Saving Lives and Property Through Improved Interoperability

In-Building/In-Tunnel User
Considerations

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August 2002

TABLE OF CONTENTS
PAGE
PREFACE .............................................................................................................................. III
1.

INTRODUCTION .............................................................................................................1
1.1 Purpose of this Report .....................................................................................................1
1.2 Coverage in Buildings and Tunnels .................................................................................1
1.3 Organization of the Report...............................................................................................2

2.

SIGNAL STRENGTH AND COVERAGE CONSIDERATIONS ..................................3

2.1 In-Building RF Coverage Considerations.........................................................................5
2.2 In-Tunnel RF Coverage Considerations ...........................................................................7
2.2.1 Tunnel Materials......................................................................................................8
2.2.2 Straight Tunnel ........................................................................................................9
2.2.3 Curved Tunnel .........................................................................................................9

3.

SOLUTION CONSIDERATIONS .................................................................................11

3.1 Technologically Simple Solutions..................................................................................11
3.1.1 Messenger..............................................................................................................11
3.1.2 Talk-around or Simplex .........................................................................................12
3.1.3 Portable Repeater...................................................................................................12
3.1.4 Bi-Directional Amplifier ........................................................................................12
3.1.5 Radiating Coaxial Cable ........................................................................................13
3.1.6 Vehicular Repeater ................................................................................................13
3.2 Technologically Complex Solutions ..............................................................................16
3.2.1 Audio Switch .........................................................................................................16
3.2.2 Fiber Optic Transmission Line ...............................................................................16
3.3 Technologically Forward-Looking Solutions .................................................................19
3.3.1 Ordinances for New Construction ..........................................................................19
3.3.2 System Level Requirements...................................................................................19
3.3.3 Hybrid System Using BDA and Fiber Optics .........................................................19

APPENDIX APHYSICS OF PROPAGATION .............................................................. A-1

A.1 Radio Frequency Propagation in Free Space ................................................................A-1
A.2 Radio Frequency Propagation in a World with Obstructions ........................................A-2
A.2.1 Multipath Fading ..................................................................................................A-2
A.2.2 Material Characteristics ........................................................................................A-2
A.3 Radio Frequency Propagation in a Confined Space ......................................................A-3
A.3.1 Radio Frequency Propagation Within a Building...................................................A-3
A.3.2 Radio Frequency Propagation within a Tunnel ......................................................A-4
APPENDIX BGLOSSARY OF TERMS ..........................................................................B-1
APPENDIX CTECHNICAL REFERENCES ................................................................. C-1

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PREFACE
The PSWN Program is a jointly sponsored initiative of the Department of Justice and the
Department of the Treasury. The program encourages interoperable communications among
wireless networks to address local, state, federal, and tribal public safety requirements. It strives
to achieve the vision it shares with the public safety communityseamless, coordinated,
integrated public safety communications for the safe, effective, efficient protection of life and
property. To support program goals and objectives, the program analyzes various aspects of
wireless communications and provides findings, conclusions, recommendations, and other
considerations from the respective analysis to the public safety community at large. This report
details considerations for agencies requiring radio communications in confined spaces. Further
detail regarding the PSWN Program and its products and services can be found at
http://www.pswn.gov.

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1. INTRODUCTION
1.1

Purpose of this Report

This report presents considerations for achieving adequate radio coverage in buildings
and in tunnels especially since public safety agencies operate radios throughout a wide range of
spectrum and each frequency has different characteristics. These considerations are provided to
assist public safety agencies in meeting their unique needs for radio coverage in such confined
spaces. It assembles a variety of information from the Public Safety Wireless Network (PSWN)
Programs experience and the experience of system planners, manufacturers, and users in the
field to help individual agencies assess their current coverage capabilities and their ability to
remedy gaps in that coverage.
1.2

Coverage in Buildings and Tunnels

A radio system must be able to propagate or transmit a signal with enough strength to be
received where needed. The system should have the capability to perform this function with a
high degree of reliability under many different conditions. Engineers thoroughly understand
free-space propagation, i.e., radio propagation between two unobstructed points in a vacuum, and
can easily predict theoretical behavior. In a realistic setting, however, obstructions such as
terrain, trees, buildings, and people, can affect signal propagation. These real-world obstructions
can create difficulties in understanding and predicting radio coverage. The task becomes even
more complex when trying to predict coverage in a confined space, such as within a building or
inside a tunnel. Under these circumstances, coverage cannot be calculated to a certainty, only
estimated.
Consider the following scenarios:
1) It is a warm summer afternoon in a metropolitan area when a 50-car freight train
carrying hazardous chemicals derails spilling more than 5,000 gallons of hydrochloric
acid into a downtown tunnel. The chemicals burst into flames and wreak havoc on
the surrounding community, bursting pipes, disrupting public utilities, and causing
black smoke to billow from holes in the pavement. First responders to the accident
estimate the internal temperature of the tunnel to be in excess of 1,500 degrees.
Limited access to the tunnel allows only a few fire personnel to enter the tunnel at a
given time. Soon after leaving the safety and relative peaceful world above, the
firefighters enter a hostile world of fire, debris, and other hazards where their
communications to backup personnel or dispatch may be hampered.
2) It is a Friday afternoon as a police officer pulls up to a building. Just moments
before, he received a radio call from a dispatcher informing him of a hostage situation
developing on the ninth floor. Unknown to the officer, the perpetrators have secured
the entire building, including the three-level parking structure beneath the tower.
Initial intelligence reports indicate the building is being held by more than a dozen
heavily armed suspects. The last thing on the officers mind is whether he or she can
communicate with his command center once inside the seized building.

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The time to be thinking about your communication systems is before events such as these
occur, not as they are developing. How does your communications network perform once you
leave the relative safety of the outside world and enter buildings and tunnels?
1.3

Organization of the Report

The considerations outlined in this report are divided into two major components.
Section 2, beginning on page 3, discusses propagation at a high level, describing expected radio
frequency (RF) signal strength in various combinations of environments, antenna heights, and
building materials. Section 3, beginning on page 11 discusses potential coverage solutions for
the scenarios addressed in Section 2. For readers interested in the characteristics of radio
propagation, a technical discussion of the physics behind confined space radio propagation is
included in Appendix A. A glossary of the technical terms used in this document, with detailed
definitions, is shown in Appendix B. The report concludes with Appendix C, which lists the
references used while developing the In-Building/In-Tunnel User Considerations.
This report does not describe every possible communications challenge for confined
environments. Instead, it provides information assembled from the PSWN Programs experience
and the experience of system planners, manufacturers, and users in the field, which may assist
the reader in solving the particular challenges they confront.

