hjr-Communication Principle (IV): Digital signal analog carrier modulation

Digital signal divided into baseband transmission and band-pass transmission

Band-pass transmission is the modulation of baseband signals

Why baseband modulation is needed:

In practice, most channels (such as wireless channels) have band-pass characteristics and cannot transmit baseband signals directly, because digital baseband signals tend to have rich low-frequency components

In order for the digital signal to be transmitted in the band communication channel, the carrier must be modulated with a digital baseband signal to match the characteristics of the signal to the channel.

The specific process is

Digital signal modulation mainly has amplitude keying (AFK), Frequency shift keying (FSK), Phase shift keying (PSK), differential phase shift keying (DPSK)

Above are three common modulated carriers, multiplied by the pulse signal and the analog carrier, shaped like S (t) *coswt,s (t) is the pulse signal an*g (T-NT)

Combined with two graphs, you can see that the amplitude of the key control is to use the amplitude has no indication of the original baseband signal high and low level

Frequency shift keying is the high and low level with 1 time times frequency and twice octave

Phase Shift PI unit amplitude is negative from positive, so the graph of phase shifted keying shows that 1 and 0 are represented by ACOSWT and-ACOSWT, respectively.