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I have a question about BFSK/M-FSK modulations in GNU Radio but in can be common problem :)

If I have a symbol 1 or 0 and it has got 64 samples (2 periods of sin/cos) and I want to modulate it with 2 different frequencies for example:

f1 = 175 KHz = 5,7 us
f2 = 225 KHz = 4,4 us

Problem is that 0 or 1 binary are same width - 64 samples but width for cos/sin periods are diferent and longer period will be cut for shorter symbol.

Do I need to generate two diferent symbols width for 0 or 1 ? what with bigger M-FSK?

perror
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Przemek Lewandowski
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1 Answers1

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No, you typically leave your symbol length alone. You're right, these symbols will then contain different amounts of periods, but that's typically not really a problem, because it doesn't matter for your demodulation.

There's some fine mathematical differences in the energy these symbols might have, but in a system where that matters, you simply can't chose arbitrary frequencies -- that's where OFDM comes into play.

You're right, however, this is just barely a GNU Radio question. Maybe you're better of on the ham radio stack exchange, or the DSP stack exchange site.

Marcus Müller
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  • Marcus - thank you very much for answering my question. I have just submitted to mailing list as you suggested. :) Back to my question, I just realized that my question was very stupid :) because sin(x) and sin(x*n) will have cross point on X axis in the same point after 2 periods of lower frequency. http://fooplot.com/plot/lbm4xcgrca Question is: will them be coherent or not :) Sorry one more time for my question, i realized my mistake later :) – Przemek Lewandowski Aug 08 '15 at 13:38
  • don't worry about your question, it wasn't bad, @PrzemekLewandowski! However, I don't really understand this new question. However, it really doesn't fit on StackOverflow. "Coherency" is not an attribute of modulation schemes, but of demodulators. Maybe re-phrase the question and ask it as a new question on http://dsp.stackexchange.com/. Unless, of course, it is a GNU Radio-specific question, but in that case, please sign up for the Mailing list and ask it there -- these aspects definitively lend themselves more to an email discussion than to StackOverflow format or Forum posts. – Marcus Müller Aug 08 '15 at 15:12