Doug Lung /
09.18.2009 09:05 AM
SBE Expresses Concern Over Medical Devices in TV Band
The Society for Broadcast Engineers (SBE) told the FCC last week that allowing the use of medical devices in portions of the TV band could pose a danger to its users.

The association filed a Reply Comment in Engineering Technology Docket 09-36 [PDF], which proposes allocating spectrum in the 413 to 457 MHz band for Medical Micro-Power Networks (MMN). While there is little concern that these low power devices would cause interference to existing users of this spectrum, SBE and others expressed concerns that their high power operations within this band would interfere with MMN. This interference could have a devastating impact--possibly life-threatening--on patients using MMNs.

In its reply comments, the SBE noted that it agreed with other comments filed by the Association of Public-Safety Communications Officials-International, Inc. (APCO), Land Mobile Communications Council (LMCC), and the Association for Maximum Service Television, (MSTV), in that this portion of the spectrum was a poor choice for MMINs.

"This is a flawed proposed allocation, and would subject MMN device users to harmful, and perhaps dangerous, interference from much higher power co-channel signals," the SBE said in its comments.

The SBE points out that while the Alfred Mann Foundation (AMF) (the organization requesting this spectrum for MMNs) proposed "redundant coding" to ensure implanted media devices would not suffer interference from higher power RF sources, it didn't provide technical detail about the effectiveness of such coding.

SBE's reply included this explanation about their concerns:

"AMF offers as the reason why the 470–698 MHz UHF TV band is unsuitable for MMN devices that it is 'occupied by high-power UHF TV transmitters, thus rendering the RF environment highly challenging for MMNs.' This indicates that AMF's redundant coding is in fact unable to reject co-channel undesired signals."

The SBE stated too that an identical argument would apply to RPU operations in portions of the 450-456 MHz band, as RPU systems generally use antennas that are close to ground levels and "as a result closer to the wearer of the implant in a horizontal plane."

The organization also supplied numbers to back up its concerns.

"As to power density, a single 100-watt station using a 25 kHz emission yields a 4 mW/Hz ratio. A megawatt ERP DTV station yields 160 mW/Hz. A 100 watt emission on a 12.5 kHz channel yields 80 mW/Hz. The present FCC rules in Part 74 and Part 90 now require new operations at the narrower bandwidth, higher density channels. This results in greater exposure to co- and adjacent-channel interference due to the effective doubling of potential users in a given slice of the spectrum."

It stated that patients could be exposed in an unpredictable and uncontrolled fashion to RPU operations and "would be subject to interference potentials and an RF environment approximating that created by the 'high power UHF transmitters' that AMF admits are a highly challenging environment for MMNs."

Broadcasters use the 450-451 MHz and 455-456 MHz part of the proposed MMNs spectrum for live audio feeds, two-way communications related to radio and TV news and programming, and for IFB communications to radio and TV reporters in the field.

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Posted by: Brian Smith
Fri, 09-18-2009 12:44 PM Report Comment
While it is true that 100 watts/25 kHz yields 4 mW/Hz, one should not use the channel bandwidth, rather the occupied bandwidth, which is at most 20 kHz, or 5 mW/Hz. The figure for a 12.5 kHz (11250 Hz occupied, if analog, less otherwise) is 8.9 mW/Hzthe 80 figure given is probably a typo. Even the new FDMA "supernarrowband" radios achieve at most 25 mW/Hz power density. However, they may be half a meter from the MMN, and a 1 MW DTV would never approach that proximity. So though its numbers are suspect, its conclusion is justified.

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