Quadrature Amplitude Modulation

If you look back at my earlier post on QAM, you will notice that in the modulator(transmitter side), you will notice that we are multiplying one signal by \sin(50x) and the other by \cos(50x). These signals are generated by separating the original message signal into two signals, and then multiplying one by \sin(50x) and the other by \cos(50x). Now, in the previous post I used a sinusoidal message signal for illustration. In reality, message signals are generally NOT perfect sinusoids.

Without further ado, here is the explanation of QAM (formulation heavily adapted from Telecommunication Breakdown by Johnson&Sethares)

Let the message m(t) be separated into two signals m_1(t) and m_2(t). Then we modulate by multiplying one by \cos and the other by \sin. In other words, if v(t) is our modulated signal, v(t) is generated by the following:

v(t) = A_c ( m_1(t) \cos(2\pi f_c t) - m_2(t) \sin(2\pi f_c t) ). This is the signal transmitted. At the receiver, in the demodulater, we duplicated the signal received: call these x_1(t) and x_2(t). x_1(t) = v(t) \cos(2 \pi f_ct) =

A_c m_1(t) \cos^2(2 \pi f_c t) - A_c m_2(t) \sin(2 \pi f_c t) \cos(2 \pi f_c t) =

\frac{A_c m_1(t)}{2} (1 + \cos(4 \pi f_c t) ) - \frac{A_c m_2(t)}{2} ( \sin(4 \pi f_c t) ). Putting this signal through a lowpass filter(filters high frequencies) gives s_1(t) = \frac{A_c m_1(t)}{2}, and we have recovered the message.

Now, x_2(t) = v(t) \sin(2 \pi f_c t) = A_c m_1(t) \cos(2 \pi f_c t) \sin(2 \pi f_c t)

- A_c m_2(t) sin^2(2 \pi f_c t) = \frac{A_c m_1(t)}{2} \sin(4 \pi f_c t) - \frac{A_c m_2(t)}{2}(1 - \cos(4 \pi f_c t) ). With a lowpass filter, we get x_2(t) = \frac{-A_c m_2(t) }{2}

We can look at some IT++ code in the future


3 responses to “Quadrature Amplitude Modulation

  1. excellent theory
    want to have a ebook on different types of multiplexing

  2. Hi Mantosh,

    Glad you liked this. While my posts on this subject might be spaced far apart, you can get a free(albeit pre-draft) version of Telecommunication Breakdown from here
    (last I checked)

  3. I did not talk about multiplexing, actually(you might have meant modulation). With Frequency Division Multiplexing using Amplitude Modulation, for example, you multiplex by using different carrier frequencies for each message signal, so that each message signal you send out is upconverted to a different carrier frequency(again, check the Telecommunication Breakdown book for details). The QAM discussion above should be approximately in Chapter 5

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