DAS readies buildings for wireless

The proliferation of wireless devices and applications shows no sign of slowing. The mantra that mobility equals increased productivity is driving adoption of everything from wireless security cameras to computerized physician order-entry services.

The problem is that within a single facility or campus, each wireless application (paradoxically) typically requires its own wired infrastructure access points or other radio sources. The result is a piecemeal, redundant infrastructure that creates a chaotic RF environment and impedes wireless applications. One shared broadband in-building passive distributed antenna system (DAS) imposes discipline on chaotic environments such as these and therefore enables quality wireless services. These systems can support all types of wireless technologies that multiple vendors provide across a broad frequency range.

The core of a passive DAS is an integrated access device (IAD). Cellular/PCS, two-way radio and paging systems connect to the IAD through a base station or an off-air repeater. The services are combined and filtered, then sent over one wired backbone or trunk, typically a 7/8 inch coaxial cable, up the riser of a building.

An engineered array of antennas branches off from the trunk on every floor of the building to transmit RF energy. The antenna array includes radiating coaxial cable, standard coaxial, directional antennas and omni-directional antennas. The array is configured for each floor to provide blanket coverage within a specified area, as well as to keep RF signals within the facility.

Applications such as wireless LAN, building automation and security systems are added to a passive DAS on a floor-by-floor basis, as they normally serve a localized area. Local applications are connected via an applications portal device that, like the IAD, combines RF energy from multiple sources and adds those signals to the antenna array. This approach requires a detailed site survey and an in-depth understanding of a building's construction and materials.

A single passive DAS can handle multiple frequencies for converged voice and data, building automation and security services. Base stations and/or off-air repeaters from multiple service providers are centralized in a main telecom room. Access points are clustered in locked intermediate distribution facilities on each floor.

One stack zone can provide blanket coverage in sites as large as 500,000 square feet. Fiber-optic repeaters provide signal to additional zones as needed for larger facilities that require a single IAD.

Essentially, a passive DAS is a large, very efficient antenna that distributes RF energy evenly throughout the building. No amplifiers or repeaters are needed. All the components of the system are passive -- they aren't mechanical and they don't require power. Therefore the system does not have to be monitored and is highly reliable. It is a utility-grade system much like electrical distribution and HVAC distribution.

DASs are compliant with the IEEE 802.11 standards. They also support standards such as Code Division Multiple Access, GSM, Integrated Digital Enhanced Network, Time Division Multiple Access, General Packet Radio Service and Universal Mobile Telecommunications Service. They can support a continuous broadband frequency range between 400 MHz and 2.5 GHz. This covers the majority of in-building wireless applications and emerging technologies.

One wireless infrastructure that supports many applications simultaneously will reduce interference and provide reliable coverage within a building.

Hermann is program manager of wireless solutions for Johnson Controls. He can be reached at james.w.hermann@jci.com.

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