Feature

Powerline communication: wireless technology

Using ac power lines to transmit data eliminates the cost of installing special wiring between distributed system elements. Still, powerline communication doesn

By Dan Strassberg, Senior Technical Editor -- EDN, 6/6/1996

New technology is slowly increasing the speeds at which powerline communication works reliably. Although several experts forecast 1-Mbps data rates before the end of the decade, vendors differ on how to achieve reliable performance at favorable cost even at lower speeds. Although companies with limited powerline experience make optimistic claims about potential speeds, designers of systems that might communicate via power lines should remain wary of approaches that have not been thoroughly tested. A vendor's demonstration may be impressive, but you should feel confident of a technology's robustness only after the system has performed well for extended periods in a variety of hostile environments.  

The allure of data transmission via ac power lines is strong; power lines are almost everywhere in the industrialized world. Unfortunately, power lines are unfriendly to data. All manner of loads from switching power supplies to lamp dimmers and universal motors inject significant noise. Powerline parameters that designers of more conventional data-transmission systems expect to be stable are anything but. Characteristic impedance (Z₀) and attenuation vs frequency can change dramatically moment to moment as various loads connect to and disconnect from the line.

If you try to transmit information over power lines for distances of miles, the power company's distribution transformers are likely to present major obstacles. Although these transformers pass ac power at 50 or 60 Hz with almost 100% efficiency, they seriously attenuate higher frequency carriers that data-transmission schemes of even modest bandwidth must use. To meet their own communications needs, power companies sometimes bridge or bypass these transformers with special high-voltage networks, and some newer schemes can recover heavily attenuated high-frequency signals from secondaries of unbypassed transformers. But, by and large, only power companies have access to such technology.

Two design philosophies

There are two basic philosophies of reliably (that is, with acceptable error rates) transmitting data over ac lines:

• Accept the poor quality of the medium. Design a system that minimizes the most common errors. Make the system recover quickly and gracefully from dropouts, so that users never notice transmission problems.
• Look for a "magic bullet"— means of getting signals across the power line unscathed, despite the hostility of the medium. Even proponents of the second philosophy acknowledge that error correction is essential to achieving acceptable error rates.

Adaptive Networks and Intellon have devoted years of R&D to schemes using the first philosophy. Both companies have repeatedly refined their technology in successive generations of products. Novell is adopting the second philosophy. Echelon, which has experience with both philosophies, prefers the second one in places where electromagnetic-compatibility (EMC) regulations permit its application. Elsewhere (in Japan and parts of Europe, for example), Echelon's LonWorks automation systems use powerline- communication products that use the first philosophy.

Both Adaptive Networks and Intellon use spread-spectrum modulation (Reference 1), but their implementations differ. Intellon uses chirp spread spectrum at frequencies from 100 to 400 kHz. The company's technology forms the basis for powerline and RF communication in the Electronic Industries Association's Consumer Electronics Bus (CEBus) home-automation standard (Reference 2). Adaptive uses what it calls, "powerline-optimized direct sequence" in approximately the same frequency range as does Intellon.

In North America, both Echelon and Novell use narrowband carriers. Both companies suggest that a carefully selected carrier frequency is a key element of their systems' reliability.

Less than 500 kHz

 Traditional powerline-communication systems (both narrowband and spread-spectrum) use carrier frequencies below the medium-wave (MW) broadcast band (that is, below ;500 kHz). The oldest commercial powerline-communication system, X-10 USA's X-10 home-control system (also sold under several other brand names) uses an amplitude-modulated, 120-kHz carrier. Although Echelon and Novell don't disclose their narrowband-system frequencies, the frequencies the companies use appear to be higher than those of more established systems.

From this choice of frequencies, you can infer that the companies' research has shown that somewhere above the MW band, an area of the spectrum is relatively free of powerline noise. This frequency band must be low enough to keep the power line from acting as a good antenna. If the power line radiates significant RF energy, the schemes run afoul of EMC regulations. EMC regulations have kept Echelon from using narrowband powerline communication in some parts of the world. Moreover, if the power line acts as a good antenna, signals remaining on the power line only a short distance from the source can be so small that amplification to a usable level is prohibitively expensive.

A newcomer to the field, Elcom Technologies, has designed four series of powerline-communication modules for use in homes. The products are just coming to market at retail prices of $99 to $299. Because each type of Elcom module uses a different frequency band, all types can work together in the same home at the same time.

One series, which routes audio from a stereo system to loudspeakers in different rooms, uses FM at 3.58 or 4.5 MHz. A second series, which distributes TV signals, operates on TV channels 3 and 4 (60 to 72 MHz). A third type allows you to use conventional wired telephones and modems in rooms without phone jacks. The telephone modules use FM at frequencies from 5.5 to 6.5 MHz. The fourth series, which uses a proprietary modulation scheme operating from 120 to 450 kHz, lets you use your home's ac wiring as the transmission medium of a LAN. This product can, for example, send signals from a computer in one room to a printer in another.

Elements of a reliable system

Carrier frequencies and modulation techniques are just part of what makes up a reliable powerline-communication system. Adaptive Networks, though close-mouthed about certain aspects of its technology, does confide that among the key elements that allow it to achieve error rates of 10^-9 are:

• adaptive line equalization that continuously monitors and corrects for the line characteristics,
• a token-passing protocol,
• a powerline-optimized version of direct-sequence spread-spectrum modulation that enables the receiver to rapidly synchronize with the incoming signal,
• a small packet, which is practical because of the rapid synchronization, and
• forward error correction.

