need help regarding how to identofy that signal is GMSK modulated
..plzz give advise in this regard..
regards,
ravi.
Help regarding identification of modulation(GMSK) type of a signal
Started by ●January 30, 2006
Reply by ●January 30, 20062006-01-30
ravi wrote:> need help regarding how to identofy that signal is GMSK modulated > ..plzz give advise in this regard.. > regards, > ravi. >Try to demodulate it and see if you succeed? GMSK signals have a nice, flat spectrum, so you can't really use that to tell them from all the other signals with nice flat spectra. GMSK is special because the signal out of the transmitter has a constant envelope. I suppose you could look for a flat spectrum and constant envelope -- but once you add noise the envelope won't be flat, and that nice flatness will go away long before the signal can no longer be successfully demodulated (not to mention that band-pass filtering will destroy the envelope flatness). Sorry for being a nay-sayer -- I hope someone has a more positive answer for you; if not post your solution when you find it. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Reply by ●January 30, 20062006-01-30
"ravi" <rkant_nitw@rediffmail.com> wrote in message news:1138641487.576500.309770@z14g2000cwz.googlegroups.com...> need help regarding how to identofy that signal is GMSK modulated > ..plzz give advise in this regard.. > regards, > ravi. >Identify relative to what? PSK? QAM? Voice? It should have a flat spectrum whose width is proportional to the bit rate and (I think) two bat ears at the mark and space frequencies, although I may be thinking of BFSK. Seems like if you FM demodulate it you should see a pretty obvious square wave signal which you won't get for psk or qam.
Reply by ●January 30, 20062006-01-30
Anonymous wrote:> "ravi" <rkant_nitw@rediffmail.com> wrote in message > news:1138641487.576500.309770@z14g2000cwz.googlegroups.com... > >>need help regarding how to identofy that signal is GMSK modulated >>..plzz give advise in this regard.. >> regards, >> ravi. >> > > > Identify relative to what? PSK? QAM? Voice? It should have a flat spectrum > whose width is proportional to the bit rateYes.> and (I think) two bat ears at > the mark and space frequencies, although I may be thinking of BFSK.You are thinking BFSK or at least some flavors -- GMSK is a flavor of BFSK. GMSK is like MSK in that it has no bat ears.> Seems > like if you FM demodulate it you should see a pretty obvious square wave > signal which you won't get for psk or qam.Uh uh. The 'G' in GMSK means 'Gaussian', which is the sort of filter through which the phase transitions are run before they are applied to the carrier. This means that you will have not-so-obvious phase transitions that would be square if they weren't so heavily filtered.> >-- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Reply by ●January 31, 20062006-01-31
I guess Ravi is interested with received signal which was GMSK modulated at transmission side - a typical mobile communication like GSM or EDGE scenario. The is related to something called "blind demodulation" where we do not know in advance what received signal is coming - GMSK/8PSK etc. I can not give detailed algorithm here. For example, a GSM burst training sequence is known a prior. Someone can exploit correlation property and calculate engery. If the engegy achieved through GMSK correlation is high enough compared to that of others (say 8PSK) then modulation type is GMSK. Cheers Santosh
Reply by ●January 31, 20062006-01-31
Nilnod wrote:> I guess Ravi is interested with received signal which was GMSK > modulated at transmission side - a typical mobile communication like > GSM or EDGE scenario. > > The is related to something called "blind demodulation" where we do not > know in advance what received signal is coming - GMSK/8PSK etc. > > I can not give detailed algorithm here. For example, a GSM burst > training sequence is known a prior. Someone can exploit correlation > property and calculate engery. If the engegy achieved through GMSK > correlation is high enough compared to that of others (say 8PSK) then > modulation type is GMSK. > Cheers > Santosh >In the GSM case it's moot -- GSM includes a frequency correction burst which is around 68kHz above the carrier for GMSK and below the carrier for 8PSK. That's pretty distinctive. Unless he just couldn't wait around for the FB he could easily tell. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Reply by ●January 31, 20062006-01-31
