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The "dB" column in WSJT-X

Discussion in 'Working Different Modes' started by KT5MR, Jun 28, 2020.

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  1. N3KE

    N3KE Ham Member QRZ Page

    Yes well the author also says right above that quote:

    "Another commonly used normalized measure of signal-to-noise ratio is Eb/N0"

    So the author appears to be of two minds on the topic. Which I understand, I'm in the same place. I do not think it is really appropriate to call Eb/No the same as SNR and I'm annoyed when sloppy figures show up in IEEE which label an axis "SNR" which is in fact actually "Eb/No". That said it is definitely a ratio of signal power to noise power with normalizations applied and it is unitless just like any given SNR is so you can see why people do that.

    Define signal to noise ratio specifically and I can then answer your question. The issue is no universally agreed upon definition of SNR.

    So yes, for a certain set of common definitions of SNR you absolutely can have a real life system with a negative signal to noise ratio. As stated in an earlier post one very common definition of SNR is signal power divided by noise power in a bandwidth equal to the instantaneous bandwidth of the signal. Using that common definition a wide variety of operational systems operate with negative SNRs. For instance a BPSK signal with a coding rate below 1/4 will operate below 0 dB SNR. This is common in satellite communications systems. Direct sequence spread spectrum systems can operate at extremely negative SNRs as well. GPS is probably the most common example but there are plenty of others including WCDMA modes used on some cellular phones, Zigbee networks that connect devices in smart homes and certain modes of WiFi.

    And also you can define SNR in ways more akin to Eb/No and depending on the fixed scaling factor you end up with in your normalization you may end up with the Shannon limit being defined as either negative dBs or positive dBs. You can see this in the document you linked where one normalization makes the Shannon limit 0 dB while the more common one makes it -1.59 dB.

    Sorry, it doesn't get anymore concrete than that.
  2. W6RZ

    W6RZ Premium Subscriber QRZ Page

    Oh, I'm fine with your answers. I don't consider myself an expert by any means. I got into Software Defined Radio and Digital Signal Processing as a hobby in 2013 after I retired from the video compression industry. Because I had that video background, I've focused on video transmission standards like DVB-S2 and DVB-T2.

    I've been struggling with the Eb/No definition for a long time and was just trying to extend the conversation here. For me, I think of signal to noise ratio as the amount of signal power divided by the noise power in a bandwidth. Basically, carrier to noise ratio.

    So negative signal to noise ratios are certainly possible in that context. Here's a DVB-S2 link running at a negative C/N. This is with an SDR transmitter running GNU Radio and a commercial receiver.


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