[Discuss-gnuradio] OFDM - on air?

[Dominik Auras]

Hello,

I'm currently trying to make your ofdm simulation work with two usrp
rev4. The scripts benchmark_ofdm_tx.py and ...rx.py are modified in
order to send (I looked on the examples in the directory
"examples/python/digital/").

Decimation rate ist 160, interpolation rate 320. Frequency 2.4G, two
Flex2400RF boards used. (Maybe my first faults lay here)

I also recorded some seconds to a file so I can now replay the same
scenario.

The first thing I observe is that the synchronisation needs to know a
value for the SNR. Fiddling around with that, the synchronizer starts
the sampler even if no signal present (at high SNR-value), but no
samples get through it if the value is lower than 8.

The correlator block thinks to have found the known symbols sometimes in
the noise. And, very confusing to me, the first ofdm symbol, after the
correlation found a peak, is always the first known symbol - even in
absence of a signal.

The message "found at search delta..." appears very often, compared to
your simulation.
It looks like the following lines (I also edited some blocks to output
debug info):

found at search delta: 1,
demod out:   38  e6  55  d1  21  93  6f  d4  2b  ea  3a  e3  41  4f  d2
9  bf  f9  f5  16  73  45  9c   a  e2         len: 25
bin 1   h_sqrd = ( 37442.839844, 1714.728149 )   power = 42633.847656
real(h)/p = 0.878242    angle = 0.045764

This is repeated until I kill the process. There are also other outputs,
with more "demod out" lines grouped together etc.

e.g. the first lines after the gr_buffer warning
bin 0   h_sqrd = ( 2.125927, 1.626051 )  power = 2.299746
real(h)/p = 0.924418    angle = 0.652948
found at search delta: 0,
demod out:   38  e6  55  d1  21  93  6f  d4  2b  ea  3a  e3  41  4f  d2
9  bf  f9  f5  16  73  45  9c   a  e2         len: 25
demod out:   18  3c  8d  b3  e7   9  5b  cf  f4  d6  52  3d  a0  d2  be
1a  a8  9c  4a  a7  a1  36  9a  85  19         len: 25
demod out:   e9  9c  88  d9  c4  93  db  c7  c6  9d  87  49  21  b1  3a
40  12  6f  27  30  de  a5  c8  97  f2         len: 25
demod out:   7f  a5  bb  14  54  b4  84  e5  e9  3f  e9  9d  f5  a5  44
54   0  fb   6  21   0  44  49  9f  fa         len: 25


Thank you very much in advance
You did a great work on gnuradio!!
Dominik