Quadrature amplitude modulation (QAM) is the name of a family of signal modulation methods widely used in modern telecommunications to transmit information. At its core, it conveys two independent analog signals by changing (modulating) the amplitudes of two differently phased versions of a single carrier wave using amplitude modulation. These paired analog signal channels may then be used either directly or to encode digital bit streams using joint amplitude-shift keying across the synchronized channels.
The two carrier waves are of the same frequency and are out of phase with each other by 90°, a condition known as orthogonality or quadrature. The transmitted signal is created by adding the two carrier waves together. At the receiver, the two waves can be coherently separated (demodulated) because of their orthogonality. Another key property is that the modulations are low-frequency/low-bandwidth waveforms compared to the carrier frequency, which is known as the narrowband assumption.
In M-ary transmission amplitude-shift keying the phase is the same but with different amplitudes, while phase-shift keying (PSK) has the same amplitude but different phases. Combining these concepts leads to QAM, where both amplitude and phase are modulated, or two binary PSK signals are combined with orthogonal carriers. QAM is being used in optical fiber systems as bit rates increase; QAM16 and QAM64 can be optically emulated with a three-path interferometer.
Demodulation
200px|right|thumb|Analog QAM: PAL color bar signal on a [[vectorscope]]
In a QAM signal, one carrier lags the other by 90°, and its amplitude modulation is customarily referred to as the in-phase component, denoted by The other modulating function is the quadrature component, So the composite waveform is mathematically modeled as:
By moving to a higher-order constellation, it is possible to transmit more bits per symbol. However, if the mean energy of the constellation is to remain the same (by way of making a fair comparison), the points must be closer together and are thus more susceptible to noise and other corruption; this results in a higher bit error rate and so higher-order QAM can deliver more data less reliably than lower-order QAM, for constant mean constellation energy. Using higher-order QAM without increasing the bit error rate requires a higher signal-to-noise ratio (SNR) by increasing signal energy, reducing noise, or both.
If data rates beyond those offered by 8-PSK are required, it is more usual to move to QAM since it achieves a greater distance between adjacent points in the I-Q plane by distributing the points more evenly. The complicating factor is that the points are no longer all the same amplitude and so the demodulator must now correctly detect both phase and amplitude, rather than just phase.
64-QAM and 256-QAM are often used in digital cable television and cable modem applications. In the United States, 64-QAM and 256-QAM are the mandated modulation schemes for digital cable (see QAM tuner) as standardised by the SCTE in the standard ANSI/SCTE 07 2013. In the UK, 64-QAM is used for digital terrestrial television (Freeview) whilst 256-QAM is used for Freeview-HD.
thumb|Bit-loading (bits per QAM constellation) on an ADSL line
Communication systems designed to achieve very high levels of spectral efficiency usually employ very dense QAM constellations. For example is ADSL technology for copper twisted pairs, whose constellation size goes up to 32768-QAM (in ADSL terminology this is referred to as bit-loading, or bit per tone, 32768-QAM being equivalent to 15 bits per tone).
Ultra-high capacity microwave backhaul systems also use 1024-QAM. With 1024-QAM vendors can obtain gigabit capacity in a single 56 MHz channel.
- Carrier/interference ratio
- Carrier-to-noise ratio
- Threshold-to-noise ratio
Technologies that increase noise resistance include adaptive coding and modulation (ACM) and XPIC.