Channel use vs. symbol transmission

Discussion in 'Wireless & RF Design' started by bonjourdaisy, Jul 27, 2013.

  1. bonjourdaisy

    Thread Starter New Member

    Jul 27, 2013
    Hey guys,

    I've seriously been struggling with these concepts for quite some time now. I took a few courses on Communications during university and every once in a while I'll look back at some of the concepts when I'm bored.
    The one issue I still struggle with is the actual definition and interpretation of things like bits/channel use. I'm really hung up on the difference between a channel use, and a symbol.

    As far as I know, the term symbol isn't really relevant until the analog stage of transmission over a channel. So what I'm having difficulty with is the equation R/Ts <= C/Tc. My source encoder is generating symbols? I was only aware the symbols refers to when you have a set of bits that are then converted to an analog pulse, which is then transmitted down a channel. The channel use stage seems to be the output of the channel encoder, so that should be related to analog pulses. But then does one channel use refer to a pulse with C bits in it? Or does it refer to the actual code word since r=k/n.

    I know this may seem confusing, but I learnt communications starting from the first stage: I have a set of bits, I convert them into a set of analog symbols that reflect a certain number of bits per symbol, and then I transmit those symbols down a band limited channel with maximum rate of 2B.

    Could anyone clarify a few concepts here? I seem to have missed something in class that has me really hung up now! 16.42.48.jpg

    Picture of my source of confusion.

    My issue is not with things like baud rate, bit rate, modulation schemes.. it's with the actual term of "channel use". What is the difference between a channel use and an actual symbol? Let me draw a few diagrams.
    So this picture has two sections: the noiseless case where I'm simply converting bits to analog symbols and then transmitting them at a given baud rate. The maximum baud rate is determined by Hartleys theorem which says I'm basically bound at 2W symbols/sec.

    The second stage is where you introduce noise and channel capacity. My symbols now are not analog anymore, but aren't they in fact digital? The actual transmission occurs as "channel uses". So what exactly are those?
  2. WBahn


    Mar 31, 2012
    I don't think I've ever heard the term "channel use" used like this before.

    How is it defined in whatever material you are reading?

    When I think of "channel use" I think in terms of "each time the channel is used". So if I have a medium access control protocol whereby I gain access to the channel and get to transmit a bunch of bits and then have to surrender the channel and wait until I can get it again, bits/channel-use would refer to something like how many bits of payload data can I transmit each time I get control of the channel.