1. INTRODUCTION

Wireless transmission of electromagnetic radiation (communication signals) has become a popular method of transmitting RF signals such as cordless, wireless and cellular telephone signals, paper signals, two way radio signals,video conferencing signals and LAN signals indoors.

Indoor wireless transmission has the advantage that building in which transmission is taking place does not have to be filled with wires or cables that are equipped to carry a multitude of signals. Wires and signals are costly to install and may require expensive upgrades when their capacity is exceeded or when new technologies require different types of wires and cables than those already installed.

Traditional indoor wireless communication systems transmit and receive signals through the use of a network of transmitters, receivers and antennas that are placed through out the interior of a building. Devices must be located such that signals must not be lost or signal strength may not get attenuated. Again a change in the existing architecture also affects the wireless transmission. Another challenge related to installation of wireless networks in buildings is the need to predict the RF propagation and coverage in the presence of complex combinations of shapes and materials in the buildings.

In general, the attenuation in buildings is larger than that in free space, requiring more cells and higher power to obtain wider coverage. Despite of all these, placement of antennas, receivers and antennas in an indoor environment is largely a process of trial and error. Hence there is need for a method and a system

for efficiently transmitting RF and microwave signals indoors without having to install an extensive system of wires and cables inside the buildings.

This paper suggests an alternative method of distributing electromagnetic signals in buildings by the recognition that every building is equipped with an RF wave guide distribution system, the HVAC ducts. The use of 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 RF is distributed more efficiently.



2. THE HVAC SYSTEM

Heating, Ventilation and Air Conditioning are ducts used in buildings designed to carry air to and from all parts of the building. In most parts of the USA and Europe almost every building is equipped with these HVAC ducts which can also function as hollow wave guides for microwave and RF signals.

Therefore, all forms of wireless transmission can in principle can be done through these waveguides. Since most of the offices and other places in buildings where people work, sit or reside are reached by this HVAC ductwork, it is also possible to provide communications between building occupants and rest of the world.

The HVAC system includes a device usually a coupler for introducing electromagnetic radiation into the duct work such that the duct acts as a wave guide. System also includes devices for enabling the electromagnetic radiation to propagate beyond the duct. In most cases ducts are largest near the central air handling equipment and become smaller as they branch out to various rooms.

Branches in the duct behaves as wave guide power splitters. Eventually RF would be radiated into the rooms through specially designed louvers. Coverage in corridors and spaces guarded from louvers could be realised by placing passive reradiators in the sides of the ducts.



The key idea behind this distribution is that low loss electromagnetic waves can propagate in hollow metallic pipes if the dimensions of the ducts are sufficiently large compared to the wavelength. Since the HVAC ducts are made of sheet metal, they are excellent waveguide candidates. the lowest frequency that can propagate in a duct depend upon the size and cross section shape of the duct. For rectangular wave guides or ducts, the cut off frequency fco for the lowest propagating mode is given

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