RF propagation in an HVAC duct system: impulse response characteristics of the channel

Usually, the heating, ventilation, and air conditioning (HVAC) duct system in buildings is a complex network of hollow metal pipes of rectangular or circular cross-section which behave as multimode waveguides when driven at RF and microwave frequencies. The impulse response in the HVAC duct system used as a wireless communication channel is shaped by three physical mechanisms: probe coupling, attenuation, and dispersion. Three types of dispersion exist in this channel that affect the RMS delay spread. In order of importance, they are multipath reflections, intermodal dispersion, and intramodal dispersion. We present a model for the power delay profile in a straight terminated duct which allows exploration of RMS delay spread parametric behavior. As an example, we calculate the delay spread as a function of distance in a straight duct and find it to be in good agreement with experimental data at distances up to 15 m. A real HVAC system has a complicated geometry, which may include bends, junctions, etc. Efficient modelling of its channel properties is a challenging task. Our model for straight ducts should be perceived as a first step towards that goal.