Very confused on benchmark_rx.py

I have difficulties benchmark_xx.py

I use USRP1 with SBX and GNU Radio 3.5.0rc0 on Ubuntu 10.10 on VMWare.

Usually I do test with following commands.

$ ./benchmakr_tx.py -f 450e6 --tx-gain=30 -r 100e3 -S 10 -m psk (or
gmsk) --tx-amplitude=0.02 (for a signal not being clipped)
$ ./benchmakr_rx.py -f 450e6 --tx-gain=50 -r 100e3 -S 10 -m psk (or
gmsk)

Of course, I modified receive_path.py to have an appropriate carrier
sensing threshold value:
# Carrier Sensing Blocks
alpha = 0.001
thresh = 30 # in dB, will have to adjust
self.probe = gr.probe_avg_mag_sqrd_c(thresh,alpha)
I changed thresh = 30 with thresh = -60 (it varies from 0 to -80 as I
want)

So, everything is fine, but when I run programs, a PC on a rx side
doesn’t
print
“ok = %5s pktno = %4d n_rcvd = %4d n_right = %4d” % (ok,
pktno,
n_rcvd, n_right)
which is in rx_callback()

When I modified following codes in receive_path.py:
# receiver
self.packet_receiver =
digital.demod_pkts(self.demodulator,
access_code=None,
callback=self._rx_callback,
threshold=-1) # by
default, threshold=-1 results into threshold=12
I changed threshold=-1 with threshold=x (x is some arbitrary negative
integer except -1)

Then, rx_callback is properly invoked. However, CRC check fails and it
prints
ok = False pktno = xxx n_rcvd = xxx n_right = xxx
and also throws an error like
(pktno,) = struct.unpack(’!H’, payload[0:2])
error: unpack requires a string argument of length 2

From all of these modification and results, I conclude that
a signal is not clipped and its spectrum looks good,
sensing a carrier is fine,
but it fails to correlate a signal with an access code which causes
rx_callback keep silent

I’m confused because this basic and fundamental example doesn’t work
well…

please give me some advices
and I appreciate to all of you in advance

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