Download Report 3: Power line Communications: State of the Art and Future

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.