Public safety: Page 3 of 5

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Design considerations
Deployment of solutions for reliable public safety in-building coverage must consider the spectrum environment, building parameters and the users’ operational needs. As with most systems for public safety, whether indoor or outdoor, reliable solutions require customization, high-quality products, good systems engineering and proper installation and maintenance. Reliable public safety in-building systems are not a “cookie cutter” design where one size fits all.

Following are some generic requirements for in-building public safety systems:

General design considerations
Annex O to NFPA 1 (National Fire Protection Association) specifies the following standards:

  • O.3.1 General: The document requires that the RES operate without causing interference to other parts of the public-safety radio system and that technologies be employed that are compatible with the public-safety radio system. Most RES employ an over-the-air broadband BDA at the donor antenna. These devices are notorious for causing harmful interference, especially if there is poor isolation between donor and coverage antennas (there should be 15 dB minimum). Annex O envisions a permitting process whereby the system design must be submitted to the authority having jurisdiction (AHJ) for approval before it can be installed.
  • O.3.2 Radio coverage:Two types of building areas are defined: critical areas and general building areas. Critical areas include emergency command centers, fire pump rooms, exit stairs, exit passageways, elevator lobbies, standpipe cabinets and other areas deemed critical by the AHJ. Critical areas must have 99% coverage, while general building areas must have 90% coverage.
    • O.3.3 Signal strength: Minimum inbound and outbound signal strengths of -95 dBm are required. Note that a typical 12.5 kHz FM radio will operate satisfactorily at -102 dBm in multipath fading. When measuring signal strength at the repeater site, the gain of any amplifiers between the antenna and receiver must be used to translate the -95 dBm threshold to an equivalent carrier-to-noise ratio at the receiver.
    • One weakness of Annex O is that it does not explicitly recognize the role of radio interference other than to say that the RES must not interfere with other parts of the public-safety radio system. In other words, the document specifies signal strength alone when it should specify a minimum carrier-to-interference-plus-noise ratio. This oversight is mitigated somewhat by specifying a minimum delivered audio quality (DAQ) in the section on testing.
    • Although not stated in the document, we must presume a building that meets the requirements of O.3.2 and O.3.3, without enhancement, does not require an RES. Similarly, enhancements only should be required on floors or in areas that do not meet radio-coverage requirements.
  • O.3.4 System frequencies: The RES must be capable of retransmitting all relevant public-safety frequencies, and these frequencies are to be provided by the public-safety agency through the AHJ.
  • O.3.5 System components: Signal boosters, including BDAs, shall have FCC type acceptance and must be operated in accordance with commission rules (for example, see CFR Title 47, Part 90.219, Use of Signal Boosters).
  • O.3.6 Power supplies: A building on fire likely will lose its prime power source, so a strict standard for secondary power is necessary. Annex O requires either 12-hour battery backup for the RES, or an automatic-starting generator and storage batteries dedicated to the generator capable of at least two hours of operation at 100% capacity. (As many of our readers can attest, generators sometimes fail to start automatically and require manual intervention.)
  • O.3.7 System monitoring: An automatic-monitoring system — with a dedicated panel in the emergency command center of the building — is required. The system must signal an alarm in the event of an antenna malfunction or signal-booster failure. A separate alarm for oscillating amplifiers is desirable. Power supply systems must, at a minimum, signal an alarm when AC power is lost, when the battery charger fails and when the battery has a low charge (defined as 70% of capacity).
    Now that we've examined the major components of Annex O, let's take a look at the building owner's and the AHJ's responsibilities regarding radio enhancement systems.
  • Responsibilities of the building owner: For new buildings, the building owner or architect should engage an RES designer to estimate the needs for enhanced coverage and to ensure that the proper infrastructure is installed to accommodate the RES. If an RES is required, the building owner or his authorized agent is responsible for system design, permitting, purchasing, testing and maintenance.
  • Responsibilities of the AHJ: The AHJ has many responsibilities and these responsibilities may be daunting unless expertise is available in-house or an outside expert is retained. First and foremost, the AHJ must decide which buildings fall under the jurisdiction of Annex O. New buildings only? All buildings of more than 100,000 square feet? Existing buildings that have undergone recent modifications?
  • The AHJ also must specify the frequencies to be amplified on the system, being mindful of any new frequencies planned by the public-safety agency in the future. Because all amplification systems introduce time delays, the AHJ must specify the maximum delay (in microseconds) that can be tolerated by the radio system. Usually, only narrowband channelized amplifiers introduce enough delay to cause problems in overlap coverage areas. The AHJ must review RES permit applications and issue permits for new systems. And finally, the AHJ must ensure qualified technicians are available to review test plans and oversee testing.
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