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This makes frequency domain equalization possible at the receiver, which is far simpler than the time-domain equalization used in conventional single-carrier modulation. In OFDM, the equalizer only has to multiply each detected subcarrier (each Fourier coefficient) in each OFDM symbol by a constant complex number , or a rarely changed value.
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 [citation needed]. QAM is used extensively as a modulation scheme for digital communications systems, such as in 802.11 Wi-Fi standards.
Amplitude and phase-shift keying (APSK) is a digital modulation scheme that conveys data by modulating both the amplitude and the phase of a carrier wave. In other words, it combines both amplitude-shift keying (ASK) and phase-shift keying (PSK).
For many common constellations including BPSK, QPSK, and 8PSK, these two methods for finding the reference give the same result, but for higher-order QAM constellations including 16QAM, Star 32QAM, 32APSK, and 64QAM the RMS average and the maximum produce different reference values.
The phase modulation (φ(t), not shown) is a non-linearly increasing function from 0 to π /2 over the interval 0 < t < 16. The two amplitude-modulated components are known as the in-phase component (I, thin blue, decreasing) and the quadrature component (Q, thin red, increasing).
The 8-VSB modulation in the ATSC system corresponds with the 64-QAM modulation of J.83b. In a 6 MHz channel, the data rate is at most 36 Mbit/s (for 64-QAM or 8-VSB); the 8-VSB ATSC achieves a data rate of 19.3926 Mbit/s while the 64-QAM J.83b achieves a data rate of 26.970 Mbit/s.
In a frequency or phase modulated signal, the signal amplitude is constant, so the points lie on a circle around the origin. The carrier representing each symbol can be created by adding together different amounts of a cosine wave representing the "I" or in-phase carrier, and a sine wave, shifted by 90° from the I carrier called the "Q" or ...
The modulation order of a digital communication scheme is determined by the number of the different symbols that can be transmitted using it. Modulation order can only be defined for digital modulations. The simplest forms of digital modulation are of second order because they can transmit only two symbols (usually denoted as "0" and "1" or as ...