Quadrature amplitude modulation (QAM) is an advanced modulation scheme widely used in Wi-Fi communication systems. It combines phase modulation and amplitude modulation.
Why Do We Need QAM?
When used for Wi-Fi digital signal modulation, QAM can achieve a higher rate than common amplitude modulation and phase modulation, which support only two types of symbols to distinguish 0 and 1.
Amplitude modulation: distinguishes 0 and 1 by changing the carrier amplitude.
Phase modulation: distinguishes 0 and 1 by changing the carrier phase. For example, binary phase shift keying (BPSK) uses phases 0° and 180° for transmitting a single bit (0 or 1) of information. In contrast, QPSK can encode 2 bits per symbol (00, 01, 10, or 11) through the following four phases: 0°, 90°, 180°, and 270°. In fact, QPSK is a special type of QAM, that is, 4-QAM.
QAM can modulate more symbols, and each symbol has its own phase and amplitude.
For example, 16-QAM can modulate symbols into 16 different waveforms, representing 0000, 0001, and so on, as shown in the following figure. This means that a total of 16 types of symbols are available, each of which may transmit 4-bit information.
16-QAM diagram
For more information, see QAM
