In communication systems, the original electrical signal at the transmitter usually has a very low frequency spectrum component, which is generally not suitable for direct transmission in the channel.Therefore, it is usually necessary to transform the original signal into a high-frequency signal with a frequency band suitable for channel transmission. This process is called modulation.[1]
Signal modulation is a process or processing method to change some characteristics of one waveform according to another waveform or signal.In radio communication, useelectromagnetic waveAs a carrier of information.[1]
After modulation, the spectrum of the original signal can be shifted. The modulated signal is called the modulated signal. The modulated signal carries information and is suitable for transmission in the channel.[1]
The information needs to undergo multiple transformations in the transmission process of the communication system.First, the source side should convert various information into original electrical signals, and then the sender should convert the original electrical signals into signal forms suitable for channel transmission (such as modulating the signals);The original electrical signal often needs to be modulated and transformed at the sending end, because the frequency of the original electrical signal is very low (we call this signal baseband signal), the transmission loss in the channel is large, and it is not suitable for direct transmission.Modulation is to convert various baseband signals into modulation signals suitable for channel transmission.[2]
Through modulation, the spectrum can be shifted, and the spectrum of the modulated signal can be shifted to the desired position, so as to transform the modulated signal into a signal suitable for channel transmission or conducive to channel multiplexing.[2]
classification
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There are many types of modulation, and the classification methods are inconsistent.The modulation style of communication signal can determine the performance of communication system, and it will affect the effectiveness and reliability of communication system.[1]
According to the form of modulation signal, it can be divided into analog modulation and digital modulation.Modulation with analog signal is called analog modulation;Modulation with data or digital signal is called digital modulation.[2]Commonly used digital modulation signals mainly include amplitude keying (ASK), frequency shift keying (FSK), phase shift keying (PSK) and quadrature amplitude modulation (QAM) signals.[1]
Compared with analog signal modulation, digital modulation has many advantages, such as: strong anti noise ability, and noise does not accumulate;It is convenient to use modern digital signal processing technology to process digital information, and can fuse signals from different sources such as sound, data and image for transmission;It has been encrypted with good security.[2]
Whether the modulation mode is linear or nonlinear depends on whether the spectrum of the signal retains the linear structure of the baseband signal spectrum after modulation;If it can be retained as linear modulation, otherwise it is called nonlinear modulation.[2]
operation
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According to different carrier signal parameters, commonly used digital modulation signals mainly include amplitude keying (ASK), frequency shift keying (FSK), phase shift keying (PSK) and quadrature amplitude modulation (QAM) signals.[1]
Amplitude keying (ASK)
Amplitude keying (ASK) is a digital modulation method that uses the amplitude of the carrier wave to change with the digital baseband signal to transmit information.In binary amplitude key (2ASK), binary information "0" and "1" correspond to two change states of carrier amplitude respectively, and 2ASK signal can be generated by analog modulation method and keying method.[1]
M-ary amplitude keying (MASK) signal carrier has M different amplitude values, among which, a symbol interval sT transmits a carrier signal of one amplitude.MASK signal can be regarded as the superposition of M 2ASK signals, and its power spectrum is the sum of the power spectrum of M 2ASK signals, so the form of its power spectrum is similar to that of 2ASK signals.[1]
Frequency shift keying (FSK)
Frequency shift keying (FSK) is a digital modulation method that uses the change of carrier frequency with digital baseband signal to transmit information.In binary frequency shift keying (2FSK), binary information "0" and "1" correspond to the two frequencies 1 f and 2 f of the carrier signal respectively.The 2FSK signal can be regarded as composed of two 2ASK signals, in which the modulation amplitude of the 2ASK signal is "0" and "1" respectively.[1]
2FSK signal can be generated by analog frequency modulation circuit and keying method.The phase change of the signal generated by the analog frequency modulation method between adjacent symbols is connected and easy to realize, but the frequency stability is poor.The phase change of the signal generated by keying between adjacent symbols is not necessarily continuous, but its conversion speed is fast and the frequency stability is high.[1]
Phase shift keying (PSK)
Phase shift keying (PSK) is a digital modulation method that uses the change of carrier phase with digital baseband signal to transmit information.It includes two modes: absolute phase keying (PSK) and relative phase keying (DPSK).[1]
The two change states 0 and π of the carrier phase of the binary phase shift keying (2PSK) signal correspond to the binary information "0" and "1" respectively, while the amplitude and frequency remain unchanged.The generation of 2PSK signal usually includes analog modulation method and keying method.[1]
Quadrature amplitude (QAM)
Quadrature amplitude modulation (QAM) is a keying method that combines amplitude and phase. Its two carriers are orthogonal at the same frequency.In the modulation process, two independent baseband signals are used to suppress the double sideband amplitude modulation of two carriers. Because the frequency spectrum of modulated signals in the same bandwidth is orthogonal, the parallel transmission of two digital information can be realized.The distribution diagram of QAM signals is usually called constellation diagram.[1]
Modulation purpose
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Modulation has four purposes:
1) The original electrical signal of the transmitter in the communication system usually has a very low frequency spectrum component, which is generally not suitable for direct transmission in the channel. Therefore, it is usually necessary to transform the original signal into a signal with a frequency band suitable for channel transmission.[1]
2) Through modulation, multiple baseband signals can be moved to different frequency spectrums, so as to make full use of the channel and improve transmission performance.[3]
3) Modulation can expand the bandwidth of wireless communication signals, thus improving the ability of anti-interference and anti fading.[3]
4) Reduce the size of transmitting and receiving antennas in wireless communication.[3]
application
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The recognition of signal modulation mode is of great significance to signal adaptive reception and intelligent communication. It can be used in many military and civilian occasions, such as electronic reconnaissance, electronic countermeasures, spectrum detection, etc.[4]
