Baseband shaping and modulation relationship

1934, American scholar Livesey put forward the concept of pulse code modulation (PCM). Since then, the era of digital communication should be said to have begun, but the rapid development of digital communication began in the 1970s. With the development of the times, users are no longer satisfied with hearing sound, but also seeing images; Communication terminals are not limited to a single telephone, but also include data terminals, such as fax machines and computers. The existing transmission medium cable, microwave relay and satellite communication will use more digital transmission. These systems all use digital modulation technology. This paper introduces the modulation method of digital signal in detail.

Digital modulation

Carrier modulation of digital signal is a part of channel coding. The reason why the channel code is inserted between the source code and the transmission channel is because the channel and the corresponding equipment have certain restrictions on the digital signal to be transmitted, and the unprocessed digital signal source cannot adapt to these restrictions. Because the frequency band resources of transmission channels are always limited, improving transmission efficiency is one of the most important indicators pursued by communication systems. It is difficult to control the transmission efficiency of analog communication. The most common single sideband amplitude modulation (SSB) or vestigial sideband amplitude modulation (VSB) can save nearly half the transmission bandwidth. Because the digital signal has only two states: "0" and "1", the digital modulation can be completely understood as the process that the operator controls the carrier with the on-off key, so the modulation mode of the digital signal is relatively simple. When the elements of the transmission channel are fully utilized, they can be combined into various modulation modes, and their mathematical models can be clearly described and expressed. Therefore, the commonly used digital modulation technologies are 2ASK, 4ASK, 8ASK, BPSK, QPSK, 8PSK, 2FSK, 4FSK and so on. , the band utilization rate ranges from 1 bit/sec/Hz to 3 bit/sec/Hz. There is QAM technology, which combines amplitude and phase modulation. At present, the frequency band utilization rate of 256QAM, which is widely used in digital microwave, can reach 8bit/s/Hz, which is 8 times that of 2ASK or BPSK. In addition, there are special modulation technologies such as MSK, which can reduce phase jump and provide powerful tools for some special application environments. In recent years, the research of high-dimensional modulation technology such as four-dimensional modulation has also developed rapidly and has been applied to high-speed modems, laying the foundation for further improving transmission efficiency. In a word, the transmission efficiency of digital communication is much higher than that of analog communication, and the types of modulation technologies are far more than that of analog communication, which greatly improves the flexibility for users to choose system configuration according to actual application requirements.

1, baseband transmission

There are two ways to transmit information: baseband transmission and modulation transmission. The signals directly generated by the signal source, whether analog or digital, are baseband signals with relatively low frequency. The so-called baseband transmission means that the digital signal generated by the source is directly sent to the line for transmission, such as local audio call and data transmission between computers. Carrier transmission is to use the original signal to change a certain parameter of the carrier to realize spectrum shift. If the carrier wave is sine wave, it is called sine wave or continuous wave modulation. Binary signal modulation is transmitted on sine wave, and its purpose is not only frequency matching, but also frequency division, time division and wavelength division multiplexing to match the capacity of source and channel.

2. Why do you want to modulate?

First of all, due to the limited frequency resources, we can't use open channels to transmit information. In addition, the ultimate goal of communication is to transmit information over a long distance. Because of transmission distortion, transmission loss and guaranteed in-band characteristics, baseband signals cannot be transmitted over long distances in wireless channels or optical fiber channels. For long-distance transmission, the digital signal must be modulated by carrier wave, and the frequency spectrum of the signal must shift to high frequency before it can be transmitted in the channel. Finally, the smaller octave also ensures good in-band characteristics. So modulation is to move the baseband signal to the specified high frequency and transmit it with less channel loss (that is, carrier transmission). The modulated baseband signal is called passband signal, and its frequency is relatively high. The main difference between carrier transmission and baseband transmission of digital signals is that modulation and demodulation are added, that is, a modulator is added after the multiplexer and a demodulator is added before the demultiplexer.

Step 3 draw pictures

There is a correspondence between information and signals representing and carrying information, which is called "mapping". The receiver extracts the information sent by the transmitter from the received signal according to the pre-agreed mapping relationship. There are many ways to map information and signals, and different communication technologies depend on their different mapping methods. In fact, the main purpose of digital modulation is to control transmission efficiency. Different digital modulation technologies are distinguished by their mapping methods, and their performance is also determined by the mapping methods.

A digital modulation process is actually realized by two independent steps: mapping and modulation, which is different from analog modulation. Mapping converts multiple binary bits into multiple symbols, which can be real signals (in ASK modulation) or two-dimensional complex signals (in PSK and QAM modulation). For example, in the mapping of QPSK modulation, every two bits are converted into quaternary symbols, corresponding to four carriers of the modulated signal. The number of elements of the multivariate symbol is equal to the capacity of the modulation constellation. In this many-to-one conversion process, band compression is realized. It should be noted that the multivariate symbols generated after mapping are still baseband digital signals. After baseband shaping and filtering, analog baseband signal is generated, but it is already the equivalent baseband form of the final desired modulation signal. If directly multiplied by the intermediate frequency carrier, the intermediate frequency modulated signal can be generated.

4. Modulation method

The modulation method mainly transmits information by changing the amplitude, phase and frequency of sine wave. Its basic principle is that the data signal is parasitic on a certain parameter of the carrier: amplitude, frequency and phase, that is, amplitude modulation, frequency modulation and phase modulation of the data signal. Digital signal has only a few discrete values, which is like using digital signal to control switches to select oscillations with different parameters. Therefore, the modulation mode of digital signal is called keying. Digital modulation can be divided into amplitude modulation, phase modulation and frequency modulation. The easiest way is to turn on the key control. When "1" appears, the carrier with amplitude a is turned on, and when "0" appears, the carrier is turned off, which is equivalent to shifting the spectrum of the original baseband signal (pulse sequence) to both sides of the carrier. If binary symbols are transmitted by changing the carrier frequency, this is the method of frequency shift keying (FSK). Low frequency when "1" appears, and high frequency when "0" appears. At this time, its frequency spectrum can be regarded as the on-off key control of low-frequency carrier by code train and the on-off key control of high-frequency carrier by inverse code of code train. If "0" and "1" are used to change the carrier phase, it is called phase shift keying (PSK). At this time, the phase jump appears at the edge of the bit period. But the phase information remains in the middle of the interval. Demodulation at the receiving end is usually carried out near its center point. Generally speaking, the performance of PSK system is better than that of switch FSK system, but synchronous detection must be adopted. In addition to the two-phase, dual-frequency and dual-amplitude systems mentioned above, various multi-phase, multi-amplitude and multi-frequency schemes can be adopted. In DVB system, QPSK is used for satellite transmission, QAM is used for cable transmission, and COFDM (Coded Orthogonal Frequency Division Multiplexing) is used for ground transmission. ASK, FSK, PSK and QAM are introduced in detail below.