Quadrature amplitude modulation (QAM) uses amplitude and phase components to provide a form of modulation that provides a high level of spectrum efficiency. QAM (Quadrature Amplitude Modulation) is widely used in wireless communication and of course has also been used for analog transmissions, such as AM stereo transmissions.
(1) Definitions
Quadrature Amplitude Modulation (QAM) is a modulation method that performs amplitude modulation on two orthogonal carriers and is widely used in wireless communications. These two carriers are usually sine waves with a phase difference of 90 degrees (2) and are therefore called quadrature carriers. QAM maps digital information to the amplitude and phase of a carrier, and its signal constellation consists of a fixed set of dots, each representing a specific binary code. By changing the amplitude and phase of the carrier, we can represent different binary codes, thus enabling the transmission of information. Hence the name given to this type of modulation. In QAM, a data signal is represented by a change in the amplitude of two carriers that are orthogonal to each other.
(2) Advantages and disadvantages
The advantage of QAM is that it can transmit multiple bits of information at the same time, so it has a high data transfer rate. But it has a number of drawbacks.
Susceptible to noise because the states are closer together, a lower level of noise is required to move the signal to a different decision point. Receivers used for phase or frequency modulation are able to use limiting amplifiers that can cancel out any amplitude noise, improving noise reliability. This is not the case with QAM.
It is related to the amplitude component of the signal. When amplifying phase-modulated or frequency-modulated signals in a radio transmitter, there is no need to use a linear amplifier, whereas when using a QAM that contains amplitude components, linearity must be maintained. However, linear amplifiers are less efficient and consume more power, which makes them less attractive for mobile applications.