O-QPSK Demodulation

Hi all,
I have to demodulate an O-QPSK signal. As usual in this kind of
modulation, symbols (chips) have an half-sine shape, and the Q channel
has a 90 degree (half simbol) offset with respect to the I channel.
The first thing I have to do (and perhaps the most difficult) is
removing the frequency offset from the received I and Q signals. I
cannot use a simple Costas loop since it doesn't work with O-QPSK
signals!
Are there dedicated loops for O-QPSK signals? Or it exists some simple
and effective way to demodultate such a signal?

Thank you in advance for any advice!
Antonio

A.D. wrote:

> Hi all,
> I have to demodulate an O-QPSK signal. As usual in this kind of
> modulation, symbols (chips) have an half-sine shape, and the Q channel
> has a 90 degree (half simbol) offset with respect to the I channel.
> The first thing I have to do (and perhaps the most difficult) is
> removing the frequency offset from the received I and Q signals. I
> cannot use a simple Costas loop since it doesn't work with O-QPSK
> signals!
> Are there dedicated loops for O-QPSK signals? Or it exists some simple
> and effective way to demodultate such a signal?

The demodulation of OQPSK is not much different from the demodulation of 
the QPSK. Think of OPSK as of two BPSK signals shifted by 1/2 bit. All 
you have to do is to jointly demodulate those two signals. Symbol sync 
and carrier sync is not any different.

Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com

A.D. wrote:

> Hi all,
> I have to demodulate an O-QPSK signal. As usual in this kind of
> modulation, symbols (chips) have an half-sine shape, and the Q channel
> has a 90 degree (half simbol) offset with respect to the I channel.
> The first thing I have to do (and perhaps the most difficult) is
> removing the frequency offset from the received I and Q signals. I
> cannot use a simple Costas loop since it doesn't work with O-QPSK
> signals!
> Are there dedicated loops for O-QPSK signals? Or it exists some simple
> and effective way to demodultate such a signal?
> 
> Thank you in advance for any advice!
> Antonio
> 
O-QPSK with half-sine shaping also goes by the name 'MSK'.  I built an 
MSK radio modem for my Master's thesis, which I have recently posted on 
the web: http://www.wescottdesign.com/articles/MSK/mskTop.html.  Chapter 
3 should give a pretty good tutorial for how to do this, and Appendix B 
is a pretty good example of just how execrable code can be while still 
functioning as intended.

-- 

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

"Vladimir Vassilevsky" <antispam_bogus@hotmail.com> ha scritto nel messaggio 
news:i9kpf.39953$tV6.22464@newssvr27.news.prodigy.net...
>
> The demodulation of OQPSK is not much different from the demodulation of 
> the QPSK. Think of OPSK as of two BPSK signals shifted by 1/2 bit. All you 
> have to do is to jointly demodulate those two signals. Symbol sync and 
> carrier sync is not any different.

Thank you Vladimir, I will try it.

Regards,
Antonio

"Tim Wescott" <tim@seemywebsite.com> ha scritto nel messaggio 
news:bMydnWHsbLEEdzjeRVn-uA@web-ster.com...
>
> O-QPSK with half-sine shaping also goes by the name 'MSK'.  I built an MSK 
> radio modem for my Master's thesis, which I have recently posted on the 
> web: http://www.wescottdesign.com/articles/MSK/mskTop.html.  Chapter 3 
> should give a pretty good tutorial for how to do this, and Appendix B is a 
> pretty good example of just how execrable code can be while still 
> functioning as intended.

Great! I will carefully read it.

Thank you very much,
Antonio 

A.D. wrote:
> Hi all,
> I have to demodulate an O-QPSK signal. As usual in this kind of
> modulation, symbols (chips) have an half-sine shape, and the Q channel
> has a 90 degree (half simbol) offset with respect to the I channel.
> The first thing I have to do (and perhaps the most difficult) is
> removing the frequency offset from the received I and Q signals. I
> cannot use a simple Costas loop since it doesn't work with O-QPSK
> signals!
> Are there dedicated loops for O-QPSK signals? Or it exists some simple
> and effective way to demodultate such a signal?
>
> Thank you in advance for any advice!
> Antonio

how big do you expect the offset to be?, I believe differential chip
detection
can handle some offset but it will cost you a little over 3dB in
sensitivity

-Lasse

<langwadt@ieee.org> ha scritto nel messaggio 
news:1135022040.250858.304390@g43g2000cwa.googlegroups.com...
>
> how big do you expect the offset to be?, I believe differential chip
> detection can handle some offset but it will cost you a little over
> 3dB in sensitivity

I'm trying to demodulate ZigBee signals, so I have a chip rate of 1Mcps on I 
and Q channel and a maximum extimated offset of about 500KHz (wrost case). 
But data are not differentially encoded, so I think that is not possible to 
use a differential detection...

Regards,
Antonio