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NIST engineer Kate Remley Credit: NIST |
By accurately re-creating the
jumbled wireless signal environment of a city business district in a
special indoor test facility, researchers at the National Institute of
Standards and Technology (NIST) have shown how the wireless industry
could lop hours off the process of testing the capabilities of new
cellular phones. The NIST techniques also could simulate complex
real-world environments for design and test of other wireless equipment.
As described in a forthcoming paper,* NIST researchers
conducted tests in downtown Denver, Colo., to measure precisely the
clustering of signal reflections from radio waves bouncing off one or
more multi-story buildings multiple times before reaching a distant
receiver. The researchers replicated this environment indoors using a
“reverberation chamber,” a room with highly reflective surfaces and a
big, slowly rotating paddle that automatically alters signal paths.
First, researchers feed a wireless transmitter’s signal into a device
called a fading simulator, which is adjusted to re-create the timing and
strength of the reflections of an outdoor urban area. The output then
is fed into the reverberation chamber, where signal reflections decay
exponentially over time, creating a cluster of signals similar to that
observed in the field tests.
Industry certification of cell phones currently
requires tests of parameters such as total radiated power using the
opposite of a reverberation chamber, a room called an anechoic chamber
that is lined with materials that absorb radio waves and reflect as
little as possible. This testing takes about a day, requiring dozens of
measurements of cell phone directional power from multiple angles. By
contrast, an equivalent set of tests could be performed in about an hour
in a reverberation chamber, according to NIST engineer Kate Remley, a
senior author of the new paper. Reverberation chambers also could be
used to measure cell phone receiver sensitivity, although currently
there would be no time savings for this test, Remley says. Many industry
testing practices are established by CTIA-The Wireless Association, the
trade group representing the wireless industry.
NIST is studying new applications for reverberation
chambers, which have typically been used to measure electronic
equipment’s immunity to radio-frequency interference. By adjusting the
reflectivity of the chamber through selective use of signal-absorbing
material, researchers have found they can “tune” the signal decay time
to simulate the conditions found in real-world environments. NIST
researchers expect the new method will be useful for test and design of
wireless devices such as cell phones, notebook computers equipped with
wireless links, as well as new technology such as wireless beacons being
developed for the emergency responder community.
The Denver tests were conducted in 2009. NIST
researchers measured the power delays between a transmitter and a
distant receiver positioned on streets lined with buildings three floors
high or taller and a flat, single-layer parking lot. Most buildings
were constructed of glass, steel, and concrete.
* H. Fielitz, K.A. Remley, C.L. Holloway, Q. Zhang, Q.
Wu and D.W. Matolak. Reverberation-chamber test environment for outdoor
urban wireless propagation studies. IEEE Antennas and Wireless
Propagation Letters. Forthcoming.
Media Contact: Laura Ost, laura.ost@nist.gov, (303) 497-4880
