Quadrature amplitude modulation
This article may be too technical for most readers to understand.(June 2020) |
Passband modulation |
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Analog modulation |
Digital modulation |
Hierarchical modulation |
Spread spectrum |
See also |
Quadrature amplitude modulation (QAM) is the name of a family of
Phase modulation (analog PM) and phase-shift keying (digital PSK) can be regarded as a special case of QAM, where the amplitude of the transmitted signal is a constant, but its phase varies. This can also be extended to frequency modulation (FM) and frequency-shift keying (FSK), for these can be regarded as a special case of phase modulation.
QAM is used extensively as a modulation scheme for digital
Demodulation
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 I(t). The other modulating function is the quadrature component, Q(t). So the composite waveform is mathematically modeled as:
- or:
-
(Eq.1)
where fc is the carrier frequency. At the receiver, a
Using standard trigonometric identities, we can write this as:
Low-pass filtering r(t) removes the high frequency terms (containing 4πfct), leaving only the I(t) term. This filtered signal is unaffected by Q(t), showing that the in-phase component can be received independently of the quadrature component. Similarly, we can multiply sc(t) by a sine wave and then low-pass filter to extract Q(t).
The addition of two sinusoids is a linear operation that creates no new frequency components. So the bandwidth of the composite signal is comparable to the bandwidth of the DSB (double-sideband) components. Effectively, the spectral redundancy of DSB enables a doubling of the information capacity using this technique. This comes at the expense of demodulation complexity. In particular, a DSB signal has zero-crossings at a regular frequency, which makes it easy to recover the phase of the carrier sinusoid. It is said to be self-clocking. But the sender and receiver of a quadrature-modulated signal must share a clock or otherwise send a clock signal. If the clock phases drift apart, the demodulated I and Q signals bleed into each other, yielding crosstalk. In this context, the clock signal is called a "phase reference". Clock synchronization is typically achieved by transmitting a burst subcarrier or a pilot signal. The phase reference for NTSC, for example, is included within its colorburst signal.
Analog QAM is used in:
- NTSC and PAL analog color television systems, where the I- and Q-signals carry the components of chroma (colour) information. The QAM carrier phase is recovered from a special colorburst transmitted at the beginning of each scan line.
- C-QUAM ("Compatible QAM") is used in AM stereo radio to carry the stereo difference information.
Fourier analysis
Applying
where denotes the Fourier transform, and and are the transforms of I(t) and Q(t). This result represents the sum of two DSB-SC signals with the same center frequency. The factor of i (= eiπ/2) represents the 90° phase shift that enables their individual demodulations.
Digital QAM
As in many digital modulation schemes, the
By moving to a higher-order constellation, it is possible to transmit more
If data rates beyond those offered by 8-
, rather than just phase.64-QAM and 256-QAM are often used in
Communication systems designed to achieve very high levels of
Ultra-high capacity microwave backhaul systems also use 1024-QAM.
Interference and noise
In moving to a higher order QAM constellation (higher data rate and mode) in hostile
- Carrier/interference ratio
- Carrier-to-noise ratio
- Threshold-to-noise ratio
See also
- asymmetric phase-shift keying(APSK)
- Carrierless amplitude phase modulation (CAP)
- Circle packing § Applications
- In-phase and quadrature components
- Modulation for other examples of modulation techniques
- Phase-shift keying
- QAM tunerfor HDTV
- Random modulation
References
- ^ "Digital Modulation Efficiencies". Barnard Microsystems. Archived from the original on 2011-04-30.
- ^ "Ciena tests 200G via 16-QAM with Japan-U.S. Cable Network". lightwave. April 17, 2014. Retrieved 7 November 2016.
- ^ Kylia products Archived July 13, 2011, at the Wayback Machine, dwdm mux demux, 90 degree optical hybrid, d(q) psk demodulatorssingle polarization
- ^ http://www.homeplug.org/media/filer_public/a1/46/a1464318-f5df-46c5-89dc-7243d8ccfcee/homeplug_av2_whitepaper_150907.pdf Homeplug_AV2 whitepaper
- ^ http://www.itu.int/rec/T-REC-G.992.3-200904-I section 8.6.3 Constellation mapper - maximum number of bits per constellation BIMAX ≤ 15
- ^ a b http://www.trangosys.com/products/point-to-point-wireless-backhaul/licensed-wireless/trangolink-apex-orion.shtml A Apex Orion
- ^ Howard Friedenberg and Sunil Naik. "Hitless Space Diversity STL Enables IP+Audio in Narrow STL Bands" (PDF). 2005 National Association of Broadcasters Annual Convention. Archived from the original (PDF) on March 23, 2006. Retrieved April 17, 2005.
Further reading
- Jonqyin (Russell) Sun "Linear diversity analysis for QAM in Rician fading channels", IEEE WOCC 2014
- John G. Proakis, "Digital Communications, 3rd Edition"
External links
- QAM Demodulation
- Interactive webdemo of QAM constellation with additive noise Institute of Telecommunicatons, University of Stuttgart
- QAM bit error rate for AWGN channel – online experiment
- How imperfections affect QAM constellation
- Microwave Phase Shifters Overview by Herley General Microwave
- Simulation of dual-polarization QPSK (DP-QPSK) for 100G optical transmission