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Evolution of terrestrial networking: circuit‐switched to packet‐switched
1
Why packetized communications?
2
CxP 70022‐01, C3I Interoperability Standards Book, Vol.1, pg. 15
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General properties of wireless networks
Class Question: Name key properties that make wireless networking different
than wired networking
Wireless Communications
• Mobility: Communicate in ways you couldn’t accomplish otherwise
• Cheaper: eliminate infrastructure of wired systems, lower cost links
• Easier to broadcast:
 Provides a relatively low cost of content distribution
 Can add users in a cost‐effective manner (point‐to‐multipoint)
3
CxP 70022‐01, C3I Interoperability Standards Book, Vol.1, pg. 15
Wireless data networks
Class Question: Let’s name the majority (all?) of the major wireless data
networks/categories:
Wireless Data Networks
• Many transmitters and receivers
• Users can be mobile (dynamic, not static networks)
• Communications are packet‐based
 10 years ago: cellular systems had lots of low‐capability devices and
focus was on voice (connection‐oriented) communications
 Now, in wireless data communications: intelligence is more
distributed
 Importantly, characteristics of wireless cannot be
compartmentalized: all OSI layers can be affected
4
CxP 70022‐01, C3I Interoperability Standards Book, Vol.1, pg. 15
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2/20/2018
Wireless comes of age
• Guglielmo Marconi invented the wireless telegraph in
1896
– Communication by encoding alphanumeric characters in analog
signal
– Sent telegraphic signals across the Atlantic Ocean
• Communications satellites launched in 1960s
• Advances in wireless technology
– Radio, television, mobile telephone, mobile data,
communication satellites
• More recently
– Wireless networking, cellular technology, mobile apps, Internet
of Things
5
Cellular Telephony
• Started as a replacement to the wired telephone
• Early generations offered voice and limited data
• Current third and fourth generation systems
–
–
–
–
–
–
–
Voice
Texting
Social networking
Mobile apps
Mobile Web
Mobile commerce
Video streaming
6
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Wireless impact
• Profound
• Shrinks the world
• Always on
• Always connected
• Changes the way people communicate
– Social networking
• Converged global wireless network
7
Some milestones in wireless communications
8
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Global cellular network(s)
• Growth
– 11 million users in 1990
– Over 7 billion today
• Mobile devices
– Convenient
– Location aware
– Only economical form of communications in some
places
9
Global cellular network(s)
• Generations
– 1G – Analog
– 2G – Digital voice
• Voice services with some moderate rate data services
– 3G – Packet networks
• Universal Mobile Phone Service (UMTS)
• CDMA2000
– 4G – New wireless approach (OFDM)
•
•
•
•
Higher spectral efficiency
100 Mbps for high mobility users
1 Gbps for low mobility access
Long Term Evolution (LTE) and LTE‐Advanced
10
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Mobile device revolution
• Originally just mobile phones
• Today’s devices
–
–
–
–
Multi‐megabit Internet access
Mobile apps
High megapixel digital cameras
Access to multiple types of wireless networks
• Wi‐Fi, Bluetooth, 3G, and 4G
– Several on‐board sensors
• Key to how many people interact with the world
around them
11
Mobile device revolution
•
•
•
•
Better use of spectrum
Decreased costs
Limited displays and input capabilities
Tablets provide balance between
smartphones and PCs
• Long distance
– Cellular 3G and 4G
• Local areas
– Wi‐Fi
• Short distance
– Bluetooth, ZigBee
12
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Future Trends
• LTE‐Advanced and gigabit Wi‐Fi now being deployed
• LTU and associated research currently very active
• Machine‐to‐machine communications
– The “Internet of Things”
– Devices interact with each other
• Healthcare, disaster recovery, energy savings, security and surveillance,
environmental awareness, education, manufacturing, and many others
– Information dissemination
• Data mining and decision support
– Automated adaptation and control
• Home sensors collaborate with home appliances, HVAC systems, lighting
systems, electric vehicle charging stations, and utility companies.
– Eventually could interact in their own forms of social networking
13
Future Trends
• Machine‐to‐machine communications
– 100‐fold increase in the number of devices
– Type of communication would involve many short
messages
– Control applications will have real‐time delay
requirements
• Much more stringent than for human interaction
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Future Trends
• Future networks
– 1000‐fold increase in data traffic by 2020
– 5G – Final definitions, standards, but envisioned by 2020
• Technologies
– Network densification – many small cells
– Device‐centric architectures ‐ focus on what a device needs
– Massive multiple‐input multiple‐output (MIMO) – 10s or 100s of
antennas
• To focus antenna beams toward intended devices
– Millimeter wave (mmWave) ‐ frequencies in the 30 GHz to 300
GHz bands
• Have much available bandwidth.