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August 2002

2. SIGNAL STRENGTH AND COVERAGE CONSIDERATIONS

This section presents general considerations for public safety agencies in understanding
in-building and in-tunnel radio coverage in relationship to frequency and distance. Coverage is
the radio systems ability to be heard by a receiver on the system and to have the receiving radio
transmit back and be heard by the system. Generally speaking, radio coverage is best when the
transmitter and receiver are within line of sight (LOS) of each other. The considerations
presented in this section focus on identifying things that affect received signal strength
considering different parameters such as area settings (i.e., urban, rural), building materials (i.e.,
glass, concrete), or variances in the transmitter-to-receiver distance. The tables in this section
provide a qualitative coverage strength indicator for each public safety frequency band as it is
affected by various situations and materials. Due to the nature of radio propagation and
environments the signal may encounter, more than one of the obstructions or scenarios within
each table may apply. For example, Table 2 shows that a 406420 megahertz (MHz) signal has
excellent penetration into low-density buildings. However, if foil insulation, concrete, or dry
wall materials are used, the received signal strength within the building will decrease. In some
cases, the received signal strength may decrease dramatically for each material encountered.
Details supporting these considerations are discussed in later sections of this document.
Free-space propagation, or propagation with an unobstructed path between the transmitter
and the receiver, is the mode to which all other modes are compared. Although theoretical in
nature, free-space propagation provides a baseline to which radio propagation in the real world
can be compared. Free-space propagation is considered theoretical because radio waves are
transmitted in a vacuum, a condition that does not occur in the real world. For real world
radio propagation, physical obstructions (some as small as airborne particulates) cause signal
loss. For a more in depth discussion on Free-space vs. real world propagation, please see
Appendix A.
Obstructions include weather, terrain, and man-made obstructions. Heavy rain or snowfall,
between the transmitter and receiver, may cause signal degradation due to absorption loss. The
magnitude of this absorption loss depends on the frequency of the signal and the amount of rain
or snowfall in the path. Further, mountainous or hilly terrain and foliage will cause shadowing,
the partial blockage of the signal, and signal scattering generating even more attenuation. For
more information on shadowing and scattering, please see Appendix B.
Man-made objects, like buildings or bridge overpasses, tend to affect radio signals in ways
similar to mountainous terrain and foliage. These effects are presented in Table 1, in which
signal strength is indicated in a range from very good coverage to poor coverage. A rating
of very little coverage or poor coverage is generally inadequate for public safety
communications. As an example, mountainous terrain causes an area to receive average signal
coverage, a rating that is much less than the very good coverage rating associated with freespace propagation. Obstructions, such as buildings, will cause radio signal strength degradation
similar to, but generally more dramatic than that caused by terrain obstructions.
Assuming constant transmit power, radio coverage is typically greater (i.e., offers
increased signal strength, and a greater coverage area) in a less dense environment, such as a

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August 2002

rural area, compared with a dense environment, such as a metropolitan area. Coverage in a
building is affected by the presence of obstructions within the path between the transmitter and
receiver, including surrounding buildings, terrain, foliage, and the materials from which the
building is constructed. Table 2 illustrates that, when holding constant all the other transmission
parameters, lesser coverage (i.e., lesser signal strength and lesser coverage area) is generated in
an urban area compared with a rural environment.
Table 1
Propagation with Natural Obstruction for Public Safety Frequencies

Weather

Mountainous Terrain

Foliage

2550

138144

148174

220222

406420

450470

764776

794806

806824

851869

Atmospheric Loss
0 to 600 ft
Atmospheric Loss
600 to 1,200 ft

Public Safety
Frequency Bands
(MHz)

Free Space

Natural Obstructions

4 = very good coverage 1 = very little coverage

3 = good coverage
0 = poor coverage
2 = average coverage

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2.1

In-Building RF Coverage Considerations

Radio propagation in a building is much more complicated than propagation in free

space. A number of factors affect radio coverage in a building. The buildings relative location
within an agencys coverage footprint may determine a major part of the buildings internal
communications capabilities. The buildings size, layout and the materials with which the
building is constructed also contribute heavily to the communications dilemma of in-building
radio coverage. In-building communications can be defined in two possible ways

Internal unit-to-unitor the ability of subscriber units to communicate with each

other within the confines of the building
Subscriber unit-to-external infrastructureor the ability of a radio unit to
communicate with infrastructure located outside of the building.

2.2.1 Building Materials

When propagating into buildings, radio signals pass through various materials before
reaching a receivers antenna. The interaction of these radio signals with building materials
usually results in lower signal strength. However, it should be noted that signals behave
differently when encountering an obstructing medium, depending on that mediums
characteristics and specific electrical properties.1 These electrical properties, which are unique
for every material, dictate the extent to which a signal can transmit through the medium. More
specifically, RF energy entering a building will be partially absorbed and partially reflected by
the building materials encountered. To illustrate this concept, a signal traveling through a simple
glass window will lose less signal strength than a similar signal traveling through a glass window
containing high concentrations of lead or other metals. In a very similar scenario, a signal will
propagate through concrete more readily than through concrete with steel re-bar. These effects
are presented in Table 2.
Shown in Table 2 is a summary of how radio signals perform in different building
environments. This table is based on conclusions drawn from research, industry experience, and
laboratory modeling. The figures are intended to provide a qualitative indication how these
frequency bands perform under the identified environments. Signal strength is indicated in a
range from very good coverage to poor coverage.
2.2.2 Receiver Heights Within a Building
Depending on the location of the receiver relative to the transmitter, signal strength will
vary due to obstructions, weather, separation distance, and reflections. It is not always practical
to maintain or establish a LOS, however, receiver height may increase the ability to
communicate.

1 The electrical properties that affect in-building and in-tunnel radio coverage are permittivity, permeability, conductivity, and
susceptibility. For further explanation of these properties, please see Appendix BGlossary of Terms.

