QAM modulation transmits two analog message signals or two digital bitstreams by varying (modulating) the amplitudes of two carrier waves using an ASK or analog AM digital modulation scheme.
Working principle
Two carrier waves of the same frequency, usually sinusoids, are out of phase with each other by 90° and are thus called quadrature carriers or quadrature components - hence the circuit's name. 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 analog case phase modulation (PM) and amplitude modulation.
Like all modulation schemes, QAM transmits data by changing some aspect of the carrier wave signal (usually a sine wave) in response to the data signal. In the case of digital QAM, multiple phase and multiple amplitude samples are used. Phase Shift Keying (PSK) is a simpler form of QAM in which the carrier amplitude is constant and only the phase shifts.
In case of warpQAM transmission, a carrier wave is a collection of two sine waves of the same frequency, 90° in phase from each other (in quadrature). These are often referred to as the "I" or in-phase component, as well as the "Q" or quadrature component. Each component wave is amplitude modulated, meaning its amplitude is changed to represent the data that must be transferred before it can be 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 with QAM by setting a suitable constellation size, limited only by noise level and link linearity.
QAM modulation is used in optical 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 constant amplitude samples, each occupying 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 analog QAM, the amplitude of each component of a sine wave changes continuouslyin time with an analog signal.
Phase modulation (analog PM) and keying (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 extended to frequency modulation (FM) and keying (FSK), since they can be considered as its subspecies.
As with many digital modulation schemes, the constellation diagram is useful for QAM. In QAM, constellation points are usually arranged in a square grid with equal vertical and horizontal spacing, although other configurations (eg Cross-QAM) are possible. Since data is usually binary in digital telecommunications, the number of points in a grid is usually 2 (2, 4, 8, …).
Because QAM is usually square, some are rare - the most common shapes are 16-QAM, 64-QAM and 256-QAM. By moving to a higher order constellation, more bits per symbol can be transmitted. However, if the average energy of the constellation remains the same (by making a fair comparison), the points should be closer together and therefore more susceptible to noise and other corruption.
This results in a higher bit error rate and therefore a higher order QAM may provide more data less reliably than a lower order QAM for a constant average constellation energy. The use of higher order QAM without increasing the bit error rate requires highersignal-to-noise ratio (SNR) by increasing signal energy, reducing noise, or both.
Technical aids
If data rates in excess of those offered by 8-PSK are required, it is more common to move to QAM as it achieves greater distance between adjacent points in the I-Q plane, distributing the points more evenly. A complicating factor is that the points no longer have the same amplitude, and so the demodulator must now correctly detect both phase and amplitude, rather than just phase.
Television
64-QAM and 256-QAM are often used in digital cable TV and cable modems. In the United States, 64-QAM and 256-QAM are authorized digital cable modulation schemes that are standardized by SCTE in the ANSI/SCTE 07 2013 standard. Note that many marketers will refer to them as QAM-64 and QAM-256. UK modulation QAM-64 is used for digital terrestrial TV (Freeview) and 256-QAM is used for Freeview-HD.
Communication systems designed to achieve very high levels of spectral efficiency typically use very dense frequencies in this series. For example, current Powerplug AV2 500-Mbit Ethernet devices use 1024-QAM and 4096-QAM devices, as well as future devices using the ITU-T G.hn standard to connect to existing home wiring.(coaxial cable, telephone lines and power lines); 4096-QAM provides 12 bits/symbol.
Another example is ADSL technology for twisted-pair copper, which constellation size reaches 32768-QAM (in ADSL terminology this is called bit-loading or bits per tone, 32768-QAM is equivalent to 15 bits per tone).
Ultra high bandwidth closed loop systems also use 1024-QAM. By using 1024-QAM, adaptive coding and modulation (ACM) and XPIC, manufacturers can achieve gigabit capacity in a single 56 MHz channel.
In SDR receiver
It is known that the 8-QAM circular frequency is the optimal 8-QAM modulation in the sense of needing the lowest average power for a given minimum Euclidean distance. The 16-QAM frequency is sub-optimal, although an optimal one can be created along the same lines as 8-QAM. These frequencies are often used when tuning an 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.
