ABSTRACT
The hottest technology in personal networking is wireless networking, which allows users to share a high-speed internet connection among multiple computers without being tethered to wires or wall jacks. Traditional indoor wireless uses a network of transmitters receiver and antennas for communication in the interior of a building. Such a placements is more a matter of trial & error and may many a times lead to ineffective communication. An easy solution to this problem using the already existing infrastructures is the HAVC duct. Heating ventilation and air conditioning ducts in building are typically hollow metal pipes which can be used as waveguides to carry signals and provide the network access to offices. Knowledge about channel properties is crucial to design such a communication system. At high frequencies this duct behaves as a multimode wave guide with a transmitting antenna coupling in and a receiving antenna coupling out. This model represents a step towards the development of a tool for planning a wireless distribution system using building HVAC ducts.
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.
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.
The operation
is described as follows. For the down link, RF signals sent from a base station
propagate through the ductwork and a small portion of electromagnetic energy is
radiated by a simple antenna inserted into the HVAC duct passing from each
room. In the uplink, the RF signal of the end-user transmitted by the laptop, handset
etc. reaches the passive antenna located in each office and propagates towards
the base station.
PROPAGATION MODEL
The HVAC channfl like all other wave guides
is a linear channel and therefore can be completely characterised by its
frequency response or transfer function. To design a wireless HVAC system, an
analytic model is necessary. This model must be valid for the ducts of
different cross sections and allow to investigate easily the frequency response
dependence on such parameters as antenna geometry, transmitter receiver
seperation distance, duct cross section size, conductivity of duct material, reflection
coefficients of terminated duct ends etc. Such a model for the HVAC duct
channel in the case of a straight multimode duct terminated at both ends is
given below.
This is a
straight HVAC duct of circular cross-section, made of metal and and terminated
at each end as shown. Two monopole probe antennas provide the coupling. Such a
duct is a double-probe wave guide with a number of propagating modes N
determined by the operating frequency and waveguide dimensions. Let the
termination loads 1&2 ie load1 and load2, have respective reflection
coefficients '1n & '2n for wave guide of mode n which can be frequency
dependent. Let ‘L’ be the distance between the two antennas and respective
distances to the terminated ends be L1 and L2.
EXPERIMENTAL VERIFICATION
To validate
the propagation model already described, verification of the frequency response
has to be done experimentally. Measurements were performed on a straight
section of a circular HVAC duct 30.5 cm in diameter made of galvanized steel.
The antennas were thin copper monopole probes 3.5cm long
and 1mm in diameter, set on a straight line along the duct length. A network
analyzer (Agilent E8358A) with an internal impedance of 50 ohms to measure the
frequency response between the probes in the 2.4 to 2.5GHz is used.
The
theoretical frequency response was computed for the case of a duct with matched
load on both the ends i.e.'= 0. The frequency response shape (number of nulls,
their depth and position) depends on the excited mode distribution, the
distance between the antennas and the distance between the terminations if any.
Three most significant excited modes in this geometry are TE61(R=16.5ohm),
TE51(R=8.6ohm), TE41(R=3.5ohm). It is mostly the interference between these
three modes that determines the specific locations of peaks and nulls. Adding
more modes increases the accuracy of the theoretical curve.
It can be seen
that the experimental frequency response curve (dashed line) and the
theoretical frequency response curve (solid line) are in good agreement.
Small-scale variations observed on the experimental curve are due partially to
surface and shape imperfections of the circular HVAC duct used for
measurements.
It can be seen
that the experimental frequency response curve (dashed line) and the
theoretical frequency response curve (solid line) are in good agreement.
Small-scale variations observed on the experimental curve are due partially to
surface and shape imperfections of the circular HVAC duct used for
measurements.
RESEARCH ISSUES
Although the
preliminary experiments described in this paper support the feasibility of the
HVAC RF distribution system, detailed research in a number of areas is needed
to develop systematic design procedures. In the following we briefly comment on
several of these.
Characterization of the RF
channel
Unlike
conventional waveguide circuits, in most cases multiple waveguide modes will be
above cutoff in ducts. This multimode environment will lead to delay spread
much like multi path in open propagation environments. Other sources of delay
spread will be reflections from the bends, junctions and end plates. In any
event, delay spread and coherence band width of such channel needs to be
explored both theoretically and experimentally.
Coupling into
multimode ducts
The existence of multiple propagating modes is a
complication usually avoided in conventional waveguide circuits. Design and
design rules are needed for realizing efficient couplers in the various sizes
and shapes of ducts that are commonly used, for each frequency bands of
interest.
Mode conversion and cross
polarization in multimode ducts
In the presence of multiple propagating modes, it is
likely that the preferred strategy is to optimize coupling into the
lowest-order, or dominant, wave guide mode. However, since HVAC ducts are not
constructed with the same precision as the actual wave guide circuits, mode
conversion is likely at joints and other imperfections. In addition to creating
delay spread as discussed above, this mode conversion could lead to signal loss
owing to excitation to orthogonally-polarized modes, as well.
Power division at branches
and tees
To obtain satisfactory power distribution throughout a
large building, it will be necessary to be able to determine and control the
power division at branches and tees.This power division is also complicated by
the 4existance of multiple propagating modes. The use of irises made using wire
screens and grids should allow independent control of power division and air
flow.
Coupling around
obstructions
Techniques are needed to couple around unavoidable
obstructions in the ducts. Design for both active and passive coupling needed
to be explored. The simplicity of passive probe couplers on either side of the
obstruction connected by low loss coax is attractive, but bidirectional
amplifiers may be needed in some instances as well. Such technique s could also
be used to couple two otherwise unconnected duct systems.
CONCLUSION
This paper
presents a new technique for high speed communication inside buildings which
seems to a viable inexpensive alternative to other existing “last mile”
technologies. Because existing infrastructure is used and the ducts exhibit
losses that are low compared with direct propagation and leaky coax, such a
system has the potential to be lower in cost and more efficient than either
conventional method. An approximate,
closed-form, propagation model for the straight HVAC duct channel in the form
of a multimode wave guide is presented here. Experimental measurements are
performed to validate this model and they are found to confirm the theoretical
results. Efficient modeling of RF propagation in a real HVAC system is a
challenging task. However this model should be perceived as a first step toward
predicting the radio coverage in ducts when designing an HVAC wireless
distribution system.
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