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Further, the receiver location within a given building with respect to the transmitter is
also a prime factor. A radio user trying to receive signals on the first floor of a building (from
outside the building or from different points within the building), in an environment with other
surrounding buildings, will more likely not have a clear LOS. Received signals on the first floor
may be blocked due to shadowing caused by the neighboring buildings and/or foliage. If a
receiver was placed on a higher story, the user might have a better chance of receiving the signal.
This improved signal would likely be a result of rooftop diffraction off nearby buildings, a higher
probability that the receiver is above the foliage, or even newly established LOS.
Below ground level, such as in basements or underground parking structures, radio users
generally experience lower signal strengths than levels above grade. This degradation occurs
because the signal must propagate through earth in addition to building materials to reach the
receiver, thus creating a large signal loss. The strength of a signal received in the basement is
significantly less than that of a signal received on higher floors within the building. These
effects are presented in Table 2.

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Table 2
In-Building Radio Propagation Considerations for Public Safety Frequencies
Receiver Heights
Within a Building
Low Density Buildings

Plain Glass

Leaded Glass

Foil Insulation

Concrete

Metal

Sheetrock

138144

148174

220222

406420

450470

764776

794806

806824

851869

4 = very good coverage

3 = good coverage
2 = average coverage

2.2

Medium Density
Buildings
High Density
Buildings

100 to 150 ft
above ground

0 to 50 ft
above ground
50 to 100 ft
above ground

2550

Public Safety
Frequency
Bands
(MHz)

Rural Setting
(low dense area)
Suburban Setting
(medium dense area)

0 to 30 ft
below ground

Building Materials2

Urban Setting
(high dense area)

Environment
Setting

1 = very little coverage

0 = poor coverage

In-Tunnel RF Coverage Considerations

It is difficult to provide reliable radio coverage within a tunnel environment. One of the
main reasons is the complex propagation environment of such enclosed structures. Every tunnel
has unique propagation characteristics because of its construction, structure, and size. Presented
in Table 3 is a summary of relative RF signal strength (i.e., coverage) in various tunnel
environments. The information provided in Table 3 is based on conclusions drawn from
research, industry experience, and laboratory modeling, as well as field testing using portable
2 For additional information on these building materials and their effect on radio communications, please see Appendix A.

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August 2002

radios and various radio test equipment (e.g., spectrum analyzers and field strength meters)
deployed in a Washington, DC, Metrorail tunnel. The study found that signals propagated better
within the 800 MHz band compared to propagation within the very high frequency (VHF) and
ultra high frequency (UHF) bands for a confined tunnel environment. This phenomenon may be
attributed to the wavelength of each frequency band. As the wavelength decreases in size, or the
frequency increases, it is more prone to be reflected within the environment. Rather than being
reflected, the lower frequency signals tend to be absorbed by the tunnel walls more readily than
the higher frequency signals. So generally speaking, higher frequency signals propagate better in
tunnel environments.
This finding is supported by other studies conducted in various venues. One such study
was conducted by independent researchers exploring wave propagation in curved tunnels to
present to the Institute for Electronics and Electrical Engineers (IEEE). The tunnels used to
study the wave propagation were located in Norway. In these studies a 925 MHz signal was
transmitted in relatively straight tunnels approximately 10 x 5 m and 4 km long. The tunnels
used in this study were constructed of materials (stone and rock) with average permittivity. In
this study, it was determined that the average attenuation of a 925 MHz radio signal, transmitted
at an effective isotropic radiated power (EIRP)3 of 45 dBm was approximately 15 db/km. The
findings of this particular study further verify the conclusion that higher frequency radio waves
propagate better than UHF and VHF in tunnels.
Another study was conducted by the United States Bureau of Reclamation, Hydroelectric
Research and Technical Service Group, in tunnels near Ephrata, Washington and Chama, New
Mexico. In this study, analysts measured the differences between the performance of 160 MHz,
400 MHz, and 900 MHz handheld units in a tunnel environment. Further, 600 MHz and 1600
MHz signals were measured in the same tunnels to calculate signal strength versus distance. In
these tests, the higher frequency (i.e., 900MHz) handheld units significantly outperformed the
lower frequency handheld units, once again supporting the conclusions drawn from the previous
two examples that higher frequency solutions are generally more suited for in-tunnel
applications.
It is important to note that like in-building communications, in-tunnel communications
can cover either unit-to-unit conversations within the tunnel, or unit-to-external conversations.
Due to the nature of tunnels, unit-to-external infrastructure communications can be quite
challenging. Often external infrastructure does not provide adequate coverage into a tunnel for
public safety communications and an alternative means of connecting in-tunnel responders to the
external infrastructure may be necessary.
2.2.1 Tunnel Materials
RF energy leaving the transmitter antenna is partially absorbed and partially reflected by
the tunnel material as the signal propagates down the tunnel. As shown in Table 3, due to the
electrical properties of the tunnel materials, a signal may propagate more efficiently in a tunnel
3 The EIRP of a transmitter is the power that the transmitter appears to have if the transmitter was an isotropic radiator, i.e., if it
radiated equally in all directions. By virtue of the gain of a radio antenna, a beam is formed that preferentially transmits
energy in one direction. EIRP is the product of the power supplied to an antenna and its gain.

In-Building/In-Tunnel User Considerations

August 2002

constructed of metal than a similar tunnel constructed with reinforced concrete. For example the
metal tunnel will reflect more energy than it will absorb. A concrete tunnel, however, will
absorb more energy than it will reflect; decreasing the distance the signal can propagate down
the tunnel.
Table 3 summarizes how radio signals perform in tunnel environments. Signal strength is
indicated in a range from very good coverage to poor coverage.
2.2.2

Straight Tunnel

In a straight tunnel, the data indicated that 800 MHz signals travel significantly farther
than VHF or UHF signals. The 800 MHz signal was acceptable throughout the entire measured
1,600 feet of the straight tunnel. According to the data, VHF coverage reached approximately
900 feet before the audio signal was severely degraded, as indicated by poor coverage in Table
3. UHF signals faded, as shown as a very little coverage indicator in Table 3, at approximately
900 feet and were severely degraded at 1,200 feet.
2.2.3 Curved Tunnel
RF signals propagating through curved tunnels experience a dramatic decrease in signal
performance compared with that in straight tunnels. VHF and UHF signals faded at
approximately 400 feet and 500 feet, respectively, in a curved tunnel. The 800 MHz signals
traveled more than twice the distance of the VHF or UHF signals. For a curved tunnel, or nonline-of-sight path to the receiver unit, the RF signal received was limited to only that signal
reflected beyond the curvature of the tunnel; thus, rendering a lower signal strength than one
might expect from a LOS transmission.

