Quadrature Amplitude Modulation (QAM) is a modulation method that performs amplitude modulation on two orthogonal carriers.These two carriers are usually sine waves with a phase difference of 90 degrees (π/2), so they are called orthogonal carriers.This modulation method gets its name.
Similar to other modulation methods, QAM transmits information by changing some parameters of the carrier.In QAM, the data signal is represented by the amplitude change of two orthogonal carriers.The waveform of quadrature amplitude modulation signal is shown in Figure 1.
Quadrature amplitude modulation
Figure 1 Quadrature amplitude modulation signal waveform
Phase modulation of analog signal and PSK of digital signal can be considered as special quadrature amplitude modulation with constant amplitude and only phase change.Therefore, analog signal frequency modulation and digital signal FSK can also be considered as special cases of QAM, because they are essentially phase modulation.This paper mainly discusses the QAM of digital signal, although analog signal QAM has many applications, for example, NTSC and PAL television systems use orthogonal carriers to transmit different color components.[1]
QAM transmit signal set
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Similar to other digital modulation methods, QAM transmission signal set can be conveniently represented by constellation diagram.Each constellation point on the constellation map corresponds to a signal in the transmission signal set.Let the transmitted signal set size of quadrature amplitude modulation be N, which is called N-QAM.Common QAM forms include 16-QAM, 64-QAM, 256-QAM, etc.[1]
other
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When the requirement for data transmission rate is higher than the upper limit provided by 8-PSK, QAM modulation mode is generally used.Because QAM constellation points are more dispersed than PSK constellation points, the distance between constellation points is larger, so it can provide better transmission performance.But the amplitude of QAM constellation points is not exactly the same, so its demodulator needs to be able to detect the phase and amplitude correctly at the same time, unlike PSK demodulation, which only needs to detect the phase, which increases the complexity of QAM demodulator.
In digital communication, the relationship between error rate (including symbol error rate and bit error rate) and SNR is often used to measure the performance of modulation and demodulation.The notation of some concepts is given below to get the expression of error rate in AWGN channel:
M=number of constellation points
Eb=average bit energy
Es=average symbolic energy=
N0=noise power spectral density
Pb=bit error rate
Pbc=bit error rate on each orthogonal carrier
Ps=symbol error rate
Psc=symbol error rate on each orthogonal carrier
The constellation of Rectangular QAM is configured as a rectangular grid.Because the minimum distance between rectangular QAM signals is not the maximum under the same energy, its BER performance is not optimal.However, considering that rectangular QAM is equivalent to the superposition of pulse amplitude modulation (PAM) on two orthogonal carriers, the modulation and demodulation of rectangular QAM is relatively simple.While the non rectangular QAM introduced later can achieve slightly better bit error rate performance, the price paid is much more difficult modulation and demodulation.
The earliest rectangular QAM is generally 16-QAM.The reason is that it is easy to see that 2-QAM and 4-QAM are actually binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK), while 8-QAM has the problem of dividing single digit bits into two carriers. 8-PSK is much easier, so 8-QAM is rarely used.[2]
Constellation diagram representation
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Similar to other digital modulation methods, the signal set transmitted by QAM can be conveniently represented by constellation diagram. Each constellation point on the constellation diagram corresponds to that point in the transmitted signal set.
Constellation points are often configured in square grids with equal spacing in horizontal and vertical directions. Of course, there are other configurations.
In digital communication, data is often represented by binary numbers. In this case, the number of constellation points is generally a power of 2.
The more constellation points, the more information each symbol can transmit.However, if the average energy of the constellation is kept unchanged, increasing the constellation points will reduce the distance between the constellation points, which will lead to an increase in the bit error rate.Therefore, the reliability of high-order constellations is worse than that of low order constellations.
With QAM modulation technology, the channel bandwidth must be at least equal to the symbol rate. For timing recovery, additional bandwidth is required, which is generally increased by about 15%.[3]
Advantages and disadvantages
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Compared with other modulation techniques, QAM coding has the advantages of taking full advantage of bandwidth and strong anti noise ability.
But the main problem of QAM modulation technology used in ADSL is how to adapt to the large performance differences between different telephone lines.To achieve ideal operating characteristics, the QAM receiver needs an input signal with the same spectrum and corresponding characteristics as the transmitter for decoding. The QAM receiver uses an adaptive equalizer to compensate for the signal distortion in the transmission process. Therefore, the complexity of the ADSL system using QAM comes from its adaptive equalizer.
