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120405 - NIST RF interference test summary


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									           Coexistence Testing for Electronic Safety Equipment
        The National Institute of Standards and Technology (NIST), funded by the
Department of Homeland Security (DHS), has been working the National Fire Protection
Association (NFPA) to devise standards and test methods for Personal Alert Safety
Systems (PASS) that rely on wireless communication technology, otherwise known as
RF PASS. Some of these RF PASS technologies operate in the unlicensed ISM bands,
and, thus, must undergo in-band RF interference or coexistence testing. This
coexistence testing is designed to introduce into the RF propagation channel types of
interference that may be found in environments where firefighters are deployed. These
tests focus on replicating RF propagation conditions for large building structures such
as office buildings, factories, convention centers and apartment buildings. Certain
wireless transmissions that may cause interference are commonly found within these
structures. For example, in offices and apartment buildings, the use of wireless local-
area networks (WLAN) or wireless personal-area networks (WPAN) is common. In
warehouses and factories, the use of Radio-Frequency Identification (RFID) technology
is common. Wireless systems such as WPAN and RFID operate in ISM frequency
bands, with frequencies and power levels specified by the FCC. Consequently, the
NIST RF interference or coexistence test is designed to test systems that operate in
ISM frequency bands using commonly encountered transmission protocols. The test
could be extended to other wireless devices such as hand-held radios, RFID, RF
locators, and wireless medical devices.
        A key aspect of the NIST coexistence test is the use of shielded, coupled
anechoic chambers, which enables the RF interference to occur in a controlled RF
environment. The base station is placed in one chamber, and the portable device is
placed in another chamber connected by a cable. The “path loss” between the
chambers is set by varying the attenuation inserted in the cable path connecting the
chambers. An calibration procedure is used to set the total path loss experienced by the
signal between wireless devices located in different chambers. As shown in the figure
below, the interfering signal is introduced into the test chamber that contains the user-
worn RF PASS. This test method allows free-field testing of a complete RF PASS
system without the use of conducted measurements or removing the antennas from the
devices-under-test. As with many emerging wireless devices, RF PASS antennas are
often integrated into the body of the portable unit, and, therefore, detachment of the
antenna is not possible. Free-field testing allows the system to be characterized with
any unusual antenna radiation pattern intact.
        The test configuration is designed to simulate the condition where a firefighter is
indoors in the presence of some other radio system. Because it is expected that the
firefighter will typically be some distance from the RF interfering source, in this test
method, the output power of the interferer is reduced by an amount equal to the free-
space path loss corresponding to 1.25 m distance. This distance was chosen as the
expected closest proximity between a firefighter and another wireless device, and falls
in the range of distances proposed in similar work on medical device RF interference
testing discussed in [1] [2]. The interfering source in this test method operates at
approximately the same output power as the RF PASS, which typically is the maximum
power allowed by the FCC.
        Another key element of the NIST coexistence testing is the concept of channel
usage by the interferer. In a typical deployment environment, the amount of RF
interference will vary from instant to instant, so the target value of interference used in
testing should be defined statistically. In this testing approach, the channel usage is
determined at the physical or RF level rather than at the data or information level of the
communication process.
        For the RF PASS coexistence testing, we define a 50 % channel usage such
that a spectrum analyzer measurement of the signal within the test chamber over a 30
second period will detect the presence of the interference source 50 % of the time. The
remaining samples in that 30 second period will measure an interference-free RF
channel. In addition, to simulate the variability of a realistic channel, over any 5 second
interval, the interference should be active between 25 % and 75 % of the time. To
measure the interferer channel usage for RF PASS testing, the spectrum analyzer
sweeps across the ISM frequency band of interest in less than 3 ms. Data acquisition
software captures the spectrum with a sampling rate of 225 ms ± 50 ms, and searches
for the maximum value within the captured spectrum. Only the interference source is
active when determining the interference channel usage, that is, there is no RF PASS
communication activity. The ratio of interference signal samples to the measured noise
samples provides the channel usage percentage.



                                             RF PASS                   Base
                                     (e.g., SCBA on its side)         Station

  A schematic of the RF interference test setup for RF PASS. The RF interference source is
  connected via a power combiner to the antenna located at the top of the chamber containing the
  RF PASS portable unit, (the Self-contained Breathing Apparatus (SCBA)).

        Note out that interference or coexistence testing has been reported in prior
literature: for the 900 MHz ISM band, see [3][4], and for the 2.4 GHz ISM band see
[2][5][6]. In addition, [6] performed laboratory-based coexistence testing in the 2.4 GHz
ISM band for medical applications. In the future, it may be possible to merge some of
the testing concepts, such as the channel occupancy (discussed here) and the
transaction “breakdown” (discussed in [6]).

[1] “American National Standard Recommended Practice for an On-Site, Ad Hoc Test
Method for Estimating Radiated Electromagnetic Immunity of Medical Devices to
Specific Radio- Frequency Transmitters,” ANSI C63.18-1997, Piscataway, NJ: IEEE,

[2] N. J. LaSorte, H. H. Refai, D. M. Witters Jr., S. J. Seidman, J. L. Silberberg,
“Wireless Medical Device Coexistence,” Medical Electronics Design, Aug. 2011.

[3] M.R. Souryal, D.R. Novotny, J.R. Guerrieri, D.G. Kuester, and K.A. Remley, “Impact
of RF interference between a passive RFID system and a frequency hopping
communications system in the 900 MHz ISM band,” IEEE EMC Symp. Dig., July 2010,
pp. 495-500.

[4] K.A. Remley, M.R. Souryal, W.F. Young, D.G. Kuester, D.R. Novotny, and J.R.
Guerrieri, “Interference tests for 900 MHz frequency-hopping public-safety wireless
devices,” IEEE EMC Symp. Dig., Aug. 2011, pp. 497-502.

[5] T. Keller, J. Modelski, "Experimental Results of Testing Interferences in 2.4 GHz ISM
Band," Microwave Conference, 2003. 33rd European , pp.1043-1046, Oct. 2003.

[6] S. Seidman, W. Kainz, P. Ruggera, and G. Mendoza, “Wireless coexistence and
EMC of Bluetooth and 802.11b devices in controlled laboratory settings,” Open Biomed.
Eng. J.,” 2011, vol. 5, pp. 74-82.

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