Ada,
>As I looked up from the documents, it was said that the signal after the
usrp source at the receiver, is baseband signal, whose central
>frequency is around 0 Hz, and already take off the carrier frequency.
But in the demodulation, there are still freq_recov and time_recov
block. >Are they at the symbol level?
Yes and there is always a freq-difference between Tx and Rx LO due to this there will always be some freq-offset