The following was posted on the Biomed Listserv this morning. Written by Rick Hampton, at Partners Healthcare,
he reports incidents of digital TV and WMTS interference, discussions
on this topic at the AAMI conference last May, and finally requests you
contact him if your facility is experiencing any interference. After
reading this, you might also want to check out a presentation that Rick
made titled, Checklists for RF: Installations and Ad hoc Testing, at MoHCA'S Summit on Electromagnetic Compatibility,
March 26, 2003. And here's an interview of Rick in 24x7 where he talks about WMTS. Emphasis in the following is mine.
disclosed information I had received from two hospitals that had
experienced interference to their WMTS telemetry systems from nearby
digital TV stations operating on TV channels 36 and 38. Some of
the information was received just a few days before leaving for the
After presenting the information, several of you approached me
afterwards to tell me you knew of other incidents of a similar, if not
identical nature. Interestingly, but perhaps not surprisingly,
FDA has no official knowledge of any events like this. [Here is a link
to the FCC about DTV and medical telemetry.] A search of the
MAUDE database turned up nothing. I also spoke with the folks
from ECRI who would have followed up on such reports and they hadn't
heard of anything either. Both of the hospitals I spoke with
requested I maintain their anonymity.
If you spoke with me regarding another interference event to WMTS
telemetry systems from nearby TV stations, please email me privately or
call me with your contact information. I would like to gather
this information and present it to FDA, FCC, ASHE, and ECRI as I
believe there are actions that could be taken to prevent this from
recurring in the future.
For those unable to make it to the AAMI conference, here are the basics of the interference events:
In mid-2002, the hospital purchased a telemetry system, installed in
September 2002, consisting of 54 channels of telemetry operating from
608.2025 - 610.7250 MHz. At the time of the initial site survey,
the nearby NTSC analog TV38 transmitter was operating at a reduced
The DTV36 transmitter was not yet activated at the time of the initial
site survey. As a result, all channels assigned were in the lower
portion of WMTS band and data from site survey used to design a
broadband antenna system.
Approximately 1 year after installation, the hospital began
experiencing intermittent, severe dropout on all channels. When
the Clinical Engineering department was unsuccessful in troubleshooting
and isolating the problem, the manufacturer was called in to diagnose
the problem. Upon performing a spectrum analysis in February of 2004,
the manufacturer found interference from both the NTSC analog TV38
transmitter operating with increased power and a newly installed DTV36
The manufacturer implemented corrective action in March 2004.
This included the installation of additional filtering and changing the
operating frequencies of all transmitters to the middle 1/3 of the WMTS
band. As a result of the additional RF filtering, the useful
portion of the WMTS band has been reduced, limiting future expansion.
New Jersey, 2004
In Mid to late 2003, the hospital upgraded its existing Part 15
telemetry system to the 608-614 MHz UHF WMTS band. A total of 96
telemetry channels were installed, with 48 channels in Telemetry and 48
additional channels in other hospital areas.
Beginning in November 2003, the hospital began experiencing excessive
dropout on multiple channels. Investigation by the manufacturer
showed high background noise and interference across the entire WMTS
In January 2004, the manufacturer of the antenna subcomponents
performed an onsite inspection and determined the amplifiers and
antennas had too much gain. The manufacturer began replacing the
amps and antennas with lower gain versions. Unfortunately, this
caused the problem to only get worse with the Telemetry unit now
experiencing serious problems. Most telemetry channels were not
functioning properly. The manufacturer then determined the
bandwidth of the new amplifiers & antennas was too broad and unable
to reject energy from the nearby DTV36 & DTV38 stations. Two
more weeks passed before the entire antenna system could be completely
redesigned and installed to preclude overload from nearby TV36 &
In the meantime, lower than usual patient census allowed the hospital
to use wired monitors and the remaining unaffected telemetry channels
to continue operation for the duration of mitigation activities.
This system was finally made fully functional across the entire 608-614
In both instances, the Chief Engineers
at the TV stations were contacted and described as being cordial, but
not particularly helpful. When the FCC was contacted, the
hospitals were redirected to ASHE, since ASHE is the official
coordinator for WMTS. When contacted, ASHE advised the hospitals
to continue working with the manufacturers to mitigate the
problems. It is important to note here, the WMTS rules contain no
regulatory recourse for institutions suffering adjacent channel
interference from nearby TV stations. It is up to the facility
and the telemetry manufacturer to solve the problem, either by
additional filtering or moving to a different band.
