What is QAM?

QAM is a modulation method of amplitude modulation on two orthogonal carriers. These two carriers are usually sine waves with a phase difference of 90 degrees (π/2), so they are called orthogonal carriers. Therefore, this modulation method is named.

abstract

Similar to other modulation methods, QAM transmits information by changing some parameters of the carrier. In QAM, the data signal is represented by the amplitude variation of two orthogonal carriers.

The phase modulation of analog signal and PSK of digital signal can be considered as a special quadrature amplitude modulation with constant amplitude and only phase change. Therefore, analog signal frequency modulation and digital signal FSK can also be considered as special cases of QAM, because they are essentially phase modulation. This paper mainly discusses digital signal QAM, although analog signal QAM also has many applications, such as NTSC and PAL TV systems, which use orthogonal carriers to transmit different color components.

Similar to other digital modulation methods, QAM transmission signal set can be conveniently represented by constellation diagram. Each constellation point on the constellation map corresponds to a signal in the transmitted signal set. Let the size of the transmission signal set of quadrature amplitude modulation be n, and call it N-QAM. 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 in binary, in which case, the number of constellation points is usually a power of 2. Common QAM forms are 16-QAM, 64-QAM, 256-QAM, etc. 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.