Tutorial 8 – All About Digital Modulation

Description: All About Modulation – Part 1

Download: mod1.pdf

26 Comments on “Tutorial 8 – All About Digital Modulation

  1. Hey,Charan! I can’t thank you enough for these tutorials.They have just increased my liking for the subject.I read this tutorial and this doubt is bothering me.

    For generating a QPSK signal, we used 2 baseband signal(I and Q) with 2 levels(+1,-1),multiplied it with 2 quadrature carriers,and got a signal with four distinct symbols. So,the question is,can we have only one baseband signal with 4 different levels and multiply it with a single carrier? Wouldn’t that,too,give us the same signal? Obviously,the baseband processing required would be more complex to have 4 levels.Is that the reason we don’t do it?

    Thank you.

    • Yes, it can be done but such a signal would not be a QPSK signal. It would have different amplitudes for th symbols. All four QPSK symbols have the same amplitude, only the phase if different. The advantage is that this signal is what is called a constant-envelope signal. This type of signal is immune to amplitude-based nonlinearities. AS such QPSK is signal created by the two I and Q signals is a much preferred signal than one created as a four level signal. Now a 16QAM signal is similar in idea to what you are talking about but again, it also has a I and Q component.

      The other issue is the speed. It is so much easier to take a single carrier source, slow the symbol rate and operate in parallel on two signals at half the speed than working on just the real signal.

      Charan Langton

  2. Hi Charan, i had a question about “all about modulation part2” which i coudnt find it in the list of tutorials, there youve mentioned that adding noise of one half of difference between the two discriminating amplitudes, will make the error, but using this amount of noise in the expression, doesnt change for example 1 to 0, it will be great if you could explain it for me!
    thanks a lot for all great tutorials

  3. On the top of page 7 there is a formula for the Phase of the signal given as “Phase of signal =arctan(I/Q). I believe it should be phase of signal = arctan (Q/I) given the way you defined the scenario with the y-axis being Q and the x-axis being I.


  4. May I know how do we compute the ideal constellation points for QPSK (0.707, 0.707, 0.707, -0.707, -0.707, -0.707, -0.707, 0.707) and 8PSK ? Is there any formula ?

    • The numbers are pairs of coordinates. Plot these and you get the constellation of four points.(you have 8 points, each a (x,y) pair. Note that they all fall on a circle of radius .707. To plot 8PSK, you need 8 equidistant points 45 degrees apart. This is easy to calculate. They all fall on a circle of radius .707 ( squaring it you get 1.0, which is the power.)

  5. Is “analog medium” means the “carrier” is analog ? Since a wire can carry both analog signal and digital signal, it will be called “analog medium” when it’s carrying analog signal. Am I right?

    • Analog just means continuous but in practice an analog signal is one based on a sinusoidal wave form source. This implies that information has been modulated onto a sinusoidal carrier, somehow (AM, FM, PSK, etc.). Hence the term analog almost always applies to this type of signal. This type of modulated signal can travel through many types of physical channels, also called medium, including wire, water or air.
      Charan Langton

      • Thanks for your reply. I was just little confused about these words in this tutorial: “Communications inside a computer are examples of pure digital communications, digital data over digital medium. LAN communications are digital data over analog medium.”
        The “physical channel”, also called medium,shouldn’t be distinguished into digital medium and analog medium.

        I think in this context, “digital data over digital medium” is talking about the ” digital baseband transmission” or “digital information transmissino”,and there is no carrier; and “digital data over analog medium” is talking about “digital information over digital carrier”

  6. Yes, that does sound confusing. Digital data is almost always sans a carrier, that is, it is at baseband and represented just by a voltage polarity of a fixed amplitude, i.e. +1V or -1V. All physical mediums are capable of transmitting such data, however not efficiently. The “wires” inside a chip do carry this digital data but because of the small distances and the property of the material, they can do do so without much distortion and loss. The actual wire such as an Ethernet cable however has a poorer frequency response and it would distort such digital signals.
    Charan Langton

    • I used Cadence SPW, Signal Processing Workstation. SPW is a wonderful program for communication system simulation, old and stable. I am right now trying to transition to Simulink but do not find it as intuitive nor as easy to use as SPW. – Charan Langton

  7. Awesome, very well written article. Many important points are covered here. Im glad that you shared this helpful information with us.
    I am really thankful to the blog owner for help us by giving valuable study materials.

  8. Interesting tutorial but I am confused by the 8PSK example on pp.33 and 34. I assume you mean Table 3 and not Table 4 but the IQ values for s8, s6, and s2 are not what are shown in that Table. There are no -0.707 values in Table 3 for example. I also can’t figure out how Figure 39 comes from the IQ mapping before it.

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