November 2007

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Can You Hear Me Now?
Cell Coverage in Buildings

       Jim Sinopoli

Jim Sinopoli PE, RCDD
Managing Principal,
Smart Buildings
Contributing Editor

Author "Smart Buildings"

If past history was all there was to the game, the richest people would be librarians.
Warren Buffett

Understanding wireless communication systems in buildings can be confusing. There’s a range of different systems, applications for the systems and radio frequencies. In addition, there is not an established business model for deploying the systems, or a clear contractual model for providing some wireless services. Buildings may need cellular coverage, a Wi-Fi system, extended wireless public safety communications, RFID systems, paging and other wireless devices. For building owners, in-building wireless systems can have several benefits: greater tenant and user satisfaction, a reduction in facility operational problems, increased public safety capabilities and improved tenant amenities.

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Among the wireless services, cellular service has probably become the most important wireless service in buildings. It’s because of the high penetration of cell phone usage in the population, the reliance by some on the cell phone as a primary phone, and the added importance of receiving and transmitting email and text. Some cell carriers report that around 60% of the telephone calls have some “indoor component” (we all may need to start getting out more). The result is that the user’s tolerance of dropped calls within a building is very low.

Cell services to most buildings are provided by the closest outdoor cell base station or tower. Several elements affect in-building cellular service: the structure and materials of the building, the distance to the nearest cell tower, and the location of the caller within the building. Users at the upper floors of a high-rise building can have problems as their handsets continuously hunt between multiple cell towers. Users in underground floors may have no coverage at all. Technically, the solution to dropped cell calls is to essentially “extend” a carrier’s cell service antenna into the building. This can be done in different ways:

Distributed Antenna System (DAS)
One method is to mount a small directional antenna on the building, pointed towards the cellular antenna belonging to a particular cellular service provider (AT&T, Verizon, Vodaphone, etc.); this is typically referred to as the “donor” cell site. The building antenna provides direct two-way communications with the cell tower. An alternative, albeit probably more expensive, is to have a “land line” connect the building and the cell tower, for example a T-1 line or metro Ethernet connection between the two locations.

From the antenna on the building, coaxial cable is run to amplifiers that are usually located in the telecommunications room. Coaxial cable is commonly used to transmit radio frequencies (RF); for example, cable television to most homes and ideally suited for transmitting radio frequencies between the outdoor building antenna and the building distribution amplifier. Because coaxial cable may run from outdoors to indoors, the installation requires proper grounding and lightning arrestors.

From the amplifier in the telecom room a distributed antenna system (DAS) is installed throughout the building. The in-building distribution system is a series of strategically located inside “omni” antennas connected through coaxial cable using signal splitters and couplers similar to a cable TV distribution system. Additional equipment such as in-line amplifiers and boosters can be used within the distribution system to address the loss of signal strength through the coaxial cable. In a campus environment, the amplified signal from the antenna can be transmitted over the campus backbone network to other buildings.

Signal strength and antenna coverage of the distribution system will be determined by the building’s structure, the power of the indoor antennas, the frequency band of the cellular carrier and the loss of signal through the coaxial. All of these factors can be accounted for in the design and engineering of the systems.

early DAS systems The early DAS systems were based on a single cell carrier and the exclusive use of coaxial cable. More recent system handles multiple carriers and distributes the signal using managed network hubs, which provides the same signal strength at every antenna.

This technical scenario works for not only cellular service but also other wireless services. For example, the radio frequencies used for public safety can be ”extended” into a building’s distributed antenna system, allowing for First Responders to readily communicate. The DAS-type infrastructure for cellular services can also be leveraged to provide in-building Wi-Fi. (Interestingly, some commercial real estate owners are starting to find that complete in-building Wi-Fi is less expensive and has fewer headaches then dealing with tenant-provided Wi-Fi systems that interfere with each other.)

Pico Cells
Pico cell systems are small radios, similar to the outdoor cellular base stations, but limited in capacity and power. They are manufactured by the major cell equipment companies such as Ericsson, Lucent, Nortel, Siemens, etc. They are best suited for apartment complexes and small office buildings. They typically are connected back to a cell base station using an IP network and the base station controls the pico cells thus managing the communication traffic to and from the small cells in the buildings.

contemporary The pico cells can compete with the outdoor macro cell for coverage. That is, a handset could possibly hunt from the small pico cells to the macro cells for service. To prevent this, the pico cells have to provide a strong and dominant signal. The strength of their signal affects the area that will be covered and the caller capacity of the system.

Multiple pico cells used in a building can provide better coverage but because each cell uses the same frequency more interference or noise is created. The result is a decreased capability of the system to carry higher data rates. Pico cells combined with a DAS system can provide better overall system performance.

Business Issues
Like most decisions with building technology systems in-building wireless is less about the technology and more about the business case. The business issues surrounding in-building wireless involve several parties: the building owner, cell phone carriers, and third-party companies that install and facilitate the improved cell services.

The key business issues are who “owns” the customer, who is making the capital investment in an in-building system, and how revenues from the system are distributed. Cellular carriers want to control their networks and their customers – some of that control may be lost depending on how an in-building system is deployed. Other business issues such as revenue sharing between carriers and third-parties, and who’s responsible for the customers’ primary contact arise.

Emerging Technology Trends
The convergence of cell service and Wi-Fi will change the dynamics of the business. Handsets capable of cell and Wi-Fi are in the marketplace, allowing users to get voice, data and video service from one handset through two different networks. Users can now choose whether to make a call via cell services or via a VOIP service, such as Skype, if within Wi-Fi coverage.

It would seem the most logical approach for most building owners would be to deploy one in-building wireless system that could at a minimum handle cellular, Wi-Fi and Public Safety. Such a system would improve tenant amenities and public safety capabilities, while possibly avoiding operational issues, resulting in added value for the building.

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