Power line communication (PLC) technology refers to a kind of communication mode that uses power line to transmit data and media signals. The technology is to load the high frequency carrying information into the current, then use the adapter which transmits and receives information by wire to separate the high frequency from the current and transmit it to the computer or telephone to realize information transmission. The biggest advantage of this technology is that it does not need to be rewired on the existing wires to realize the carrying of multi services such as data voice and video, and realize the” four networks in one” .end-user only needs to plug in the power plug to realize the Internet access to TV channels to receive programs and make calls or video calls.
In the transmission, the user data is modulated by the modulation technology, the high frequency carrying information is loaded on the current, and then transmitted on the power line; at the receiving end, the modulated signal is taken out by the filter first, and then demodulated, the original communication signal can be obtained, and transmitted to the computer or telephone to realize the information transmission. PLC equipment can be divided into Center Office End and modem, the Center Office End is responsible for communication with internal PLC modem and connection with external network. In communication, the data from the user enters the modem for modulation, and then it is transmitted to the equipment at the office through the distribution line of the user. The office demodulates the signal and then transfers it to the external Internet. The specific design idea of bidirectional transmission module of the power line carrier : the transmission module is composed of modulator, oscillator, power amplifier, t / R steering switch, coupling circuit and demodulator, in which the oscillator provides a carrier signal for the modulator. When transmitting data, the signal to be sent out from the TXD end is modulated by the modulator, then the modulated signal is sent to the power amplifier stage for amplification, and then the modulated signal is loaded to the power line through the T / R steering switch and coupling circuit. When receiving the data, the modulated signal sent by the transmitting module enters the demodulator through the coupling circuit and T / R steering switch, and after demodulated by the demodulator, the original signal is extracted, and being sent from the RXD end to the next level of digital equipment.
Modulation mode of power line communication
OFDM is usually used in power line communication, that is, OFDM is a technical measure to ensure stable and complete data transmission in severe communication environment. The specification of hpme plug 1.0 covers 4-21mhz communication frequency band, in which 84 OFDM communication channels are divided. The principle of OFDM is that several communication channels are divided according to 90 degree phase. As a result, when a certain channel waveform passes zero, the waveform of adjacent channels is exactly the maximum value of amplitude, so as to ensure that the waveforms between channels will not overlap or cross interfere due to external interference.
Network interface of power line communication
The common interfaces of power line communication equipment are as follows:
RJ-45 interface is the most commonly used interface for Ethernet. RJ-45 is a commonly used name, which refers to the modular jack or plug with 8 positions (8 pins) according to the international connector and plug-in standard,which standardized by IEC (60) 603-7.
USB (Universal Serial Bus) is a new peripheral interface standard initiated by Intel, Microsoft, Compaq, IBM, NEC and other major manufacturers. The transmission speed of USB1.1 is 12mbps, and USB2.0 can reach 480mbps; the maximum length of cable is 5m. The USB cable has 4 lines, including 2 signal lines and 2 power lines, which can provide 5 volt power supply. USB cable can also be divided into shielded and unshielded. The transmission speed of shielded cable can reach 12mbps, and the price is relatively expensive. The speed of unshielded cable is 1.5mbps, but the price is cheap. USB can connect 127 devices in series at most. Plug-and-play and hot plug are supported.（https://www.shhjnet.com/index.html）