Hi,
I was trying to use the OFDM Mod block to create a OFDM signal with FFT
size=8192, occupied tones=6818 and CP=2048
However I keep getting this error:
File “/usr/local/lib/python2.7/dist-packages/gnuradio/digital/ofdm.py”,
line 106, in init
padded_preambles)
File
“/usr/local/lib/python2.7/dist-packages/gnuradio/digital/digital_swig.py”,
line 4517, in ofdm_insert_preamble
return _digital_swig.ofdm_insert_preamble(*args, **kwargs)
RuntimeError: digital_ofdm_insert_preamble: invalid length for preamble
symbol
If I use a lower number of occupied tones this error disappears but I
was
interested in this number because it is close to the DVB-T 8k mode.
Can you explain me what am I doing wrong or if there is a workaround for
this problem
Thank you and Regards,
Francisco
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Hi Francisco - The code in question is, as the error says, at line 106
of the Python file “ofdm.py”:
self.preambles = digital_swig.ofdm_insert_preamble(self._fft_length,
padded_preambles)
This code calls the C++ code found in the file
digital_ofdm_insert_preamble.cc, which throws an error when any valid
preamble’s length is not equal to the FFT size. This throwing has
nothing to do with the cyclic prefix length or # occupied tones. You
should be able set no preambles, according to the code, by sending in a
zero-length vector (I think). The preamble must be a vector of vectors,
with the first dimension being the number of preambles and the second
dimension must be the FFT length. Check out which preamble you’re using
would be my recommended next step. Good luck. - MLD
Thanks for the response. I am using the GRC and I let the preamble
size=0.
When I use a lower number of occupied tones this error doesn’t occur.
However if I used the values specified in the first post that message
appears
Michael D.-3 wrote:
to do with the cyclic prefix length or # occupied tones. You should be
However I keep getting this error:
Discuss-gnuradio Info Page
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Hi Francisco - If you read through “gr-digital/python/ofdm.py” carefully
(lines 68:81), you’ll find that the number of “occupied tones” cannot be
more than 4512 because of the way the known preamble symbols are
pre-generated; feel free to regenerate these symbols to as many as you
want, but make sure to use the new symbols on both the Tx and Rx for
proper transmission. There is no check for this value, and the Python
script will set the resulting preamble to length 4512 when the number of
occupied tones is greater than 4512. So, even if you set in GRC to not
use preambles, one is always created through the known symbols. It
wouldn’t be too difficult to tweak this Python script to properly expand
the known symbols’ part of the preamble to be of the correct length;
please feel free to do this tweaking & submit a patch. Good luck. - MLD