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Blind Channel phase offset recovery

Started by Ali A Nasir August 13, 2009
Hi!
 
     I have implemented blind equalization technique like CMA (Constant
Modulus Algorithm) and Dispersion Minimization algorithm but they are
working only for real channel taps. 
    
     Can anyone tell me that if the channel taps are complex i.e. channel
is introducing phase offset, then how we can implement blind phase offset
recovery ? I would be grateful if someone could hint me about some good
research paper on this issue. What I have searched works in Decision
directed mode but is there any technique which do recover the phase offset
blindly ?

Regards
On Aug 13, 7:48&#2013266080;am, "Ali A Nasir" <aliarsha...@hotmail.com> wrote:
> Hi! > > &#2013266080; &#2013266080; &#2013266080;I have implemented blind equalization technique like CMA (Constant > Modulus Algorithm) and Dispersion Minimization algorithm but they are > working only for real channel taps. > > &#2013266080; &#2013266080; &#2013266080;Can anyone tell me that if the channel taps are complex i.e. channel > is introducing phase offset, then how we can implement blind phase offset > recovery ? I would be grateful if someone could hint me about some good > research paper on this issue. What I have searched works in Decision > directed mode but is there any technique which do recover the phase offset > blindly ? > > Regards
If by "blind" you have no prior information of how the signal or data should be, then this is impossible. For example, a QPSK constellation rotated by pi/2, or pi, or 3pi/2, all look the same. In most practical systems you'll have a preamble to solve this ambiguity. Does that make sense or did I misunderstand your question? Julius
>On Aug 13, 7:48=A0am, "Ali A Nasir" <aliarsha...@hotmail.com> wrote: >> Hi! >> >> =A0 =A0 =A0I have implemented blind equalization technique like CMA
(Cons=
>tant >> Modulus Algorithm) and Dispersion Minimization algorithm but they are >> working only for real channel taps. >> >> =A0 =A0 =A0Can anyone tell me that if the channel taps are complex i.e.
c=
>hannel >> is introducing phase offset, then how we can implement blind phase
offset
>> recovery ? I would be grateful if someone could hint me about some
good
>> research paper on this issue. What I have searched works in Decision >> directed mode but is there any technique which do recover the phase
offse=
>t >> blindly ? >> >> Regards > >If by "blind" you have no prior information of how the signal >or data should be, then this is impossible. For example, a >QPSK constellation rotated by pi/2, or pi, or 3pi/2, all look >the same. > >In most practical systems you'll have a preamble to >solve this ambiguity. > >Does that make sense or did I misunderstand your question? >Julius >
I have the information about the constellation that I am using but not allowed to use any training or preamble ( since its blind recovery ). Let us say I am using pi/4 QPSK
On Aug 13, 10:14&#2013266080;am, "Ali A Nasir" <aliarsha...@hotmail.com> wrote:
> >On Aug 13, 7:48=A0am, "Ali A Nasir" <aliarsha...@hotmail.com> wrote: > >> Hi! > > >> =A0 =A0 =A0I have implemented blind equalization technique like CMA > (Cons= > >tant > >> Modulus Algorithm) and Dispersion Minimization algorithm but they are > >> working only for real channel taps. > > >> =A0 =A0 =A0Can anyone tell me that if the channel taps are complex i.e. > c= > >hannel > >> is introducing phase offset, then how we can implement blind phase > offset > >> recovery ? I would be grateful if someone could hint me about some > good > >> research paper on this issue. What I have searched works in Decision > >> directed mode but is there any technique which do recover the phase > offse= > >t > >> blindly ? > > >> Regards > > >If by "blind" you have no prior information of how the signal > >or data should be, then this is impossible. &#2013266080;For example, a > >QPSK constellation rotated by pi/2, or pi, or 3pi/2, all look > >the same. > > >In most practical systems you'll have a preamble to > >solve this ambiguity. > > >Does that make sense or did I misunderstand your question? > >Julius > > &#2013266080; &#2013266080;I have the information about the constellation that I am using but not > allowed to use any training or preamble ( since its blind recovery ). Let > us say I am using pi/4 QPSK
Then you are out of luck, unless the data is differentially encoded or something. You'll have to send some sort of sync word to solve the ambiguity. Julius
On 8/13/2009 8:05 AM, julius wrote:
> On Aug 13, 10:14 am, "Ali A Nasir"<aliarsha...@hotmail.com> wrote: >>> On Aug 13, 7:48=A0am, "Ali A Nasir"<aliarsha...@hotmail.com> wrote: >>>> Hi! >>>> =A0 =A0 =A0I have implemented blind equalization technique like CMA >> (Cons= >>> tant >>>> Modulus Algorithm) and Dispersion Minimization algorithm but they are >>>> working only for real channel taps. >>>> =A0 =A0 =A0Can anyone tell me that if the channel taps are complex i.e. >> c= >>> hannel >>>> is introducing phase offset, then how we can implement blind phase >> offset >>>> recovery ? I would be grateful if someone could hint me about some >> good >>>> research paper on this issue. What I have searched works in Decision >>>> directed mode but is there any technique which do recover the phase >> offse= >>> t >>>> blindly ? >>>> Regards >>> If by "blind" you have no prior information of how the signal >>> or data should be, then this is impossible. For example, a >>> QPSK constellation rotated by pi/2, or pi, or 3pi/2, all look >>> the same. >>> In most practical systems you'll have a preamble to >>> solve this ambiguity. >>> Does that make sense or did I misunderstand your question? >>> Julius >> I have the information about the constellation that I am using but not >> allowed to use any training or preamble ( since its blind recovery ). Let >> us say I am using pi/4 QPSK > > Then you are out of luck, unless the data is differentially encoded or > something. You'll have to send some sort of sync word to solve the > ambiguity. > > Julius
If FEC is used sometimes you can use a FEC lock indicator to resolve the phase ambiguity. It adds a little time to the acquisition process, but it can often do the job. -- Eric Jacobsen Minister of Algorithms Abineau Communications http://www.abineau.com

