quadrature amplitude modulation (QAM)
Quadrature amplitude modulation (QAM) is a modulation method for high transmission density that combines amplitude mod ulation and phase modulation. QAM modulation has higher effectiveness than amplitude modulation because the signal is modulated in both amplitude and phase.
Quadrature amplitude modulation uses two carrier signals with the same carrier frequency that are 90° out of phase. These carrier frequencies are designated as in-phase and quadrature - "I" and "Q" for short. This is why they are also called IQ signals. The two carrier signals are amplitude modulated with two or more different signals and then combined. During demodulation, the carrier signal is fed back in two phase positions offset by 90°.
The different QAM processes
In the various QAM methods, several bits are combined in groups of 4, 8, 16, 32 or 64 bits as a dibit, tribit or quadbit and modulated with two, four or more different amplitudes with both carrier signals offset by 90 degrees. In 8QAM, for example, two different amplitudes are modulated in four phases. The modulation result represents a matrix in which each bit combination is assigned to a specific level and phase. With high bit groups, the level and phase angles of the individual bit groups to each other decrease, which leads to an increased susceptibility to interference of the QAM modulation.
All ITU- TS recommendations for high-speed modems such as V.32, V.32terbo or V.34 use quadrature amplitude modulation in different variants, starting with 8QAM via 16QAM, 64QAM and 256QAM up to 1024QAM, which is used in 802.11ax, for example. The same applies to DSL methods, WLANs according to 802.11 and various other transmission technologies that use quadrature amplitude modulation.
Double Square Quadrature( DSQ) is another quadrature amplitude modulation with higher modulation density. The DSQ method achieves the high modulation density by using two offset quadrature amplitude modulations.