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8-QAM modulation question

  1. Jun 13, 2007 #1
    1. The problem statement, all variables and given/known data
    Illustrate in wave forms for data “101001000111010011110100” by using the following modulation techniques:
    (a) 8-QAM (2 bit ASK + 1 bit PSK)
    (b) 8-QAM (2 bit PSK + 1 bit ASK)
    (c) 2 bit PSK + 1 bit FSK

    2. The attempt at a solution
    *attached* apparently I got it all wrong. Im such a noob :confused: Would appreciate any help on this. thanks :D
     

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  3. Jun 13, 2007 #2

    berkeman

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    Staff: Mentor

    First, can you draw the I-Q diagrams for each of those encoding schemes? I don't think I could draw the time-domain waveforms without first sketching the I-Q diagrams....
     
  4. Jun 13, 2007 #3
    we werent given the I-Q diagrams. I only know how to do the ASK, PSK and FSK diagrams.
     
  5. Jun 13, 2007 #4

    berkeman

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    Staff: Mentor

  6. Jun 20, 2007 #5
    According to the email from my lecturer, he said
    how do i know what wave form to represent for each group of 3 datas? do i just assign it myself. And what is the difference between 1 bit ASK and 2 bit ASK? Help! im really desperate to get the answer to this question. *cries*
     
  7. Jun 20, 2007 #6

    berkeman

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    Staff: Mentor

    1 bit ASK is on-off keying. So a 0 might be represented by a flat line, and a 1 represented by one period of a sine wave. 2-bit ASK means that the amplitude can take on 4 values, say from zero to full scale in even steps. Although, as a practical matter, it is best not to use zero amplitude for the bottom of the ASK. You typically will pick a maximum modulation depth less than 100%. So for 1-bit ASK, you might pick amplitudes of 0.5 and 1.0 for the single cycle sine waves. And so on. There are optimal values for these amplitudes, based on channel characteristics. But in your current problem set, I don't think they are asking you to optimize much -- just understand the basic concepts.

    So for 8-PSK, you would draw a single cycle sine wave for the 000 value, and phase shift it by 2PI/8 for the 001 value, etc. I'm not sure how they want you to connect the individual timeslots in your waveforms -- you can see that some of the transitions make for some pretty ugly overall time domain waveforms. But there are ways to handle that also (have you learned those yet?).


    EDIT -- fixed "2PI/8"
     
  8. Jun 20, 2007 #7

    chroot

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    Staff Emeritus
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    Well, in truth, he didn't give you a necessary piece of data: the map between a symbol (like 101) and the corresponding point in the constellation.

    He probably wouldn't mind if you assign symbols to points in the constellation yourself, but, honestly, he didn't give you enough information to unambiguously answer the question.

    - Warren
     
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