July 14, 2009
Wireless Mics and Public Safety Frequencies

The DTV transition last month ushered in a new era of digital television by closing the door on full power analog television broadcasts and clearing the UHF-TV spectrum between 698-806 MHz. Much of that "700 MHz Band" was assigned to telecommunications giants such as Qualcomm, AT&T and Verizon in a highly publicized auction last year, but it is perhaps not as well known that significant portions were also assigned for use by Public Safety agencies.

For the many wireless mic users who own and continue to use wireless equipment that transmits in this band, there appears to be a decidedly mixed blessing here: On the one hand, the spectrum is clear and the wireless systems can therefore transmit farther and more reliably than ever. On the other hand, as the winners of last year's auction begin to fire up transmitters in their spectrum, wireless mic users may suddenly and without warning find themselves unable to transmit reliably beyond three feet! There simply is no way to predict when the carriers will begin testing and deploying base station transceivers in the spectrum they paid such a premium for. Having said that, it would appear that Qualcomm is ready to deploy almost immediately. In addition, Verizon has begun limited testing of LTE technology in some markets.

When interference from one of these Advanced Wireless Services (AWS) does occur, either the wireless mic user has to change frequencies or the AWS subscribers will have to move out of the range of the wireless mic (or both). A nuisance to be sure, but not life threatening.

This is not the case with Public Safety communications, however. There are already a number of 700 MHz mobile radio systems in full operation across the country, and more on the way after the DTV transition completed in June.

 

Ralph Haller,
Chair of the National Public Safety Telecommunications Council

 

According to Ralph Haller, Chair of the National Public Safety Telecommunications Council and former Private Radio Bureau Chief at the FCC, "Users of wireless microphones in this band should know that their current systems could become unusable as more public safety agencies move to 700 MHz.  More serious is the potential for critical first responder information to be blocked by wireless microphones, resulting in loss of life or property."

Although it seems hard to imagine given the relative differences of RF output level and bandwidth, a recent engineering study filed with the FCC by V-Comm Engineering came to the following conclusion:

Operation of wireless microphones in the same spectrum as CMRS (Commercial Mobile Radio Services) and Public Safety will result in significant harmful interference to all parties.

  • In most cases, the interference will NOT be mutual.  It will impact one user or the other user (not both at the same time).  It will impact either the Wireless Microphone system or the CMRS/PS system, not both at the same time. The interference depends on proximity between the systems and uplink/downlink spectrum band used.
     
  • Since the interference is not mutual, there is no incentive to resolve the interference caused to the other party.
     
  • Both types of systems operate with sufficient power levels to cause interference to each other; they cannot co-exist on a co-channel basis in the 700 MHz spectrum.
 

 

(L) Henry Cohen, Production Radio Rentals
(R) Don Root, San Diego County Sheriff's Department

As Henry Cohen of Production Radio Rentals explains, “if a [First Responder] is at the fringe of their reception area, either due to distance or structure attenuation, a nearby wireless mic will overpower the Public Safety signal if the mic is tuned to a frequency that encompasses the PS frequency within its channel bandwidth.”

Don Root, Operations Manager for the San Diego County – Imperial County (CA) Regional Communications System is concerned that if the wireless mic “manages to cover the control channel of the public safety trunk system in the venue where the paramedic is trying to communicate with the hospital while working on a critical patient, (they) will have no idea why his or her radio doesn't 'work' there."

There are a couple of ways to avoid this catastrophic interference:

  1. Take advantage of manufacturer's rebate offers.
    All of the major wireless microphone manufacturers offer a rebate incentive to trade these units in for systems that operate below 698 MHz. In many cases, this will result in a substantial savings towards the cost of a new system.

  2. Re-band existing units.
    Some manufacturers are also offering to re-band units at a discounted rate (compared to their normal service charges). This gives you the opportunity to move into a favorable frequency range for less then the cost of even the discounted systems available with a rebate.

  3. Stay away from Public Safety and CMRS frequencies!

If must use your systems above 698 MHz, be aware of the following:

  • As previously stated, the 700 MHz band will become increasingly and unpredictably crowded.

  • It is likely that the FCC will ban the use of wireless microphones in the 700 MHz band. Unlike their current policy of, in the words of Nady System's John Nady, "tacit allowance and benign neglect" toward unlicensed wireless mics, the FCC will take enforcement action in cases of interference to public safety. The FCC Enforcement Bureau has even dropped by film sets.

  • Until the FCC officially bans wireless mic/intercom operation in the 700 MHz band, here are some guidelines to help choose frequencies and stay away from Public Safety radios:

  •  
    • DO NOT use frequencies between 763-775 MHz and 793-805 MHz.

    • Since no one successfully bid on Block "D" in the 700 MHz auctions last year, you can temporarily set your systems to "squat" there between 758-763 MHz and 788-793 MHz.

Of course, once the FCC does ban radio mics from the 700 MHz band, you must shut down and replace your systems. Verizon has announced that Boston and Seattle will be the first cities to lose 700 MHz

The T-Band Problem

Wireless microphone and intercom manufacturers also offer systems that operate between 470-512 MHz. These frequencies are allocated nationwide for UHF television stations (hence the name "T-Band"), and are also allocated in thirteen “Major Economic Areas” to commercial and Public Safety entities for use with land mobile radio services (walkie-talkies, repeaters, etc.), and says Haller, "there are similar interference concerns in that band."

Fortunately, the FCC maintains a free online database of all assigned frequencies, searchable by frequency range and geographical coordinates. Enter your frequency range and choose "Geo-Search," then enter your address and radius. Stay away from the resulting frequencies by at least 150 KHz.

Between Public Safety allocations, the impending deployment of Advanced Wireless Services, and White Space-TV Band Devices on the horizon, there is no doubt that this a tough time to predict which frequencies will be clear in the long run.  RF spectrum policy is caught between three competing forces:

  • The traditional "scant resource" model, which says that spectrum is just like any other natural resource and needs to be tightly managed.
     
  • The "market forces" philosophy which says that private enterprise and capitalism are best equipped to determine the most efficient use of the spectrum.
     
  • The "Open Spectrum" approach, which defines interference as a cognitive and computational problem as opposed to a frequency problem.

Whether you believe RF spectrum management should be approached from a scientific, economic, or political philosophy, one thing is clear: Stay away from Public Safety frequencies. As Henry Cohen noted in his presentation at Gotham Sound: "The situation they are responding to could be the one that you're involved in."


With special thanks to Henry Cohen, Ralph Haller and Don Root.

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Wireless Mics and Public Safety Frequencies
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