• But require more transmit power and have higher attenuation due to
obstructions
– Native support for machine to machine communication
• Sustained low data rates, massive number of devices, and very low
delays.
15
What is 5G?
(eMBB)
(Massive M2M)
Low‐latency,
ultra‐reliable
(LLUR)
16
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Mobile wireless communications market statistics
o Mobile will represent 20% of total IP traffic by 2021
o The average global mobile connection speed will surpass 20 Mbps by 2021
o Smartphones will comprise 86% of mobile data traffic by 2021
o By 2021 there will be 1.5 mobile devices per capita (phones, phablets)
o By 2021, 4G/LTE will be 53% of connections and 79% of total traffic
o By 2021, 5G will be 0.2% of connections (25M) and 1.5% of total traffic
o The average smartphone will generate 6.8 GB of traffic per month by 2021, a four‐fold
increase the 2016 average of 1.6 GB per month
17
The trouble with wireless
• Wireless is convenient and less expensive, but not
perfect
• Limitations and political and technical difficulties inhibit
wireless technologies
• Wireless channel
– Line‐of‐sight is best but not required
– Signals can still be received
•
•
•
•
Transmission through objects
Reflections off of objects
Scattering of signals
Diffraction around edges of objects
18
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Characteristics of wireless data transmission
• Very high signal attenuation (degradation)
• Transmission is very noisy; subject to high BER
• Broadcast channel is inherently insecure; no physical
security to prevent spoofing
• Wireless channel is not necessarily symmetric and are
not transitive
– Remember the physical channel is symmetric
– But transmitters and receivers are not symmetric because
of electronics, purpose, etc.
• If A can talk to B, it does not mean B can talk to A (symmetic)
• If A can talk to B, and B can talk to C, it does not mean A can talk to
C (transitive)
19
Characteristics of wireless data transmission
• Reflections can cause multiple copies of the signal to arrive
– At different times and attenuations
– Creates the problem of multipath fading
– Signals add together to degrade the final signal
• Nodes are mobile so that the topology of the network is
changing
– Causes intermittent link connectivity
– Doppler spread caused by movement
• Interference from other users
• Batteries, power management issues to account for
• Radio spectrum is regulated
20
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Focus of this course
•
Wireless Environment – In‐depth understanding of general wireless
communications
•
Modulation – use a signal format to send as many bits as possible
•
Error control coding – add extra bits so errors are detected/corrected
•
Adaptive modulation and coding – dynamically adjust modulation and coding to
current channel conditions
•
Equalization – counteract the multipath effects of the channel
•
Multiple‐input multiple‐output systems – use multiple antennas
– Point signals strongly in certain directions
– Send parallel streams of data
•
Direct sequence spread spectrum – expand the signal bandwidth
•
Orthogonal frequency division multiplexing – break a signal into many lower rate bit
streams
– Each is less susceptible to multipath problems
21
Electromagnetic spectrum of telecommunications
22
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Political difficulties
• Between companies
– Need common standards so products interoperate
– Some areas have well agreed‐upon standards
• Wi‐Fi, LTE
• Not true for Internet of Things technologies
• Spectrum regulations
– Governments dictate how spectrum is used
• Many different types of uses and users
– Some frequencies have somewhat restrictive
bandwidths and power levels
• Others have much more bandwidth available
23
Signals & Decibels
24
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Electromagnetic Signals
• Function of time, e.g., for a general sine wave:
s(t ) = A sin(2πft + ϕ)
• Can also be expressed as a function of frequency
– Signal consists of components of different frequencies
– This is an important insight for telecommunications
RF understanding
25
Signals: Time domain concepts
• Analog signal ‐ signal intensity varies in a smooth
fashion over time
– No breaks or discontinuities in the signal
• Digital signal ‐ signal intensity maintains a constant
level for some period of time and then changes to
another constant level
• Periodic signal ‐ analog or digital signal pattern that
repeats over time
s(t +T) = s(t)
‐∞ < t < +∞
• where T is the period of the signal
26
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Signals: Analog and Digital waveforms
27
Signals: Time‐domain concepts
• Aperiodic signal ‐ analog
or digital signal pattern
that doesn't repeat over
time
• Peak amplitude (A) ‐
maximum value or
strength of the signal over
time; typically measured
in volts
• Frequency (f)