Keeping the packets small increases the probability of squeezing packets between noise bursts, which, if they occur during a packet, corrupt the data. (Here, noise burst refers not just to noise voltages on the line, but also to sudden drops in line impedance that attenuate signals for brief periods.) To make the small packet work, Adaptive keeps the overhead in packet headers to a minimum. Nevertheless, despite the short headers, each of Adaptive's networks can comprise over 65,000 nodes.

Finally, by correcting errors as they occur, forward error correction reduces the number of packets requiring retransmission because the receiving node finds them to be corrupted. In Adaptive's case, the redundancy that forward error correction introduces requires transmitting faster than 250 kbps to obtain a useful data rate of 100 kbps. Nevertheless, the overall effect on throughput is positive. Without forward error correction, the noisy powerline environment would force retransmission of so many packets that the useful data rate would be substantially lower than 100 kbps.

According to Intellon, one technique that is a key to the reliability of its spread-spectrum system is a very wide dynamic range. Early versions of Intellon's system encountered problems when narrowband noise sources caused amplifiers to saturate, blocking data. About a year ago, Intellon introduced new versions of its ICs that are backward-compatible with its older chips but are much less susceptible to such problems.

Home automation

 Home automation is one area that has embraced powerline communication. Al-though the low cost of a "wireless" approach is im-portant in homes, avoiding the mess and disruption of installing special wiring matters, too. In industrial and commercial applications, the power line's main attractions are low installation and hardware cost and quick network setup. Another attraction is freedom from problems that plague RF systems in some locations. Walls and partitions attenuate high-frequency RF signals. Filing cabinets and other metal objects act as reflectors. So, an RF system that at first appears easy to set up can prove troublesome.

Outside the home, products in which powerline communication has enjoyed success include hospital patient-data-management systems, point-of-sale terminals, and vending machines. Networked vending ma-chines automatically report malfunctions and continually update inventory databases, letting the machine operator minimize inventory and avoid unnecessary trips for restocking. Few vending machines are near telephone lines, however, so, without powerline communication, networking a vending machine usually requires installing a phone line. Of course, the machine must also incorporate a telephone modem.

Including powerline-communication functions in products such as point-of-sale terminals and vending machines requires designing in embedded communications subsystems. Both Adaptive Networks and Intellon offer ICs that designers of high-volume products can mount on boards of their own design.

Adaptive's three-chip AN1000CS 100-kbps chip set costs less than $30 in OEM quantities. The set includes an ASIC built around a 65C02 µP core. Some of Adaptive's customers have ported their main application code to the 65C02, which has processor cycles to spare. By letting this core processor act as the system controller, these customers save the cost of a separate µC. Among Intellon's ICs is the SSC P400, which will be available this summer. The 32-pin PLCC, which costs less than $10 in OEM quantities, includes a network-interface controller and a powerline transceiver.

Intellon and Adaptive also offer board- and module-level units that are useful in low-volume products and in prototypes of higher volume products. Adaptive's board-level AN1000 modules cost $135 (1000). Intellon's CCM PLMON1 (for 120V lines) and CCM PLMON2 (for 240V lines) are powerline-communication modules that include powerline modems. You can use the $439 modules with higher level products, or you can integrate the modules into such products.

Finally, for development work, Adaptive offers pairs of fully encased RS-232C-interfaced powerline modems bundled with software for $3200. Intellon offers a choice of CEBus design stations, including units based on the 68HC11 and 8051 µCs. With either processor, there are units for 120 and 240V power lines. All types cost $8995.

Powerline communication does not apply only to consumer and commercial products. Electric-power utilities use similar technology. Power companies have used very low-speed (hundreds of bits-per-second) powerline communication along high-voltage transmission lines for several decades. Now, fiber-optic communication offers much higher bandwidth. When a power company installs a fiber link along with copper or aluminum power conductors, the added cost is negligible. Also, the fiber's electrical insulation properties, diminish the need to pass the signals through a very high-voltage isolation barrier.

On medium-voltage (;20- to 100-kV) lines between substations and local distribution transformers, the situation is different. Power companies need to retrofit circuits with communication capabilities that offer greater bandwidth than that currently available. The greater bandwidth is useful for such functions as remote reading of meters and demand-side management (switching loads such as air conditioners and water heaters on and off to mitigate peak demand).

Although fiber optics could work on such medium-voltage circuits, the difficulty and cost of the retrofits has encouraged the power industry to look for a different approach. The Electric Power Research Institute has developed and will soon test a powerline-communication system that should achieve 19.2-kbps data rates and pass signals through distribution transformers without the need for bridging networks. For EEs not employed by power companies, such systems are mainly of academic interest, however.

What most EEs need to know is that if they have an application that needs more bandwidth than about 100 kbps and an RF system that works acceptably, RF is usually a better choice than powerline communication; RF systems transmit more bits per second/dollar. On the other hand, if extra bandwidth isn't needed, powerline systems' lower cost saves money. Moreover, if the bandwidth is adequate but only a few network nodes need to be wireless, a powerline system can still be a good choice. Although you can't stay connected to a powerline network as you roam over thousands of feet carrying your notebook PC, a few RF or IR links integrated into what is mostly a powerline network can provide the needed connectivity.

You can reach Senior Technical Editor Dan Strassberg at (617) 558-4205, fax (617) 928-4205, e-mail ednstrassberg@cahners.com.

References

1. Strassberg, Dan, "Spread-spectrum communication rises from military roots to star in a wireless world," EDN, Dec 22, 1994, pg 59.
2. Strassberg, Dan, "Home-automation buses: Protocols really hit home," EDN, April 13, 1995, pg 69.