The i-Zone

A cellular carrier's key cell site in Seattle goes off-line randomly for no apparent reason. A PCS carrier's frequency is blocked repeatedly, and a police station sits a mile away in Albuquerque, NM. Coincidences? Not likely. These carriers have entered the i-zone.

No doubt you, too, have experienced a frightening interference nightmare. Your spine tingles and the goosebumps grow when you think that after inadequate coverage, interference is the biggest cause of poor-quality calls. It chills your bones to know that interference can plague your systems unexpectedly and inexplicably.

The most frightening thing is that many interference problems come not from your own facilities, but from your proximity to other high-power RF installations. But never fear, you can beat even the scariest cases of interference.

A MICROWAVE MESSEven though PCS carriers have relocated microwave links so they could launch their services, Harold Gwin, Powertel vice president of network operations for Alabama and Mississippi, said microwave problems still exist, especially in rural environments or areas where there are only one or two carriers.

Western Wireless faced a huge interference challenge at sites in Albuquerque, NM. Microwave systems already in place caused so much interference that some sites had alarming dropped-call rates of 14 percent to 18 percent, according to Sue Smith, Telecom Wireless Solutions (TWS) executive vice president of engineering and network operations. Western Wireless discovered that because a nearby police station hadn't cleared some microwave, the signals were radiating on the same band and interfering with the base stations.

Unfortunately, you never can get rid of microwave links completely.

"(Engineers) know where the microwave link is, from point A to point B, so they won't use the frequency in that area," Smith said. "(But) just because you have a link from one place to another place doesn't mean that the energy stops at the receiver."

The microwave frequency extends past the receiver, and carriers don't always realize that not only can they not use the frequency close to the link, but they cannot use it for miles beyond, either.

A safe distance depends on how high powered the microwave dishes are and how narrow the microwave beam is. This distance is probably at least double the length of the link. For instance, if it's a 10-mile link, you can't use that frequency for at least another 10 miles.

Although you can't use the frequency between the dishes, you can use the narrow, 1- or 2-degree beamwidth on either side of them, Smith said.

In Albuquerque, TWS helped Western Wireless retune by moving its sites off frequencies that the microwave links occupied. According to Smith, retuning is an effective way to get around external interference quickly. But moving a site away from the interference is only a short-term solution because you haven't solved the problem, you've only moved the frequency off the site. Eventually, you must relocate those microwave links to a different band. Retuning gives you and the interferer time to complete the relocation.

Gwin said Powertel faced a similar problem when it discovered that gas and electric companies previously had used its spectrum for microwave networks.

"We had to launch our network, and at the same time we had to attempt to move the old microwave to different frequency bands," he said. "The other systems were still on the air, and we were attempting to come on the air, too, so there was some interference."

In Birmingham, AL, the incumbent microwave operator was the Jefferson County police. According to Gwin, the county's system operated out of Powertel's band, but its equipment was old, and it was receiving the carrier's signal. To solve the problem, Powertel installed filters on the police station's equipment.

"We had talked with them previously ... so they knew that we were coming on the air," Gwin explained. "When we did come on the air and interfered with them, we simply bought the equipment for them to do away with the interference."

OUT-OF-BAND OBSTACLESSMR and cellular equipment often are not a good mix. According to David Chase, Superconductor Technologies director of system engineering, because the SMR band is just above the cellular B-receive band, SMR signals can interfere with A and B carriers.

For example, one cellular carrier wanted to expand coverage in a rural area, but SMR transmitters in the area were not clean. An amplifier was installed, but spurious signals began showing up in-band. An extensive search found an SMR carrier as the source. Cooperation between both carriers was the key to finding a solution.

If the SMR interferer is well within its band, then the cell-site owner is responsible for putting in the necessary filtering so it doesn't overload the front end. In this case, however, the cellular carrier couldn't use filters to solve the problem because the interference was in-band. The system doesn't know the difference between a spurious signal and one that belongs in the band legitimately, Chase said. Receive filtering doesn't solve those problems, but transmit filtering on the SMR side might. Another solution required the SMR carrier to clean its transmitters or put filters on them.

"If you have an SMR transmitter or something that's out-of-band, it comes on and then disappears after a couple of seconds because it has finished transmission," Chase said. "It may be during those few seconds that you've created some interference and you've dropped your calls."

The only thing you see, he explained, is an increase in dropped calls, and you don't really know why. Chase suggested using a low-loss combination filter LNA that is specific to the A band to solve such predicaments. Tower-top amplifiers may not work in these situations because the filtering is broad, and although they will improve range, tower-top amplifiers also can allow interference in your system.

