- Buyers Guide
ISM vs. Spread Spectrum -- Avoiding the FCC
Fletcher Heald & Hildreth PLC
The Federal Communications Commission (FCC) recognizes two types of devices that intentionally emit RF energy: communications equipment and everything else. The first category includes AM/FM, TV, point-to-point, satellite, cell phones, taxi radios, CB, garage door openers and anything else that uses radio waves to convey intelligence, broadly construed. The everything-else category is properly called industrial, scientific and medical (ISM) equipment. It includes devices used for heating, ionization of gases, mechanical vibration, particle acceleration, hair removal and any other noncommunications application of radio waves. Household examples include microwave ovens, jewelry cleaners and ultrasonic humidifiers. Industrial applications cover a wide range of cooking, heating and ionization devices.
CONTENDING FOR SPECTRUM
Both ISM and radio communications require spectrum, and both obtain it from the FCC. For decades after the FCC opened in 1935, enough bandwidth was available to give exclusive allocations to all who wanted them. Isolating ISM into bands of its own made particular sense, because some ISM equipment generates high power radio noise that otherwise threatens sensitive communications equipment.
As communications technologies evolved toward higher frequencies, so too did ISM. Today, 11 distinct ISM bands range from 7 MHz to 245 GHz. ISM equipment can legally operate at any power level in these bands. Indeed, ISM is the only FCC-regulated service having no in-band limits whatsoever (other than RF safety limits). ISM equipment is also permitted to operate outside the designated bands; however, there its RF emissions are limited to relatively low levels.
The demand for spectrum has accelerated, and now far exceeds the supply. During the 1980s, the FCC allocated the last available frequencies below 1 GHz, and since then has filled the gaps up to approximately 40 GHz. Many applications prefer the 0.5 to 5 GHz range, where antenna size, propagation and cost factors are favorable for communications. When demand continued to mount after all of those frequencies were assigned, the FCC had little choice but to require that services share their spectrum allocations.
One form of sharing has been commonplace for decades. The FCC's Part 15 rules have long allowed unlicensed communications transmitters to operate at low power throughout the spectrum, except for certain restricted bands. Early examples included garage door openers and remote-control toys and, later, cordless telephones. Traditional Part 15 devices operate at a few microwatts, with a useful range of only a few meters.
The main benefit of Part 15 operation is flexibility. Where licensed users must usually file with the FCC to move or install a transmitter, a Part 15 user simply powers up the equipment. But the flexibility comes at a price. A Part 15 device must accept incoming interference, no matter how severe, from any other service. Moreover, a Part 15 device must shut down if it causes harmful interference to almost any other service. These rules discouraged applications that require any significant reliability from using Part 15. Licensing was slow and inconvenient, but it bought protection from interference.
SPREAD SPECTRUM ARRIVES
In 1985, the FCC approved spread spectrum modulation under Part 15. A spread spectrum transmitter distributes its signal over more spectrum than it otherwise would need to carry the data payload. Because the spread-out signal puts low average energy onto any one frequency, it causes little interference to other users. In addition, because the receiver is active over a wide bandwidth, it is relatively immune to interference from a conventional narrowband transmitter.
The FCC authorized spread spectrum at the unprecedented (for Part 15) output power of 1 W and put it in the 915, 2450 and 5800 MHz ISM bands. These three bands, sometimes called the "junk bands," would be crowded even without spread spectrum. They are heavily used by ISM, including tens of millions of microwave ovens and industrial devices. All three bands are allocated for amateur use and for conventional Part 15 operations at milliwatt levels. A vehicle location service operates throughout the 915 MHz band. Various parts of the 2450 MHz band are used for local positioning, private land mobile radio and downlinks in the Mobile Satellite Service. The US government has significant radar and military operations in all three bands.
Spread spectrum radio was slow to catch on. The FCC revised the rules in 1990 and again in 1997, each time opening new options for manufacturers. Prices gradually came down, while data rates went up to 2 Mbps. The mid-1990s saw a developing market in niche applications. The picture changed suddenly in 1998, when an FCC rule interpretation pushed data speeds to 11 Mbps. Overnight, spread spectrum became competitive with wire-in-the-wall office local area networks.
