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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  . 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. Power Combiner External Attenuator RF Interference Source 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 , and for the 2.4 GHz ISM band see . In addition,  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 ).  “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, 1997.  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.  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.  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.  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.  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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