Measure, measure and then measure
Ensuring radio frequency, or RF, coverage in enclosed structures — such as parking garages, buildings and tunnels — is a challenging task because some in-building RF distribution systems, if improperly installed, have the potential to introduce and amplify interference as they extend coverage into dead areas. Consequently, system designers are faced with a delicate balancing act that is complicated by the special challenge of providing temporary in-building coverage for first responders during emergencies.
One of the keys to designing and implementing a successful RF distribution system is to measure the existing environment before any work is done. “Measure before you bid on a job. Measure before system installation. And measure before the system is turned on,” said George Potter, an engineering manager for ComTech Communications. “You want to make sure you don’t bite people once you turn it on. … It doesn’t take days to take quick measurements, only an hour or two.”
Initial RF measurements are taken with a field strength meter to determine the power of the desired signal in the building. When the system is being installed and tested, the field strength meter is complemented by a spectrum analyzer, which is used to identify any potentially unwanted signals that may have been introduced into the building environment. In-building systems can increase desired signals but also have the potential to deliver and amplify unwanted energies that could cause other problems. Such unwanted signals “will bite you,” Potter said, noting that they can diminish the effectiveness of a bi-directional amplifier (BDA), particularly those that are improperly installed. Consequently, it’s best to choose a BDA with the necessary bandwidth and filtering to minimize undesired carriers, he said.
Such preparation can reveal surprising characteristics in the RF environment. “Buildings are often not what they seem,” said Potter, who has been designing RF distribution systems since 1999. “Reinforced concrete will often pass RF. Clear glass sometimes doesn’t.”
Modern buildings glass-coated with metal oxides to reflect sunlight and/or keep in heat can interfere with the ability to get signals in and out of a site.
On the other hand, buildings constructed with a lot of steel may actually need fewer antennas and cabling than similar structures lacking such reinforcement. “Metal pans used on each floor act as waveguides,” Potter said. “This isolates each floor, so you need to have an antenna on each one. It also means you don’t have interference between antennas, so you need fewer of them.”
“Another problem area concerns the time it takes for signals to move through the BDA,” Potter said. “Most broadband-type BDAs don’t introduce significant delays, but channelized or digital signal processor-based devices could cause problems, as processing from analog to digital and back again adds to the time it takes for signals to move in and out of the building. Such delays can interfere with radio and cell phone coverage, so installers need to consult with the system manufacturer concerning the type of radio to be used and the distance to the outside antenna.”
RF-over-fiber — fiber-optic technology used to distribute signals — is finding increased use for in-building distribution systems. But it has its limits, according to RF consultant Jack Daniel, owner of Jack Daniel Co.
“Fiber-optic technology can be used to replace long runs of coax,” Daniel said. “RF is converted to light at the input, and light is converted to RF at the output. [But] it is an analog modulation and cannot be run over multimode fiber, only single-mode fiber. And you can’t run RF on the same fiber as digital devices, such as routers or repeaters.”
The inability to share a single strand of fiber-optic cable with data equipment isn’t typically a problem because builders install plenty of fiber in new constructions to support telecommunications and data networks. “Single-mode [fiber] costs almost as much as multimode, so people just put in single mode,” Daniel said. However, while it typically is easy for an RF planner to find the appropriate type of unused fiber-optic cable in a new building, older buildings can become more expensive projects due to the cost of putting in new cable.
RF-over-fiber has been used in unique ways to distribute signals while avoiding interference with existing broadcast infrastructure. Daniel used the technology in downtown Los Angeles to provide lower-floor building coverage for the city government in a low-rise structure surrounded by larger towers (see graphic).
An antenna on one of the adjacent towers was used to pick up and transmit signals. Fiber was then used to carry the signals from the rooftop to underground fiber, where it made a 3-block journey into the basement of the government building. There it was converted back into RF and fed into a BDA. According to Daniel, the upper floors of the building effectively block interference and feedback from an antenna on an adjacent building and avoid the problems that would have resulted from placing an antenna on the roof of the government building.
When in-building RF distributions systems don’t exist, or may not be functioning because of a disaster, first responders have to bring their own. Portable or suitcase-sized repeaters allow public-safety agencies to immediately provide extended coverage for a floor above and below where the repeater is deployed. They also can be used to provide coverage to the outside of a building if there’s enough time to set up an antenna near or through a window.
However, portable solutions must be toted into a building along with other rescue equipment, adding to the workload of first responders. “A temporary in-building solution can weigh 30 to 50 pounds,” said John Facella, M/A-COM’s director of public-safety markets. Another problem: Onboard batteries only provide airtime for a limited number of hours. Facella said one method that might extend the reach of in-building repeaters is to position them near elevator doors.
“The elevator shaft may act like a waveguide,” he said. “[But] placement may be critical and will vary by building. It must be pre-tested [before use].”
Another alternative for temporary in-building coverage is to use vehicle-mounted repeaters, typically mounted on fire-fighting apparatus. Vehicle-mounted repeaters offer greatly extended service life because they can use the power from the vehicle for operations, but they have to be used carefully because multiple devices in close proximity to each other can cause interference or operational problems.
“Newer systems eliminate this problem. … [They] are smart enough to recognize each other on-scene and figure out which one should be active while the others go to stand-by,” Facella said.
