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In this paper, we report theoretical and experimental channel-capacity estimates of heating, ventilation, and air conditioning (HVAC) ducts based on multicarrier transmission that uses M-ary quadrature amplitude modulation and measured channel responses at the 2.4-GHz industrial, scientific, and medical band. It is shown theoretically that data rates in excess of 1 Gb/s are possible over distances up to 500 m in straight ducts in which reflections have been suppressed. Our experimental results also show that even in the case of more complex HVAC duct networks (i.e., HVAC duct networks that include bends, tees, etc.) data rates over 2 Gb/s are possible. Our estimations in this case are valid for distances of up to 22 m, which was the maximum distance of our experimental setup. These experimental results, measured with a large-scale testbed set up at Carnegie Mellon University, Pittsburgh, PA, albeit limited in terms of transmitter-receiver separation distance, provide further evidence on the potential of HVAC systems as an attractive solution for providing communications in indoor wireless networks. Index Terms--Heating, ventilation, and air conditioning (HVAC) systems for wireless transmission, indoor propagation, internet access, multicarrier transmission, orthogonal frequency-division multiplexing (OFDM).

Heating, ventilation, and air conditioning (HVAC) ducts in buildings behave as multimode waveguides when excited at radio frequencies and thus, can be used to distribute radio signals. The channel properties of the ducts are different from the properties of a usual indoor propagation channel. In this paper, we describe physical mechanisms which affect the HVAC channel impulse response and analyze their influence on the delay spread. Those mechanisms include antenna coupling, attenuation, and three types of dispersion: intramodal, intermodal, and multipath. We analyze each type separately and explore the behavior of the delay spread as a function of distance in straight ducts. Experimental channel measurements taken on real ducts confirm the validity of our model. Index Terms--Dispersive channels, indoor radio communication, multimode waveguides, wireless local-area network (LAN).