Although the networks that buildings use to distribute signals for public safety radio communications and for commercial cellular wireless communications share the common name of distributed antenna system (DAS), they really should be considered to be two different types of wireless infrastructure, according to Dino Giordano. A principal RF design engineer, Giordano handles wireless products for Microwave Photonic Systems, where he directs the design and development of DAS products. He spoke at the 2017 Network Infrastructure Forum, a part of the International Wireless Communications Expo. His talk was titled “Convergence of Public Safety & Commercial DAS.”
Figure 1 (below) compares portable radios of the type used by emergency first responders and mobile handsets of the type used by consumers who buy wireless service from the likes of Verizon, AT&T Mobility, T-Mobile US and Sprint. Giordano said that portable radios transmit as much as 5 watts of radio frequency (RF), which puts a lot of power into the air, and it is at a fixed (nonvariable) level — unlike lower-powered cellular handsets. The radios use frequency bands from VHF to 900 MHz.
The lower frequencies used for public safety radio contrast with frequencies used for cellular, which range from 700 MHz to 2150 MHz and higher. The antenna density suitable for VHF won’t work for a 2-GHz system, Giordano said, requiring a different way of thinking about the two systems.
Both systems have two options for obtaining the signals they carry: taking them as a direct feed from a repeater or base station, or using an antenna to receive the signals over the air. The use of an antenna is much more common for public safety radio than for cellular signals.
With some buildings, the cellular carriers install their own small cell or other type of base station, Giordano said. Not many municipalities bring public safety radio repeaters to buildings, so the DAS developer will have to use an antenna and then condition the received signal to feed the DAS. When base stations feed cellular signals directly into the DAS headend, conditioning the signals is not necessary.
Giordano said that public safety radio system designs emphasize coverage over capacity, and it’s the reverse for cellular systems. He gave as an example a police radio system with 14 channels, which is the number that defines its capacity. “The worst case would happen when 14 or more users keyed their radios at the same time,” Giordano said. “Most likely, that’s not going to happen because if you have 14 people in a distressed situation and they key up, they’re not going to key up in the same place. They’re going to disperse. You can’t hear each other if everyone’s yelling into their portables. So, your worst-case scenario might be five people keying up.”
With commercial cellular service, thousands of people could attempt to use their handsets at the same time during a special event that brought them together. Giordano said that means capacity becomes a big issue.
Another issue Giordano identified is the need for cellular systems to offload traffic from the DAS to a Wi-Fi network, another method for increasing capacity. “With a public safety system, you don’t even care about that,” he said. “You want to make sure you have coverage. You want to make sure, if an officer or firefighter falls behind an obstruction and an emergency medical service (EMS) technician responds, he can key his radio and despite building loss and hip loss, he has seamless communications. For commercial services, if something happened to attract a crowd, such as David Copperfield walking into the room, and if everyone wanted to call their friends and send video of him, they’re going to hit the network hard and load it down.”
Elevator shafts and elevator cars are additional locations in buildings that public safety systems and commercial cellular systems can treat differently. A commercial system can leave out coverage for elevator shafts, Giordano said, but communicating in elevator shafts may be critical for firefighters.
Another difference with public safety systems involves frequency plans. To give an example, Giordano said the 700-, 800- and 900-MHz public safety services use block frequency assignments much the way commercial cellular systems have block frequency assignments. The uplink and downlink frequencies are in blocks.
“With VHF and UHF communications, it’s the Wild West,” Giordano said. “Assignments change per location. Requirements change per location. In one building or another in the same city, the city may only require radio coverage for the fire department, and in other buildings they may require coverage for the police and emergency medical service, too.”
VHF and UHF public safety frequency band plans sometimes have interleaved frequencies with alternating uplink and downlink frequency assignments. Giordano showed an example of a frequency scheme for an in-building communications system with interleaved frequencies (see Figure 2). “You see how they interleave uplink and downlink frequencies,” he said. “How do you accommodate that? One way would be to use a digital type of configuration where you can take a bandwidth and differentiate between each channel. Or you can use a quad type of configuration with chunks of broadband — a 2-megahertz chunk, and then a 3-megahertz chunk and so on and so forth to accommodate something like that.” Giordano said VHF public safety frequencies have similar interleaving that calls for challenging separations.
In contrast, the public safety communications frequency bands at 700 MHz, 800 MHz and 900 MHz and the commercial services have guard bands that make isolating the uplinks from the downlinks more effective. Giordano said designers of public safety DA must spend considerable time on achieving isolation between frequency bands and, instead, designers of commercial systems spend time designing offload capacity.
Regulatory requirements differ between the two types of systems, Giordano said. Public safety systems must meet regulatory requirements from the National Fire Protection Association and the International Code Council with its International Fire Code. A public safety DAS must deliver coverage throughout the entire building and meet a minimum acceptable deliverable audio quality. “Public safety systems require redundancy for certain headend equipment and require backup power,” Giordano said. “Commercial systems have requirements of their own, but not of this magnitude, so why would you put a commercial service under the scrutiny given to a public safety system if you don’t have to?”
Wireless carriers could be placed in a position where they would be guaranteeing that the shared infrastructure will work for life-critical communications, Giordano said. “That is the big element,” he said. “No one wants to get a lawsuit because a public safety system doesn’t work for a life-critical situation.”
Satisfactory convergence of public safety and commercial DAS may lie in the monitoring, Giordano said. Although he favors separate hardware for the systems, he said maybe a third party could monitor both systems. “The hardware has many tap-off points, and commercial systems have many received signal strength indication (RSSI) and other measurements,” he said. “That could all be gathered into one third-party network operations center where the systems could be monitored or controlled from one remote location. In an IP type of environment, it would be possible to verify that the public safety system is working fine and to develop demographics for the commercial system, all on the same software base. With the monitoring and control, it could be possible to have a nice system with separate hardware and possibly separate coaxial infrastructures, monitored under the same centralized service.”
Giordano said he sees more separate public safety and commercial DAS, and he said one reason is because different manufacturers make them. He said some manufacturers offer converged systems, but the majority of buildings use systems from separate manufacturers, or they have partnerships. “I’ve been with partnerships where you go to a big entity and find that its public safety realm is covered by strategic partners,” he said. “I would say about 90 percent of in-building enhancements are using separate hardware.”
Don Bishop is the Executive Editor of AGL Magazine He joined AGL Media Group in 2004. He was the founding editor of AGL Magazine, the AGL Bulletin email newsletter (now AGL eDigest) and AGL Small Cell Magazine. He is a frequent moderator and host for AGL Conferences. Don writes and otherwise obtains editorial content published in AGL Magazine, AGL eDigest and the AGL Media Group website.