[Discuss-gnuradio] OFDM TX/RX

[guillaume.tochou]

Hello all,

I want to make a flow graph for transmitting and receiving OFDM modulated data. To be precise, I want to do a LTE-M "like" transmission/reception, so I am trying to transmit data with a BW of 1.08 MHz OFDM.
To do so, I use the following configuration : samp_rate = 1.92 MHz, fft length = 128, number of sub carriers = 72 , cp = 32

I modified the tx_ofdm.grc example file (/gnuradio/gr-digital/examples/ofdm/tx_ofdm.grc)
To summary, I change the occupied carriers : range(-36, -21) + range(-20, -7) + range(-6, 0) + range(1, 7) + range(8, 21) + range(22, 37),) ; pilot carriers and symbols unchanged ; double the sync words to be equal to fft length.

The problem is at the output of the "Tag debug" I have just those messages :
INFO: Parser returned #f
INFO: Detected an invalid packet at item 1496
......
INFO: Parser returned #f
INFO: Detected an invalid packet at item 681901

And the spectrum isn't "stable" at 1.08 MHz BW.
 

What am I doing wrong ? How the sync words are determine ? Should I add pilot/symbol carriers ?

Best Regards and thanks by advance,

Guillaume Tochou

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tx_ofdm.grc
Description: application/gnuradio-grc