QAM modulation transmits two analog message signals or two digital bitstreams by changing (modulating) the amplitudes of two carrier waves using an Amplified Keying (ASK) digital modulation circuit or analog AM.
Principle of operation
Two carrier waves of the same frequency, usually sine waves, are 90 ° out of phase with each other and are thus called quadrature carriers or quadrature components - hence the name of the circuit. The modulated waves are summed, and the final waveform is a combination of both phase shift keying (PSK) and amplitude shift keying (ASK), or in the analogous case of phase modulation (PM) and amplitude modulation.
Like all modulation schemes, QAM transmits data by changing some aspect of the carrier wave signal (usually a sinusoid) in response to the data signal. In the case of digital QAM, several discrete phase and multiple discrete amplitude values are used. Phase Shift Keying (PSK) is a simpler form of QAM in which the carrier amplitude is constant and only the phase is shifted.
In the case of the basis of QAM transmission, the carrier wave is a combination of two sine waves with the same frequency, 90 ° in phase from each other (in quadrature). They are often called the “I” or in-phase component, as well as the “Q” or quadrature component. Each component wave is modulated in amplitude, that is, its amplitude is changed to represent the data that must be transferred before they are combined together.
Application
The inscription decision boundaries in the photo above indicates the boundary of the surface (or "decision boundary", literally).
QAM (quadrature amplitude modulation) is widely used as a modulation scheme for digital telecommunication systems such as 802.11 Wi-Fi standards. Arbitrary high spectral efficiency can be achieved using QAM by setting the appropriate size of the constellation, limited only by the noise level and linearity of the communication channel.
QAM modulation is used in fiber systems as the bit rate increases. QAM16 and QAM64 can be optically emulated with a 3-channel interferometer.
Digital technology
In digital QAM, each component wave consists of samples of constant amplitude, each of which occupies a single time interval, and the amplitude is quantized, limited to one of a finite number of levels representing one or more binary digits (bits) of a digital bit. In an analog QAM, the amplitude of each component of a sine wave continuously changes over time with an analog signal.
Phase modulation (analog PM) and manipulation (digital PSK) can be considered as a special case of QAM, where the magnitude of the modulating signal is constant, with only the phase changing. Quadrature modulation can also be expanded to frequency modulation (FM) and manipulation (FSK), since they can be considered as its subspecies.
As with many digital modulation schemes, the constellation diagram is useful for QAM. In QAM, the constellation points are usually arranged in a square grid with equal vertical and horizontal distances, although other configurations are possible (for example, Cross-QAM). Since in digital telecommunications the data is usually binary, the number of points in the grid is usually 2 (2, 4, 8, ...).
Because QAM is usually square, some are rare - the most common forms are 16-QAM, 64-QAM, and 256-QAM. Moving to a higher constellation of a higher order, you can transmit more bits per character. However, if the average energy of the constellation remains unchanged (through a fair comparison), the points should be closer to each other and, therefore, more susceptible to noise and other corruption.
This results in a higher bit error rate, and therefore higher order QAMs may provide more data less reliably than lower order QAMs for a constant average constellation energy. Using a higher order QAM without increasing bit error rate requires a higher signal to noise ratio (SNR) by increasing signal energy, reducing noise, or both.
Technical devices
If data rates exceeding the rates offered by 8-PSK are required, it is more common to switch to QAM, since it reaches a greater distance between adjacent points in the IQ plane, distributing the points more evenly. A complicating factor is that the points no longer have the same amplitude, and therefore the demodulator must now correctly determine both the phase and the amplitude, and not just the phase.
TV
64-QAM and 256-QAM are often used in digital cable television and cable modems. In the United States, 64-QAM and 256-QAM are authorized modulation schemes for digital cable, which are standardized by SCTE in standard ANSI / SCTE 07 2013. Note that many marketers will refer to them as QAM-64 and QAM-256 . UK QAM-64 modulation is used for digital terrestrial television (Freeview), while 256-QAM is used for Freeview-HD.
Communication systems designed to achieve very high levels of spectral efficiency usually use very dense frequencies from this series. For example, modern Powerplug AV2 500-Mbit Ethernet devices use 1024-QAM and 4096-QAM devices, as well as future devices that use the ITU-T G.hn standard to connect to existing home wiring (coaxial cable, telephone lines, and power lines) ; 4096-QAM provides 12 bits / character.
Another example is ADSL technology for twisted-pair copper, the constellation of which reaches 32768-QAM (in ADSL terminology this is called bit loading or bit per tone, 32768-QAM is equivalent to 15 bits per tone).
Feedback ultra-high bandwidth systems also use 1024-QAM. Using 1024-QAM, adaptive coding and modulation (ACM) and XPIC, manufacturers can get gigabit capacity in a single channel with a frequency of 56 MHz.
In the SDR receiver
The 8-QAM circular frequency is known to be the optimal 8-QAM modulation in the sense of the need for the lowest average power for a given minimum Euclidean distance. The frequency of 16-QAM is suboptimal, although the optimum can be created according to the same principles as 8-QAM. These frequencies are often used when tuning the SDR receiver. Other frequencies can be recreated by manipulating similar (or similar) frequencies. These qualities are actively used in modern SDR receivers and transceivers, routers, routers.