for CPM and the projection onto basis functions (correlation > > with basis functions) is equivalent to the matched filters, am I right? > > yes; however see the comment i made about the colored noise and the > detection vs detection/estimation in my earlier post > > > Go back to the GMSK transceiver I mentioned in the file > > > > http://gnuradio.org/trac/browser/gnuradio/branches/releases/3.2/gnuradio-core/src/python/gnuradio/blks2impl/gmsk.py > > > > The modulator structure is NRZ mapping --> Gaussian filter --> FM modulator > > And the demodulator structure is FM demodulator --> Timing recovery --> > > detector > > > > I think after the FM demodulator the signal can be viewed as a PAM > > modulated signal and what if I add a matched filter before the timing > > recovery? Can I improve the BER performance? I think the matched filter > > can maximize the output SNR. Will it make the detector work better? > > you are already suboptimal when you do FM demodulation... > > > Best regards, > > Shizheng Li > > > > > > > > On Thu, Jul 30, 2009 at 2:07 PM, Achilleas Anastasopoulos > > <address@hidden <mailto:address@hidden>> wrote: > > > > > > There is no reason why you should not use a matched filter. > > However make sure you understand that a symbol-spaced MF > > generates sufficient statistics only for detection, > > ie, not for (epoch) synchronization. > > Also note that in the case of GMSK (CPM in general) a bank of MFs > > will generate colored noise. > > Another appropriate implementation of a front end projects the > > entire oversampled signal to a set of orthonormal basis functions > > which has the advantage of generating white noise samples for > > (simpler) further processing. > > > > Take a look at how a generic receiver for an arbitrary CPM > > is developed in > > > > http://gnuradio.org/svn/gnuradio/trunk/gr-trellis/src/examples/test_cpm.py > > > > There, the signal is first projected to its basis functions (which > > is calculated by a helper python application in "fsm_utils.py") > > to generate a sufficient statistic which is then used in conjunction > > with trellis decoding to do soft-decision sequence detection. > > What is missing though is epoch and phase syncronization (to do at > > some point...) > > > > Achilleas > > > > > > _______________________________________________ > > Discuss-gnuradio mailing list > > address@hidden <mailto:address@hidden> > > http://lists.gnu.org/mailman/listinfo/discuss-gnuradio > > > > > > > > > > -- > > Best Regards, > > Shizheng Li > > Ph.D. Student and Research Assistant > > Department of Electrical and Computer Engineering > > Iowa State University > > > > > > _______________________________________________ > Discuss-gnuradio mailing list > address@hidden > http://lists.gnu.org/mailman/listinfo/discuss-gnuradio Bing™ brings you maps, menus, and reviews organized in one place. Try it now. |
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