"Tim Wescott" <tim@seemywebsite.com> wrote in message news:LOmdnb-GFvGROkLenZ2dnUVZ_tqdnZ2d@web-ster.com...> Nilnod wrote: > > > I guess Ravi is interested with received signal which was GMSK > > modulated at transmission side - a typical mobile communication like > > GSM or EDGE scenario. > > > > The is related to something called "blind demodulation" where we do not > > know in advance what received signal is coming - GMSK/8PSK etc. > > > > I can not give detailed algorithm here. For example, a GSM burst > > training sequence is known a prior. Someone can exploit correlation > > property and calculate engery. If the engegy achieved through GMSK > > correlation is high enough compared to that of others (say 8PSK) then > > modulation type is GMSK. > > Cheers > > Santosh > > > In the GSM case it's moot -- GSM includes a frequency correction burst > which is around 68kHz above the carrier for GMSK and below the carrier > for 8PSK. That's pretty distinctive. Unless he just couldn't wait > around for the FB he could easily tell.I'm fairly sure a FB uses GMSK strictly. So you'd only get the 68kHz offset above the carrier. 8PSK is not used in FBs (frequency correction bursts). You could try to see the phase transitions in the signal. 90 degree phase changes will be GMSK and smaller changes would be 8PSK. But I'm not sure if the OP wants to distinguish GMSK and 8PSK or just find GMSK amongst *any* time of incoming demodulations. In the latter case, I'd say, try to demodulate assuming it is GMSK...and if it is GSM, you can try to see if you have a TSC in your bursts. If you get a match, then you have GMSK. Cheers Bhaskar> -- > > Tim Wescott > Wescott Design Services > http://www.wescottdesign.com
Reply by ●January 31, 20062006-01-31
Bhaskar Thiagarajan wrote:> "Tim Wescott" <tim@seemywebsite.com> wrote in message > news:LOmdnb-GFvGROkLenZ2dnUVZ_tqdnZ2d@web-ster.com... > >>Nilnod wrote: >> >> >>>I guess Ravi is interested with received signal which was GMSK >>>modulated at transmission side - a typical mobile communication like >>>GSM or EDGE scenario. >>> >>>The is related to something called "blind demodulation" where we do not >>>know in advance what received signal is coming - GMSK/8PSK etc. >>> >>>I can not give detailed algorithm here. For example, a GSM burst >>>training sequence is known a prior. Someone can exploit correlation >>>property and calculate engery. If the engegy achieved through GMSK >>>correlation is high enough compared to that of others (say 8PSK) then >>>modulation type is GMSK. >>>Cheers >>>Santosh >>> >> >>In the GSM case it's moot -- GSM includes a frequency correction burst >>which is around 68kHz above the carrier for GMSK and below the carrier >>for 8PSK. That's pretty distinctive. Unless he just couldn't wait >>around for the FB he could easily tell. > > > I'm fairly sure a FB uses GMSK strictly. So you'd only get the 68kHz offset > above the carrier. 8PSK is not used in FBs (frequency correction bursts).See page 19 of GSM 05.02 version 8.4.1 Release 1999, section 5.2.4 at the bottom where it says "NOTE:". I could be mistaken about the meaning of the "COMPACT" format -- if it means 8PSK then my statement is correct.> > You could try to see the phase transitions in the signal. 90 degree phase > changes will be GMSK and smaller changes would be 8PSK. But I'm not sure if > the OP wants to distinguish GMSK and 8PSK or just find GMSK amongst *any* > time of incoming demodulations. In the latter case, I'd say, try to > demodulate assuming it is GMSK...and if it is GSM, you can try to see if you > have a TSC in your bursts. If you get a match, then you have GMSK. >Yea, if he just wants to distinguish GMSK from anything else then I think that's what he's got to do. It gets hard if it has an arbitrary baud rate, or lots of noise, though. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Reply by ●January 31, 20062006-01-31
