I know that QAM modulation uses a change in phase and amplitude to send data. But We use a series of frequency spaces as bandwidth to send data.

However, we know that different frequencies oscillate differently. So, if we modulate data using several frequency spaces together, how are data laid out?

Is it that size of data in each frequency is different? Or is it something different?



2 Answers 2


I THINK this addresses your main question

  • My understanding is that all points in a QAM "constellation" are liable to be populated equally probably for random long term data .

However, the material quotes below certainly should if you spend the required time going through it.

If the above comment and the material below does not cover your question would you please explain the question in more detail.

For starters the following provides an excellent feel for a multi amplitude multi phase modulation scheme.

The two diagrams below, when watched together, give you a superb idea of how basic QAM functions.This is 4-QAM which is the "entry level".
Diagrams from this superb national instruments QAM tutorial

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There is a good attempt to graphically display what is happening on this page but not as good as above.

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Wikipedia provides this excellent overview with "constellations" for square and circular QAM at various N values. (If those terms do not make sense, read the article).

Related, Wikipedia QAM TV

Very good Comparison of 8-QAM, 16-QAM, 32-QAM, 64-QAM 128-QAM, 256-QAM, etc Useful definition o=f sorts from here

  • QAM (quadrature amplitude modulation) is a method of combining two amplitude-modulated (AM) signals into a single channel, thereby doubling the effective bandwidth. QAM is used with pulse amplitude modulation (PAM) in digital systems, especially in wireless applications.

  • In a QAM signal, there are two carriers, each having the same frequency but differing in phase by 90 degrees (one quarter of a cycle, from which the term quadrature arises). One signal is called the I signal, and the other is called the Q signal. Mathematically, one of the signals can be represented by a sine wave, and the other by a cosine wave. The two modulated carriers are combined at the source for transmission. At the destination, the carriers are separated, the data is extracted from each, and then the data is combined into the original modulating information.


In QAM, data rate is defined independently of the frequencies, and when more frequencies are used together, then they have got the same raw data rate.

I know QAM used in digital tv broadcast in Europe and other countries around the world (DVB family of standards) and Wi-Fi. In there, date rate / symbol rate / bit rate is defined independently of the transmitter's frequency and it is the same in all frequencies used together.

For example DVB-T (terrestrial tv broadcast) has a data rate approx. 1000 baud (i.e. 1000 symbols per second), which is by far much lower than the base transmitting frequency (which is 470-862 MHz), and this is the same for all frequencies. The symbols can start and end in arbitraty phase positions, not just in zero. (DVB-T and WiFi add OFDM modulation scheme to it, but that's out of scope of this question.)

Another example may be satellite tv DVB-S which uses just a single frequency with a very high data rate. But again: The data rate is irrelevant of the frequency.


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