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Old 02-02-2010, 07:34 PM
tanmoykar
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Default Difference between pi/4 QPSK and pi/4 DQPSK

Hi,
Can anyone please explain me the difference between pi/4 QPSK and pi/4
DQPSK? Recently i came accross these 2 termilogies. If You have any good
reference explaining these 2 Modulation types, can you please share the
same?


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Old 02-02-2010, 07:56 PM
Tim Wescott
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Default Re: Difference between pi/4 QPSK and pi/4 DQPSK

On Tue, 02 Feb 2010 13:34:26 -0600, tanmoykar wrote:

> Hi,
> Can anyone please explain me the difference between pi/4 QPSK and pi/4
> DQPSK? Recently i came accross these 2 termilogies. If You have any good
> reference explaining these 2 Modulation types, can you please share the
> same?


'True' QPSK encodes two bits as the absolute phase of the signal vs. the
carrier -- so 0 degrees shift from carrier == 00, 90 degrees shift == 01,
etc.

Differential QPSK ('DQPSK') encodes two bits as the phase _jump_ of the
signal -- so if the phase doesn't jump that means 00, +90 means 01, etc.
(there are different mappings).

QPSK enjoys a theoretical advantage, but in practice it requires some
clever way to figure out what the carrier really is, or it requires a
carrier reference on a side tone of some sort.

DQPSK enjoys a practical advantage that if you lose carrier
synchronization for a symbol or two recovery is much quicker.

--
www.wescottdesign.com
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  #3 (permalink)  
Old 02-02-2010, 10:18 PM
Eric Jacobsen
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Default Re: Difference between pi/4 QPSK and pi/4 DQPSK

On 2/2/2010 12:56 PM, Tim Wescott wrote:
> On Tue, 02 Feb 2010 13:34:26 -0600, tanmoykar wrote:
>
>> Hi,
>> Can anyone please explain me the difference between pi/4 QPSK and pi/4
>> DQPSK? Recently i came accross these 2 termilogies. If You have any good
>> reference explaining these 2 Modulation types, can you please share the
>> same?

>
> 'True' QPSK encodes two bits as the absolute phase of the signal vs. the
> carrier -- so 0 degrees shift from carrier == 00, 90 degrees shift == 01,
> etc.
>
> Differential QPSK ('DQPSK') encodes two bits as the phase _jump_ of the
> signal -- so if the phase doesn't jump that means 00, +90 means 01, etc.
> (there are different mappings).
>
> QPSK enjoys a theoretical advantage, but in practice it requires some
> clever way to figure out what the carrier really is, or it requires a
> carrier reference on a side tone of some sort.


Yup. Usually the phase is resolved with the help of the FEC lock
indicator, i.e., just try them all until one locks.

> DQPSK enjoys a practical advantage that if you lose carrier
> synchronization for a symbol or two recovery is much quicker.


Yup. Often due to crappy oscillators with bad stability or phase noise
or Doppler fading.

--
Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.abineau.com
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