18-09-2012, 05:26 PM
Broadband Communications over Power Lines(PLC)
Broadband Communications.ppt (Size: 329.5 KB / Downloads: 39)
What is PLC?
PLC (also known as BPL) is a technique for sending high speed data through the medium voltage power distribution network
The idea of sending data through the power distribution network is not new
Utilities use LF for network control and telemetry (typically < 0.200 MHz)
Schools have used carrier current system for “campus radio” systems that operate in the AM band (0.530 – 1.700 MHz)
What makes BPL different is that it uses frequencies between 2 and 80 MHz
Types of PLC
There are three major categories of PLC:
Access PLC uses electrical distribution lines, overhead or underground, to provide broadband Internet access to homes and businesses.
This is currently in the developmental stage
In-building PLC uses the electrical wiring within a building to network computers.
Most systems use the HomePlug standard which provides protection for amateur frequencies
Control PLC operates below 500 kHz, and is used by electric-utility companies to control their equipment using the power-lines as transmission lines.
These systems are not a problem, although it is interesting to note that utilities successfully lobbied against an amateur VLF allocation, claiming that amateur transmissions could disrupt their communications.
PLC Modulation Techniques
Access PLC
Access PLC systems are presently under development. Current systems use OFDM (orthogonal frequency division multiplexing) in combination with some type of PSK .
Carrier frequencies can be between 2 and 30 MHz.
No filtering provisions are made to protect other users of the HF/VHF spectrum
Data throughput > 10 Mb/sec
HomePlug
HomePlug uses a combination of OFDM (orthogonal frequency division multiplexing) and DQPSK (differential quadrature phase shift keying) to send data through power lines within the home.
Carrier frequencies are between 4.5 and 21 MHz.
The modem output has notches at 3.5, 7.0, 10.1, 14.0, and 18.1 MHz to reduce interference to amateurs.
Data throughput > 10 Mb/sec
Software in the receiver continuously determines the transfer function of the medium and carrier frequencies are changed to use the best available spectrum.
HomePlug signal power is spread over the entire 4.5 – 21 MHz bandwidth as the carrier frequencies are changed.
HomePlug signals can pass through a residential service entry panel with less than 10 dB loss.
RFI from Access Line PLC
Tests involving PLC systems in Japan and Europe indicate that PLC can cause significant interference to other users of the HF spectrum.
Medium voltage power distribution lines look very much like antennas at HF wavelengths.
The next slide shows the far-field pattern of a 328 ft (100 m) segment of a typical MV distibution line. The conductors are at a height of 10m (32 ft 10 in) and they are spaced 1.5 m (4ft 5in) apart. The load is assumed to be the primary of a neighborhood transformer, which is assumed to have an impedance of 10 – j25 ohms.
RFI from HomePlug PLC
HomePlug uses a spectral mask that provides 30 dB of filtering in the amateur HF bands.
At current RF brightness levels of –80 dBm/Hz, HomePlug signals can cause interference to antennas (especially indoor) that run within 2 – 3 m of household power lines.
HomePlug signals can also leak into other households with < 10 dB of attenuation, causing problems for all homeowners sharing a common transformer.