Digital modulation can be divided into two categories: linear modulation and nonlinear modulation. Can you be specific?

Digital modulation is an important way of modern communication, which has many advantages compared with analog modulation. Digital modulation has better anti-interference performance, stronger anti-channel loss and better security; Error control technology can be used in digital transmission system to support complex signal conditions and processing technologies, such as source coding, encryption technology and equalization.

Common digital modulation methods, such as:

Ask- amplitude shift keying modulation, which represents binary symbols 0 and 1 with different amplitudes respectively.

FSK- frequency shift keying modulation, that is, different frequencies represent different symbols. For example, 2KHz means 0 and 3KHz means 1.

PSK phase shift keying modulation, through the binary symbols 0 and 1 to judge the front and back phases of the signal. Such as 1 with π phase, 0 with 0 phase.

GFSK- Gaussian frequency shift keying, which limits the spectrum width of the signal by Gaussian low-pass filter before modulation.

GMSK ——? Gaussian filter minimum shift keying, a modulation technique used in GSM system.

Extended data

Like analog modulation, digital modulation takes sine wave as carrier and has three basic modes: amplitude modulation, frequency modulation and phase modulation. In the current general terminology, digital modulation is called "keying", that is, the pulse sequence of information symbols to be transmitted is regarded as "electric key" to control some parameters of the carrier.

So with amplitude shift keying (ASK), frequency shift keying (FSK) and phase shift keying (PSK), various forms of numerical control can be derived. In binary digital modulation, there are only two changing states of the amplitude, frequency or phase of the carrier, namely 2ASK, 2FSK and 2PSK.

Digital modulation signal can be expressed as the combination of carrier in-phase component and quadrature component, and the time domain expression of modulation signal is: s(t)=A(t)cos[ωct+φ(t)].

Where the amplitude (envelope) of A(t)-s(t), the phase of φ (t)-s(t) and the carrier frequency of ωc-s (t). The formula can be expanded as:

s(t)=[A(t)cosφ(t)]cosωCT-[A(t)sinφ(t)]sinωCT？ (formula 1)

=I(t)cosωct-Q(t)sinωct

Where I(t) is the in-phase component, and I (t) = a (t) cos φ (t); Q(t)- orthogonal component, Q(t)=A(t)sinφ(t). I(t) and Q(t) are low-pass signals because their frequency components are concentrated in low frequencies.

The above formula shows that the general generation method of keying signal is orthogonal modulation, that is, the digital signal is converted into an appropriate baseband waveform, and then multiplied by two carriers with orthogonal phases and superimposed.

In this way, the keying signal can be described by baseband waveforms I(t) and Q(t), and the differences between various modulations are reflected in the baseband pulse forms of I(t) and Q(t) and their relative timing. It can be seen that the equation 1 provides a method to analyze various modulation techniques.

Baidu encyclopedia-digital modulation