– Rate, in cycles per second,
or Hertz (Hz) at which the
signal repeats
28
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Signals: Time‐domain concepts
• Period (T) ‐ amount of time
it takes for one repetition of
the signal
– T = 1/f
• Phase (ϕ) ‐ measure of the
relative position in time
within a single period of a
signal
• Wavelength (λ) ‐ distance
occupied by a single cycle of
the signal
– Or, the distance between two
points of corresponding phase
of two consecutive cycles
29
Signals: Sine Wave Parameters
• General sine wave
– s(t ) = A sin(2πft + ϕ)
• Slide‐30 shows the effect of varying each of the three
parameters
–
–
–
–
(a) A = 1, f = 1 Hz, ϕ = 0; thus T = 1 s
(b) Reduced peak amplitude; A=0.5
(c) Increased frequency; f = 2, thus T = ½
(d) Phase shift; ϕ = π/4 radians (45 degrees)
• Note: 2π radians = 360° = 1 period
30
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Signals: Sine Wave Parameters
s(t) = A sin (2πft + ϕ)
31
Signals: Frequency Domain Concepts
• Fundamental frequency ‐ when all frequency
components of a signal are integer multiples of one
frequency, it’s referred to as the fundamental frequency
• Spectrum ‐ range of frequencies that a signal contains
• Absolute bandwidth ‐ width of the spectrum of a signal
• Effective bandwidth (or just bandwidth) ‐ narrow band of
frequencies that most of the signal’s energy is contained
in
32
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Signals: Addition of frequency Components(T = 1/f)
33
Signals: Frequency Domain Concepts
• Any electromagnetic
signal can be shown to
consist of a collection of
periodic analog signals
(sine waves) at different
amplitudes, frequencies,
and phases
• The period of the total
signal is equal to the
period of the
fundamental frequency
Frequency Components of Square Wave
34
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Signals: Acoustic spectrum of speech and music
35
Signals: Relationship between Data Rate and Bandwidth
• The greater the bandwidth, the higher the
information‐carrying capacity
• Conclusions
– Any digital waveform will have infinite bandwidth (Why?)
– BUT the transmission system will limit the bandwidth that
can be transmitted
– AND, for any given medium, the greater the bandwidth
transmitted, the greater the cost
– HOWEVER, limiting the bandwidth creates distortions
TRANSMISSION
FUNDAMENTALS 2‐36
36
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Signals: Attenuation of Digital Signals
37
Data Communication Terms
• Data ‐ entities that convey meaning, or
information
• Signals ‐ electric or electromagnetic
representations of data
• Transmission ‐ communication of data by the
propagation and processing of signals
38
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Examples of Analog and Digital Data
• Analog
• Video
• Audio
• Digital
• Text
• Integers
39
Analog Signals
• A continuously varying electromagnetic wave that
may be propagated over a variety of media,
depending on frequency
• Examples of media:
– Copper wire media (twisted pair and coaxial cable)
– Fiber optic cable
– Atmosphere or space propagation (wireless!)
• Analog signals can propagate analog and digital data
40
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Digital Signals
• A sequence of voltage pulses that may be
transmitted over a copper wire medium
• Generally cheaper than analog signaling
• Less susceptible to noise interference
• Suffer more from attenuation
• Digital signals can propagate analog and
digital data
41
Reasons for Choosing Data and Signal Combinations
• Digital data, digital signal
– Equipment for encoding is less expensive than digital‐to‐
analog equipment
• Analog data, digital signal
– Conversion permits use of modern digital transmission and
switching equipment
• Digital data, analog signal (Wireless Systems focus!)
– Some transmission media will only propagate analog signals
– Examples include optical fiber, wireless, and satellite
• Analog data, analog signal
– Analog data easily converted to analog signal
42
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Analog Transmission
• Transmit analog signals without regard to content
• Attenuation limits length of transmission link
• Cascaded amplifiers boost signal’s energy for longer
distances but cause distortion
– Analog data can tolerate distortion
– Introduces errors in digital data
43
Digital Transmission
• Concerned with the content of the signal
• Attenuation endangers integrity of data
• Digital Signal
– Repeaters achieve greater distance
– Repeaters recover the signal and retransmit
• Analog signal carrying digital data
– Retransmission device recovers the digital data from
analog signal
– Generates new, clean analog signal
44
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Wireless Systems focus: Maximize channel capacity
• Impairments, such as noise, limit data rate
that can be achieved
• For digital data, to what extent do
impairments limit data rate?
