Re: de-rotation of symbols

On May 30, 6:38 am, "fishman2001" <d...@xxxxxxxxxxxxxx> wrote:
I have a quick question, but I'm not sure about how to ask it.
in the reception of GMSK, or in fact QPSK signals, the symbols allways get
de-rotated? can any one explain why this is necessary and exactly what is
happening in this derotation process?

Thanks.

Symbol rotation arises because the incoming carrier frequency and the
receiver's local oscillator (LO) are not synchronized. The lack of
synchronization creates a small residual frequency error. It is at
this residual rate that the symbols rotate around the origin of the
complex plane. When decisions are made, the complex plane is typically
sliced into quadrants. Imagine a constant symbol 1+j is transmitted to
you. Without synchronization, that symbol rotates around, turning into
-1+j, -1-j, and 1-j.

The synchronizer has to not only stop the rotation, but remove any
fixed offset to put the symbol back at 1+j. A PLL accomplishes this by
estimating the LO vs carrier phase error (by some nonlinear operation
to wipe the modulation off the signal), filtering the error, and
adjusting the phase of the LO to bring it into phase and frequency
lock with the incoming signal. Unfortunately, the phase detector
typically has an ambiguity, so that there are multiple lock points (4
in the case of QPSK). This can be resolved using a known pattern in
the data.

John
.

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