Hi
   thanks everyone for the valueble suggestions. actually my problem is
i'm trying to incorporate a soft decoding stage with a blind MIMO detector
(with interference cancellation) which produces a soft estimate on
transmitted bits and get a Turbo structure going (i.e. iteratively
exchange information between decoder and detector stage to improve
performance).
    this blind (without training sequence) MIMO detector (or for that
matter any blind detector) has a weakness that it produces a arbitrary
phase ambiguity in the estimated symbols. now, if i try to incorporate
differential encoding to resolve phase ambiguity then it is not possible
to do more than one iteration loop between detector and decoder, because
it is not possible to replicate the encoding operation at the receiver
without error propogation.
    therefore i need to find some way to resolve this ambiguity without
using differential encoding. 

thanx
vivek 


>On 22 Mar 2005 06:21:04 -0800, "Mark" <makolber@yahoo.com> wrote:
>
>>vgoel2 wrote:
>>> Hi all,
>>>        can anybody suggest some technique other than differential
>>encoding
>>> or using training sequence to remove phase ambiguity of 180 degree
>>when
>>> using BPSK and phase ambiguity of 90,180,270 when using 4-QAM for
>>> modulation.
>>>        i'm using these modulation schemes for blind detection in MIMO
>>> systems , where the received signal has this arbitrary phase
>>ambiguity.
>>> any help about this problem will be greatly appreciated.
>>>
>>> thanks
>>> vivek goel
>>
>>there is the obvious strategy of having the receiver "scan" through the
>>2 or 4 possible carrier phases looking for the one that produces
>>correct messages i.e. that the FEC errors are low
>>
>>Mark
>
>That's pretty commonly done, actually, especially if it's not a bursty
>system.
>
>The OP put some tough constraints on by not wanting a training
>sequence (which I took to mean pilot symbols as well) or differential
>encoding.  Without either of those the only thing I would know to do
>would be follow Clay's suggestion and actually decode all
>possibilities and pick the one with the best decoding metrics.   That
>seems awfully complex to me, though, so maybe just an ambiguity
>resolving state machine would be better (i.e., cycle through the
>ambiguities, stop when you get decoder lock).
>
>
>Eric Jacobsen
>Minister of Algorithms, Intel Corp.
>My opinions may not be Intel's opinions.
>http://www.ericjacobsen.org
>


		
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On 22 Mar 2005 06:21:04 -0800, "Mark" <makolber@yahoo.com> wrote:

>vgoel2 wrote:
>> Hi all,
>>        can anybody suggest some technique other than differential
>encoding
>> or using training sequence to remove phase ambiguity of 180 degree
>when
>> using BPSK and phase ambiguity of 90,180,270 when using 4-QAM for
>> modulation.
>>        i'm using these modulation schemes for blind detection in MIMO
>> systems , where the received signal has this arbitrary phase
>ambiguity.
>> any help about this problem will be greatly appreciated.
>>
>> thanks
>> vivek goel
>
>there is the obvious strategy of having the receiver "scan" through the
>2 or 4 possible carrier phases looking for the one that produces
>correct messages i.e. that the FEC errors are low
>
>Mark

That's pretty commonly done, actually, especially if it's not a bursty
system.

The OP put some tough constraints on by not wanting a training
sequence (which I took to mean pilot symbols as well) or differential
encoding.  Without either of those the only thing I would know to do
would be follow Clay's suggestion and actually decode all
possibilities and pick the one with the best decoding metrics.   That
seems awfully complex to me, though, so maybe just an ambiguity
resolving state machine would be better (i.e., cycle through the
ambiguities, stop when you get decoder lock).


