Hello all:
We have been working on an APCO P25 project at my university, and are
fortunate enough to have 4 USRP N210's all equipped with the WBX boards.
As the project has progressed we have accomplished many of our goals.
However, one thing that has haunted us throughout the entire project is
transmission from USRP to USRP results in very high bit errors. We also
have 2 P25 handsets available and when Tx'ing from a handset and receiving
from a USRP or Tx'ing from the USRP and receiving from a handset, everything
is fine, we have no perceivable bit errors (we haven't really dug into the
exact bit error measurements, however, we are working with a DVSI AMBE
vocoder/FEC, which implies the bit errors are large enough to screw up the
error correction, which, no matter how you cut it, shouldn't happen with two
USRP's 2 feet from each other).
So we ran some tests with the 4 USRP's
We used a two-tone test at +1kHz and -2kHz. We used GNU Radio and GRC with
a fairly simple set up that consisted of reading the MATLAB generated
two-tone sample data using the "file source" block into the "UHD USRP sink."
On the Rx end, it was the same, but reversed. I have supplied figures of
the received data, but guessed the GRC setup code isn't necessary.
In the first figure, we saw that using the same USRP for Tx and switching
USRP's on the Rx end resulted in very odd data. In the second, we used USRP
1 to Tx and 2 to Rx (what we believe to be the "worst" USRP's in the bundle)
and attached them to an external clock. It can be seen that as far as the
two tone test goes, the peaks were right on the money.
Another thing we noted is that by changing the gain on the Tx end, the
harmonics shown don't scale with the power. At low power, the harmonics are
far too close to the main peaks, which is worrying (initially we had the
gain of the USRPs marginally under the maximum gain because we initially
thought the errors were caused by the RF front end going into some kind of
saturation state. From this data we see this isn't the case). Also of note
is that at the time the first figure was generated, the USRP's were
approximately 2 feet from each other. In the latter figure, they were about
5 feet apart. It is obvious that the harmonics in the second figure are
higher, relative to the main peaks, than the first. I don't really have a
solid question to ask other than is this behavior normal?
It is apparent that the poor results in the first figure are caused by clock
drift, but the harmonics are also very worrying. Especially USRP 4 in the
first figure, which shows a relatively high harmonic right next to the main
peaks. Since the time we have sampled the supplied data, we have been
progressing forward in the project, so we haven't been able to test the P25
waveform from USRP to USRP, and can't verify that the initial bit error
problems are alleviated by getting rid of the clock drift, or if they are
caused by the harmonics. Is there something we can do to remedy this
problem, or, again, is this behavior normal?
http://old.nabble.com/file/p32726685/Rx_DualTone_1.jpg
http://old.nabble.com/file/p32726685/external_clock_dual_tone.jpg