• Channel Capacity – the maximum rate at
which data can be transmitted over a given
communication path, or channel, under given
conditions
45
Effects of Noise on a Digital Signal
46
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Concepts Related to Channel Capacity
• Data rate ‐ rate at which data can be communicated
(bps)
• Bandwidth ‐ the bandwidth of the transmitted signal
as constrained by the transmitter and the nature of
the transmission medium (Hertz)
• Noise ‐ average level of noise over the
communications path
• Error rate ‐ rate at which errors occur
– Error = transmit 1 and receive 0; transmit 0 and receive 1
47
Nyquist Bandwidth
• For binary signals (two voltage levels)
– C = 2B
• With multilevel signaling
– C = 2B log2 M
• M = number of discrete signal or voltage levels
48
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Signal‐to‐Noise Ratio: Most important wireless communications metric
• Ratio of the power in a signal to the power contained in
the noise that is present at a particular point in the
transmission
• Typically measured at a receiver
• Signal‐to‐noise ratio (SNR, or S/N)
• A high SNR means a high‐quality signal, low number of
required intermediate repeaters
• SNR sets upper bound on achievable data rate
49
Shannon Capacity Formula
• Equation:
• Represents theoretical maximum that can be
achieved
• In practice, only much lower rates achieved
– Formula assumes white noise (thermal noise)
– Impulse noise is not accounted for
– Attenuation distortion or delay distortion not accounted
for
50
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Example of Nyquist and Shannon Formulations
• Spectrum of a channel between 3 MHz and 4
MHz ; SNRdB = 24 dB
• Using Shannon’s formula
51
Example of Nyquist and Shannon Formulations
• How many signaling levels are required?
52
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Classifications of Transmission Media
• Transmission Medium
– Physical path between transmitter and receiver
• Guided Media
– Waves are guided along a solid medium
– E.g., copper twisted pair, copper coaxial cable, optical fiber
• Unguided Media
– Provides means of transmission but does not guide
electromagnetic signals
– Usually referred to as wireless transmission
– E.g., atmosphere, outer space
53
Unguided Media
• Transmission and reception are achieved by
means of an antenna
• Configurations for wireless transmission
– Directional
– Omnidirectional
54
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Electromagnetic spectrum of telecommunications
55
General Frequency Ranges
• Microwave frequency range
–
–
–
–
1 GHz to 40 GHz
Directional beams possible
Suitable for point‐to‐point transmission
Used for satellite communications
• Radio frequency range
– 30 MHz to 1 GHz
– Suitable for omnidirectional applications
• Infrared frequency range
– Roughly, 3x1011 to 2x1014 Hz
– Useful in local point‐to‐point multipoint applications
within confined areas
56
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Terrestrial Microwave
• Description of common microwave antenna
–
–
–
–
Parabolic "dish", 3 m in diameter
Fixed rigidly and focuses a narrow beam
Achieves line‐of‐sight transmission to receiving antenna
Located at substantial heights above ground level
• Applications
– Long haul telecommunications service
– Short point‐to‐point links between buildings
57
Satellite Microwave
• Description of communication satellite
– Microwave relay station
– Used to link two or more ground‐based microwave
transmitter/receivers
– Receives transmissions on one frequency band (uplink),
amplifies or repeats the signal, and transmits it on another
frequency (downlink)
• Applications
– Television distribution
– Long‐distance telephone transmission
– Private business networks
58
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Broadcast Radio
• Description of broadcast radio antennas
– Omnidirectional
– Antennas not required to be dish‐shaped
– Antennas need not be rigidly mounted to a precise
alignment
• Applications
– Broadcast radio
• VHF and part of the UHF band; 30 MHZ to 1GHz
• Covers FM radio and UHF and VHF television
59
Decibels Review
Decibels are defined as:
dB = 10 Log10 (Pout/Pin)
NOTE – Decibels in the above formula always represents a Power
Ratio
You can add and subtract dBs to represent just about any power
ratio without resorting to a calculator by remembering the rules:
• Positive dBs mean multiply (or gain).
• Negative dBs mean divide (or attenuate).
• Memorize one dB value!
60
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Decibels Review
There are only two dB conversions you ever really need:
+3 dB means 2 times bigger (multiply by 2)
+10 dB means 10 times bigger (multiply by 10)
And by corollary:
‐3 dB means 2 times smaller (divide by 2)
‐10 dB means 10 times smaller (divide by 10)
Now consider the obvious you already know, like 2 x 2 = 4. Since
dB’s add for multiplication, then 4x means +3 dB +3 dB = +6 dB, 8x
means +3 dB +3 dB +3 dB = 9 dB. Likewise 10x is +10 dB and 100x is
+20 dB. Remember that attenuation is negative dB’s. So, 1/100th
the power would be ‐20 dB and 1/1000th the power is ‐30 dB.
