Expanded conditional license opportunities, greater tolerance for adaptive modulation in the works

Microwave radio serves as the nation’s nervous system. (Microwave ovens, a different technology, take care of the stomach.) These radios operate through the ubiquitous sideways-facing dishes and domes on radio towers, water towers, and tall buildings. Their signals regulate the movements of railroad trains, control the electric grid and natural gas and oil pipelines, carry long-distance telephone calls and Internet traffic, transport TV programming to cable systems, send 911 messages to the local police station, deliver cell phone calls to the towers, tell the ATM your bank balance, and carry vast amounts of data that fuel ordinary businesses nationwide.

Many of these systems operate at availability levels in excess of 99.9999% (“six nines,” in industry parlance). This allows for outages adding up to no more than 30 seconds per year – not bad for systems that sit high up outdoors, exposed to the weather year round.

The FCC is considering three changes that would help improve the operation of these systems.

Conditional Licensing

To obtain a microwave license is time-consuming. The applicant must: (1) design the system; (2) go through frequency coordination, which limits harmful interference to other systems and warns the applicant about any harmful interference coming in; (3) file an application with the FCC; and (4) wait for the FCC to issue the license. 

But there is a shortcut. Under a procedure called “conditional licensing,” the applicant can flip the ON switch after step (3).  The system can thus provide service while the application wends its way through FCC processing, conditioned (hence the name) on having to shut off if the application is turned down. That rarely happens.

Demands for service can arise quickly, while the FCC – and we say this with the deepest affection – sometimes moves slowly. This makes conditional licensing an important tool for microwave service providers.

Two of the changes pending at the FCC are intended to make conditional licensing easier.

Of the several frequency bands allocated for microwave operations, the 6 GHz band is the long-haul workhorse. Low frequencies go farther; and in much of the country, 6 GHz is the lowest available. For links that must span tens of miles or more, 6 GHz is often the only choice.

The band has two segments, called the “Lower 6” and the “Upper 6,” which differ in two important ways. The Lower 6, shared with over four thousand C-band satellite uplink earth stations, tends to be more congested than the Upper 6, which has no earth stations. On the other hand, the Lower 6 allows microwave channels up to 30 MHz wide, while the Upper 6 maximum is only 10 MHz. An operator who needs 30 MHz, but cannot find room in the Lower 6, is not entirely out of luck. It can ask the FCC for a waiver to use 30 MHz in the Upper 6. Such waivers are usually granted. But an application that needs a waiver does not qualify for conditional licensing, and so the system cannot turn on until after several weeks of FCC processing.

The Fixed Wireless Communications Coalition (FWCC) asked the FCC to amend its rules to add 30 MHz authorization to the Upper 6.

The FWCC also had a request concerning the 23 GHz band. Suitable for shorter distances, this band is ideal for transporting cell phone and mobile Internet signals to and from cell towers. The catch (there always seems to be one) is the federal government, which shares the band. On the one hand, the government long ago set aside four channels in the band for conditional licensing. On the other hand, those are everyone’s first choice; they tend to be crowded, and are not available everywhere. An applicant for any other channel does not qualify for conditional licensing, and so cannot operate until the FCC grants the license. In addition to the usual FCC processing, a non-conditional 23 GHz application requires government sign-off, which further delays the grant.

The industry was pleased back in 2007 when the government freed up two additional channels for conditional licensing. But they were of no immediate benefit. The conditional-licensing channels are listed in the FCC rules, so adding two more requires a full-scale rulemaking proceeding. The FWCC formally requested that change in November 2007.

The FCC has now issued a Notice of Proposed Rulemaking to accommodate the FWCC’s requests in both the 6 GHz and 23 GHz bands. Comment and reply dates will be announced on publication in the Federal Register, which has not yet occurred. The language of the NPRM suggests that the FCC intends to adopt both of the requested rules, barring unexpected surprises. The docket is WT Docket No. 09-114.

On the 23 GHz item, the FCC took an additional and very welcome step: it granted a request from the FWCC for a waiver during the rulemaking. This makes the two additional channels available for conditional licensing now, even before the FCC has received and reviewed comments. In the unlikely event that the FCC declines to add the two channels to the rules, an applicant then operating on conditional authority would have to shut off until the license issues.

Adaptive Modulation

To a microwave signal, air is tricky stuff. Depending on the frequency band and atmospheric conditions, air can severely weaken a radio beam, or even bend it away from the receiver. “Fades,” as these conditions are called, can interrupt communications and make it harder to achieve “six nines” availability, or whatever the application needs. A modern microwave system automatically steps up the power to counter a fade. But power increases can only go so far, and if used to excess, can cause interference to other systems.

There is another approach, similar to one we use every day. When talking to someone in a noisy room, or shouting over a distance, people instinctively speak slowly. We know from experience that a slower message is more likely to get through. The same is also true of data carried via radio: slowing the data rate – the bits per second – makes communications more reliable. (This is embodied in Shannon’s Theorem, the central tenet of communications theory.) In addition to boosting the power, a microwave operator can combat an atmospheric fade by throttling back the data rate.

In a given system, the data rate depends primarily on the “modulation,” the technique used to impress payload data on the radio signal. A system that automatically changes to a slower modulation when needed is said to be using “adaptive modulation.”

But there is a catch, as usual. The FCC, seeking to ensure that spectrum is used efficiently, requires a microwave system to be capable of carrying certain amounts of data. In the 6 GHz band, for example, a channel 30 MHz wide must be able to carry at least 134.1 million bits per second. (For comparison, an Internet signal this fast would download an entire movie in about four minutes.) Adaptive modulation, to be useful, must sometimes drop the data rate below the specified minimum. Does that violate the FCC rules?

The FWCC, together with several microwave manufacturers and service providers, thinks not, and has asked the FCC to interpret the data-rate requirement in a way that allows the use of adaptive modulation. In a word, they want the FCC to treat the minimum in the rules as an average, not an instantaneous value. That would let a system occasionally drop below the minimum, for brief periods, so long as normal operation between fades meets the minimum, as does average operation over a period of time.

This makes sense. A system without adaptive modulation runs a greater risk of having a fade cut off communications altogether. That gives a data rate of zero, hardly efficient use of the spectrum. Worse, the system may need several minutes to resynchronize, during which critical payload communications cannot get through. Data transfer at a reduced rate is better than none at all.

The FCC has issued a public notice soliciting public input on the request posed by FWCC et al. Comments are due on July 27, and reply comments on August 11, in WT Docket No. 09-106.