Eric Jacobsen
Minister of Algorithms, Intel Corp.
My opinions may not be Intel's opinions.
http://www.ericjacobsen.org

Clay S. Turner wrote:

> "vgoel2" <vivek.goel@gmail.com> wrote in message 
> news:YPOdnew7rMjFE6LfRVn-hQ@giganews.com...
> 
>>Hi all,
>>      can anybody suggest some technique other than differential encoding
>>or using training sequence to remove phase ambiguity of 180 degree when
>>using BPSK and phase ambiguity of 90,180,270 when using 4-QAM for
>>modulation.
>>      i'm using these modulation schemes for blind detection in MIMO
>>systems , where the received signal has this arbitrary phase ambiguity.
>>any help about this problem will be greatly appreciated.
>>
>>thanks
>>vivek goel
>>
> 
> 
> Hello Vivek,
> 
> You can also use pilot symbols. However when these are used, usually a known 
> synch is used also. You may also use trellis encoding and a Viterbi decoder 
> and process the data for all possible starting states and choose the case 
> with least total accumulated error. However most communication systems are 
> designed with either a differential encoding or an absolute phase reference 
> and some use both.
> 
> IHTH,
> 
> Clay
> 
> 
If you use the trellis encoding/Viterbi decoder technique in a noisy 
environment and your bit phase PLL slips a phase you create a flood of 
bit errors.  If the same thing happens with a training sequence you'd 
lose the packet, which wouldn't be death if you weren't using FEC 
anyway.  If you use differential modulation you lose one bit.

I learned this from someone who learned it the hard way -- but I don't 
know how good their radio was.

-- 

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

vgoel2 wrote:
> Hi all,
>        can anybody suggest some technique other than differential
encoding
> or using training sequence to remove phase ambiguity of 180 degree
when
> using BPSK and phase ambiguity of 90,180,270 when using 4-QAM for
> modulation.
>        i'm using these modulation schemes for blind detection in MIMO
> systems , where the received signal has this arbitrary phase
ambiguity.
> any help about this problem will be greatly appreciated.
>
> thanks
> vivek goel
>
>
>
> This message was sent using the Comp.DSP web interface on
> www.DSPRelated.com


there is the obvious strategy of having the receiver "scan" through the
2 or 4 possible carrier phases looking for the one that produces
correct messages i.e. that the FEC errors are low

Mark

"vgoel2" <vivek.goel@gmail.com> wrote in message 
news:YPOdnew7rMjFE6LfRVn-hQ@giganews.com...
> Hi all,
>       can anybody suggest some technique other than differential encoding
> or using training sequence to remove phase ambiguity of 180 degree when
> using BPSK and phase ambiguity of 90,180,270 when using 4-QAM for
> modulation.
>       i'm using these modulation schemes for blind detection in MIMO
> systems , where the received signal has this arbitrary phase ambiguity.
> any help about this problem will be greatly appreciated.
>
> thanks
> vivek goel
>

Hello Vivek,

You can also use pilot symbols. However when these are used, usually a known 
synch is used also. You may also use trellis encoding and a Viterbi decoder 
and process the data for all possible starting states and choose the case 
with least total accumulated error. However most communication systems are 
designed with either a differential encoding or an absolute phase reference 
and some use both.

IHTH,

Clay

"vgoel2" <vivek.goel@gmail.com> writes:

> Hi all,
>        can anybody suggest some technique other than differential encoding
> or using training sequence to remove phase ambiguity of 180 degree when
> using BPSK and phase ambiguity of 90,180,270 when using 4-QAM for
> modulation.
>        i'm using these modulation schemes for blind detection in MIMO
> systems , where the received signal has this arbitrary phase ambiguity.
> any help about this problem will be greatly appreciated.

You may be able to synchronize based on the statistics of the data. For
example, if you know that 00 is the highest probability of occurrence out
of the four 4-QAM (AKA QPSK), then measure the received symbols for
some period and make the required adjustment.

Other than these types of heuristic tricks, I'm not sure what else
one could do.
-- 
Randy Yates
Sony Ericsson Mobile Communications
Research Triangle Park, NC, USA
randy.yates@sonyericsson.com, 919-472-1124

Hi all,
       can anybody suggest some technique other than differential encoding
or using training sequence to remove phase ambiguity of 180 degree when
using BPSK and phase ambiguity of 90,180,270 when using 4-QAM for
modulation.
       i'm using these modulation schemes for blind detection in MIMO
systems , where the received signal has this arbitrary phase ambiguity.
any help about this problem will be greatly appreciated.

thanks
vivek goel   
 

		
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