Nearby non-linear objects also can affect cell sites negatively. Chase said that one rural carrier thought an out-of-band signal was overloading the receiver's front end and causing interference. The carrier would come on for a short time, and then under certain circumstances, all of its digital calls would disappear. Initially, to solve the problem it decided to use the site solely for analog calls instead of digital, so calls were noisy, but at least they weren't dropped. But Chase said that wasn't a real fix.

After getting rid of the out-of-band interferer, the carrier found that the in-band interference still was a problem. After some detective work, it found the source.

"There was a water tower nearby, and the high-power signals ended up mixing inadvertently in the rusty tower and generating a signal in-band," Chase said.

The only way to solve the problem was to move the antenna so it no longer illuminated the RF non-linearity.

IN-BAND INCONVENIENCESAccording to Gregg Clausius, AirTouch director of network operations for the Northwest region, an old or defective radio or transmitter has caused in-band interference with AirTouch's sites, resulting in spurious emissions radiated across the band and jamming digital networks.

The carrier has a seasoned group of field engineers that are dispatched immediately when these problems arise. Armed with Yagi antennas and spectrum analyzers, they hunt down the source of the interference. Then they work with the owner to mitigate the problem quickly.

For example, a public service provider in North Seattle had a defective transmitter that was radiating and jamming two or three of AirTouch's digital cell sites, causing a 60 percent origination failure on those sites, Clausius said.

"This was a unique one because it was hitting more than one cell site," he said. "Usually when we have a 'jammer' of this nature, it just hits one site or sector, but this one was fairly widespread."

AirTouch quickly dispatched engineers and technicians, and located the defective transmitter source within 50 minutes.

AirTouch contacted the source immediately and mitigated the problem within 90 minutes, working directly with the interferer's local engineer. Initially, it was hard to turn the source off because public services have to be in operation. But eventually the organization turned it off because the device wasn't working correctly and needed to be replaced.

Clausius said cooperation is key in these situations. In this case, AirTouch had several follow-up discussions with the public service engineer about installing certain filter types.

"(He) kept us in the loop and said, 'These are the kind of filters I'm going to use in the future. Could you keep an eye out to see if it is working,'" Clausius said.

Doug Blake, director of network operations for the Midwest region, said U.S. Cellular also encounters in-band interference, and when it does, cooperation often leads to a solution.

Customers in Galesburg, IL, complained about troubles with call origination when they were on the edge of the cell-site service area. After conducting a cell-site maintenance certification and checking the antennas, the radio alignment, coaxial cable and grounding, a U.S. Cellular technician found the problem at the -95dB level.

"The farther away you get from the cell site, the lower your signal is, so when you got to that -95, -100 threshold, the mobile would have a hard time setting up to the cell site," Blake said.

U.S. Cellular used a spectrum analyzer to monitor the entire spectrum and found the problem. A radio station, 92.7, was producing in its receive band, which is 834MHz. AirTouch talked to the station engineer. Blake said he's not sure what the station did to solve the problem, but soon after, the interference and customer complaints stopped.

CAN YOU ESCAPE?To escape the i-zone, Blake said frequency coordination is critical, especially for co-location. Powertel's Gwin and AirTouch's Clausius agreed that communication and cooperation between competitors operating in the same area is essential for curbing interference.

TWS' Smith said you should use site surveys to pre-calculate possible problems before you launch service. That way, you can put your antennas at a certain angle or vertical height so they don't face directly into your competitor's transmitter.

"As the towers are getting squished (closer) and (closer) together, we're getting more receiver blocking problems," Smith said. "The more signals and the more carriers you havein one place, then the more mixes and the greater the chances for ... interference."

In many cases, you can avoid the i-zone with good engineering design, good maintenance and extra filtering.

Problem: External interference sources

Solution: Scan band to find source and improve filtering. If source is out of regulated band, inform the FCC.

Problem: Transmitter spurious emissions

Solution: Add filtering to transmitter.

Problem: Receiver blocking

Solution: Add filtering to receive path and increase isolation via antenna spacing or duplexor.

Problem: Intermodulation products in transmitter

Solution: Improve transmit-to-transmit isolation, add filters to transmit path and retune.

Problem: Intermodulation products in receiver

Solution: Improve transmit-to-receive isolation, add filters to receive path, and separate transmit and receive antennas on tower.