Broadband Over Power Lines - Application Concerns

Application Concerns

BPL may offer benefits over regular cable modem or digital subscriber line (DSL) connections: the extensive infrastructure already available appears to allow people in remote locations to access the Internet with relatively little equipment investment by the utility. Cost of running wires such as Ethernet in many buildings can be prohibitive; Relying on wireless has a number of predictable problems including security, limited maximum throughput and inability to power devices efficiently.

But variations in the physical characteristics of the electricity network and the lack of standards mean that provisioning of the service is far from being a standard, repeatable process. And, the bit rate a power line system can provide compared to cable and wireless is in question. The prospect of BPL was predicted to possibly motivate DSL and cable operators to more quickly serve rural communities.

PLC modems transmit in medium and high frequency (1.6 to 80 MHz electric carrier). The asymmetric speed in the modem is generally from 256 kbit/s to 2.7 Mbit/s. In the repeater situated in the meter room the speed is up to 45 Mbit/s and can be connected to 256 PLC modems. In the medium voltage stations, the speed from the head ends to the Internet is up to 135 Mbit/s. To connect to the Internet, utilities can use optical fiber backbone or wireless link.

Deployment of BPL has illustrated a number of fundamental challenges, the primary one being that power lines are inherently a very noisy environment. Every time a device turns on or off, it introduces a pop or click into the line. Switching power supplies often introduce noisy harmonics into the line. And unlike coaxial cable or twisted-pair, the wiring has no inherent noise rejection. The system must be designed to deal with these natural signaling disruptions and work around them. For these reasons BPL can be thought of as a compromise between wireless transmission (where likewise there is little control of the medium through which signals propagate) and wired transmission (but not requiring any new cables).

Broadband over power lines has developed faster in Europe than in the United States due to a historical difference in power system design philosophies. Power distribution uses step-down transformers to reduce the voltage for use by customers. BPL signals cannot readily pass through transformers, as their high inductance makes them act as low-pass filters, blocking high-frequency signals. So, repeaters must be attached to the transformers. In the U.S., it is common for a small transformer hung from a utility pole to service a single house or a small number of houses. In Europe, it is more common for a somewhat larger transformer to service 10 or 100 houses. This makes little difference for power distribution. But delivering BPL in a typical U.S. city requires an order of magnitude more repeaters than in a comparable European city. On the other hand, since bandwidth to the transformer is limited, this can increase the speed at which each household can connect, due to fewer people sharing the same line. One possible solution is to use BPL as the backhaul for wireless communications, for instance by hanging Wi-Fi access points or cellphone base stations on utility poles, thus allowing end-users within a certain range to connect with equipment they already have.

The second major issue is Electromagnetic Compatibility (EMC) with main parameters the signal strength and operating frequency. The system was expected to use frequencies of 10 to 30 MHz in the High Frequency (HF) range, which has been used for many decades by a variety of communications systems (military, aeronautical, amateur radio, etc.) and by international and regional shortwave broadcasters. Power lines are unshielded and will act as antennas for the signals they carry, and they will cause interference to high frequency radio communications and broadcasting. Modern BPL systems use orthogonal frequency-division multiplexing (|OFDM), which allows them to mitigate interference with specific radio services by not using specific frequencies for data transmission subcarriers. A 2001 joint study by the American Radio Relay League (ARRL) and HomePlug Powerline Alliance showed that for 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 at the notched frequencies" and interference only happened when the "antenna was physically close to the power lines" (however other frequencies still suffer from interference). What the effects of large scale deployment on PLT modems in house will do to the notching has still to be defined, however in lab tests the notches appear to fill in due to intermodulation between modems.