What do QPSK, 16QAM and 64QAM in LTE mean?

Quadrature phase-shifted Yin Ke (QPSK) is a digital modulation method. It can be divided into absolute phase shift and relative phase shift. Because of the phase ambiguity in absolute phase shift mode, relative phase shift mode DQPSK is mainly used in practice. At present, it has been widely used in wireless communication and has become a very important modulation and demodulation method in modern communication.

QAM is the abbreviation of English quadrature amplitude modulation, which means quadrature amplitude modulation and is a digital modulation method. 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, 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. 16QAM means 16 symbol QAM modulation. 64QAM refers to a QAM modulation method with 64 symbols.