WiMax, C-band: Unpeaceful Co-Existence

The mystery at CBS-owned KYW Philadelphia began last February.

Something was interfering so badly with the network feed coming off its C-band satellite downlink that the picture and sound were freezing up several times a minute — not a good thing in primetime.

Fortunately, the station found that it could escape the interference by switching to an alternative feed on another transponder at a higher frequency.

After a couple of weeks, the interference disappeared, only to return in full force in the middle of September.

This time, KYW called the FCC's field office in Philadelphia. The spectrum police visited the station, confirmed the interference and began making phone calls.

The interference abruptly stopped on Sept. 24. So far, it has not returned.

As a matter of policy, FCC officials declined to discuss the case.

But based on his conversations with the FCC officials and his own analysis of the interfering signal, KYW Chief Engineer Rich Paleski believes he has solved the riddle and, in so doing, discovered a problem that should concern every station that imports programming via C-band satellite, which is to say just about every station in the United States.

According to Paleski, the interference emanated from a WiMax service provider testing equipment in the 3650-3700 MHz band that the FCC authorized for WiMax and other wireless broadcast access services just last year.

The upper end of the 50 MHz band abuts the lower end of the satellite C-band downlink frequencies, which run from 3700 MHz to 4200 MHz.

WiMax is a souped up version of WiFi, the ubiquitous wireless routers that allow people to tap into the Internet without having to run wires to their computers. But whereas a WiFi signal covers only a single home or office, a WiMax signal covers large portion of a city.

WiMax offers users broadband access speeds ranging from 1 Mbps to 5 Mbps.

Today, most licensed WiMax service is provided in the 2.5 GHz band, far away from any service that might cause broadcasters trouble.

But in May 2007 the FCC opened up the 3650-3700 band for WiMax operators, which have already begun registering to use it.

Among the early providers: Next Phase, in Southern California; Nth Air, in Las Vegas, Los Angeles, the Bay Area and Denver; and Rapid Link, in St. Louis, Los Angeles, Dallas and Sacramento, Calif.

Under the FCC's non-exclusive, "regulation-lite" licensing scheme, WiMax operators, once qualified, need only register their base stations in the FCC Universal Licensing System. However, they are obliged to abide by certain power limitations and coordinate their installation with licensed earth stations to insure against interference.

Paleski and others doubt that C-band satellite and WiMax can co-exist happily in adjacent bands. "This service could be a difficult neighbor," he said.

According to Paleski, the WiMax signal is pernicious. Some of the interfering signals were 30-40 dB above the satellite signal and they "seemed to be coming from everywhere."

His examination also left little doubt that the offending signals were being used for broadband access.

"Looking at it on a spectrum analyzer, it was definitely some type of digital data delivery system," he said. "From the signature of the interference, you could tell there was some two-way communications going on in a digital fashion."

Paleski said that he tried to remedy the problem by installing a second filter between the feedhorn and LNB of his satellite dish. The filters are designed to scrub the signals coming off the dish of all signals except the desired satellite frequencies.

"There is a little slop on either side of the filter," Paleski said. "The [WiMax] service is so close to the edge of that filter that the filter is not as effective as it would be if it was maybe another 200 or 300 MHz further away."

According to Paleski, ABC's Philadelphia O&O WPVI experienced the same interference at the same time as KYW. (WPVI's chief engineer could not be reached for comment.)

Paleski said that he has surveyed the CBS O&Os in other markets, but none reported trouble.

Nonetheless, Paleski believes the WiMax interference from the 3650-3700 MHz frequencies could evolve into a major problem not only for broadcasters, but also for cable operators and other heavy C-band users, as more and more WiMax operators move into the band.

"We are just waiting to get our butts kicked by this service," he said.

TV stations need to be alert to this type of interference, he added. And, at the very least, he said, they ought to make sure they register their downlinks at the FCC so that WiMax operators know they are there as they plot their base-station locations.

If it comes to a battle between C-band satellite users and WiMax operators, broadcasters will have an ally in the satellite industry, which has much at stake in protecting its communications links.

Robert Ames, president of the Satellite Users Interference Reduction Group, is well aware of the problem and says it is a global one. In some places, WiMax and C-band share spectrum.

Shielding C-band satellite from wireless broadband access providers like WiMax "is our No. 1 priority next year," he said.

Two years ago, during a World Cup soccer match in Bolivia, an experimental WiMax service turned on and knocked out TV in 60 percent of the country. "There was damn near a riot," Ames said.

The terrestrial WiMax signals can be thousands or tens of thousands of times more powerful than the satellite signal, Ames said.

"So what happens is that you will get some of the spillover that saturated the front end of a satellite dish," he said. "Even though it may be adjacent to your operating frequency, it is still going to wipe you out."

WiMax operators are hungry for spectrum, Ames said.

Prior to the World Radiocommunications Conference in 2007, SUIRG conducted tests that clearly showed the potential for WiMax interference.

Although the conference agreed to protect satellite frequencies in general, Ames said, it left the door open for potentially disruptive interference with a footnote full of exceptions for various frequencies in various countries.

"The WiMax people are saying, look at all the money you can make," said Ames. "People don't understand dBs, but they sure understand money. [emphases mine - TMC]"

Rob Kubik, vice chairman of the regulatory working group for the WiMax Forum, an advocacy and standard-setting group, said that operators in the 3650-3700 band must protect "legacy" satellite earth stations using frequencies inside the band, but not those using frequencies in adjacent bands.

"As long as you are complying with the power and antenna limits of the FCC, I don't think there are any additional measures that the operators in 3650 [MHz] have to take to enter the band," he said.