Power lines are formed using a variety of conductors, joined together almost at random. At high frequencies (1 MHz to 30 MHz), the powerline channels are characterized by variable attenuation with frequency as a result of physical attenuation and delay spread (multipath) due to impedance mismatches. Figure 1 shows an example of powerline channel frequency response. While a typical channel may present an average attenuation of approximately 40 dB, it is not uncommon for portions of the bands to experience attenuation greater than 60 dB. Similarly, while most in-home powerline channels have a delay spread of 1 µs to 2 µs, some channels may exhibit a delay spread of larger than 5 µs.

The major sources of noise on the powerline are from electrical appliances, which generate noise components that extend into the high-frequency spectrum. One of the unique characteristics of this man-made noise is its cyclic variation with respect to the ac line cycle. Electric appliances may turn on and off, and/or draw electric power as a function of the ac line cycle. During this process, they generate noise that also changes with the ac line cycle. Typically, the least amount of noise is present at the ac line cycle zero crossing, while the ac line cycle peaks experiences the highest amount of noise. Impulse noise is common over powerlines. Furthermore, the location of impulse noise is generally tied to the underlying ac line cycle. Figure 2a-2c shows periodic impulse noise from halogen lamps, yard light and dimmer switches, as compared with the PLC data signal (the small rectangular shapes) and the ac line cycle. Devices that contain brush motors (Figure 1d) produce impulse noise of a random nature. Because much of the noise experienced by each node may be highly localized due to attenuation, powerline channels typically are not symmetric. In addition to noise from appliances, induced radio frequency (RF) signals lso impair certain frequency bands.

Apart from dealing with the harsh channel conditions, PLC systems must operate with regulatory constraints on the frequency band to use and the maximum transmit power. For example, within the United States, PLC systems operate under FCC Part 15 rules using the frequency band between 1.8 MHz to 30 MHz, at a maximum power spectral density (PSD) of -50 dBm/Hz. Several sub-bands within this range have to be notched out to prevent interference with licensed services. Moreover, the regulatory environment is in flux: aeronautical bands may be added in the United States, power levels are under debate in Europe, and Japan is considering introduction of regulation that would allow PLC. This unstable international regulatory environment requires that PLC systems be flexible to adapt with changing regulations.

The HomePlug AV specification covers the lower two layers of the International Standards Organization's (ISO) Open System Interconnect (OSI) model. The following sections provide a brief overview of the physical layer (PHY) and medium-access control (MAC) layer of HomePlug AV.

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