Re: Help regarding identification of modulation(GMSK) type of a signal

"Tim Wescott" <tim@xxxxxxxxxxxxxxxx> wrote in message
news:4b2dnXXsMMyCVELenZ2dnUVZ_tqdnZ2d@xxxxxxxxxxxxxxx
> Bhaskar Thiagarajan wrote:
>
> > "Tim Wescott" <tim@xxxxxxxxxxxxxxxx> wrote in message
> > news:LOmdnb-GFvGROkLenZ2dnUVZ_tqdnZ2d@xxxxxxxxxxxxxxx
> >
> >>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.

> > 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


.

Relevant Pages

  • Re: GMSK MODEM
    ... The method of FM the carrier with the gaussian filtered data is nice and ... And the discriminator work so nicely. ... You can generate the GMSK directly ...
    (comp.dsp)
  • Re: Help regarding identification of modulation(GMSK) type of a signal
    ... modulated at transmission side - a typical mobile communication like GSM or EDGE scenario. ... If the engegy achieved through GMSK ... modulation type is GMSK. ... 8PSK is not used in FBs (frequency correction bursts). ...
    (comp.dsp)