Louisiana, February 2005
At least one other possible interference event was reported to me, but
with no details. Based on a search of FCC's database, the most
likely areas where this would have occurred is in the vicinity of
Alexandria, New Orleans, West Monroe, or New Iberia. I've sent an
email to all Louisiana hospitals registered with ASHE, however the
report remains unconfirmed. I have received only a handful of
replies, all indicating no knowledge of any such event.
Based on what I've learned from the
manufacturers and hospital CEs, the two main contributing issues
leading up to these events are inadequate antenna design and a
misunderstanding of the WMTS regulations.
In both cases, the WMTS installation utilized a highly amplified,
broadband antenna system, with filtering inadequate to attenuate the
out-of-band radio energy from the nearby TV stations. The antenna
design is a holdover from Part 15 days when telemetry allocations
allowed operation on unused TV channels from 174 MHz - 668 MHz.
In those days, interference from a new nearby station could be remedied
simply by tuning the receivers and transmitters to a new, unused TV
channel further away from the new transmitter. Now, however, with
the WMTS allocation, there is no place to move without requiring the
wholesale removal and replacement of the affected system.
Further, the filters commonly used in these antenna systems are
inadequate for the job. Of the two events described, one hospital
used a legacy antenna system installed some time ago, while the other
had a new antenna installed with the old design. It should be
pointed out here that simply retuning the transmitters and receivers to
the UHF WMTS band may not provide adequate protection from digital TV
interference with this type of antenna system.
The second contributing factor was an inadequate understanding of the
WMTS rules by both the hospital CEs and, apparently, the manufacturers.
While one of the CEs stated he received notification letters from the
TV stations announcing the pending activation of the TV transmitters
(the other has no recollection of receiving the letters), both CEs
state they did not realize remedial action might be necessary to
prevent interference to WMTS systems. I
wish to point out here there is a common misunderstanding of what it
means for the WMTS to be "protected" by FCC. Many people believe
1) all WMTS systems are designed to prevent interference, 2)
interference to WMTS, of ANY kind, will not be tolerated by the FCC,
and 3) the FCC will take action against any interloper. It is
obvious from examining the rules and these cases, none of these beliefs
In light of these events, I strongly recommend any hospital utilizing UHF WMTS telemetry systems take the following steps:
1) Foremost, don't panic. While being near a TV36 or TV38 station
certainly increases your risk dramatically, it does not guarantee you
will experience interference, as there are several factors that enter
into the interference equation. There are currently 29 DTV36/38
stations on the air with allocations for another 55, a number sure to
grow. There is time for a levelheaded collection of information.
2) Review all coordination letters you may have received from nearby TV
stations to ensure there are no current plans for activation of a TV36
or TV38 station in your vicinity.
3) Because the FCC's Media Bureau is constantly processing requests for
new DTV allocations and requests for moving from one channel to
another, you should visit their website on a monthly basis to ensure
you haven't missed any coordination letters. You can perform a
directed search for your area at the "TVQ TV Database Query" at http://www.fcc.gov/mb/video/tvq.html. Alternatively, you can check the updated list of DTV stations at http://www.fcc.gov/oet/dtv/start/dtv2-69.txt.
(Note: Pay close attention to the preparation date of any prepared
lists you reference. Many links on the FDA and FCC websites are out of
date and lead back to the original allocation list dated 1998.)
4) Contact the manufacturer of your WMTS telemetry system to ascertain
the type of antenna system and receivers in your system. Request
the manufacturer provide you with a detailed list of upgrades, along
with their costs, needed to prevent your system from suffering from
"adjacent channel interference" from TV36 and TV38 signals.
5) Should you find a new TV36 or TV38 station near activation in your
vicinity, contact the TV station and respectfully request they delay
beginning broadcasts until you have had a chance to perform remediation
activities to your system, if needed. I would suggest you copy
your correspondence to ASHE, FDA, and FCC.
6) If your facility has already experienced interference to your WMTS
system, report it to the FDA and ECRI. If, for whatever reason,
your administration wishes not to report such issues to FDA, PLEASE
consider reporting them to ECRI or, failing that, to me. I will
honor requests for anonymity as I have for the other two hospitals, but
understand it is vitally important for these types of events to be
If you have any questions, please feel free to contact me.
Wireless Communications Manager
Partners HealthCare System
One Constitution Center, OCC210
Charlestown, MA 02129
UPDATE: I confirmed with Rick that all of the above interference
problems were with channelized WMTS, not the frequency hopping (GE) or
DECT-based (Philips) "smart-hopping" WMTS implementations more recently
available. It's also worth noting that DTV produces "side lobe
interference" where signal from channels 36 and 38 bleed over into the
WMTS band -- all allowable by the FCC. As a result, markets with both
TV channels face additional pressure in implementing more than a modest
number of fixed channels in WMTS.