Taking the Toll Off Testing

National Institute of Standards and Technology (NIST) researchers conducted
tests in downtown Denver, to measure precisely the clustering of signal
reflections from radio waves-bouncing off one or more multistory 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.

By accurately recreating the jumbled wireless signal environment of a city
business district in a special indoor test facility, researchers at 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
equipments.

First of all, researchers feed a wireless transmitter's signal into a device
called fading simulator-which is adjusted to recreate 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.

The industry certification of cell phones require 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.
Reverberation chambers also could be used to measure cell phone receiver
sensitivity, although currently there would be no time savings for this test.
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, notebooks equipped with wireless
links, as well as new technologies such as wireless beacons being developed for
the emergency responder community.

Archana Singh

archanasi@cybermedia.co.in