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Does Your Building Need a Passive or Active DAS?

At RF Solutions, we design public safety and cellular enhancement distributed antenna systems (DAS) for high-rise buildings and other large venues. Cell DAS installations are almost always active solutions, but public safety systems can be passive, active, or hybrid. Before we begin to design a system, there are many factors to be considered. The optimal system design will depend on the unique characteristics of the building in which it will live and the communications requirements of local first responders. These factors include the coverage area, the design of the building, the building materials, signal source(s), and the project timeline. Once we’ve gathered the building particulars, we can determine whether the system should be passive, active, or a hybrid of the two. 


A passive system uses “passive” components like coaxial cable and splitters to distribute RF signals. Using coax to distribute the signal will produce a higher signal loss than an active solution. The further the antennas are located from the head end, the higher the signal loss, which results in lower downlink output power. These restrictions are the reasons why a passive system may not be appropriate for a specific building. 


An active system uses fiber optic cable to distribute signal between a centralized signal source (head end) and remote nodes placed strategically throughout the building. A hybrid solution is built upon an active system using fiber to distribute signal but relies on a passive DAS design for most signal distribution. Buildings with unique challenges might benefit from a hybrid solution. 


When determining if a building needs an active system, there is no magic size (square footage) number per se, but it usually falls around the 1million sq. ft. mark. However, there are many sub-1mm sq ft buildings where we’d choose an active system. You could easily have a 750,000 sq ft legacy hospital facility with dense walls and floors that will need to use an active system to meet the required coverage because RF signals will have poor penetration. Many older (pre-war) buildings present these same issues. A new, smaller building might require an active system if it’s steel reinforced. A warehouse with heavy shelving and dense stored materials may need one as well. Conversely, a +1mm sq. ft football stadium might not require an active system due to the open architecture. That’s why it’s so important to understand the propagation characteristics, like wall density, building composition, and building age, before starting the design. 


You might ask why not always use an active solution? The simple answer is cost. Active systems are expensive, but an added benefit of using a fiber DAS is that it’s future-proofed. Using fiber provides the unique ability to upgrade when new technologies or frequencies must be added to a system. Converting the radio signals to light over fiber allows the use of multiple distribution points (nodes) with little or no loss in the transport system.


So, you see, there isn’t a hard and fast rule you can use to determine which way to go with your public safety DAS but knowing all the building’s characteristics and technology requirements beforehand will steer you in the right direction.