Constellation representation of quadrature amplitude modulation

Similar to other digital modulation methods, the signal set transmitted by QAM can be conveniently represented by a constellation diagram, and each constellation point on the constellation diagram corresponds to that point in the transmitted signal set.

Constellation points are often arranged in a square grid with equal spacing in horizontal and vertical directions, of course, there are other configurations.

In digital communication, data is usually represented by binary numbers. In this case, the number of constellation points is usually a power of 2.

The more constellation points, the more information each symbol can transmit. However, if the average energy of the constellation remains unchanged, the distance between constellation points will become smaller, resulting in an increase in bit error rate. Therefore, the reliability of high-order constellation is worse than that of low-order constellation.

Using QAM modulation technology, the channel bandwidth should be at least equal to the symbol rate. In order to restore the timing, additional bandwidth is needed, which is generally increased by about 15%.