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.
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Ralph Haller,
Chair of the National Public Safety Telecommunications Council
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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.
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(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:
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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.
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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.
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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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