Today, approximately $1.5 B worth of spread spectrum wireless equipment operates in every sector of the economy, including retail, transportation, utilities, manufacturing, health care and finance. Half the transactions on the New York Stock Exchange are mediated by spread spectrum wireless terminals. Spread spectrum Internet access offers broadband speeds across 40 km distances. In consumer markets, spread spectrum cordless phones are commonplace, and in-home video and audio distribution systems will soon follow. The Bluetooth spread spectrum protocol will be incorporated into most wireless phones, laptops, computer peripherals, personal digital assistants and other consumer devices beginning later this year.
SPREAD SPECTRUM VS. ISM
Although relegated to last priority in the band, spread spectrum designers achieve respectable levels of reliability. Some of the applications listed above demand it. Hospitals, stock exchanges and wireless phone companies (among others) are sophisticated users who do not knowingly risk service interruptions.
On the other hand, no one can harden a communications receiver against unlimited interference. Equipment designers necessarily make assumptions about the RF environment in which their products will operate. Not unreasonably, Part 15 designers assume an environment populated with devices that comply with the FCC rules. Spread spectrum receivers today are reasonably well insulated against interference from other communications equipment and from existing categories of ISM devices, such as microwave ovens. But they remain potentially vulnerable to interference from new and different kinds of ISM equipment, such as RF lights.
In principle, ISM's priority in the band protects it absolutely against Part 15. However, in practice, the ISM industry has expressed two fears about the growth of Part 15 spread spectrum. First, some are concerned that spread spectrum may acquire enough economic and political power to impose limitations on ISM. Second, some ISM vendors worry that customers may reject ISM products that interfere with spread spectrum operation.
Both concerns are largely hypothetical so far. The only ISM product to date that raises any serious concern among Part 15 interests is RF lighting. Spread spectrum interests fear these lights will operate continuously from positions overhead and emit high levels of RF. Therefore, some manufacturers have asked the FCC to limit emissions from RF lamps. Moreover, one lighting manufacturer has told the FCC that potential customers are avoiding the product because they fear interference into their spread spectrum equipment. Other new ISM applications may raise some of the same issues.
Both ISM and Part 15 will benefit if they can resolve the technical incompatibilities and competitive issues between them. However, they should do it in private, away from the FCC. Part 15 has little to gain at the FCC, whose rules make it unambiguously subordinate to ISM. No one expects the FCC to regulate any ISM product out of existence, not even RF lights. In addition, the FCC will not listen to Part 15 complaints about interference from existing ISM products that comply with the rules. At most, the FCC might limit new ISM products that pose exceptional interference threats but, even then, it will try to ensure that the products remain commercially viable.
The FCC also will not address ISM's concerns. If customers come to believe they cannot use both ISM and spread spectrum, the FCC will not stop them from choosing spread spectrum. Although Part 15 devices must accept ISM interference, Part 15 customers do not. They are free to eliminate the interference by rejecting ISM products if they think it necessary to protect their communications.
Instead of contending at the FCC, both industries will achieve better results by negotiating directly with each other. Together they can seek standards that permit ISM products to operate properly, as well as provide an RF environment hospitable to spread spectrum operations. If communications engineers can help to set the characteristics of ISM equipment in the vicinity, they can design systems that tolerate the interference well. That in turn will enable ISM vendors to assure customers that their products will not degrade spread spectrum performance. The spread spectrum industry can assist those ISM marketing efforts by certifying suitable ISM devices for compatibility with spread spectrum communications devices.
In short, customers should not be forced to choose between the two types of products. Both industries come out ahead if customers can buy and use both successfully. *
Mitchell Lazarus received his BSEE and MSEE from McGill University and MIT, respectively, and his PhD in experimental psychology from MIT. He also holds a law degree from Georgetown University Law Center. Currently, Lazarus is a telecommunications attorney with the firm of Fletcher Heald & Hildreth PLC in Arlington, VA. He can be reached at (703) 812-0440 or MLazarus@alum.MIT.edu.