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Power line communication (PLC)
Saturday, 20 January 2007
Power line communication (PLC) is a technique transmitting voice or data at a rapid speed through a power line in a house, an office, a building, and a factory, etc. Power-line communication systems have become increasingly viewed employed as an attractive alternative to conventional hard-wired communication systems, which require dedicated communication wiring, and wireless systems, which involve complex and costly transmitter and receiver circuits. In power line communication, the existing alternating current (AC) power wires serve as a transmission medium by which information is relayed from a transmitter or control station to one or more receivers or loads connected downstream from an AC source. There is a significant advantage to the use of alternating current (AC) power lines for purpose of communication between electronic devices using a power line communication system. Since no new wires are required to implement the function of communication, power-line communication systems greatly reduce the complexity and effort of installation, particularly in building retrofit applications in which it is highly desirable to be able to install an energy control system with little or no alteration of the existing electrical wiring. The powerline communication technology utilizes a power line as a signal transmission channel. This technology makes it possible to perform communication between a plurality of communication devices by connecting each of those devices to a receptacle installed in each room in a house. Use of existing power lines as a communication medium eliminates installation costs for adding dedicated communication wiring to existing structures. Such a power line communication system can be used for monitoring and controlling basic function including energy management, security, and safety control in applications including homes, factories, offices, automobiles and aircraft. Electric utility companies utilize power line communication systems to provide a means for a central station to communicate command signals to remote receivers which are located at the sites of electric energy consumers.

The infrastructure for providing broadband Internet access is presently insufficient to meet demand. With some modification, the infrastructure of the existing power distribution systems can be used to provide data communication in addition to power delivery, thereby forming a power line communication (PLC) system. Development of a power line communication system would therefore provide more users with high-speed telecommunications access. The PLC communication may be applied to various fields such as superhigh-speed Internet communication, Internet phone, home networking system, home automation and remote metering system just by connecting a plug of a computer to a power source without using LAN or a telephone. The usual power distribution system in a building is a system of wires, electric outlet receptacles, fuses and/or circuit breakers, switches and controls and permanently wired fixtures and appliances, installed for the purpose of distributing low frequency AC power in the building. In the PLC system, a radio frequency communication signal of a few hundreds Hz to a few tens MHz is transmitted together with an alternating power to a power line supplying the alternating power having frequencies of 50 to 60 Hz to houses, and a private access device receives only the communication signal for communicating. A power line communication system typically operates by superimposing a modulated carrier frequency on the AC signal carried on a power line. A basic PLC system consists of a transmitter unit capable of adding the communication signal to the AC power line signal and a receiver unit capable of separating the communication signal from the AC power component signal. In a PLC system, a transmitter generates modulated RF carrier signals at one location in the building, which are coupled to the existing power distribution system via an appropriate coupling network and a receiver at another location receives and demodulates the RF carrier providing the desired transmission of voice and/or data signals from the one location to the other. A power line communication modem is a device that makes it possible to use these power lines as a communication line. The PLC modem converts digital data from an information processing unit such as a personal computer to analog line data, overlaps the analog line data with a power line of a commercial power source, or converts the analog line data inputted through the power line to digital signals and transfers them to the information processing unit. Power line modems have a wide range of uses, including allowing personal computers to communicate with each other, or with other household devices, without the need for separate data cables. A PLC communication system comprises a plurality of communication stations connected to a single communication network. Each of the communication stations comprises a PLC for exchanging rapid data through the single communication network.

Power transmission lines are ideal for communication as they extend from the power generation site to the remote sites without intervening obstacles. Use of the distribution power lines, however, has been slower to develop, as the distribution power lines are susceptible to electrical noise and interference, such as voltage spikes and Gaussian white and colored noise, and the need to avoid introducing radio-frequency (RF) interference into the lines. Specifically, the equipment connected to the indoor power line can generate noise or lowers an impedance of the power line, which can result in an increase in the error rate to cause communication failures in the power line communication. Furthermore, power lines inherently attenuate RF signals heavily, and are not designed to support efficient transfer of higher frequency signals. Power line communication is typically performed by using normal mode signals. Therefore, in the power line communication, communication failures may occur from normal mode noise in particular among the noises generated by the electrical equipment. The increasing desirability of being able to selectively and remotely control electrical loads on the distribution system, and the availability of low cost encoders for automatic meter reading, have produced a flurry of activity in the use of distribution power lines for communication purposes. When serious attenuation or distortion is generated in the PLC channel due to a load having a poor channel property, the communication quality is considerably deteriorated. A number of different techniques are known for mitigating the problems caused by communicating data over power lines. These techniques include error correction, differential and adaptive shift code keying (DCSK and ACSK), and direct sequence spread sequence transmission.