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I am trying to simulate a QPSK transmission using GNU Radio following mpsk_stage6.grc example in Gnuradio tutorials. enter image description here

Currently I have no idea where is the problem and everytime I execute the flow graph i get this on a.txt file. enter image description here

I am not sure why this flow graph is not correct. Thank you in advance.

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  • $\begingroup$ there's no byte alignment (framing) done here, so how would your receiver know whether the first QPSK symbol it receives are the first two bits of a byte? Send something simpler, and look at the bits you're getting, not at a text file. $\endgroup$ Nov 9 '20 at 22:44
  • $\begingroup$ @MarcusMüller I already sent something simpler like a vector but i need to send atleast a text file in the simulation to try the same scheme using an USRP. How should I do this byte alignment you mentioned earlier? Which blocks do you suggest to accomplish this?. Thanks for the answer. $\endgroup$ Nov 9 '20 at 23:04
  • $\begingroup$ well, then write a block yourself! It's not hard :) prepend (or periodically insert) some known sequence of bits to allow your block to detect a start, discard that sequence, and after that, just forward the bits :) $\endgroup$ Nov 9 '20 at 23:11
  • $\begingroup$ @MarcusMüller, what if the prepended known sequence (i think this is called a "preamble" or a "frame synchronization marker") turns up simply from the data? that would be a false frame synchronization marker, but how would the receiver know? $\endgroup$ Nov 10 '20 at 19:32
  • $\begingroup$ a) you could bit-stuff your data so that it couldn't b) your receiver wouldn't just look for the sequence always, but only when it hasn't locked on to the bitstream already (otherwise it'd just look for the preamble in the right places to verify it's still locked. Do that!). If you have bad luck and there's a pseudo-preamble before you observe a real one, you'd be losing another frame. $\endgroup$ Nov 10 '20 at 19:46

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