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EMI problem


Electromagnetic interference (EMI) is a problem for all wireless engineers. If you aren’t generating it and having to control it, then you’re trying to overcome the EMI from other sources. As a wireless engineer, your eyes have probably gone bad reading the fine print in your dog-eared copy of Part 15 of the Federal Communications Commission rules and regs, and you have sweated out the certification of your wireless designs by outside testing houses. You also have grudgingly learned that while software is the more challenging part of a wireless design, the testing and EMI reduction eat up the majority of the design time and effort.

Just when you think you’ve learned how to get your EMI under control, you’re going to have to worry about a whole new source of EMI. It isn’t simply a radiating local oscillator or harmonics from a switching power amp, but an external source that you have no control over. Welcome to BPL.

Broadband Over Power-Line (BPL)
BPL, also known as power-line communications (PLC), provides broadband Internet connections over the installed base of ac power lines. Your electrical power utility becomes your ISP. The infrastructure is already there, with jillions of miles of ac power lines. Just modulate the data onto one or more RF carriers, and let the signal ride on top of the 60-Hz wave. It seems to make sense because that technique has been used for years for in-home control (remember X10?) and currently for home networking.

It also provides broadband services to areas that lack cable TV or DSL lines. Believe it or not, more than a few places are still like that in the U.S. I can drive 30 miles outside of my Austin, Texas home and easily find an area where dial-up is as good as it gets. Furthermore, BPL theoretically should be cheaper since little or no infrastructure investment is needed and the technology is already on the shelf. Finally, BPL could bring broadband to the masses in developing countries. What’s not to like?

The main problem is that the RF signals radiate freely from the power lines. Most BPL uses orthogonal frequency-division multiplexing (OFDM) with multiple carriers in the 2- to 50-MHz range. These RF carriers are low-level, but with the power lines ultimately being the effective antenna, they radiate all over the place. Any radio service in the HF and VHF bands is subject to massive EMI. If you’re within a few hundred feet of a power line carrying BPL, what will appear as a very high noise floor will effectively wipe out your low-signal reception. That interference can be detrimental to signals up to several miles away, too.

We don’t have radiation problems from other OFDM services, like DSL, on our local loop telephone lines because the twisted pair is… well… twisted. Therefore, it provides heavy signal cancellation. Besides, lots of it is buried. There’s no radiation from cable TV connections either because coax is inherently self-shielding—but not power lines, which are right out there like a 1920s transatlantic antenna array.

You’re probably asking why this is such a big deal. Who uses those HF bands anyway? More people use HF than you think. The two biggest groups affected are amateur radio operators and shortwave listeners (SWL). Hams and SWLs with an antenna near a BPL power line have no hope of engaging in their hobby. Both of these services use highly sensitive low-noise communications receivers that are effectively rendered useless by a BPL signal. The EMI is at what hams call the S9+ level, meaning about 10 to 20 dB greater than the usual ambient noise level.

I spoke in depth with Ed Hare, the laboratory manager of the American Radio Relay League (ARRL), about BPL. The ARRL, the national association for hams, has taken the lead in complaining to the FCC about BPL and attempting to get the FCC to back off or solve the problem. It’s making progress, but the solutions aren’t there yet. According to FCC rules, the utilities companies planning to offer broadband services via BPL are responsible for fixing any EMI problems and dealing with local EMI complaints. Good luck with that.

I don’t recall when I have been so ambivalent about a broadband technology. BPL is logical and has its benefits. But with such a major EMI problem, I just cannot get behind it. I am a ham and SWL, and it would tick me off if I couldn’t participate in my hobby in which I have invested thousands of dollars in transceivers and high-tech radios over the years. Besides, my guess is that there are more hams and SWLs in the U.S. than there are BPL subscribers. Who is the majority here? This is not to say that BPL systems won’t proliferate and add many subscribers. But hopefully they would only after solving their EMI nightmare.





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