29-07-2010, 06:57 PM
Prepared by:
Christopher P. Diehl, Benjamin E. Henty, Nikhil Kanodia, and Daniel D. Stancil
Abstract
An alternative method of distributing RF in buildings is proposed in which the heating and ventilation ducts are used as waveguides. Because of the relatively low waveguide loss, this method may lead to more efficient RF distribution than possible with radiation through walls or the use of leaky coax. Further, the use of existing infrastructure could lead to a lower-cost system. Initial experimental results are presented that demonstrate duct-assisted propagation between nearby offices in a university building. An example method is described for obtaining efficient coupling between coax and 8”x12” rectangular duct over the 902-928 MHz ISM band.
Introduction
One of the challenges related to the installation of wireless networks in buildings is the need to predict RF propagation and coverage in the presence of complex combinations of shapes and materials in a building environment. In general, the attenuation in buildings is higher than in free space, requiring more cells and higher power to obtain adequate coverage. Over the past several years, an extensive wireless data network has been installed at Carnegie Mellon that provides coverage to about one half of the campus with raw speeds of two megabits per second. With more than 100 access points, it is believed to be the largest local area network (LAN) installation anywhere. The effort required to provide a detailed description of the geometry and material composition of buildings on such a scale often causes system designers to resort to trial-and-error layouts. An alternative to relying on direct propagation throughout a building is to install leaky coaxial cable[5]. Although this method lends itself to a systematic design procedure, the cost of the coax and its installation may be prohibitive. An alternative method of distributing RF in buildings is suggested by the recognition that every building is equipped with an RF waveguide distribution system—the HVAC ducts. The use of the HVAC ducts is also amenable to a systematic design procedure but should be significantly less expensive than other approaches since existing infrastructure is used and the RF is distributed more efficiently.
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