Broadband Over Power Line (BPL)
The second major issue is signal strength and operating frequency. The system
is expected to use frequencies in the 10 to 30 MHz range, which has been used
for decades by amateur radio operators, as well as international shortwave
broadcasters and a variety of communications systems (military, aeronautical,
etc.). Power lines are unshielded and will act as antennas for the signals they
carry, and have the potential to completely wipe out the usefulness of the 10 to
30 MHz range for shortwave communications purposes.
Modern BPL systems use OFDM modulation which allows the mitigation of
interference with radio services by removing specific frequencies used. A 2001
joint study by the ARRL and HomePlug powerline alliance showed that modems using
this technique "in general that with moderate separation of the antenna from the
structure containing the HomePlug signal that interference was barely
perceptible" and interference only happened when the "antenna was physically
close to the power lines".
Much higher speed transmissions using microwave frequencies transmitted via a
newly discovered surface wave propagation mechanism called E-Line have been
demonstrated using only a single power line conductor. These systems have shown
the potential for symmetric and full duplex communication well in excess of 1
Gbit/s in each direction. Multiple WiFi channels with simultaneous analog
television in the 2.4 and 5.3 GHz unlicensed bands have been demonstrated
operating over a single medium voltage line. Furthermore, because it can operate
anywhere in the 100 MHz - 10 GHz region, this technology can completely avoid
the interference issues associated with utilizing shared spectrum while offering
the greater flexibility for modulation and protocols found for any other type of
microwave system.