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Table 3
4
In-Tunnel Radio Propagation Considerations for Public Safety Frequencies

Public
Safety
Frequency
Bands
(MHz)

600 to 900 ft

900 to 1,200 ft

1,200 to 1,500 ft

0 to 300 ft

300 to 600 ft

600 to 900 ft

Concrete

All Metal

Construction
Material

300 to 600 ft

Curved Tunnel

0 to 300 ft

Straight Tunnel

2550

138144

148174

220222

406420

450470

764776

794806

806824

851869

4 = very good coverage

3 = good coverage
2 = average coverage

1 = very little coverage

0 = poor coverage

4 The information provided in Table 3 is based on conclusions drawn from research, industry experience, and laboratory
modeling, as well as field testing using portable radios and various radio test equipment (e.g., spectrum analyzers and field
strength meters) deployed in a Washington, DC, Metrorail tunnel.

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3. SOLUTION CONSIDERATIONS
This section presents options to assist public safety agencies in providing in-building and
in-tunnel radio coverage. The following pages provide sample solutions that address a variety of
constraints. The choice for the solution implemented can be influenced by RF interference
effects on associated systems, and budget resources. The solutions presented in this section are
divided into three categories based on the respective technology of the solution: simple,
complex, and forward-looking. Further, where possible, approximate costs have been included
in the summary tables to give the reader a rough estimate of cost impacts. For example, the first
solution, a messenger, has been categorized as a technologically simple solution that has minimal
cost.
This section describes individual solutions; however, a combination of solutions may
serve an agency better than any single solution. For example, to achieve adequate in-building
coverage for an emergency response at a high-rise building, an emergency operations plan may
call for the use of an audio switch, mobile command post, and portable repeater, in addition to a
backup plan of messengers. Other combinations of solutions could support mission requirements
for ad hoc emergency response as well as for fixed, known coverage trouble spots. For example,
an agency may wish to develop a portable audio switch solution in conjunction with a bidirectional amplifier network that is installed in the downtown district of the city.
It is important to note, however, that if agencies using disparate systems have already
developed an interoperability solution for use outside buildings and tunnels, then they may only
need to implement a similar interoperability solution inside the buildings and tunnels.
Interoperability outside of buildings and tunnels does not always translate into in-building or intunnel interoperability. For example, if each agency uses a switch based system for
interoperability, then a similar switch solution for in-building or in-tunnel interoperability may
be required.
3.1

Technologically Simple Solutions

This section addresses the technologically simple solutions summarized in Table 4.

These solutions are generally the most basic options an agency can implement. For the purposes
of this document, technologically simple solutions are those solutions that do not require any
specialized training or skills to implement and understand.
3.1.1 Messenger
In 490 BC, a military commander dispatched an unknown runner to Athens to inform the
council that the Persians had been defeated on the plains of Marathon. Since the advent of land
mobile radio (LMR) and other wireless devices, the need to dispatch messengers has been all but
eliminated. However, when communications fail or are simply unavailable, dispatching
messenger personnel to relay information from the responders to the incident commanders is
sometimes the only means of transferring information. Dispatching a messenger does not require
an installation or establishing common frequency bands that other solutions may require. While
this solution may provide benefits, it assumes personnel are available to relay messages, and is

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not practical over an extended period of time. Furthermore, information integrity may be put at
greater risk as the number of personnel increases from the point of origin to the destination.
3.1.2 Talk-around or Simplex
Although the messenger solution may be the least costly option, the talk-around or
simplex option is a close second. Because it is highly portable, users may be able to employ this
solution in the confined environments of buildings and tunnels. This solution requires both
parties to possess radios that operate with the same technology, in the same frequency band, and
that have a simplex or talk-around capability. As long as each radio is within coverage range of
the other radios being used, this solution can be employed in just about every environment. For
example, responders operating within a confined space and are within range of other portable or
mobile radios can use the simplex feature of their radio to communicate with each other.
Further, this solution may be used in a user relay format, much like the game of telephone, to
reach the external infrastructure. This solution is limited by available power output of the radio
and by the electrical properties of the confined space.
3.1.3 Portable Repeater
When the situation requires a more robust solution, the talk-around or simplex option is
generally too limited to provide the needed services. Portable repeaters, however, afford the
luxury of a more powerful system without the complex installation of a larger system. Also, the
nature of a portable repeater enables an organization to install it for use in a temporary
assignment. For instance, an executive protection detail can deploy a series of portable repeaters
for temporary communications as the person the detail is protecting moves from location to
location. While this solution may sound ideal, it too has its drawbacks. Both in-building and intunnel scenarios could call for implementation of this solution depending on the requirements of
the response units. If the repeater is confined to a large case or vehicle it may not be feasible to
use the unit in some situations such as in a collapsed tunnel or sub-basement.
Portable repeaters were designed to provide ad hoc coverage in areas where existing
LMR infrastructure does not exist to radio users operating on the frequency for which the
portable repeater is licensed. These repeaters can be used in conventional and trunked systems
and are mostly limited by the interference they may cause with existing systems (e.g., local
public safety or commercial networks) in the area. In some instances, portable repeaters may be
used to extend the coverage area of an agency or provide a semi-permanent solution until a more
permanent solution becomes available. Other shortcomings include limitations tied to available
portable power supplies and insufficient capacity.
3.1.4 Bi-Directional Amplifier
The bi-directional amplifier (BDA) is perhaps one of the most common solutions to the
in-building or in-tunnel dilemma. Originally designed to provide supplemental radio coverage in
difficult coverage environments, the bi-directional amplifier has become a valuable tool in
providing agencies with an in-building or in-tunnel projection of their radio network. A BDA
system consists of one or more amplifiers located inside a confined environment and is

