Quardrature Amplitude Modulation

[DrKareem]

Quardrature Amplitude Modulation.

I know the basic techniques, like when you have 1 point and only two signals are to be multiplexed. But the constellations got me a bit confused. My text-book doesn't explain it thoroughly, and there isn't much deep information about it on the internet.

"The number of "dots" in the constellation is given as a number before the QAM, and is always an integer power of two -- from 2^1 (2QAM) to 2^12 (4096QAM)."

This is from wikipedia site. I'm just wondering, each point adds two signals? or multiply the number of signals mulitplexed by two??


This is an example exercise:

An analog passband channel can pass a maximum of 20 KHz. If the channel is used for transmission of digital data, what is the minimum number of points in a QAM constellation if 5 ADPCM signals are to be multiplexed in an the analog channel?

In both cases of my interpretation, the answer would be 3 points, no??

 

[DrKareem]

erm, that would be 2^3=8 points.

8 points have the ability to multiplex 16 signals or 2^8= 256??

 

[ravenprp]

Quadrature Amplitude Modulation (QAM) is a modulation technique used in communication systems to transmit digital signals over an analog channel. It combines both amplitude and phase modulation to transmit multiple signals simultaneously, increasing the data rate and spectral efficiency of the transmission.

The constellation in QAM refers to the pattern of points on a signal diagram, which represents the different combinations of amplitude and phase of the transmitted signals. The number before QAM (e.g. 2QAM, 16QAM, etc.) indicates the number of points in the constellation, which is always an integer power of two.

In terms of your question about the number of signals being multiplexed, each point in the constellation represents multiple signals. For example, in 2QAM, each point represents two signals, one for the in-phase (I) component and one for the quadrature (Q) component. In 16QAM, each point represents 16 signals, 8 for the I component and 8 for the Q component.

In the example exercise given, the minimum number of points in the QAM constellation would depend on the specific modulation scheme and the bandwidth of the channel. Without this information, it is not possible to determine the minimum number of points in the constellation. However, in general, the more points in the constellation, the higher the data rate that can be achieved.