"Tim Wescott" <tim@seemywebsite.com> wrote in message news:4b2dnXXsMMyCVELenZ2dnUVZ_tqdnZ2d@web-ster.com...> Bhaskar Thiagarajan wrote: > > > "Tim Wescott" <tim@seemywebsite.com> wrote in message > > news:LOmdnb-GFvGROkLenZ2dnUVZ_tqdnZ2d@web-ster.com... > > > >>Nilnod wrote: > >> > >> > >>>I guess Ravi is interested with received signal which was GMSK > >>>modulated at transmission side - a typical mobile communication like > >>>GSM or EDGE scenario. > >>> > >>>The is related to something called "blind demodulation" where we do not > >>>know in advance what received signal is coming - GMSK/8PSK etc. > >>> > >>>I can not give detailed algorithm here. For example, a GSM burst > >>>training sequence is known a prior. Someone can exploit correlation > >>>property and calculate engery. If the engegy achieved through GMSK > >>>correlation is high enough compared to that of others (say 8PSK) then > >>>modulation type is GMSK. > >>>Cheers > >>>Santosh > >>> > >> > >>In the GSM case it's moot -- GSM includes a frequency correction burst > >>which is around 68kHz above the carrier for GMSK and below the carrier > >>for 8PSK. That's pretty distinctive. Unless he just couldn't wait > >>around for the FB he could easily tell. > > > > > > I'm fairly sure a FB uses GMSK strictly. So you'd only get the 68kHzoffset> > above the carrier. 8PSK is not used in FBs (frequency correctionbursts).> > See page 19 of GSM 05.02 version 8.4.1 Release 1999, section 5.2.4 at > the bottom where it says "NOTE:". I could be mistaken about the meaning > of the "COMPACT" format -- if it means 8PSK then my statement is correct.No - COMPACT doesn't mean 8PSK. Usually, if something uses 8PSK, the bit definitions will be shown in groups of 3 (to show 3 bits per symbol). See the previous section 5.2.3 for an example in the Normal burst definition. COMPACT is just a slightly different access method with minor differences in frame structure, etc - not sure of the exact details but it doesn't seem like it is widely used.> > You could try to see the phase transitions in the signal. 90 degreephase> > changes will be GMSK and smaller changes would be 8PSK. But I'm not sureif> > the OP wants to distinguish GMSK and 8PSK or just find GMSK amongst*any*> > time of incoming demodulations. In the latter case, I'd say, try to > > demodulate assuming it is GMSK...and if it is GSM, you can try to see ifyou> > have a TSC in your bursts. If you get a match, then you have GMSK. > > > Yea, if he just wants to distinguish GMSK from anything else then I > think that's what he's got to do. > > It gets hard if it has an arbitrary baud rate, or lots of noise, though. > > -- > > Tim Wescott > Wescott Design Services > http://www.wescottdesign.com
Reply by ●January 31, 20062006-01-31
Bhaskar Thiagarajan wrote:> "Tim Wescott" <tim@seemywebsite.com> wrote in message > news:4b2dnXXsMMyCVELenZ2dnUVZ_tqdnZ2d@web-ster.com... > >>Bhaskar Thiagarajan wrote: >> >> >>>"Tim Wescott" <tim@seemywebsite.com> wrote in message >>>news:LOmdnb-GFvGROkLenZ2dnUVZ_tqdnZ2d@web-ster.com... >>> >>> >>>>Nilnod wrote: >>>> >>>> >>>> >>>>>I guess Ravi is interested with received signal which was GMSK >>>>>modulated at transmission side - a typical mobile communication like >>>>>GSM or EDGE scenario. >>>>> >>>>>The is related to something called "blind demodulation" where we do not >>>>>know in advance what received signal is coming - GMSK/8PSK etc. >>>>> >>>>>I can not give detailed algorithm here. For example, a GSM burst >>>>>training sequence is known a prior. Someone can exploit correlation >>>>>property and calculate engery. If the engegy achieved through GMSK >>>>>correlation is high enough compared to that of others (say 8PSK) then >>>>>modulation type is GMSK. >>>>>Cheers >>>>>Santosh >>>>> >>>> >>>>In the GSM case it's moot -- GSM includes a frequency correction burst >>>>which is around 68kHz above the carrier for GMSK and below the carrier >>>>for 8PSK. That's pretty distinctive. Unless he just couldn't wait >>>>around for the FB he could easily tell. >>> >>> >>>I'm fairly sure a FB uses GMSK strictly. So you'd only get the 68kHz > > offset > >>>above the carrier. 8PSK is not used in FBs (frequency correction > > bursts). > >>See page 19 of GSM 05.02 version 8.4.1 Release 1999, section 5.2.4 at >>the bottom where it says "NOTE:". I could be mistaken about the meaning >>of the "COMPACT" format -- if it means 8PSK then my statement is correct. > > > No - COMPACT doesn't mean 8PSK. > Usually, if something uses 8PSK, the bit definitions will be shown in groups > of 3 (to show 3 bits per symbol). See the previous section 5.2.3 for an > example in the Normal burst definition. > COMPACT is just a slightly different access method with minor differences in > frame structure, etc - not sure of the exact details but it doesn't seem > like it is widely used. >I stand corrected. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
