The FCC has proposed a "spectrum etiquette" to govern unlicensed operation in the 902-928 MHz band, and possibly in the 2.4 and 5.7 GHz bands as well. If adopted, the etiquette would amount to the biggest change in unlicensed usage since the approval of high-power spread spectrum 22 years ago.
As explained below, the proposal threatens to split unlicensed users into warring camps. If adopted as proposed, the etiquette would have its worst impact on users who operate at high power all the time, primarily the wireless Internet service providers, or WISPs. The main beneficiaries would be applications whose transmissions are of very low power and/or short duration. But the vast majority of unlicensed users fall somewhere in between. They, and the manufacturers who supply them, should carefully work through the effect of the FCC’s proposals on their own operations.
Comments are due on October 15, and reply comments on November 14. Companies that care about the outcome can have the most influence by setting out their positions in the first round of comments, and using the second round to challenge those who disagree.
An Etiquette — Who Needs It?
The term "etiquette" in this context is similar to its near-synonym "protocol," as used in telecommunications. Both words refer to pre-arranged steps for setting up a communications link, although an etiquette tends to be less interactive.
The need for an etiquette — if there is one — arises from worsening congestion in the three unlicensed bands earmarked for the relatively high power of one watt: 902-928 MHz, 2400-2483.5 MHz, and 5725-5850 MHz. The first two of these carry most of Wi-Fi, all of Bluetooth, ZigBee, other industrial controls, active RFID, meter-reading equipment, wireless Internet access, amateur radio, and various other licensed services, not to mention countless consumer goods: microwave ovens, cordless phones, nursery monitors, wireless audio gear, and much more. A typical home might have half a dozen or more of these products.
The FCC regulates unlicensed devices mostly on their overall properties, as by setting maximum power and minimum bandwidth. With exceptions, the rules are silent on the details of operation. Yet, despite the near-lawless environment and vast numbers of users, unlicensed communications work pretty well. Manufacturers usually throttle back the power to far less than the rules allow, mainly to extend battery life; but as a side benefit, this practice also limits potential interference to a small area. Another way to save the battery is by keeping transmissions short, and this likewise cuts down interference. Some unlicensed technologies, including Wi-Fi, automatically shift their communications to less crowded parts of the band, enabling multiple users to work around each others’ signals and operate successfully even when close together.
Relatively few users, such as the WISPs, exploit the maximum permitted power, and also transmit continuously. The WISPs are mostly small companies that deliver broadband Internet to homes and businesses, particularly in areas beyond the reach of DSL and cable. In the past they have received encouragement from the FCC, which favors widespread, competitive broadband services.
The WISPs generally use directional antennas that focus the transmitter power into a narrow beam, enabling them to reach distant customers. But the resulting high energy can affect other unlicensed devices in the antenna path, sometimes miles away. Similar problems can arise from Wi-Fi access points, especially the higher-powered units installed in some public outdoor areas. Being non-directional, these affect other devices in all directions, although over a smaller radius.
As WISP operations have expanded in recent years, other unlicensed users have increasingly experienced interference. Today the most popular WISP transmitters occupy 1/3 or less of the band. But nothing in the rules prevents a single user from taking up all of the frequencies across the entire band. That would effectively close off the band to others across a large area.
An unlicensed user suffering interference, regardless of its origin, has little recourse. The label on every cordless phone and Wi-Fi laptop says it all: "This device must accept any interference received, including interference that may cause undesired operation." Anyone complaining to the FCC will get a polite response, but no help.
Despite that rule, the newly proposed spectrum etiquette is meant to help protect some unlicensed users — those that interfere less — by reining in some of the applications likely to cause them harm. At the same time, of course, those measures will cause harm to whatever applications they restrict. Finding the right balance will not be easy.
Questions to Ponder
The FCC’s inquiry begins with three overarching questions. First, is an etiquette necessary? Second: if so, to what bands should it apply? The FCC’s first thought is to impose an etiquette, if at all, only at 902-928 MHz. This is the favorite band of the WISPs, as these frequencies can penetrate most buildings and some terrain. The band is also home to a lot of RFID and control equipment used for pipelines, electrical utilities, railroads, and other safety-conscious industries. But the FCC is also open to requiring an etiquette for 2400-2483.5 and 5725-5850 MHz. Third, asks the FCC: what kinds of signals should be subject to an etiquette? Most high-powered unlicensed signals are of three types: frequency hopping, direct sequence, or what the FCC confusingly calls "digital modulation," which includes the OFDM waveforms used for high-speed Wi-Fi. The FCC is inclined to exempt frequency-hopping transmitters from an etiquette, considering that the rules already prohibit them spending more than a small fraction of their time on any one frequency.
In case it decides to go forward, the FCC has listed several ways in which it might implement an etiquette. Any or all of these could turn up in the final rules.
The most controversial element would limit the "duty cycle" –the percentage "on" time — for higher-powered units. As one example, the FCC posits that a transmitter operating at 1 Watt (the maximum) might be allowed to run only 10% of the time. A unit at or below 1 milliwatt (1/1000 Watt) might permitted to operate continuously. In between, the percentage would scale upward for lower powers, according to some formula. All percentages would be averaged over 0.4 seconds, a number that already figures prominently in the technical rules.
No form of this proposal is likely to sit well with the WISPs, as it would drastically cut the data speed they can deliver to subscribers. It would also impair devices such as the now-ubiquitous Wi-Fi access points. Because these typically operate at a few tens of milliwatts, the proposed etiquette would silence them for some fraction of every 0.4 seconds. This may not be noticeable in a home network. But in an office or coffee shop, where an overworked access point might download simultaneously to a dozen or more laptops, a restriction on duty cycle would result in longer waits for email and web pages.
If the FCC does limit duty cycle, further questions arise. Should a transmitter be allowed to vary its power and duty cycle, so long as the two collectively comply? (Sure.) Should several high-powered units, each on a limited duty cycle, be allowed to synchronize their transmissions? Perhaps not. This would enable unit A to transmit during the required quiet times for units B-J, and so on for B and the others. A user could bypass the rule simply by collocating multiple transmitters.
Another proposal would require an unlicensed device to have "listen-before-talk" capability, so as to avoid stepping on other users’ transmissions. Wi-Fi transmitters already have this feature, but they use it for selfish purposes, to help ensure that the sender’s transmissions get through. The proposal would require a more altruistic implementation, to avoid interrupting other people’s communications. The idea is not new. The FCC requires listen-before-talk in unlicensed radios using parts of the 5 GHz U-NII band, to protect the federal radars left behind when the band was reallocated to private use.
Surprisingly omitted from the FCC’s proposals is "automatic power control," which continuously adjusts transmitter power to the minimum needed for communication. The technology has long been standard in cell phones, both to extend battery life and to let providers adjust cell sizes as needed, and is also required in the U-NII band. Automatic power control has the unique advantage of minimizing interference to others without any impairment of the device’s own performance, and it extends battery life to boot. The chief downside is a more complex circuit design, which translates to higher costs for the device.
Let the Debate Begin
Proceedings on the unlicensed rules tend to be controversial. The vast diversity of users and manufacturers lead to widely divergent interests. The licensed users in the bands, particularly the amateur radio community, tend to have strong views as well. If the past is any guide, the FCC is likely to receive thousands of strongly-worded comments, all advocating inconsistent positions, each one predicting the downfall of Western civilization if the FCC fails to follow the author’s advice.
The folks at the FCC will try very hard to reach a workable result. But the quality of their ultimate decision depends on the quality of the information they receive. Those with an interest in these bands who fail to speak up have no right to complain about the outcome.