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 of common interest. HVAC ducts can be used as a wireless communication channel for providing high-speed network access to offices [1]. An example of an HVAC system used for signal distribution is shown in Figure 1. The access point is connected to an antenna which excites waveguide modes in the duct system. These modes propagate through the duct system with different group velocities and attenuation constants, experiencing multiple reflections from terminations and non-uniformities, and are received by the office antenna. Users access the system either directly (via a cable connected to the office antenna) or wirelessly (the office antenna re-radiates into the office space).