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connected to an internal and external antenna network. The external antenna, usually located on
the roof of the building, or mouth of the tunnel, needing coverage, receives the signal coming
from the radio site. The BDA amplifies the signal and retransmits it into the building or tunnel.
A subscriber unit within the building can use the BDA to extend his portable radio coverage and
communicate with his external system. The BDA listens for incoming traffic inside the confined
space, amplifies it and retransmits it to the external system, hence bi-directional. A BDA can be
relatively inexpensive. However, it is the supporting infrastructure of cabling, antennas, filters
and power supplies that puts this solution in the medium cost category. Furthermore, unless
BDAs are adjusted correctly, they can create interference issueswith themselves, through
negative feedback; with other BDAs; or with the agencys existing radio system.
3.1.5 Radiating Coaxial Cable
Radiating coaxial cable, also referred to as leaky coax, is installed in subway tunnels,
ships, and buildings around the world. The low profile nature of this solution makes leaky coax
attractive for building and tunnel applications. It can be used where a BDA is impractical or
unsuitable, such as in subway tunnels where a low-profile antenna is required to avoid physical
interference with passing passenger trains. The design of the radiating coaxial cable provides
uniform coverage throughout the tunnel (where installed). In addition, radiating coaxial cable
has provided coverage benefits for a wide band of frequencies. It is important, however, to note
that radiating coaxial cables are not perfect solutions for every environment. Radiating coaxial
cables are passive devices. They can be used in conjunction with BDAs or repeater systems to
increase a systems in-building or in-tunnel coverage. Leaky coax is highly susceptible to
electromagnetic interference in high electromagnetic environments such as rail tunnels used in
conjunction with diesel locomotives. The electromagnetic fields created by the locomotives
generators can easily overwhelm a leaky coax solution.
3.1.6 Vehicular Repeater
A vehicular repeater is a component used in conjunction with a mobile radio, which
effectively expands the range of a portable radio in the field. To illustrate this concept, as an
officer leaves his/her vehicle and begins transmitting on his/her portable radio, the 3-5 W
portable radio signal is boosted through the vehicular repeater, thus enabling transmission at
much greater distances and the enhanced ability to penetrate in-building or in-tunnel. For inbuilding or in-tunnel scenarios, the vehicular repeater can be brought to the scene to improve the
localized communications in the emergency response area. The vehicular repeater typically is
not limited by a power source and is highly mobile. However, a disadvantage can be limited
versatility in confined or remote environments.

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Is portable
Can operate within any
environment
Is usable by most radios

Is portable
Has a flexible installation
Is a temporary solution

Not dependent on
available spectrum
Has a lower cost than
implementing a physical
solution
Can be used as a
temporary solution
Is the simplest solution

Advantages

In-Building/In-Tunnel User Considerations

Portable Repeater

Talk-around or
Simplex

Human Runner

Solution(s)

14

Has limited versatility in

confined environments
Is limited by power
supplies
Requires radios to be
within coverage of each
other
Requires mutual aid
frequencies between
radios
Can be difficult to find
antenna mountings
Requires power source
Provides a single
channel with limited
capacity
Is limited by logistical
concerns such as who
receives these radios, or
how other radios can be
integrated into the
system

Requires dedicating
human resources to the
task
Can result in a possible
loss of message integrity
Is not practical as a
permanent solution

Disadvantages

Both

Both

Both

In-Building or
In-Tunnel
Solution

Table 4
Technologically Simple Solutions

<$25,000

Talk-around or
simplex available in
existing radios

Human resource,
material cost minimal

Approximate
Cost

August 2002

Used by security
organizations to establish
temporary communications,
used in public safety events
to increase communication
capabilities, such as county
fairs, or festivals.

Used by personnel to
establish communications
when access to a repeater is
not available or desired

Any situation where wireless

communications have failed
or are not available

Examples of Use

Propagates uniformly
Has a low profile
Can be installed where
omni-directional or
directional antennas are
not suitable

Is mobile
Transmits at a higher
power than portable radios
Is not usually limited by
power supplies
Can boost the radio
coverage in the crisis area

Can be used with

directional antennas to
provide improved
coverage

Advantages

In-Building/In-Tunnel User Considerations

Vehicular
Repeater

Radiating Coaxial
Cable

Bi-Directional
Amplifier (BDA)

Solution(s)

15

Has limited versatility in

confined environments
May not provide
adequate in-building
coverage because of low
RF penetration
Limited selection of
hardware vendors

Has poor performance in

high electromagnetic
environments
Susceptible to
interference

May cause interference

with existing system
Must be adjusted to
prevent destructive
interference from
feedback or other BDAs

Disadvantages

In-building
primarily, intunnel where
possible

Both

Both

In-Building or
In-Tunnel
Solution

$4,000 to $100,000

$3-$7/foot +
installation hardware

>$20,000

Approximate
Cost

August 2002

Used to amplify the signal

from a portable radio

Used within tunnels, ships,

and buildings where it may
not be feasible to use BDAs
with directional antennas

Commonly used in buildings

and tunnels to boost the
coverage

Examples of Use

3.2

Technologically Complex Solutions

This section presents the in-building/in-tunnel solutions that are technologically more
complex than the solutions presented in the previous section. For the purposes of this document,
technologically complex solutions are those solutions that require specialized training or skills to
implement and understand. These solutions are summarized in Table 5.
3.2.1 Audio Switch
An audio switch is a device generally used in public safety to connect radio systems. In
most cases, a radio from one agency is connected to the switch. The switch patches the audio
signal from the first radio through to another radio. Then the other radio retransmits the patched
audio on its own system. Audio switches can vary in complexity from patching audio to a single
radio, to very complex switches capable of connecting several radios, phone lines, satellite and
cell phones together. Advanced features as specialized call tones and encryption may not be
available with some switches.
Similar to a portable repeater an audio switch can be inserted into a confined space to act
as a relay between users inside the building or tunnel as well as between users inside and
external to the building or tunnel. Like the portable repeater option, this solution can be as
portable as its installation allows.
In addition to potentially providing extended radio coverage for a systems users, the
audio switch has become a staple in interoperability solutions by providing a means of
connecting disparate radios together to achieve interoperability. Unlike the repeater option, the
audio switch can be used to interface multiple users regardless of the frequency band on which
their systems operate.
This solution is limited by the capacity of the switchthe number of units it can service.
Depending on the size of the crisis area, several units may be required to cover the operational
envelope. Furthermore, the unit depends heavily on a steady power supply to maintain
connectivity. And finally, this solution generally requires software programming for each
additional radio added to the switch.
3.2.2 Fiber Optic Transmission Line
While leaky coax is ideal for some tunnel applications, it is not always the best choice,
especially when considering environments that have high level of electromagnetic interference
(EMI) (e.g., train tunnels used with diesel engines). In such environments, one option to
consider is a RF transport medium not susceptible to EMI, such as fiber optic cable. This
solution, however, is best used in conjunction with other in-building or in-tunnel solutions such
as BDAs. In order to use a fiber optic transmission line, additional equipment is required to
translate the radio signal into digital light pulses for transmission on the fiber optic line. Thus
rendering fiber optic cables a point-to-point technology. Fiber optic lines can be used in a
multiplexing environment. Multiplexing is sending multiple signals or streams of information on

