Re: QPSK carrier recovery problems

HI, thanks for your reply.

Why are you considering the 4th power for carrier recovery?

I thought that blind synchronization method is the simplest way to
synchronize to carrier.
One of simplest blind methods is 4th power for QPSK signal. But it
seems that it doesn't
work well when you have RRC pulse shaping in baseband.

What is your objective here, to recover the carrier or to decode
the signal?

Of course my objective is to decode data. In my solution carrier phase
is synchronized
with timing in transmitter, so when I recover carrier, I also have
recovered timing.

How much clock and phase offset do you have in your system?
Just like most questions, it doesn't make much sense unless
you include this type of information.

Here are parameters of all of the system:
- objective is to send digital data via semiduplex(one way only) audio
(300Hz-3000Hz@-3dB) channel, where minimum S/N ratio is 10dB
- acceptable data rate is from 3.7 to 5 kbit/s with BER not worst than
1E-4. Given data rate parameter is acceptable in project architecture,
but after choosing transmission parameters, data rate will be fixed to
value from given range. Of course higher data rate value is better
because of transmission error correction possible in future
- transmitter and receiver are separate devices
- transmitter is put at the one end of channel, receiver at the other
- crystal oscillators running DSP parts in transmitter and receiver is
not synchronized to each other. Maximum difference between crystal
oscillator frequencies in transmitter is 500ppm.
- receiver synchronization to received signal must not take longer
than 150ms
- there could be some signal loss, but not longer than for 10ms and
not more often than 5 times per second
- transmitter power amplifier is linear.

According to above parameters I have chosen 1.85kS/s QPSK with
fcarrier=1.85kHz, RxADC/ TxDAC sampling freq=18.5kHz, RRC shaping beta
0.24: one filter in transmitter and one in receiver. Every symbol in
transmitter starts in the same phase of carrier. As I wrote above, I
thought that when I lock to a carrier, I will also lock to symbol
timing and voila - I know when sample the symbols. I have chosen this
because of simplicity of implementation.

Of course I am open on other modulation/implementation proposals.
Please advise.

Best regards,
Adam Turowski






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