In this paper, we developed a 3-dimensional (3D) ray-tracing simulator using MATLAB for establishing the viability of heating, ventilation, and air conditioning (HVAC) ducts as a reliable communication channel for indoor communication at millimeter-wave (mm-wave) frequencies. We present theoretical analysis of image theory ray-tracing and provide the equations for total electric field due to different rays undergoing reflections at the duct walls. We also computed the received signal strength indicator (RSSI) for the dry and moisture-laden air flowing through the HVAC ducts. The ray-tracing results are compared with the experimental and theoretical results we obtained for the HVAC ducts. With transmitter effective isotropic radiated power (EIRP) of 7 dBm, we obtain RSSI which varies between -34 dBm and -53 dBm for dry atmospheric pressure and temperature of 1013.25 hPa and 294.26 K, respectively, and duct lengths of up to 8 m at 60 GHz.
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