【 Weak current College 】 Powerline Internet technology in depth analysis

«Ò» ¡ ¢ PLC technology elements 1. Power line network unit (PNU) which is responsible for the control of the power line network and traffic from the unit distribution network integration. Through appropriate telecommunications trunk interface, PNU and traffic transmission to feed network. 　　Under feed network different media in use, PNU also convertible from low-voltage distribution network for data traffic. 2. the power line network Terminal (PNT) it for end user PC or other user with the appropriate interfaces such as Ethernet or USB. 　　In order to reduce costs, a separate device to PC or other device. 3. the coupling devices (CouplingUnit) it is the signal for incoming lines and filtering noise. At present it is still a bolt into the electrical socket of relatively independent of device, it may in the future and PLC modem integrated in one. 　　PLC modem and PC coupled devices within a collection of one day will enable PC can run directly on the Internet. Distribution network is a shared media, i.e., all the users connected to share the same "cables". In a typical urban configuration, it is converted to a transformer is connected to about 100 to 200 users. PLC system best able to 1Mbps transfer rates to support 80 users, this proportion is sufficient. By PLC technical support customer, you need to have a specification that has a strong capacity of bandwidth allocation of media access control (Mac) layer. This makes the power line network not only to support 80 Internet user data switching back and forth, and the flexibility to adapt to different rates of transmission of the uplink and downlink data. Second, data signal transmission technology 1, digital spread spectrum technology (SST) in current practice, in order to achieve for family or economic product communication and control network, the need for more robust multi-user environment PL communication technology, spread spectrum communication technology came into being.
Spread spectrum communication relative to the case of narrow-band communication with a certain technical advantages, mainly reflected in anti-jamming. Because spread spectrum signal bandwidth is usually relatively large (several decades to several hundred KHz), so its interference in the frequency range relative to reduce the proportion, in other words, that is all kinds of noise only affect a small portion of the transmission of signals, and most of the signal will be able to complete, the correct destination, for various types of interference is strong resistance. 　　For the most common impulse noise, while narrowband communication receiver has a narrower band, making only a small portion of noise into the sink, but since this class receives the device filter has a high-quality factors, instantaneous pulse noise will make it happen, since interference caused it to transmit the signal produced by mistake; with a low quality factors filter makes passband bandwidth increase, resulting in more noise into the sink, narrow-band communication on impulse noise resistance. 　　However use spread spectrum technology, when it receives a greater energy noise signal, the receiver will be part of the noise of high energy arrives automatically stop work, receiving only a small part of the affected signal decoding error correction can be; in addition, spread spectrum receiving device uses filter with low quality factors, and thus do not cause interference to the system, so spread spectrum technology with strong thresholding. 　　Generally speaking, the current implementation of spread spectrum there are three ways: direct sequence modulation, frequency hopping carrier and use a carrier frequency Chirps. 　　1) direct sequence modulation (Direct-Sequence Modulation) technology is the energy of the signal is distributed equally across the whole band, and adopted the pseudo-random sequence will flow more to make the signal to spread spectrum, this series has several times in the transmission signal binary data bit rate of symbol rate. 2)-hop line (Frequency-Hopping) that spread spectrum signals in a frequency with continued for a period of time, to represent the data of one, several or a part of. 　　When the signal at a frequency interference, signal can be switched to spread spectrum bandwidth within other frequencies, thus significantly reducing the extent of its interference, this approach for CW interference with strong resistance. 3) using the scan frequency Chirps for carrier this method usually used is similar to the Ethernet network, it uses CSMA a series of short, can be synchronized scan frequency chirps as carrier, each 100 us chirps General continued, it represents the most basic communication symbol time (UST). These chirps cover 100-400 KHz frequency bands, and always to 200-400 Khz frequency, then to 100-200 KHz frequency. As a result of the linear scan chirps signal bandwidth than signal bandwidth to a much larger, its linear acceleration is relatively high, while the CW interference frequency acceleration is generally stable, so long as the filter is designed to only a specific angular acceleration signals, CW interference can be excluded. 　　In addition, this kind of chirps waveform also has very strong autocorrelation, this kind of fuzzy logic in all of the connections in the decision a device on the network, you can also identify any device from the Internet of this unique waveform, and does not need to send and receive synchronization between devices. Power line digital spread spectrum technology can make full use of the transmission band, broadband, high-speed data transfer. 　　Spread spectrum communications can overcome the narrow-band noise and multipath effects, therefore very suitable power line communications environment. SST technology easier to implement, automatically select high SNR band against transient disturbances; but the code interference between serious and requires non-linear equalizer.
2, orthogonal frequency division multiplexing (OFDM) orthogonal frequency division multiplexing using multi-channel narrowband orthogonal subcarrier, transmission of multi-ports data, each signal code element for a longer time, you can avoid interference between code element. 　　Select available through dynamic subchannel, this technology can reduce the frequency of narrow-band interference and Valley point. OFDM technology can be traced back to the century 6Ten years, mainly for military HF communication system. However, an OFDM system structure is very complex, thus limiting its further promotion. Until the 1970s, people made use of discrete Fourier transform to implement multiple carrier modulation to implement complex software method for OFDM, simplified system architecture, which makes it more practical use OFDM technology. 　　In recent years, as digital signal processing (DSP) technology for the rapid development of OFDM as a waveform can be effective against interference between high-speed transmission technology has been widely used in civil telecommunications system. OFDM technology has been applied to a high-speed MODEM and wireless FM channels of broadband data transmission. Fourth-generation mobile communications (4G) will use OFDM technology that allows data transfer rate can reach 10Mbit/s, wireless LAN also have adopted the technology. 　　Is in the preparation of digital terrestrial television and the wave is on the development of high speed wireless LAN "IEEE 802.11a" is scheduled to adopt this new technology. Orthogonal frequency division multiplexing technology can improve the quality of the power line network transmissions, even in the distribution of serious disruptions, OFDM also provides the high bandwidth and bandwidth transfer efficiency, and the appropriate error correction technology to ensure reliable data transmission. In OFDM systems each child channel carrier mutually orthogonal, so their spectrum is overlapping, which not only reduces the Intercarrier interference and improve spectrum utilization, you can resist the constant-amplitude wave interference. But OFDM recipient machine complex, costly, and request the delivery of a large dynamic range, linear amplification on transient disturbance sensitive. Three, compared with other access technology, Powerline broadband access network has the following advantages: 1) to take full advantage of existing low-voltage distribution network infrastructure without any cabling, is a kind of "No New Wires" technology, saving resources. 　　No need to dig trenches and pass through the holes and avoiding on buildings and public facilities, while also saving manpower. 　　2) can provide users with high-speed Internet access services, voice services, thus contributing to the user on the Internet and phone adds new options, is conducive to other telecommunications service providers improve service, lower prices. 　　3) provides support for home networking so that people can enjoy by PLC technology brings home audio and video network, more people fighting games, and other entertainment. 　　4) is the main force, home automation throughout all rooms through to the wall outlet, the intelligent home appliances networking early to enjoy the digital family comfort and convenience. 5) using a permanent online connection PLC, construction of fire, theft, anti toxic gas leakage, etc. of security monitoring system for Office workers of mind; construction of medical emergency system, with the elderly, children and patients ' families feel more at ease. 　　Using PLC can provide independent digital communities services and e-business, Home Office and remote appliance control. 6) remote automatic readout of water, electricity, gas table data, the utility companies save significant costs and convenience to users.