Overview of Quadrature Amplitude Modulation (QAM)

This chapter mainly introduces the basic computer network communication technologies such as local area network and wide area network, as well as the main functions and working principles of each layer of OSI, and also introduces the commonly used technical indicators and parameters in computer network data communication. These are the basis and premise of our usual network engineering construction and system design. This part is QAM quadrature amplitude modulation in modulation mode. 4. Quadrature amplitude modulation.

QAM (Quadrature Amplitude Modulation) is to amplitude modulate two carriers with the same frequency and orthogonal phase with two modulation signals, and then superimpose the modulated signals for transmission or transmission. When chroma signals are formed in NTSC and PAL systems, two chroma signals are modulated onto chroma subcarriers by quadrature amplitude modulation.

QAM can also be used for digital modulation. Digital QAM has 4QAM, 8QAM, 16QAM, 32QAM and other modulation methods. Among them, 16QAM and 32QAM are widely used in digital cable TV systems. Take 16QAM as an example to introduce its principle.

Figure 3-34 shows the block diagram and constellation diagram of 16QAM modulator. An input binary data stream as a modulation signal is converted into four parallel data streams after serial-parallel conversion. These four channels of data, combined in pairs, enter two level converters respectively and are converted into two channels of 4-level data. For example, 00 is converted to–3,0 1, 10 is converted to1,1is converted to 3. Two channels of 4-level data g 1(t) and g2(t) modulate the carriers cos2πfct and sin2πfct respectively, and then add them to get the 16QAM signal.

QAM has high modulation efficiency and high signal-to-noise ratio of transmission path, which is suitable for cable TV transmission. In the United States, quadrature amplitude modulation is usually used for terrestrial microwave links, not for domestic satellites. In Europe, QAM modulation is used for cable digital TV, while in Canada, quadrature amplitude modulation is used for satellites. QAM is a combined modulation technique of amplitude and phase, which uses the amplitude and phase of carrier wave to transmit information bits. Therefore, in the case of the same minimum distance, more constellation points can be accommodated in the QAM constellation diagram, and higher frequency band utilization can be realized. At present, the maximum number of QAM constellation points has reached 256QAM.

PSK only uses the carrier phase, and all its constellation points can only be distributed on a circle with the same radius. When there are many constellation points, the minimum distance between constellation points will be very dense, which is very vulnerable to noise interference. The reliability of modulation technology can be measured by the minimum distance between adjacent constellation points. The greater the minimum distance, the stronger the ability to resist noise and other interference, provided, of course, that the average power of signals is the same. When the amplitude of interference such as noise is less than 1/2 of the minimum distance, the demodulator will not misjudge, that is, it will not transmit error code; When the amplitude of interference such as noise is greater than 1/2 of the minimum distance, an error code will be sent. Therefore, PSK is generally only used below 8PSK, and commonly used are BIT/SK and QPSK. When the number of constellation points is further increased, that is, when higher frequency band utilization is needed, QAM modulation will be adopted. In PSK, I signal and Q signal are not independent of each other. In order to get a constant envelope signal, their values are limited, which is the basic characteristic of PSK signal. If this limitation is removed, quadrature amplitude modulation QAM can be obtained. As a special case, QAM is exactly the same as 4-PSK when each orthogonal signal has only two values. When the signal constellation of M & gtQAM is distributed in a square at 4 o'clock, it is not distributed along a fixed circle like PSK.