The purpose of modulation is to: 1) facilitate wireless transmission and reduce antenna size;2) Frequency division multiplexing to improve communication capacity;3) Improve signal anti-interference ability.In order to make full use ofchannel capacity , meet the different needs of users,communication signals Differentmodulation mode 。With the rapid development of electronic technology and the continuous improvement of users' requirements for information transmission, the modulation methods of communication signals have experienced a change fromsimulationFrom simple to complex.[1]
Chinese name
Modulated signal
Foreign name
modulating signal
Role
Low frequency signal transformed from original information
stayWireless transmissionMedium, the signal is based onelectromagnetic waveRadiating into space through an antenna.In order to obtain higherRadiation efficiencyThe size of the antenna should generally be greater than one fourth of the wavelength of the transmitted signal.andbaseband signal The wavelength of the lower frequency component is longer, which makes the antenna too long to realize.Through modulation, the frequency spectrum of baseband signal is moved to a highercarrier frequency The size of the radiation antenna can be greatly reduced.In addition, modulation can move multiple baseband signals to different carrier frequencies to realize channel multiplexing and improve channel utilization.Finally, modulation can expand the signal bandwidth, improve the anti-interference and anti fading capabilities of the system, and improve the signal-to-noise ratio of transmission.The improvement of SNR is at the expense of transmission bandwidth.Therefore, in the communication system, the key is to choose the appropriate modulation mode.[2]
development
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In the 1960s, C.S. Waver et alStanford UniversityAfter publishing a paper entitled The Automatic Classification of Modulation Types by Pattern Recognition, the research on signal modulation recognition technology has gradually developed.
In 1984, Liedtke F F proposed a modulation recognition method for digital signals using signal amplitude, frequency, differential phase histogram, amplitude, frequency variance and other characteristic parameters.
In 1988, DeSimio M P et alspectrumAnd extracted from the envelopecharacteristic parameter The adaptive method is used to recognize the modulation of the signal.
In 1990, Polydoros A et al. proposed a method to identify MPSK signals using quasi optimal log likelihood ratio and decision theory.Subsequently, Nandi A K et al. proposed thatneural networkAndDecision theoryThe method of pattern recognition of analog and digital signals using multiple characteristic parameters.
In 2008, Wong M L et al. proposed a semi blind recognition method using the maximum likelihood criterion, with good results.[3]
analog signalIt means that the signal changes continuously with time. In analog signals, the signal values at different time points can change continuously.Compared with digital signal, analog signal processing method is simple, does not need special signal processor, and has higher resolution and information density, without quantization error. Therefore, analog signal can be used to more accurately describe the value of physical quantities in nature.The main disadvantage of analog signals is that they are vulnerable to noise interference, especially when the signal is retransmitted many times or transmitted over a long distance, the impact of these random noises may become very significant.
Amplitude modulation principle
Amplitude modulation of signal refers tobaseband signal To control the amplitude change of the carrier wave.Amplitude modulation is divided into:amplitude modulation(AM), which carries the carrier frequency signal;Double sideband modulation(DSB) to suppress the double sideband modulation of the carrier;Single sideband modulation(SSB), SSB modulation is divided into upper sideband modulation and lower sideband modulation.
Frequency modulation principle
Frequency modulation (FM) is a kind of angle modulation. It refers to the modulation method that makes the carrier frequency change with the modulation signal, that is, the modulation process in which the instantaneous angular frequency of the modulated signal changes with the baseband signal.Unlike AM, FM belongs toNonlinear modulationThe modulated signal spectrum is a nonlinear transformation of the modulated signal spectrum.[3]
digital system
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digital modulation Technology inModern communicationThe system has a wide range of applications. Compared with analog modulation technology, digital modulation technology has the advantages of strong noise resistance, high robustness, high flexibility, and good security. The main disadvantage is that the required transmission bandwidth is large and the equipment is usually complex.However, the use of digital compression technology, large-scale integrated circuit technology, optical fiber and other high-capacity transmission media has greatly reduced the difficulty and complexity of digital system applications, and digital systems are increasingly popular.
Amplitude keying principle
Amplitude keyingIt is the earliest digital modulation method, which transmits digital information through the amplitude change of carrier wave.Binary amplitude keying (2ASK) means that the modulated signal isBinary digitAmplitude keying of baseband signals.For 2ASK, when the signal is "1", the carrier passes through the switch and is a large signal;When the signal is "0", the carrier is cut off, there is no transmission signal, only background noise.
Principle of frequency shift keying
stayFrequency shift keyingIn, the frequency of the sinusoidal carrier follows the numberbaseband signal Change, the transmission of digital information is realized by the change of carrier frequency.If theDigital baseband signalFor binary digital signals, binary frequency shift keying (2FSK) is generated.In the 2FSK signal, when the binary baseband signal is "1", the carrier frequency is
When the signal is "0", the carrier frequency becomes
。
Phase shift keying principle
stayPhase-shift keyingIn, the phase of the sinusoidal carrier follows the numberbaseband signal Change, transmission of digital information throughcarrier phase While the amplitude and frequency of the signal remain unchanged.If the digital baseband signal in phase shift keying is a binary digital signal, binary phase shift keying (2PSK) is generated.In 2PSK, binary signals "0" and "1" usually correspond to the initial phase respectively
And 0.
Phase shift keying can be divided into absolute phase shift and relative phase shift.The phase modulation with the phase of the unmodulated carrier as the reference is called absolute phase shift.Taking binary phase modulation as an example, when the symbol is "1", the modulated carrier is in phase with the unmodulated carrier;When the symbol is taken as "0", the modulated carrier and unmodulated carrier are in phase inversion;The carrier phase difference after modulation is 1800 at "1" and "0".
Quadrature amplitude modulation principle
Quadrature amplitude modulationalso known asamplitudeAnd phase joint keying, which is obtained by using two independent baseband waveforms to suppress the carrier double band modulation of two mutually orthogonal co frequency carriers, and the modulated signals are orthogonal in frequency spectrum within the same bandwidth, so the transmission of two parallel digital information can be realized.MQAM performs amplitude and phase modulation at the same time, which has stronger anti-interference ability and higher spectrum utilization.[3]
application
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Signal modulation is widely used in civil and military fields.
In the civil field, it is mainly used forRadio spectrumManage signal identification and interference confirmation.In order to prevent criminals fromRadio spectrumIn order to ensure the normal operation of legal communication, it is necessary to monitor and manage the communication spectrum for illegal use and interference of.Using modulation identification technology in spectrum monitoring equipment can help managers distinguish users of different natures and determine the unknowninterference signalThe nature of.
In the military field, it is mainly used forElectronic warfareAnd military software radio technology.In the design of EW communication intelligence interception receiver, the modulation mode of the received communication signal is obtained, which provides a reference basis for the demodulator to select the demodulation algorithm, and helps to select the best interference mode or interference cancellation algorithm in electronic warfare, so as to ensure friendly communication, while breaking and inhibiting enemy communication, and achieveCommunication countermeasureThe purpose of.[4]
