Real-World RF Obstacles

In a perfect world, subscribers would live and work in a suburban environment with short buildings and few trees and use their phones only when they're outside. They don't. As a result, providing seamless, reliable coverage indoors and out means working around interference, metallic window tinting, water, foliage and concrete canyons, to name just a few of the gremlins that can put RF in the wrong places and keep it out of the right places.

Some of the biggest gremlins look the most benign. Water is one example. When a signal hits a lake or river, it can skip 30km or more. If it lands in another part of the network, results can include interference such as pilot pollution or dropped calls because the distant signal isn't strong enough to support a call.

Coastal highways can be especially problematic. In one case, a site covering a shoreline was beaming the signal perpendicular to the road.

"It was covering that road, but it was also shooting across the water and causing a tremendous amount of interference on the other side," said Neal Buren, Agilent Technologies product-planning engineer, wireless-service products. "The solution was to reorient the antennas so that they ran parallel with the shoreline. That brought down the interference on the opposite side."

The lesson: Too much coverage can cause more problems than too little. One example is pilot pollution, where too many strong signals confuse the phone and cause it to hand off repeatedly.

"In one case, we had 59 handoffs in a mile-and-a-quarter stretch of road," said Jed Dunbar, Transcept director of product management. "Every time you have a handoff, you take up overhead at the base station, (and that) saps capacity. So a high-capacity CDMA base station essentially becomes no better than an AMPS base station because it has all this overhead allocated to handoffs."

Another natural gremlin is foliage. In the summer, leaves can reduce coverage, and in winter, their absence can free signals to travel farther than they're supposed to. Even pine needles can become resonant at the right length.

"The site definitely needs to be above the treetops because it significantly increases the losses when you deploy below foliage," said Frederic Miran, Nortel Networks product-marketing advisor, GSM. "Even when you deploy fairly high above the treetops, you start having coverage breathing, where the coverage varies a bit during the season. It's not significant, but it can create some issues if you don't optimize for both seasons. You might not have the right neighbors declared at the right locations to perform the proper handovers, or you may see new interference created in this location."

URBAN RENEWAL Covering a dense-urban environment can be even more daunting. Buildings frequently shadow signals and increase multipath, so simply siting on top of the tallest buildings doesn't result in solid coverage on streets. One option is to site on shorter buildings and downtilt the antennas into the concrete canyons.

"We picked some of the buildings with lower rooftops and angled the antenna at one end of the canyon so it bounced off the next building and went down the street," said Gordon Davidson, Ericsson CDMA optimization services group senior manager for business development. "By doing that, we also provided in-building coverage. At the other end, we pointed another antenna at the opposite angle so these two were meshing together. They were able to take advantage of CDMA's multipath abilities."

Finding a suitable rooftop could become less of a challenge, thanks to lighter site equipment that doesn't require a location that can support a 2,000-pound cabinet. In Salt Lake City, for example, US West Wireless is using Ericsson gear that weighs about 78 pounds. "It's really easy to get on a rooftop now," said Joe Hannan, US West Wireless director of network planning and engineering.

Good coverage in concrete canyons doesn't always translate into good coverage inside adjacent buildings. One solution is to increase the power on nearby outdoor sites, but more often than not, that approach also increases interference and saps capacity in other parts of the network, to name just a few side effects.

"In CDMA, even if you're not creating a lot of interference, you may end up using up a lot of your forward-link capacity in order to penetrate the building," said Agilent's Buren. "You can't carry nearly as many calls on a particular sector as your network design would allow. (One carrier), in terms of the number of channel elements in the base station, had the ability to carry 20-some calls on a particular sector. But because they were using so much of the forward-link power to penetrate the building, they were carrying a maximum of six or seven calls at a time."

WHAT'S INSIDE COUNTS Concrete and granite aren't the only building materials that attenuate signals. Metallic window tinting, common in modern office buildings, can cut signals by as much as 60dB. But tinting actually can be an asset because it blocks outdoor signals, which might interfere with indoor signals or be too weak to support a call, and keeps indoor signals from leaking out and causing interference.

"It allows you more isolation between the inside and the outside and allows you to more reliably connect to the indoor cell," said Nortel's Miran.

One traditional drawback to in-building solutions is installation hassles such as getting landlord permission and running cables. But that appears to be changing. To use Transcept's PCS-over-cable solution, WirelessNorth will run cable through the Duluth, MN, skyway, a network of corridors that link downtown buildings to shield pedestrians from the elements.

"There are some key buildings that our marketing people want us to hit, so we're pulling it through the ceiling of the skyway to hit these buildings and using either two or four antennas off of each Cable Microcell Integrator (CMI) to improve in-building coverage," said Laura Slobotski, WirelessNorth engineering manager. As for landlords, "nobody has had any concerns. Our real-estate person would bring up the CMI box and show them the size and antennas and the cable. It's really non-intrusive."

Installations of fiber-optic and twisted-pair distributed-antenna systems also are getting faster, especially when they leverage existing infrastructure and know-how.

"The advantage is you don't need traditional installers," said Ashraf Abdu, Nortel Networks product-marketing advisor, GSM. "You can use installers that are used to installing LAN infrastructure. That reduces the cost because those types of infrastructures are less expensive because they're supported by a broader technology. If the building is already wired for fiber, it's usually a couple of days to wire it up as long as you have a good idea of where you want to put the antennas."

Costs also are falling. Take Radio Frequency Systems' (RFS') MicroBDA.

"Somebody could probably get the antennas, cable and Micro- BDA for $5,000," said Andy Singer, director of marketing and technical services at RFS, a company created by the merger of Celwave and Cablewave. "It doesn't take that many calls or satisfied customers to justify $5,000 to get better coverage inside a small building."