julius wrote:

> On Aug 13, 7:48 am, "Ali A Nasir" <aliarsha...@hotmail.com> wrote: > >>Hi! >> >> I have implemented blind equalization technique like CMA (Constant >>Modulus Algorithm) and Dispersion Minimization algorithm but they are >>working only for real channel taps. >> >> Can anyone tell me that if the channel taps are complex i.e. channel >>is introducing phase offset, then how we can implement blind phase offset >>recovery ? I would be grateful if someone could hint me about some good >>research paper on this issue. What I have searched works in Decision >>directed mode but is there any technique which do recover the phase offset >>blindly ?
This is quite possible if the eye diagram has at least a slight opening.
> If by "blind" you have no prior information of how the signal > or data should be, then this is impossible. For example, a > QPSK constellation rotated by pi/2, or pi, or 3pi/2, all look > the same.
You don't have to worry about N * Pi/2 ambiguity if your goal is just to equalize the phase offset. It could be equalized to the nearest multiple of Pi/2.
> In most practical systems you'll have a preamble to > solve this ambiguity.
Data recovery is a different problem. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
>Hi! > > I have implemented blind equalization technique like CMA (Constant >Modulus Algorithm) and Dispersion Minimization algorithm but they are >working only for real channel taps. > > Can anyone tell me that if the channel taps are complex i.e.
channel
>is introducing phase offset, then how we can implement blind phase
offset
>recovery ? I would be grateful if someone could hint me about some good >research paper on this issue. What I have searched works in Decision >directed mode but is there any technique which do recover the phase
offset
>blindly ? > >Regards >
I have recently tried adaptive DFE with carrier phase recovery (provided symbol synch is achieved) on real-time signals. It worked for me but I had to play with step size of NLMS and loop constant for phase correction. This is explained in Proakis Comms. page 700. Chintan
>On 8/13/2009 8:05 AM, julius wrote: >> On Aug 13, 10:14 am, "Ali A Nasir"<aliarsha...@hotmail.com> wrote: >>>> On Aug 13, 7:48=A0am, "Ali A Nasir"<aliarsha...@hotmail.com> wrote: >>>>> Hi! >>>>> =A0 =A0 =A0I have implemented blind equalization technique like CMA >>> (Cons= >>>> tant >>>>> Modulus Algorithm) and Dispersion Minimization algorithm but they
are
>>>>> working only for real channel taps. >>>>> =A0 =A0 =A0Can anyone tell me that if the channel taps are complex
i.e.
>>> c= >>>> hannel >>>>> is introducing phase offset, then how we can implement blind phase >>> offset >>>>> recovery ? I would be grateful if someone could hint me about some >>> good >>>>> research paper on this issue. What I have searched works in
Decision
>>>>> directed mode but is there any technique which do recover the phase >>> offse= >>>> t >>>>> blindly ? >>>>> Regards >>>> If by "blind" you have no prior information of how the signal >>>> or data should be, then this is impossible. For example, a >>>> QPSK constellation rotated by pi/2, or pi, or 3pi/2, all look >>>> the same. >>>> In most practical systems you'll have a preamble to >>>> solve this ambiguity. >>>> Does that make sense or did I misunderstand your question? >>>> Julius >>> I have the information about the constellation that I am using but
not
>>> allowed to use any training or preamble ( since its blind recovery ).
Let
>>> us say I am using pi/4 QPSK >> >> Then you are out of luck, unless the data is differentially encoded or >> something. You'll have to send some sort of sync word to solve the >> ambiguity. >> >> Julius > >If FEC is used sometimes you can use a FEC lock indicator to resolve the
>phase ambiguity. It adds a little time to the acquisition process, but