In-Building/In-Tunnel User Considerations

16

August 2002

a carrier at the same time in the form of a single, complex signal and then recovering the separate
signals at the receiving end. Digital signals are commonly multiplexed using time-division
multiplexing, in which the multiple signals are carried over the same channel in alternating time
slots. In some optical fiber networks, multiple signals are carried together as separate
wavelengths of light in a multiplexed signal using dense wavelength division multiplexing.
Like leaky coax, fiber optic lines have a low installation profile to avoid physical
interference with their environment. The installation and supporting hardware required to use
fiber optic transmission lines, however, is generally more expensive than a typical leaky coaxial
cable or other transmission line. Cost drivers for fiber optic cabling are that they are components
of digital systems and require converters at each end of the transmission line.

In-Building/In-Tunnel User Considerations

17

August 2002

Is not affected by high

electromagnetic
environments
Has a low profile
Allows signal
multiplexing

Can interface with

multiple users
Is portable
Extends existing
infrastructure

Advantages

In-Building/In-Tunnel User Considerations

Fiber Optic Transmission Line

Audio Switch

Solution(s)

18

Is limited by the number

of users it can service
Requires a power
source
May require several
units depending on the
size of the building or
tunnel, and the
operational envelope
May require
programming for each
radio added to the
system
Must be used in
conjunction with a
transmitter/receiver and
optical-to-electrical
converter
Requires installation
Is more expensive than
radiating coaxial cable
Requires analog-todigital converter for use
with analog systems

Disadvantages

In-tunnels

Both

In-Building
or In-Tunnel
Solution

Table 5
Technologically Complex Solutions

>$20,000

$5,000 to
$60,000

Approximate
Cost

August 2002

Used by railroads within

tunnels because of high
electromagnetic
environment created by
the locomotives. Also
used in hazardous
materials environments
where radio frequency
energy can be dangerous

Used by emergency
personnel assisting with
the rescue and recovery
operation after the
Pentagon attack

Examples of Use

3.3

Technologically Forward-Looking Solutions

This section addresses the solutions that are technologically forward looking. For the
purposes of this document, technologically forward-looking solutions are those solutions that
take into consideration the prospect of newer technologies. Generally these solutions will make
the transition to future technologies less cumbersome. These solutions summarized in Table 6.
3.3.1 Ordinances for New Construction
An ordinance for improved public safety communications in new building construction is
an option that many municipalities are beginning to implement. The purpose of an ordinance is
to mandate radio-friendly infrastructure inside new construction. The major advantage is that the
radio coverage is designed into the structure from the start. Although not directly associated
with the cost of a system, this solution is included to identify another means of ensuring adequate
in-building or in-tunnel coverage when implementing new systems that are otherwise inherently
costly.
3.3.2 System Level Requirements
As system planners address the next-generation communications network, in-building or
in-tunnel radio coverage should be viewed as a system requirement. In addition, the system can
be designed to cover known coverage trouble spots within buildings and tunnels. These
benefits, however, do not come without a cost. More stringent requirements for building and
tunnel coverage generally increase the number of radio sites a designer uses, and thus
significantly increase the total cost of the project. Replacing a recently implemented system with
a newer system generally is not feasible. However, system planners should include building and
tunnel coverage criteria into future procurements. The City of Mesa, Arizona, for example,
insisted on providing a minimum level of in-building coverage with its new system. For
coverage trouble spots, the city allocated additional funding to address those areas with a
lower cost solution such as a BDA.
3.3.3 Hybrid System Using BDA and Fiber Optics
Using a hybrid system of BDAs and fiber optic transmission lines combines the
advantages gained by utilizing a high bandwidth medium, not susceptible to EMI, with the
functionality of the BDA. As stated, an ideal use for this solution is in a railroad tunnel in which
there is a high level of EMI. Another environment in which this option makes an excellent
solution is in hazardous material environments in which the transmission of RF energy can be
dangerous.

In-Building/In-Tunnel User Considerations

19

August 2002

Can be designed to
meet the in-building or
in-tunnel requirements
of the agency
Can minimize
interference issues
Can be designed to
cover multiple
buildings and tunnels if
the need arises
Combines the
functionality of the
BDA with the
electromagnetic
benefits of the fiber
opticsideal in
environments where
radio propagation will
cause some concern
or will be effectively
neutralized by the
environment

Establishes a vehicle
for improved coverage
in future construction
Can be a low-cost
solution if the city is
interoperable

Advantages

In-Building/In-Tunnel User Considerations

Hybrid System Using BDAs

and Fiber Optics

New System Designed

Explicitly to Include Coverage
in Buildings and Tunnels

City Ordinances for New

Buildings to Provide Access
to Public Safety
Communications Network

Solution(s)

20

Is more expensive than

traditional BDA systems

Has a very high cost

Must consider the life
cycle of current system
and make arrangements
to include in-building or
in-tunnel coverage in
new system(s)
Can be difficult to specify
adequate coverage for
buildings and tunnels,
including room for
expansion

Does not address older

structures
Requires legislative
changes to city code

Disadvantages

Primarily in
tunnels

Both

In-building

In-Building
or In-Tunnel
Solution

Table 6
Technologically Forward-Looking Solutions

>$40,000

> $1,000,000

Costs of
materials and
construction
are dependent
on the
Ordinance

Approximate
Costs

August 2002

Used by railroads within

tunnels because of high
electromagnetic
environment created by
locomotives

City of Mesa, AZ, designed

its system with in-building
coverage in mind and
allocated additional
funding for problem spots

