As we move about the public safety sector, we often hear a common misconception, which is that distributed antenna systems (DAS) and bidirectional amplifier (BDA) systems are the same. But while they generally serve the same purpose—which is to boost radio frequency (RF) signals to enhance in-building coverage—they are quite different. Let’s explore the basic differences.
A DAS is designed to enhance coverage in large areas, such as high-rise buildings, hospitals and arenas. The system converts the RF signal captured by the rooftop directional donor antennas or provided by small base stations to an optical signal, which then travels via fiber-optic cable to a series of amplifiers positioned on various levels of the structure. At this point, the signal is amplified and converted back to an RF signal, which then is distributed via the building’s internal antenna system. A DAS requires complex engineering and installation, and a lot of equipment and testing.
In contrast, a BDA system is much simpler, consisting only of the rooftop directional donor antenna, a signal booster, and coaxial cable. Once RF signal is captured by the rooftop antenna, it travels from there to the signal booster via the cable; once amplified, the signal travels via the building’s internal antenna system, which distributes it to portable radios being used in the structure. A BDA system is targeted to much smaller coverage areas, such as small office buildings and garages.
One thing that DAS and BDA systems have in common is that the National Fire Protection Association (NFPA) and International Code Council (ICC) has developed standards that govern their design, installation and use, NFPA 1221 and the International Building Code (IBC)/International Fire Code (IFC), respectively. However, commercial cellular DAS and BDA systems only are required to meet standard building codes. In contrast, DAS and BDA systems designed to support a public safety LMR system are held to higher standards regarding performance, i.e., minimum coverage levels, ruggedness and fire resistance. For example, the backup power requirement for a commercial cellular DAS or BDA system might be only a couple of hours, which would be completely inadequate for a public safety application.
There are other challenges. One is that municipal codes don’t always incorporate the latest NFPA and/or ICC standards. Another is that municipal codes often are not consistently applied or interpreted. Yet another is that such codes sometimes are impractical or simply don’t make sense. For example, in one project we encountered, the fiber-optic cable was subjected to a two-hour fire rating, which was achieved through a combination of stainless-steel tubing and insulation, which was very expensive and added a lot of unnecessary cost to the project. The cost proved to be unnecessary because right next to the fiber-optic cable was the power cord to the amplifier, which completely was unprotected. So, if fire entered that room, the fiber-optic cable would survive but the power cord would burn up, taking the amplifier off-line.
There are many other nuances and obstacles to deploying DAS and BDA systems, which are essential to reliable and effective public safety communications. MCP’s subject-matter experts are prepared to help you navigate these differences and determine which path is right for your building and coverage needs—please let us know how we can help.
Paul Levitsky is an MCP communications consultant; he can be emailed at PaulLevitsky@MissionCriticalPartners.com.