Inside Addition

With today's in-building solutions, your wireless calls can sound as if you were sitting outdoors.

Walking around today's urban and suburban communities, it does not take long to realize the enormous number of wireless antennas. Considering the number of antennas dotting the landscape, it might seem hard to believe that there are still significant coverage challenges facing every major wireless operator in its urban and suburban markets.

But customers now want to use their phones in places the wireless carriers never intended their networks to cover.

“Although today's business professionals have landline phone systems, they are actually using their cellular phones as much, if not more, than their office phones,” said Raeleen Matlock, Alltel (www.alltel.com) RF engineer. “This may become a primary reason for choosing or leaving a provider. As a result, engineering plans today must consider the benefits of in-building coverage.”

Greg Evans, Agilent Technologies (www.agilent.com) manager of engineering services, specializes in providing engineering assistance to wireless carriers to help solve their in-building and other coverage and capacity issues.

“Today, about 78% of wireless voice calls actually originate from inside a building, so even if the demand for better quality inside remained static, it is a massive market need and, obviously, a complex technical challenge in almost all cases,” Evans said.

In fact, in-building demand is not expected to remain static. As data services are rolled out, wireless carriers will have the ability to offer the true “wireless office,” and the potential of total office portability will become reality.

“It is obvious that adequate in-building coverage, which will allow for optimal data rates in addition to coverage and capacity for voice services, will be an absolute essential aspect of any carrier's build-out plans,” Evans said.

Varying Approaches

Similar to the transition carriers had to make as high-transmit-powered bag and car phones were replaced with low-transmit-powered portables, carriers must adapt their networks creatively to the market demands. Several infrastructure companies have designed specific solutions for in-building and specialized coverage challenges. These challenges range from getting adequate coverage inside a small office building in a suburban community to covering the four major tunnels of the New York City metropolitan area to the 47,000-seat brick, concrete and steel Safeco Field in Seattle.

Because there is a difference between getting coverage inside a relatively light-use, small office building, a densely packed metropolitan airport, a 100-story skyscraper and the space-, power- and maintenance-restricted tunnel under the Hudson River, there are different approaches that cater to these specific needs.

The initial challenge is getting the RF signals into and out of the structure's walls or barriers. Once the RF signals are inside, the important issue is signal distribution.

In deciding how to get the signals inside, capacity is the consideration. In dense, high-capacity-requirement situations, one or more dedicated base stations located physically inside the structure makes the most sense. Options range from suitcase-sized, low-power-draw picocells to full-fledged macro base stations, depending on the capacity and size requirements. An alternative is to dedicate the capacity of a surrounding site by remotely locating the antenna of one or more sectors. Depending on the distance, the antenna could be fed with standard coaxial cable; however, fiber-optic cable, which allows for longer distances, is becoming widely used.

If capacity is not a concern, there are other options. First, a carrier could attempt to increase the performance of the serving base stations surrounding the in-building challenge by using higher-power transmit amplifiers on the downlink and superconductor-based low-loss filters and low-noise amplifiers on the uplink, thus creating more robust signal paths that would be able to penetrate into and out of the building. If the primary in-building need is on higher floors, this is often adequate because RF penetration is significantly better on higher floors. Many buildings, however, do have needs on the lower floors, particularly in urban areas where garages often are located in basements. Here, the serving RF could come from an off-the-air repeater, which essentially is a low-noise amplifier that relays signals from the user to the serving base station and vice versa.

RF Distribution

Once the RF is through the walls, the real difficulty begins. Two leading suppliers of in-building antenna-distribution technology and services are AeroComm (www.aerocomminc.com ) and LGC Wireless (www.lgcwireless.com). Both companies use fiber optics extensively to distribute the RF from the source (either a base station or a repeater) to the various antennas located throughout the structure. Although the hardware integration would seem to be the primary challenge in these installations, at least half of the in-building deployment cost can be labor, according to Lee Masoian, AeroComm president.

“In addition, most of the biggest hurdles faced generally deal with issues not regularly dealt with by wireless operators — such as the lack of space and electric power in most tunnel applications,” he said.

Easing the complexity of the installation also is a major issue for Enrique Cuellar, LGC Wireless vice president of marketing. With more than 1,000 systems shipped, LGC Wireless has been involved with in-building projects such as Newark Airport, the Venetian Hotel-Resort-Casino in Las Vegas, the Metro subway system in Santiago, Chile, and Petronas Towers in Kuala Lumpur, Malaysia.

“Our system uses flexible and affordable twisted-pair (CAT-5) unshielded or shielded (UTP/STP) cable and fiber-optic cable to move into tight spots easily,” Cuellar said. “Most technicians have a lot of experience working with these cables, and, in fact, many times the cabling for the wireless in-building system can actually be laid by the computer-network installers at the same time.”

Essentially, most of the systems use the same general concept of converting the RF signals to optical signals to distribute them across distances that would be too lossy and cost prohibitive if coaxial cable were used. Current optical technology can allow for transport of signals effectively up to three miles, enough to reach the tops of the highest skyscrapers and across the largest casinos.

Once the optical signal reaches the location where it will be radiated, it is converted back to RF frequencies and sent to the antenna, or sometimes to the radiating cable, as is the case in most tunnel applications. Often small-profile antennas are used that can be hidden easily from view.

Technology Agnostic

The RF-to-optic and optic-back-to-RF conversions are not specific to a particular wireless technology. Although design engineers must consider issues such as handoffs, co-channel interference, pilot pollution and other similar performance-engineering issues dependent on the technology, the antenna-distribution systems only deal with the actual RF signals and are independent of the technology. They will work the same whether the carrier is using IS-136, GSM, CDMA, AMPS or even a combination. This adds to the value of such systems in that multiple technologies — as many carriers are or will be dealing with shortly — will not be an issue, and the distribution systems will be compatible with next-generation technologies.

The demand for better in-building coverage will be driven by trends such as wireless handsets replacing landline, the desire for office portability and the outcry by public-safety officials for coverage in high-rise buildings and parking garages. Carriers must deal with the complex task of supplying this in-building service and likely will rely heavily on specialized engineering service and solution companies to accomplish such tasks in the future.

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Miceli (amiceli@suptech.com) is Superconductor Technologies regional sales manager and author of Wireless Technician's Handbook.