Noisy Neighbors

For build-out in urban markets, most wireless carriers prefer to co-locate on rooftops. Interference can arise, though, if the party gets too crowded.

In urban jungles, towers are not an option for delivering wireless service. All those skyscrapers, though, provide carriers a ready-made replacement. Howard Richard, Alltel staff manager for property management, called rooftops and buildings the only viable option for carriers in urban markets.

For most carriers, sharing a rooftop with competition isn't a problem, so long as everyone keeps their signals to themselves. Interference occasionally arises when antennas are too close together, especially on a rooftop, where it is a challenge to create vertical separation.

"I'd rather have vertical separation being on the same tower only five feet above other antennas than being 100 yards away at the same elevation," said Keith Radousky, Cingular executive director, engineering.

Hey, Tall Guy in Front! Currently, interference at co-located rooftop sites isn't an epidemic. Dave Snyder, Crown Castle CTO, said interference on rooftop sites is a rare nuisance that can increase dropped calls and degrade call quality. Frequent or not, rooftop interference can create notable problems.

Alltel's Richard said that rooftop interference usually stems from obstruction.

"Think of it as a rooftop crowded with people," he said. "Can you see with a guy standing in front of you? That guy is six feet tall and you're 5'11", and you're looking at the back of his head."

The key is to cooperate with your neighbors to ensure enough horizontal separation between the antennas. The distance between them varies depending on the size of the rooftop, but can range from a cramped two feet to the more ideal 10 feet, Richard said.

Alltel and Nextel have grown acquainted with one another on rooftops, and have managed to peacefully cohabit.

"Nextel is an SMR carrier, and they operate in the same frequency band (800MHz) as we do," Richard said. "At times they can get too near and cause problems with interference because it bleeds over into our bandwidth, so we filter them."

Ultimately, rooftop interference is a manageable problem for Alltel.

"It's just a matter of planning," Richard said. "Does it cause problems? Yeah, from time to time if it's not done right."

When it comes to picking the right rooftop, Cingular finds that the cupboard is almost bare.

That makes co-location all the more critical and interference intolerable.

"Generally speaking, there's one or two buildings that will typically meet our needs," Radousky said. "We don't have much choice to not accept a site because we can't make it work with the other antennas; somehow we find a way to make it work."

Radousky outlined a 10% rule whereby if Cingular determines the ideal location for a new site in New York City is 14th and Peachtree Street, a usable rooftop must exist within 10% of the proposed site's serving radius.

Even if the carrier is lucky enough to find a rooftop in that radius, strict standards for antenna placement must be met as well. Radousky said Cingular's 40-some pages of guidelines for antenna placement are in the hands of those selecting the site and placing the antennas.

When interference does occur, it isn't long before a red flag rises.

"We identify it via a report from the network that tells us when we have disturbed channels at the cell sites," Radousky said. "That will trigger an investigation to determine whether the interference is coming from our own network or an external source. If it turns out to be from another operator, we go to the location with an antenna and spectrum analyzer, and determine which set of antennas the emissions are coming from."

Radousky said that interference typically comes from a malfunctioning RF transmitter. If it's not from a malfunction, then some sort of filtering is required.

What started as courtesy has become requisite for any carrier looking to co-locate on a rooftop site. Rooftop contracts call for a new carrier to conduct tests to prove it won't interfere with its potential neighbors.

"If I go to contract with a rooftop, the contractor is going to say that I've got to test either by virtue of a radio-propagation test or intermod study to determine intermodulation-type interference issues," said Alltel's Richard. "Contractually, if someone lets me on a rooftop, I want them to be bound that they aren't going to let someone come in and disturb (my transmission). It's like a tenancy.

Any changes to your design or frequency, and you're at the back of the line, Richard added. Should interference arise from an existing carrier, though, it's fix it or move out.

The Future Is Crowded As carriers move to 3G, the volume of rooftop interference is going to increase.

"It's an increasing trend, absolutely, especially with 3G coming," said Cingular's Radousky. "We're all going to be deploying additional RF transmitters for third generation."

With each transmitter added to a rooftop site, the chance for interference exponentially increases, according to Brad Deats, Summitek director of engineering. The second factor is the increase in data transmissions.

"People are much less tolerant to errors in data," Deats said. "You can still understand someone if they are talking, even if they sound like Mickey Mouse; people will put up with that a little bit. But if they are downloading a file into their PDA and it becomes corrupted three quarters of the way through, they are not going to be very happy."

Who or what is causing the interference often comes as a surprise to carriers, according to Deats, who said Summitek gets a call a week from carriers experiencing rooftop interference problems.

"Normally it's from something other than a wireless carrier, although we test the other carriers," he said. "The wireless carriers' equipment tends to be newer and maintained very well by an ongoing staff. The problematic area in rooftops is where you are surrounded by aging metallic joints that are easily illuminated and generate intermodulation."

Now carriers don't have to send their wisest engineers to every rooftop that is experiencing interference. Products such as Summitek's BaseStar bring interference testing to the masses.

BaseStar listens to all transmitters in the area of the carrier that is experiencing interference to identify its potential sources. With one transmitter isolated on the carrier, another on the potential malfunctioning transmitter and a third to where the interference should occur, BaseStar listens for an event correlation.

Moving forward, carriers will need to evaluate every interaction that could occur on a co-located rooftop site before moving into the neighborhood.

"One of the reactions a customer had when we went onto a congested rooftop was `Wow, I didn't know they could be a problem, or that those two transmitters could interact,'" Deats said.

Wireless carriers have rooftop co-location down; cooperation has made for many friendly neighborhoods. However, now the challenge will be to keep the noise down.