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Report 3: Power line Communications: State of the Art and Future Trends Niovi Pavlidou, IEEE Communications, April 2003. pp34-39 Electrical distribution circuits constitute a universal wiring system, but they were not built for communication purposes. Therefore, the transmission capability is restricted resulting to varying impedance, considerable noise and high attenuation; the channel is even considered to be worse than that of mobile communications. The characteristics of the channel can be both time and frequency dependent. The impedance is highly varying with frequency and ranges between a few ohms and a few kilo-ohms with peaks at some frequencies where the network behaves like a parallel resonant circuit. Some indoor and outdoor characteristic of attenuation have been reported in the literature. Measurement at voltage 0.35V on in-house lines resulted in about 15dB attenuation. The noise curve is very much dependent of location and time and is generated by many sources: motors, radio signals, and power supply. Generally, channel noise varies strongly with frequency, load, time of the day, and geographical location. Noise sources close to the receiver have the greatest effect on the received noise due to the fact that noise as well as signal is subject to attenuation. The approaches to modeling the transfer characteristics of power lines are classified in two categories: - the hardware approach (based on impedance of cables and network topology) - the communication approach (the channel is modeled by its attenuation, phase shift or noise sources, …) This model permits to use classical communication technologies and tools to estimate expected performance. Communications techniques: Different modulation techniques have been proposed such as frequency shift keying, code-division multiple access, and orthogonal frequency-division multiplexing. Each of these presents certain advantages depending on the target application. However, the use of spread spectrum (SS) techniques completely changed the electric power distribution networks. A combination of M-ary FSK modulation and coding can provide a constant envelope modulation signal, frequency spreading to avoid bad parts of the frequency spectrum, and time spreading to facilitate correction of frequency disturbances and impulse noise simultaneously. Most medium access control techniques (fixed access, dynamic protocols …) are candidates for Power Lines Communication. Polling and Aloha are among the most studied protocols for medium access. For errors, application of automatic repeat request and hybrid schemes can avoid the influence of short term disturbances while dynamic strategies for capacity allocation can successfully confront long-term disturbances. Two services that are necessary for power line networks are confidentiality and identity authentication due to their open insecure bus structure. However, the most crucial issues remain the treatment of emissions of high-frequency signals through the line and the standardization procedures.