Hi all, continuing on the difference previously stated, I just have a quick
Q about the demodulation side of the the DQPSK, at the encoder im using the
mapping:
delta_phi(i) = phi(i) - phi(i-1)
with QPSK mapping:
B1 B2 phi
0 0 0
0 1 pi/2
1 1 pi
1 0 3*pi/2
and then i transmit only delta_phi across the channel,at the demodulator i
use:
phi(i) = delta_phi(i) + phi(i-1)
Where I assume that phi(-1) = 0 at the transmitter and the receiver to get
started initially. The mapping works perfectly when theres no channel
effects, but say at the receiver, if theres 1 error in the received
delta_phi, then everything received after that is wrong, looking at the
demodulator, it makes sense since 1 wrong delta_phi(i) makes phi(i)
incorrect and the incorrect phi(i) becomes an incorrect phi(i-1) in the
next step, so even if delta_phi(i) is correct in the next step, phi(i) will
still be incorrect, and its all downhill from there. I just dont see how to
fix this problem at the moment, any insight would be greatly appreciated.
Regards,
Allan De Freitas
Reply by Eric Jacobsen●February 2, 20102010-02-02
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
Reply by Tim Wescott●February 2, 20102010-02-02
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
Reply by tanmoykar●February 2, 20102010-02-02
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?