If a cellular fiber DAS has ports where I can simply plug in my Public Safety service and share some of the infrastructure, then it seems like a no-brainer to take that route, right? The answer is NO, not necessarily and here’s why:

Consider the following before jumping into what could be a costly mistake:

Code – Do your homework before considering a shared DAS? Contact your AHJ to research code restrictions early in your planning process. Some jurisdictions don’t allow a shared DAS.

Filtering – If your Public Safety DAS, regardless of whether fiber based or passive, is to be installed in the same building as a cellular DAS, then there is a high probability that additional filtering will be required. The size and cost of the filtering can be drastically different depending on isolation. Typical colocation filter costs can range from $1000-$2000 per filter. If a converged solution is preferred, expect to add an extra $500 to $1000 per filter per remote locations, due primarily to the lack of port-to-port isolation offered by most combining devices. If, however, a separate passive DAS is used, generally 2 to 3 times more isolation can be achieved by physically separating antennas, resulting in a smaller, lower cost filter.

Keep in mind that two filters are generally required; one that reduces downlink noise and must be in-line with the Public Safety DAS, and the other, to be placed in-line with the cellular DAS and functions to reject strong out-of-band Public Safety signals.

Coverage – Want to see your iBwave heat map suffer a major set-back, just add an external colocation filter to protect against strong Public Safety signals. If the desire is convergence, plan on reducing your remote’s output power by 50% to 75% because of the additional insertion loss from the filters. Yep, you guessed it, double the number of cellular remotes needed for your design.

The added benefit of designing separate cellular and Public Safety DAS is more isolation, which translates to relaxed filtering requirements. For example, colocation filters with 30dB of Public Safety reject will have considerably less insertion loss than those used for converged designs where upwards of 80dB may be required.

Did you know that the density of antennas needed to build-out a Public Safety DAS is about 1 antenna to every 4-5 cellular antennas?  That said, building a standalone Public Safety DAS might not be as pricey as you think, especially if you consider battery requirements. Speaking of batteries…

Battery Backup – It gets worse. According to most codes, if a Public Safety in-building coverage enhancement system shares any active components with a cellular DAS, then those cellular components must comply with code as well. This applies to both headend and remotes systems. Here is where things get serious. Let’s consider a typical case where a fully loaded 5W cellular remote unit that consumes 500 Watts of power is compared to a typical Public Safety 2W remote consuming <150 Watts. The BBU cost for the Public Safety remote (separate DAS) is generally around $4,000, whereas one can expect to pay over $10,000 to backup each cellular remote (shared active components). So not only is the BBU for power hungry cellular remotes more than twice the price to that of Public Safety only remotes, but compound that with the limited coverage footprint of a cellular remotes and you’ll find that twice as many BBU’s are required than would otherwise be required by standalone Public Safety DAS.

Environmental – Code calls for all active components to be housed in NEME4 enclosures. Some cellular DAS vendors do offer fiber remote units that comply with this part of the code, however few offer headend systems in NEMA4 enclosures.  Therefore, be aware of the extra costs involved in meeting this section of the code.

In summary, there are many variables that come into play when considering whether to adopt a converged DAS, completely separated DAS, or a hybrid approach where only the passive DAS is shared. Worth noting is the price of labor , which can vary from region to region, as well as by application type. The purpose of this tech note was to bring awareness to some of major “gotchas” that could impact your bottom-line.

Comba Telecom, Inc. offers a public safety Fiber DAS that is available in Class A or Class B 700/800MHz single or dual configurations, FirstNet™ ready and fully compliant with IFC and NFPA (National Fire Protection Association). The channelized uplink squelch and channelized uplink AGC functions ensure minimum interference to base stations and optimizes communications quality. Donor signal is directly fed to the Master Unit of the CriticalPoint Fiber DAS; additional BDA for signal source is NOT required.