Re: [Discuss-gnuradio] Running tunnel.py/benchmark_tx.py (OFDM) with XCVR2450?

[Marcus D. Leech]

On 31/01/12 08:28 AM, Florian Schlembach wrote:
>> But fiddling with gain values is often useful; even if you've already
>> done that I recommend trying again, by reducing tx-amplitude and the
>> actual gain values, shifting the terminals around (perhaps they're too
>> close?).
>>     
> We have now found out that we need a sampling rate of at least 2Msps
> which means we have to set the bandwidth to at least 2MHz (I read sth
> about that the USRPs have problems with higher decimation rates):
>
> ./benchmark_tx.py -f 2.400G -A TX/RX --tx-amplitude=0.2 -v -W 2M
> ./benchmark_rx.py -f 2.400G -A TX/RX --rx-gain=35 -v -W 2M
>
>
>  The OFDM (bpsk) example is now working and all packets seems to be
> transmitted. Unfortunately, not all of the packets could be
> demodulated correctly as they are marked as "ok: False" - lets say a
> quarter of them. This would yield a really bad performance in terms of
> a reliable transmission. We also played around with the distance and
> the alignment of the omni antennas but ultimately, we could not get
> rid of the false packets.
>
> Have you encountered a similar bad performance? Have you also
> encountered such a strange behavior regarding the lower sampling rate?
> What else could we try?
>
> ______________________________________________
>   
Frequency offset is the usual cause of such problems.

The master oscillators on a USRP vary between about 10PPM and 20PPM,
which at higher frequencies
  means on offset of several kHz.     A narrow-band signal suffers much
more from frequency offset issues
  that a wideband one--the frequency error constitutes a larger fraction
of the overall signal.

Frequency offset errors are normal in any radio communications
system--since master oscillator frequencies
  are *never* perfect.  Real-world systems typically have an FLL or AFT
somewhere in the receive
  chain to compensate.

This is why the last IF filter on a narrowband FM receiver is usually
wider than would be suggested by
  the modulation bandwidth, and why there's usually some kind of AFT
feedback.




-- 
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org