Introduction
Similar to other modulation methods, QAM transmits information through the change of certain parameters of the carrier. In QAM, the data signal is represented by amplitude variations of two carriers orthogonal. The orthogonal amplitude modulation signal waveform is shown in Figure 1.
Figure 1 orthogonal amplitude modulation signal waveform
The phase modulation of the analog signal and the PSK of the digital signal can be considered to be the same, only a special orthogonal amplitude of the phase change modulation. Thus, analog signal frequency modulation and digital signal FSK can also be considered a special case of QAM, because they are essentially phase modulation. The QAM of the digital signal is mainly discussed, although analog signal QAM also has many applications, such as NTSC and PAL-based television systems use orthogonal carriers to transmit different color components.
QAM Transmit Signal Set
Similar to other digital modulation, the QAM transmit signal set can be easily represented by the constellation diagram. The constellation map corresponds to a signal in the transmitted signal. The transmit signal set size of the orthogonal amplitude modulation is N, referred to as N-QAM. Common QAM forms have 16-QAM, 64-QAM, 256-QAM, etc.
Other
When the required data transmission rate is high, the QAM modulation method is generally employed. Because the QAM's constellation point is more dispersed than the constellation point of the PSK, the distance between the constellation points is even greater, so it can provide better transmission performance. However, the magnitude of the QAM constellation is not exactly the same, so its demodulator needs to correctly detect the phase and amplitude, unlike PSK demodulation only need to detect the phase, which increases the complexity of the QAM demodulator.
Digital communication often uses the error rate (including the wrong symbol rate and my bit error rate) and the signal-to-noise ratio to measure the performance of the modulation and demodulation. The following is given below to obtain an expression of the error rate under the AWGN channel:
m = number of constellation points
EB = average bit energy
es = average symbol energy =
N0 = noise power spectral density
PB = error bit rate
PBC = per orthogonal wave Misror bit rate
ps = error symbol
PSC = Mistant rate value of each orthogonal carrier
Rectangular Qam (Rectangular QAM) Rectangular grid configuration. Because the minimum distance between the rectangular QAM signal is not the largest of the same energy, its error rate performance is not optimal. However, considering the superposition of the rectangular QAM equivalent to pulse amplitude modulation (PAM) on two orthogonal waves, the modulation demodulation of the rectangular QAM is relatively simple. The non-rectangular QAM described later can reach slightly some bit error performance, but the cost of pay is difficult to modulate and demodulate.
The earliest rectangular QAM is generally 16-QAM. The reason is very easy to see that 2-QAM and 4-QAM are actually binary phase shift keying (BPSK) and orthogonal phase shift keying (QPSK), and 8-QAM has a single bit from the single bit For two carriers, 8-PSK is much easier, so 8-qam is rarely used.
Constellation diagram Represents
Similar to other digital modulation methods, the QAM transmitted signal set can be conveniently represented by the constellation map, each constellation point corresponds to the transmitted signal set a little.
The constellation point often uses a square mesh configuration of the horizontal and vertical direction, and of course there are other configuration methods.
Data often uses a binary number in digital communication, and the number of constellation points is generally 2 power.
The more constellation points, the greater the amount of information that can be transmitted. However, if the constellation point is increased if the average energy of the constellation is unchanged, the distance between the constellation points becomes small, causing the bit error rate to rise. Therefore, the reliability of the high-order constellation is poor than low order.
Adopt QAM modulation technology, the channel bandwidth is at least equal to the symbol rate, in order to recover timed, additional bandwidth is required, generally to increase by about 15%.
Advantages and Disadvantages
Compared to other modulation techniques, QAM encoding has the advantages of sufficient use of bandwidth, strong anti-noise ability.
However, the main problem with the QAM modulation technique for ADSL is how to adapt to large performance differences between different telephone lines. To achieve more ideal work characteristics, the QAM receiver requires one and the input signal having the same spectrum and the corresponding characteristics of the sender for decoding, the QAM receiver uses the adaptive equalizer to compensate for the distortion generated during the transmission process, so The complexity of the QAM ADSL system comes from its adaptive equalizer.