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Questions regarding DAS's

Markb

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Hi All,

I am working with my Fire Prevention folks on Distributed Amplifier Systems in the city in which I work.
I recently assisted with/observed coverage testing at a new concrete tilt-up industrial complex being built.

A little background first:

Law enforcement is on a Motorola P25 700MHz countywide trunked system

Fire is on a city-owned repeated analog VHF system with on-scene tactical communications in simplex

The coverage tests are coming back as failing (predicted based on completion of construction and occupation) on the uplink side. I am assuming at this point that it is the 700 trunked system that is failing, absent specific information from the vendor.

The vendor is stating that the -95db uplink signal strength requirement is being waived by some other jurisdictions around us. Can anyone explain that rationale? It seems to me that being able to transmit out of a building would be just as important as receiving into the building. Also, out of curiosity, why the -95db threshold? Is it related in particular to digital communications - sufficient DAQ or is it important to analog as well? It can't be an arbitrary number.

Thanks in advance,
Mark

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mmckenna

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-95dB is in the NFPA standards as the minimum signal strength for reliable coverage/bit error rate, etc. My understanding is that it applies to the systems specified by the Agency Having Jurisdiction (AHJ). Doesn't matter if it's analog or digital.
 

Markb

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Yeah, I was just curious how they arrived at -95.

...and in turn the ramifications of excepting buildings from that requirement. Seems like it would be a hard figure and anything less would mean degraded or no coverage.
 
Last edited:

MTS2000des

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Keyword is minimum. I am not keen on how NFPA arrived at -95, but in the real world when implementing a DAS/BDA, a better figure to shoot for is -80 to -85, this gives you a 10db fade margin to work with in the link budget, assuring that, even if a subscriber radio is slightly out of alignment, user doing stupid stuff (like wearing the radio too close to the body, unapproved antennas, etc) they will still fall within the -95dbm and not get high BER (digital) or static (analog) and be able to talk.

Any BDA/DAS should be properly designed by a professional with experience, not some "get er done" hack. BDA/DAS design is a detailed art, and not something someone can just go buy off Amazon, throw into a building, and walk away- that is, unless one wants problems with expensive consequences.
 

mmckenna

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Any BDA/DAS should be properly designed by a professional with experience, not some "get er done" hack. BDA/DAS design is a detailed art, and not something someone can just go buy off Amazon, throw into a building, and walk away- that is, unless one wants problems with expensive consequences.
I wanted to add something along the lines of that to my reply above.

An installer saying that they need a waiver to get the system to pass is a huge red flag, with flashing red lights, and a little guy in and orange vest yelling "danger, danger, danger" all at the same time.
There is no shortage of "installers" who will claim they can/know how to install BDA systems. Some of them actually know what they are doing. Some do not. Some simply took a quick course put on by a manufacturer and are now self proclaimed "ex-spurts, profeshunuls". A system designed by an engineer with the proper certifications should not need waivers to pass. Installers who are properly trained and backed up should not even mention the term "waiver", they should be fixing the issue with the assistance of the engineer. Since these system require periodic retesting/certification by most agencies, getting a waiver now can be a problem later on. The system -will- have issues down the road, and you're likely to get another company that'll come in to "fix" it and you'll get screwed over because they'll have to fix all the issues created by the original installer.

Sounds like a total Cluster-#### in the making.
 

Markb

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Thank you guys for the input. We are requiring developers to use licensed companies to assess for and install DAS/BDA's where required. We adopted our code requirement (consistent with NFPA) from a large neigboring agency with a lot of dialed comm folks, so that's why I was wondering about the donor signal exemption.
I am leaning towards recommending that we deny. I am just a rank-and-file guy so we'll see where that goes.

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radioman2001

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Other than Part 90 certified systems there is no license installers. Most as is ours is Part 15, and as Part 15 states must not generate interference and must accept interference that may cause degradation of service. IMO BDA's are old technology, DAS is the better option with more control of inbound and outbound RF. The best is an actual base/repeater located within the dead zone, then distributed out to all areas of building/tunnel etc.
 
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I was talking with a friend who has done many DAS installs from the engineering side. I asked if the IFC or NFPA had requirements for testing relating to time of day, building occupancy or even something like doors closed or open. A test in a new building sans furniture and folks is bound to be much better than a real world situation.

He likes to keep the SeeHawk antenna no higher than waist high to give a more accurate test. He mentioned a firefighter calling a mayday for a fallen FF could be on his/hers hands and knees with a non vertical antenna orientation which the testing does not take into account.
 
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