It is defintely do-able in real time software. I have done some work in the past that continuosly decoded all 8 timeslots in real time on a 1GHz PPC. Also, the processor usage goes way down when you only look at 1 of the 8 timeslots, which is usually all that is needed. Ben
On 6/6/08, Long, Jeffrey P. <[EMAIL PROTECTED]> wrote: > > I agree with Steven, while there are definitely more "optimal" > solutions like MLSE they don't always make sense for every application. > GSM phones and the like have dedicated DSP resources to run a Viterbi > algorithm but is this doable on the typical gnu users computer? I will > defer to the experts on this but from my own low power(< 20 mA) > portable wireless experience the ad-hoc techniques sometimes make more > sense. > > -Jeff > > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:discuss-gnuradio-bounces+jplong<discuss-gnuradio-bounces%2Bjplong> > [EMAIL PROTECTED] On Behalf Of > Steven Clark > Sent: Friday, June 06, 2008 10:58 AM > To: Ben Wojtowicz > Cc: Discuss-gnuradio@gnu.org > Subject: Re: [Discuss-gnuradio] gsm gmsk demodulation > > > On 6/6/08, Bob McGwier <[EMAIL PROTECTED]> wrote: > >> > >> This is not my professional experience. The sounding data is used > to find > >> the channel and then the data symbols are soft detected through a > "viterbi > >> equalizer" in every implementation I am aware of that is any good at > all > >> with the exception of one I wrote several years ago which estimates > the data > >> given the channel and then restimates the channel and then the data > and then > >> the channel and then the data, etc. MMSE and not MLE is the goal > and this > >> was a suboptimal implementation of the EM algorithm. It was > suboptimal > >> since it did not estimate the data bauds using ALL observations but > only > >> those between sounding data. Further, assumptions that the > conditional > >> distributions of the data given the observations could be described > in 1st > >> and 2nd product moments (not Gaussian but having similar > properties). This > >> has been published by many. The computational complexity is on a > par with > >> the viterbi equalizer and it outperforms it. > >> > >> Most of the cell phones I know use the Viterbi equalizer. > >> > >> Bob > > On Fri, Jun 6, 2008 at 9:41 AM, Ben Wojtowicz <[EMAIL PROTECTED]> > wrote: > > I agree with Bob, most gsm demodulators I have seen use a viterbi > equalizer > > (sometimes called MLSE equalization). > > Ben > > > > > > Ok, good to hear from guys with more experience. So you would have a > viterbi equalizer to mitigate ISI, and then wrap that in a layer of > forward error correction? Is this computationally feasible for > cpu-based software radio? Sounds like it could get computationally > expensive pretty quick... > > -Steven > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > http://lists.gnu.org/mailman/listinfo/discuss-gnuradio >
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