What’s the right infrastructure for public-safety DAS?

There are differences between the deployment goals of a public-safety DAS and a cellular DAS. A public-safety DAS is primarily concerned with comprehensive (everywhere in the structure) coverage that provides clear and consistent voice communications for first responders. With a cellular DAS, the goal is to provide coverage in public-use areas and also enough data capacity for smartphone users.

A public-safety DAS requires redundant equipment and fiber paths, along with overlapping coverage to achieve an appropriate level of system reliability. Commercial DAS networks do not require redundancy. Trying to combine a redundant and non-redundant system in one configuration is not consistent with good engineering practices and raises the cost of the deployment.

Interference within a radio network can bring down an otherwise properly working system. As more frequencies are added to a network, the likelihood of interference grows dramatically. An integrated public-safety/cellular DAS carries many frequencies and warrants much deeper intermodulation distortion analysis to determine and understand where direct hits—and, therefore, interference—may occur.

It is time consuming and expensive to understand and manage these non-linearities. Within a combined system, an appropriate safe operating mode may not be achievable. Trying to combine public-safety and cellular frequencies within a single system presents an unnecessary risk of interference.

In addition, public-safety and cellular DAS systems have different handset power control and coverage standards. A public-safety radio handset transmits full power at all times, while a cellular handset adjusts its power level according to the demands of a specific connection. Therefore, an integrated DAS is more difficult to optimize, because of the differences in antenna layout, potential uplink overload issues and overall dynamic range issues demanded by these operating differences. And suboptimal antenna layouts used to address these differences can lead to higher system costs.

Antenna placement is another mismatch between public safety DAS and cellular DAS. Public safety systems generally use lower frequency bands—for example, spectrum from 150-450 MHz—and it takes fewer antennas to cover a building for these services than it does for a cellular DAS operating at frequencies as high as 2100 MHz.

Putting the two systems together makes it overkill from a public-safety perspective, in terms of the number of antennas (and associated “upstream” equipment) needed. Antennas for public-safety DAS also will also required in areas where commercial use is typically not required, such as stairwells, back rooms, and spaces for utility and HVAC equipment. These areas are typically not covered in cellular DAS deployments.  

If the commercial DAS is sectorized (as is often the case in larger venues, such as airports), the combined cellular/public safety system will be burdened with the need to add more antennas and cabling not normally required of a public-safety system alone. More infrastructure means more cost, and additional materials means a more complicated network and higher long-term maintenance expenses.

Finally, combining a cellular and public-safety DAS is economically inefficient, because it requires the deployment of equipment throughout the DAS that wouldn’t be necessary for either of the DAS systems alone. For example, one regulatory requirement for public-safety DAS is that there be battery backup for the system for 8-12 hours. If the building has converged cellular/public safety DAS, it will have to provide backup for the entire infrastructure, which can cost 3-5 times what it would cost to back up the public-safety DAS alone.