GMSK/MSK constellation derotation and Channel impulse response estimation
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From: ane on 7 Aug 2008 14:31 I am trying to implement a GMSK receiver I have a block diagram of the receiver which is like this Received IQ Samples - > constellation derotation -> CIR estimation from training sequence -> MLSE (viterbi) equalizer The constellation derotation is supposed to collapse the four end points of GMSK constellations (+1, +j,-1,-j) to a two point (+1,-1) constellation. My question is how exactly are the constellation derotation and CIR estimation are supposed to performed? From what I have figured it out so far, the constellation derotation is supposed to by a multiplicaiton by exp{-j*n*pi/2} for n=1,2,3,4 i.e. every four received IQ samples are multiplied by {-j,-1,+j,1}. The goal is to limit the binary symbols to 2 (+1,-1) so that the MLSE equlaizer has 2^L states. However, this multiplication by exp{-j*n*pi/2} seem to totally change the pulse sequence. For example, if we skip the filtering for a moment (MSK modulation) and consider a pulse train +1,+1,-1,-1,+1,+1,+1,-1,-1,... and starting phase 0, then the modulated IQ samples are +j,-1,+j,+1,+j,-1,+j,+1,...multiplication by - j,-1,+j,+1,... results in the derotated sequence of +1,+1,-1,+1,+1,-1,+1,+1... Which, while limited to symbols(+1,-1), is a different pulse train sequence than the input. Would it not mess up the CIR calculation based on a known training sequence? Would it not result in incorrect sequence estimation by the viterbi equalizer?
From: ane on 7 Aug 2008 16:22 On Aug 7, 3:09 pm, "cpshah99" <cpsha...(a)rediffmail.com> wrote: > Hi > > >My question is how exactly are the constellation derotation and CIR > >estimation are supposed to performed? > > I dont knw abt constellation derotation but to estimate CIR, u need to > corss correlate the o/p constellation derotation(as per ur diagram) with > the training sequence. > > But can u tell more abt the structure of packet? i.e. what is the training > sequence? is it BPSK or QPSK? > > Generally, after the I-Q down conversion, the o/p is xcorrelated with > known training sequence. > > Hope this helps. > > Chintan The modulation is GMSK. The training sequence is alternating sequence of 00s and 11s (00110011...) or (-1-1+1+1-1-1+1+1...) . The question is regardlessof whther you use cross correlation, deconvolution, least squares, LMS or any other method for CIR estimation what would you use as the known training sequence? (-1-1+1+1...) or the ideal derotated version of it(without gaussian filtering)?
From: Tim Wescott on 8 Aug 2008 02:58
On Thu, 07 Aug 2008 11:31:48 -0700, ane wrote: > I am trying to implement a GMSK receiver I have a block diagram of the > receiver which is like this > > Received IQ Samples - > constellation derotation -> CIR estimation from > training sequence -> MLSE (viterbi) equalizer > > The constellation derotation is supposed to collapse the four end points > of GMSK constellations (+1, +j,-1,-j) to a two point (+1,-1) > constellation. > > > My question is how exactly are the constellation derotation and CIR > estimation are supposed to performed? From what I have figured it out > so far, the constellation derotation is supposed to by a multiplicaiton > by exp{-j*n*pi/2} for n=1,2,3,4 i.e. every four received IQ samples > are multiplied by {-j,-1,+j,1}. The goal is to limit the binary symbols > to 2 (+1,-1) so that the MLSE equlaizer has 2^L states. However, this > multiplication by exp{-j*n*pi/2} seem to totally change the pulse > sequence. For example, if we skip the filtering for a moment (MSK > modulation) and consider a pulse train +1,+1,-1,-1,+1,+1,+1,-1,-1,... > and starting phase 0, then the modulated IQ samples are > +j,-1,+j,+1,+j,-1,+j,+1,...multiplication by - j,-1,+j,+1,... results in > the derotated sequence of +1,+1,-1,+1,+1,-1,+1,+1... Which, while > limited to symbols(+1,-1), is a different pulse train sequence than the > input. Would it not mess up the CIR calculation based on a known > training sequence? Would it not result in incorrect sequence estimation > by the viterbi equalizer? Could they be using differential GMSK, where a transmitted '1' means a forward rotation of the phase, and a transmitted '0' means a reverse rotation of the phase? Seems like after derotation you'd still have a differential BPSK-ish signal to decode... -- Tim Wescott Control systems and communications consulting http://www.wescottdesign.com Need to learn how to apply control theory in your embedded system? "Applied Control Theory for Embedded Systems" by Tim Wescott Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html |