OFDM modulator o/p sampling rate

i am trying to transmit sound through OFDM system using the USRP
i was able to transmit and receive audio in OFDM system without the USRP
and also was able to transmit and receive audio using the USRP without

when i try to merge the 2 system using the scope i see that there is a
signal before the OFDM demod and no signal exactly after it and receive
a timeout message

i am thinking that this happen because the sampling rate before the USRP
is not adjusted right for transmission(i am using 1 USRP and transmit
and receive using a basic Tx and a basic Rx)

so is there a way to know the output sampling rate exactly after the
ofdm modulation?

and if this was not the problem what may cause this problem?

the ofdm modulator parameters
fft length: 512
occupied tones:200
cyclic prefix:128
payload:0 (0=automatic)

and the i/p is 32000 samples