Just a quick question as i am yet to find any useful info on a website
i thought this would be the best place to ask.

I am implementing DQPSK and was wondering if this means you
differentially encode the data prior to serial to parallel conversion
or after?

I am working on the basis that DQPSK is similiar to DBPSK which the
differential encoding is covered on www.complextoreal.com.

I have designed a working model but when put onto my FPGA the data
being modulated isn't being demodulated by another Commercial
demodulator.

I think its prior to the data stream being split but then how can you
demodulate it non-coherently?

Regards

Mark

On 18 Feb 2004 10:35:27 -0800, mark@markio.co.uk (Mark Irwin) wrote:

>Just a quick question as i am yet to find any useful info on a website
>i thought this would be the best place to ask.
>
>I am implementing DQPSK and was wondering if this means you
>differentially encode the data prior to serial to parallel conversion
>or after?
>
>I am working on the basis that DQPSK is similiar to DBPSK which the
>differential encoding is covered on www.complextoreal.com.
>
>I have designed a working model but when put onto my FPGA the data
>being modulated isn't being demodulated by another Commercial
>demodulator.
>
>I think its prior to the data stream being split but then how can you
>demodulate it non-coherently?

Why not follow the standard?  If you are required to interwork with a
"commercial demodulator" then it's likely that the type of
differential encoding is described exactly by a standard.

Which standard are you using?  (Perhaps the question should be: which
standard are you meant to be using?)  Getting "information" from a
website is not a substitute for having real specifications.

You might like to look at some modem standards, e.g. ITU-T V.26

Regards,
Allan.

Hi Mark,

mark@markio.co.uk (Mark Irwin) wrote in message news:<2eb104f0.0402181035.12887938@posting.google.com>...
> Just a quick question as i am yet to find any useful info on a website
> i thought this would be the best place to ask.
> 
> I am implementing DQPSK and was wondering if this means you
> differentially encode the data prior to serial to parallel conversion
> or after?
> 
> I am working on the basis that DQPSK is similiar to DBPSK which the
> differential encoding is covered on www.complextoreal.com.
> 
> I have designed a working model but when put onto my FPGA the data
> being modulated isn't being demodulated by another Commercial
> demodulator.
> 
> I think its prior to the data stream being split but then how can you
> demodulate it non-coherently?
> 
> Regards
> 
> Mark

It is the transmitted symbols which are differentially encoded. That
is, a received symbol is not define its absolute phase, but by its
phase shift from the previous symbol. This allows the receiver to
avoid determining the absolute phase of the transmitted signal. The
symbols are usually grey coded, so if your estimated phase shift is
one step too great or too small you only make a 1 bit error in the
recovered data. Unfortunately, on the next symbol you will probably
reverse the error and get another one bit error. This is a key
drawback of differential encoding - errors normally occur in pairs.

Regards,
Steve