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From: CronusZA on 27 May 2010 06:48
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



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