(1) Source coding and decoding
Two basic functions: one is to improve the effectiveness of information transmission, that is, through some kind of compression coding technology to try to reduce the number of symbols to reduce the symbol rate. The second is to complete the analog/digital (A/D) conversion, that is, when the information source gives an analog signal, the source encoder converts it into a digital signal to achieve the digital transmission of the analog signal.
(2) Channel coding and decoding
Function: Error control. Digital signal will be affected by noise and other errors in the transmission process. In order to reduce errors, the channel encoder and the transmitted information element add protection components (supervisory elements) according to certain rules to form the so-called “anti-interference coding”. The channel decoder at the receiving end decodes according to the corresponding inverse rules to find or correct errors and improve the reliability of the communication system.
(3) Encryption and decryption
In order to ensure the security of the transmitted information, the digital sequence is artificially disturbed by the transmission, that is, the password is added, this process is called encryption. Restoring the original message is decryption.
(4) Digital modulation and demodulation
Digital modulation: The frequency spectrum of the digital baseband signal is moved to the high frequency to form a band communication signal suitable for transmission in the channel. At the receiving end, the digital baseband signal can be restored by coherent demodulation or non-coherent demodulation.
(5) Synchronization
Synchronization: It is to keep the signals at both ends of the receiving and receiving in step with each other in time, and it is a prerequisite for ensuring the orderly, accurate and reliable work of the digital communication system.
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