[Discuss-gnuradio] USRP Receiver config problem

[satarkar]

Hi,

I am trying to run ALOHA MAC on gnu radio. The modulation scheme used is
GMSK. The Tx and Rx chains are as follows:

Tx Chain:

src -> framer -> crc enoder -> fec encoder -> preamble -> nrz ->
interpolator -> Gauus filter -> FM modulator -> gain -> usrp_tx

Rx Chain

usrp_rx -> filter -> demod -> integrator filter -> correlator -> fec
decoder -> crc decoder -> extract frame

For some reason, I am unable to receive the frame when I run my python
scripts. I tested USRP boards by passing a simple sine-wave and it is
received correctly. I checked the output of usrp_tx on oscilloscope and it
does display the frame going through. To test, whether there was any
problem in the signalblocks I replaced usrp_tx by a filesink and dumped
the output of Tx chain in a file. I replaced the usrp_rx by this file and
the frame was correctly received. I think this means that there is problem
of configuration with usrp_tx and usrp_rx. Following are the configuration
scripts:

usrp_tx:

cordic_freq =29.32e6
sw_interp = 8
usrp_interp = int (128e6 / sw_interp / data_rate)  #160
fs = 128e6 / usrp_interp  #128e6/160 = 800e3

u = usrp.sink_c (0, usrp_interp)
u.set_tx_freq (0, cordic_freq)

usrp_rx:

r_cordic_freq = 29.32e6         # IF frequency
r_data_rate = 40e3
r_samples_per_symbol = 8

r_usrp_decim = int (64e6 / r_samples_per_symbol / r_data_rate)  # 80

r_fs = r_data_rate * r_samples_per_symbol                       # 800 e3

#  initialize USRP
r_u = usrp.source_c (0, r_usrp_decim)
r_u.set_rx_freq (0, -r_cordic_freq)

Can someone help me with this?

thanks and regards,

-Sumit