City of Burbank, CA, has

passed legislation
requiring new structures to
provide radio coverage for
public safety agencies

Examples of Use

APPENDIX APHYSICS OF PROPAGATION

APPENDIX APHYSICS OF PROPAGATION

This section presents a discussion of the physics of radio wave propagation that must be
considered when planning for in-building and in-tunnel radio coverage. A glossary of terms is
provided in Appendix B to further explain the technical details of this discussion.
A.1 Radio Frequency Propagation in Free Space
When radio signals propagate in an environment free of obstructions (i.e., free space),
one can predict their behavior by subtracting radio signal losses from gains. Gains enhance or
increase signal strength while losses attenuate or reduce that strength. The results from summing
the gains minus losses define the effective strength of the signal, and ultimately, whether the
signal is strong enough for a receiver to recognize. To determine radio propagation in free space,
the following factors must be considered:

Gains

Antenna GainThe gain of an antenna is the ratio of its radiation intensity to

that of an ideal isotropic antenna (i.e., a hypothetical perfect antenna that radiates
equally in all directions).

Receiver SensitivityThe magnitude of the received signal necessary to produce

objective bit error rate or channel noise performance.

Transmit PowerFor the purposes of this document, transmit power will be

defined as the power transmitted from the antenna, also known as the effective
radiated power (ERP).

Losses

Atmospheric Attenuation EffectsThe atmosphere offers resistance to radio

signals and lowers their strength. Changing atmospheric conditions, such as
heavy rain or temperature fluctuations, can affect signal propagation. The effect
atmospheric conditions can have on a signal can depend on the signals
wavelength. Generally, the higher the frequency, the more a signal is attenuated
due to atmospheric absorption loss.

Path Loss Due to the Separation DistanceElectromagnetic waves radiate in

all directions. Ideally (i.e., in free space) the signal will propagate from the
transmitter without obstructions that might cause the signal strength to weaken.
However, the signal will lose power as the distance it travels increases. Due to
the Law of Conservation of Energy, as waves travel outward from an emitting
source, the occupied area increases, but because energy is conserved, the energy
per unit area must decrease. A signal strength loss of approximately 6 dB occurs
as the distance doubles between the source and the receiver.

In-Building/In-Tunnel User Considerations

A-1

August 2002

A.2 Radio Frequency Propagation in a World with Obstructions

In theoretical free space, one can determine radio signal strength through simple
calculations. Radio propagation in the real world, however, is significantly different from
theoretical free space. Many real-world factors hamper radio propagation. These factors
include, but are not limited to, atmospheric absorption, multipath fading, signal power loss due to
terrain obstructions, and signal power losses due to manmade obstacles. Generally, the more
obstacles a wave encounters, the weaker the signal will be when it reaches the receiver.
Manmade obstructions, such as buildings and bridges, make much more abrupt changes
than natural obstacles such as hills and trees. Because of these abrupt changes, more shadow
(see Appendix B for more information on shadowing) loss occurs in and around buildings,
reducing the signal strength in the region behind the obstacle.
A.2.1 Multipath Fading
An important factor to consider is multipath fading. In practice, transmitters and
receivers are surrounded by objects. These objects constantly reflect and scatter the transmitted
signal, causing several waves to arrive at the receiver at different times via different routes. As
the signal is refracted and reflected off of various obstacles the power received at any given point
varies. As a radio moves from point to point, the signal strength varies due to multipath fading.
Depending on the frequency, a user may or may not notice the effects of multipath fading.
Lower frequency signals have a longer wavelength (a 100 MHz signal has a wavelength of
approximately 9.25 feet, whereas a 800 MHz signal has a wavelength of 1.25 feet) and would
require the user to travel a greater distance to notice a discernable difference. Furthermore, the
higher frequency signals generally reflect and refract more than the lower frequency signals
(another function of a shorter wavelength), which may result in additional transmission paths.
This phenomenon has been observed as an individual walks through a building with a portable
radio and observes the signal strength fluctuating from point to point.
A.2.2 Material Characteristics
Each material has its own unique electrical properties, and each material will affect a
signal differently. A signals electric and magnetic field strengths diminish as the wave travels
through a medium. As a signal passes through a material, some of the energy is absorbed and
converted to heat. This is referred to as absorption loss. To further clarify, consider the theory
associated with absorption loss and a practical example of this loss. Theory states the magnitude
of these losses depends on the materials thickness and electrical properties. As evidence, a
signal that passes through a thin wall will have stronger field strength after traveling through the
medium than a signal that passes through a thicker wall of the same material and construction.
Table A-1 lists the average signal loss for radio paths obstructed by common building materials.
This table is intended to give relative losses per unit thickness for each of the materials listed.

In-Building/In-Tunnel User Considerations

A-2

August 2002

Table A-1
Average Signal Loss for Radio Paths Obstructed by Common Building Materials
Material Type
Wall constructed of metal plate
Aluminum siding
Foil insulation
2.7 x 2.7 square reinforced concrete pillar
Concrete block wall
Sheetrock (3/8 in)2 sheets

Loss
(decibels)
26
20.4
3.9
1214
13
2

A.3 Radio Frequency Propagation in a Confined Space

The previous sections discussed the effects of obstructions on radio signals; however,
propagating radio signals reliably inside confined spaces adds an entirely new dimension. Due
to the proximity of obstructions in a building environment behavior such as reflections,
diffraction around sharp corners, or scattering from walls, ceilings, or floor surfaces will occur.
A.3.1 Radio Frequency Propagation Within a Building
Numerous variables complicate radio coverage in a building environment. To determine
radio coverage inside a building, a system designer or planner needs to have crucial information
about the buildings construction, density, and the specific locations where communication is
required. The orientation to the transmitter will affect signal coverage within the building in
several ways. First, underground areas may not receive the signal without implementing specific
underground coverage solutions. Secondly, multistory buildings will have coverage that varies
from floor to floor. It is not uncommon for a 30th floor office to have better radio reception than
a similar office on the 1st floor.
In a building environment, obstructions are classified into two categorieshard and soft
partitions. Hard partitions are the physical and structural components of a building such as the
building layout, room dimensions, doorway openings, and window locations. On the other hand,
obstacles formed by the office furniture and fixed or movable structures that do not extend to a
buildings ceilings are considered soft partitions. Radio signals effectively penetrate both kinds
of obstructions in ways that are difficult to predict. Each time the signal passes through an
obstacle, the signal strength is reduced. This is also true for floor-to-floor transmissions and
underground transmissions. As indicated in the discussion in Section A.2.2Material
Characteristics, a general rule of thumb is that as the thickness of the obstacle increases, the
successful transmission of energy through the obstacle will decrease.
Coverage prediction is complicated further by movement of people and objects within the
building. Multiradio S.A. found a study discussing the effects on radio coverage due to crowds
of people. Tests were conducted with a point source antenna distribution system in a building to
determine coverage requirements for 800 MHz and 1.9 gigahertz (GHz) systems. The study
found that changes in the density of people caused signal variations as high as 30 decibels (dB).

