Communication Theory (3) - Modulation
Ⅰ. Introduction
Modulation : The pinned to the message signal a parameter of a carrier to form a modulated signal.
Demodulation : reverse of modulation, the recovery message signal from the modulated signal.
A modulated object
- In wireless communication, channel characteristics matching, increase the frequency of the transmitted signal , reducing the antenna size;
- Spectrum shifting , a simultaneous transmission of multiple signal channels, multiplexing, improved channel utilization;
- Expanded signal bandwidth , improving system anti-interference capability;
- Implementing bandwidth and SNR interchange (validity and reliability);
- Use telephone lines to a PC access to Internet, to translate analog / digital signals.
Second, the modulation classification
1. The signal according to:
- Message signal , also known as a modulation signal, a baseband signal;
- Carrier : vehicle, used is a sine wave, a pulse sequence;
- Modulated signal : the modulated carrier signal carrying the information message, a variety of forms.
2. can be classified from different perspectives:
- Type modulated signal by: modulating the analog / digital modulation
- Press modulated signal spectral structure: linear modulation / nonlinear modulation
- Modulated by a sinusoidal carrier parameters: amplitude modulation / frequency modulation / phase modulation
- By type of carrier signals: continuous wave modulation / pulse modulation
Ⅱ. Amplitude modulation
First, the general model
1. Rationale: Fourier transform
2. General model
Amplitude Modulation: a message signal to control the amplitude of the sinusoidal carrier .
Methods: a message signal by multiplying device multiplying the carrier signal through the band-pass filter (time domain convolution filter characteristic).
Examples: AM, DSB, SSB, VSB.
Second, the conventional double-sideband amplitude modulation AM
t Domain: modulated waveform, the modulation / demodulation method of a signal
f Domain: spectrum modulated signal, the bandwidth B
- AM signal envelope is proportional to the law of message signals, it is possible to use a simple envelope detection method ** (incoherent demodulation) ** demodulator;
- Spectrum by the carrier, the upper sideband USB, lower sideband LSB components. Bandwidth B AM =. 2F H ;
- Also known linear modulation amplitude modulation;
- Application: shortwave AM broadcasting.
- Disadvantages: use of low power, up to reach 50%
Third, the double-sideband suppressed carrier DSB
- The spectrum of the upper sideband USB, lower sideband LSB composition, no carrier components. The bandwidth B the DSB = B AM =. 2F H ;
- Modulation efficiency of up to 100% .
- Using coherent demodulation :
- Methods: a message signal multiplied by the multiplier like coherent carrier signals, and then through the low-pass filter (time domain convolution filter characteristic).
- Requirements: carrier synchronization (and coherent carrier signals with the same frequency carrier)
Fourth, SSB single sideband modulation
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Transmitting only one sideband, high bandwidth efficiency . Bandwidth B SSB = B AM / F 2 = H ;
Crowded spectrum communication applications, such as short-wave communications, multi-channel carrier telephone system.
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Low power consumption . For use in a mobile communication system.
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Disadvantages: complex equipment, there are technical difficulties, require coherent demodulation .
V. vestigial sideband modulation VSB
- Vestigial sideband filter characteristics: complementary symmetry properties of the carrier frequency;
- SSB interposed between the program and the double sideband.
Ⅲ. Angle modulation
Sinusoidal carrier has three parameters: amplitude, frequency, phase. You can carry the message signal.
Wherein the frequency (FM) and phase (PM) is called angle modulation.
Frequency modulation (FM)
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Constant amplitude of the instantaneous phase of t differentiating to give the instantaneous angular frequency .
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FM carrier spectrum component by w C infinite number of side bands on both sides of w C ± NW m , whose amplitude depends on the MF;
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In theory, the FM bandwidth is infinite;
In practice, by Carson formula calculate FM Bandwidth: B FM = 2 (m F + 1'd) F m . F m is the maximum frequency of the modulation signal
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FM modulation is linear modulation. Also known as FM demodulation discriminator , the differential circuit using envelope detector implemented +.
FM features and applications
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Features: constant amplitude, constant envelope.
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Advantages: Strong noise immunity;
Consideration: occupy large channel bandwidth, spectrum utilization rate;
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Application: large channel noise or a high quality applications. The satellite communication, mobile communication, microwave communication.
IV. Noise immunity
Performance indicators: output signal to noise ratio, system gain
Input SNR: N I = n- 0 B. n- 0 is a unilateral noise power spectral density, B =. 2F H is the bandwidth, twice the baseband bandwidth.
AM DSB SSB VSB (Amplitude Modulation)
- Coherent demodulator: linear demodulation, signal and noise can be dealt with separately.
- Double sideband and single sideband modulation same noise immunity.
- Small noise ratio, signal to interference noise, generating a threshold effect . The reason is that the nonlinear effect demodulation envelope detection.
- Fixed-noise ratio.
FM (angle modulation)
FM systems can increase the transmission bandwidth to improve noise performance (SNR).
to sum up
- Spectrum utilization SSB> VSB> DSB / AM> FM
- Anti-noise performance: FM> DSB / SSB> VSB> AM
- Equipment complexity: AM simplest, DSB / FM followed, SSB and most complex
