Hi All,
I have a question regarding Costas loop for QPSK phase synchronization. I had read some papers on it and worked on the math behind it. I understood that the costas loop takes the QPSK modulated signal (in which the original carrier signal is hidden), multiples with a locally generated carrier (which may be phase shifted wrt to transmitter's carrier) does some addition and multiplication between I and Q components and finally gives the phase shift between the transmitter's carrier and the receiver's carrier.
It does this by cancelling the effect of I and Q data on the received signal (because while working out the math there was a I^2 term and Q^2 term which usually is = 1). My question is when the transmitter uses a pulse shaping filter like RRC, how does the square of I and Q data get cancelled? As in any phase recovery technique where no pilot symbols are used, one must remove the effect of data from the received signal to find the phase difference as done in case PLL is used.
I too found another implementation of costas loop which uses limiters. But here there was a strong assumption that the phase shift is less that 45 degrees. So in the case when the initial phase (i.e before loop lock) shift is more than 45 degrees, then the constellation would be n times 90 degree rotated and hence all data would be detected incorrectly.
Which type is implemented in the existing GNU radio block?
Thank you
Sakthivel