In-Building/In-Tunnel User Considerations

A-3

August 2002

A.3.2 Radio Frequency Propagation within a Tunnel

Similar to in-building coverage, in-tunnel coverage is difficult, at best, to predict with
certainty. Some important factors in determining tunnel radio coverage are the configuration of
the tunnel, the materials used to build the tunnel, and the relative orientation of the receiver to
the transmitter when the transmitter is located outside the tunnel.
The configuration of the tunnel plays a crucial role in determining the radio coverage. If
the tunnel is generally straight and the antenna is located in the tunnel, the signals primary
component will be a result of line of sight (LOS) transmission. As the tunnel changes direction,
the signal experiences more loss due to reflections and scattering. The more abruptly the tunnel
changes direction, the greater the multipath loss is, and the lower the signal level will be.
Furthermore, the losses the signal will experience will be driven by the electrical characteristics
of the materials used in the tunnel construction.

In-Building/In-Tunnel User Considerations

A-4

August 2002

APPENDIX BGLOSSARY OF TERMS

APPENDIX BGLOSSARY OF TERMS

To understand the problems associated with in-building or in-tunnel radio coverage, it is
important to know the vocabulary used to describe the phenomena.

AbsorptionFigure B-1 illustrates absorption, which occurs when a radio wave

encounters an obstacle that allows RF to pass through, to some degree, to radio
waves. When a radio wave strikes the obstacle, part of the radio signals energy
dissipates as heat. This is called absorption. When a radio wave reaches an obstacle
such as a wall, the obstacles material absorbs and reflects portions of the radio
frequency (RF) energy.

reflected ray

Figure B-1
Absorption

ConductivityThe ratio of current density in a conductor to the electric field

causing the current to flow, the ability to transmit electricity.

DecibelThis unit is commonly used to express relative difference in power or

intensity, usually between two signals, equal to 10 times the common logarithm of the
ratio of the 2 levels. The decibel is usually abbreviated as dB.

In-Building/In-Tunnel User Considerations

B-1

August 2002

DiffractionFigure B-2 illustrates diffraction, which occurs when the transmission

path between the transmitter and the receiver is obstructed by a sharp edge, such as a
wall or doorway. Once the wave strikes the surface edge, diffraction occurs (i.e., the
wave bends). The resultant signal coverage past the point where the diffraction
occurred is now defined by shadowing.

Figure B-2
Diffraction Around a Corner

FrequencyThe number of complete cycles per unit time of a complete waveform,

usually measured in Hertz. Hertz is a unit of measure that means cycles per
second. So, frequency equals the number of complete cycles occurring in one
second.

PermeabilityThe ratio of the magnetic flux density in a material to the external

field strength. The permeability of free space is also called the magnetic constant.

PermittivityA measure of the ability of a material to resist the formation of an

electric field within the material. Also called dielectric constant, relative permittivity.

In-Building/In-Tunnel User Considerations

B-2

August 2002

ReflectionFigure B-3 illustrates reflection, which occurs when a propagating

electromagnetic wave strikes an object that is very large (e.g., the surface of the
Earth, buildings, or walls) compared with the wavelength of the propagating wave.

Figure B-3
Reflection
ScatteringFigure B-4 illustrates scattering, which occurs when a propagating
electromagnetic wave strikes an object that is very small (e.g., foliage, street signs,
and lampposts) compared with the wavelength of the propagating wave. Scattered
waves are produced by rough surfaces, small objects, or by other irregular
obstructions. The nature of this phenomenon is similar to reflection, except that the
radio waves are scattered in many directions. Of all the previously mentioned
phenomena, predictions of scattering effects are the most complex.5

Figure B-4
Scattering Due to a Rough Surface

5 This graphic has been included for illustrative purposes and is not drawn to scale.

In-Building/In-Tunnel User Considerations

B-3

August 2002

ShadowingFigure B-5 illustrates shadowing, which is the result of an

electromagnetic wave being diffracted by an obstruction. The angle of incidence will
determine the angle of diffraction and how the wave propagates behind the object.
The area immediately behind the object is said to be in the shadow.

Figure B-5
Shadowing behind an object

SusceptibilityThe dimensionless quantity describing the electromagnetic effect on

a material when subjected to an electromagnetic field. A high susceptibility rating
makes a coaxial cable a poor choice for a distribution system when used in an intense
electromagnetic environment.

WavelengthThis is the measure of the distance between one peak or crest of a

wave of light, heat, or other energy and the next corresponding peak or crest.

In-Building/In-Tunnel User Considerations

B-4

August 2002

APPENDIX CTECHNICAL REFERENCES

APPENDIX CTECHNICAL REFERENCES

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Probability of Error for Noncoherent and Differentially Coherent Modulations Over
Generalized Fading Channels. IEEE Transactions on Communications, vol. 46, no. 12,
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Aragon, Alejandro. (August 2000). MCU Programmable RF Transmitter. Centre for
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Bhatti, Saleem. (March 1995). The Electromagnetic Spectrum; Propagation in Free-Space and
the Atmosphere; Noise in Free-Space. University College London, 14.
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Burt, Dennis. (no date). Creating Better Coverage in Buildings and Tunnels. Multiradio S.A.
Online, 1-6. http://www.multiradio.com/Notas/Nota-andrew3.html
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DuBroff, Richard E., Marshall, Stanley V., and Skiteck, Gabriel G. (1996). Electromagnetic
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Laitinen, Heikki. (1999) Verification of a Stochastic Radio Channel Model Using Wideband
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Mohan, Ananda and Suzuki, Hajime. (July 2000). Measurement and Prediction of High Spatial
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