Re: qpsk demodulation questions

On Jan 13, 7:06 pm, sten...@xxxxxxxxx wrote:
Hi,

I'm currently working on a QPSK demodulator, but I'm seeing some
things that don't look right, and I wanted to run them by some of the
people in this group. Is there a site where I can post up some
screenshots to show you what I'm seeing

1...The recovered signals on the I and Q branches are the inverse of
the actual transmitted data. A transmitted '1' is received as a zero.
Is this expected?

There is a 4 way ambiguity in the recovered phase, so this is
possible, as are other combinations. You can use a preamble or
differential encoding to resolve.


2...Both the recovered I and Q signals are very similar to the
summation of the I and Q channels in the modulator. I would have
expected the received I data to look like the transmitted I data and
similarly for Q.

One would expect them to look like I and Q in the modulator, allowing
for phase ambiguity.


3...The signals on the I and Q branches are not clean, After mixing
and low pass filtering to remove the carrier, the signals still
contain spikes which are causing errors in the data. I read about
using a matched filter/averaging filter to clean them up. What about
an RRC at the output of the modulator and one on each of the
demodulator I and Q branches prior to the mixer....any suggestions?


Match the receiver filter to the transmit pulse shape. If it is
rectangular, use a rectangular filter in receiver.

4...Is any transmitted data normally scrambled with a PN code to
improve timing? The reason I'm asking is my TXed data contains strngs
of continuous ones and zeros. This is causing the PLL to lose lock. Is
it more usual to send a preamble sequence. The only problem I can see
with the preamble is that it would have to be continually transmitted.
Anybody been down this road before, I'd appreciate your advice.


Scrambling is common in continuous modems like the old V.XX telephone
standards. Preambles are used in burst modems. This should not affect
the carrier PLL though.

John

.

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