>it can often do the job. > > >-- >Eric Jacobsen >Minister of Algorithms >Abineau Communications >http://www.abineau.com >
@ Eric Thanks for your guidance, Could you please explain some thing about FEC lock indicator. Is it Frequency Error Correction sort of thing used in Frequency Locked Loops ?
> > >julius wrote: > >> On Aug 13, 7:48 am, "Ali A Nasir" <aliarsha...@hotmail.com> wrote: >> >>>Hi! >>> >>> I have implemented blind equalization technique like CMA
(Constant
>>>Modulus Algorithm) and Dispersion Minimization algorithm but they are >>>working only for real channel taps. >>> >>> Can anyone tell me that if the channel taps are complex i.e.
channel
>>>is introducing phase offset, then how we can implement blind phase
offset
>>>recovery ? I would be grateful if someone could hint me about some
good
>>>research paper on this issue. What I have searched works in Decision >>>directed mode but is there any technique which do recover the phase
offset
>>>blindly ? > >This is quite possible if the eye diagram has at least a slight opening. > >> If by "blind" you have no prior information of how the signal >> or data should be, then this is impossible. For example, a >> QPSK constellation rotated by pi/2, or pi, or 3pi/2, all look >> the same. > >You don't have to worry about N * Pi/2 ambiguity if your goal is just to
>equalize the phase offset. It could be equalized to the nearest multiple
>of Pi/2. > >> In most practical systems you'll have a preamble to >> solve this ambiguity. > >Data recovery is a different problem. > > >Vladimir Vassilevsky >DSP and Mixed Signal Design Consultant >http://www.abvolt.com > >
@ Vladimir Thanks for your reply. The blind equalization algorithm (Dispersion Minimization) about which I talked before remove the ISI ( opens the eye even for maximum ISI ) but in case of channel phase offsets, the constellation is rotated in accordance with the phase offset. But that phase offset results in wrong decision at slicer during decoding. So my question is what could be done to mitigate that phase offset ( say after the ISI is removed ) ? Can I find the channel phase offset blindly and the rotate my equalized constellation accordingly to get accurate detection ?
>>Hi! >> >> I have implemented blind equalization technique like CMA (Constant >>Modulus Algorithm) and Dispersion Minimization algorithm but they are >>working only for real channel taps. >> >> Can anyone tell me that if the channel taps are complex i.e. >channel >>is introducing phase offset, then how we can implement blind phase >offset >>recovery ? I would be grateful if someone could hint me about some good >>research paper on this issue. What I have searched works in Decision >>directed mode but is there any technique which do recover the phase >offset >>blindly ? >> >>Regards >> > >I have recently tried adaptive DFE with carrier phase recovery (provided >symbol synch is achieved) on real-time signals. It worked for me but I
had
>to play with step size of NLMS and loop constant for phase correction.
This
>is explained in Proakis Comms. page 700. > >Chintan >
@ Chintan, Thanks for your guidance. Yup, symbol synchronization is achieved in my case. Have you implemented DFE in a system which was initially trained with some training preamble and then you used DFE to adapt the equalizer coefficients in decision directed mode ? Actually I have to equalize in blind environment ( with no training ). The situation is that after ISI removal and equalization for the channel amplitude, the resultant constellation is rotated in accordance with the channel phase offset. But that phase offset results in wrong decision at slicer during decoding. So my question is what could be done to mitigate that phase offset ? Can I find the channel phase offset blindly and then rotate my equalized constellation accordingly to get accurate detection ?