61
Decibels Review
You can always make a table like this whenever you need to convert to dBs
Some Examples:
The ratio of 16 times = 2 x 2 x 2 x 2 which is
+3 dB + 3 dB + 3 dB + 3 dB = + 12 dB.
A gain of 500 is simply 1000 divided by 2 or
+30 dB ‐ 3 dB = 27 dB.
1/2000 is ‐ 30 dB – 3 dB = ‐ 33 dB.
‐14 dB = ‐20 dB + 3 dB + 3 dB or ‐20 dB + 6 dB
which is 1/100 x 4 = 1/25th.
Make up some of your own and test it with a
calculator.
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Decibels Review
When dB’s are absolute values and not ratios:
The use of dBm, dBμ, dBw, etc. is really an abbreviation
An exception to using dB notation for pure ratios is a "shorthand" scheme for
indicating a ratio of power compared to a given defined level.
One example is the common artifice of using a subscript such as dBm to indicate
Power compared to one milliwatt. Therefore, ‐3dBm means 1/2 of one milliwatt or 3 dB
below 1 milliwatt. Similar notation is used with the Greek letter mu (μ) for dBs
compared to a microwatt, as in 10 dBμ to mean 10 microwatts or 1/100th of a
milliwatt.
Therefore, ‐20 dBm = +10 dBμ
Get used to the above‐‐get really comfortable with dBs‐‐as you will encounter all this
again in Optical Communications, Satellite, and Wireless courses and FOR THE REST OF
YOUR CAREER.
63
Antenna Fundamentals
64
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Antenna Fundamentals
• An antenna is an electrical conductor or
system of conductors
– Transmission ‐ radiates electromagnetic energy
into space
– Reception ‐ collects electromagnetic energy from
space
• In two‐way communication, the same antenna
can be used for transmission and reception
65
Antenna Radiation Patterns
• Radiation pattern
– Graphical representation of radiation properties of an
antenna
– Depicted as two‐dimensional cross section
• Beam width (or half‐power beam width)
– Measure of directivity of antenna
• Reception pattern
– Receiving antenna’s equivalent to radiation pattern
• Sidelobes
– Extra energy in directions outside the mainlobe
• Nulls
– Very low energy in between mainlobe and sidelobes
66
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Antenna Radiation Patterns
Also termed
isotropic
radiation
67
Types of Antennas
• Isotropic antenna (idealized)
– Radiates power equally in all directions
• Dipole antennas
– Half‐wave dipole antenna (or Hertz antenna)
– Quarter‐wave vertical antenna (or Marconi antenna)
• Parabolic Reflective Antenna
• Directional Antennas
– Arrays of antennas
• In a linear array or other configuration
– Signal amplitudes and phases to each antenna are adjusted to
create a directional pattern
– Very useful in modern systems
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Simple Antennas
69
Radiation Pattern in Three Dimensions
70
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Parabolic Reflective Antennas
71
Antenna Gain
• Antenna gain
– Power output, in a particular direction, compared
to that produced in any direction by a perfect
omnidirectional antenna (isotropic antenna)
• Effective area
– Related to physical size and shape of antenna
72
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Antenna Gain
• Relationship between antenna gain and effective
area
•
•
•
•
•
G = antenna gain
Ae = effective area
f = carrier frequency
c = speed of light 3  108 m/s)
λ = carrier wavelength
73
Spectrum Considerations for Signal Transmission
• Controlled by regulatory bodies
– Carrier frequency
– Signal Power
– Multiple Access Scheme
• Divide into time slots –Time Division Multiple Access
(TDMA)
• Divide into frequency bands – Frequency Division
Multiple Access (FDMA)
• Different signal encodings – Code Division Multiple
Access (CDMA)
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Spectrum Considerations for Signal Transmission
• Federal Communications Commission (FCC) in the
United States regulates spectrum
–
–
–
–
–
–
Military
Broadcasting
Public Safety
Mobile
Amateur
Government exclusive, non‐government exclusive, or
both
– Many other categories
75
Spectrum Considerations for Signal Transmission
• Industrial, Scientific, and Medical (ISM) bands
– Can be used without a license
– As long as power and spread spectrum regulations
are followed
• ISM bands are used for
– WLANs
– Wireless Personal Area networks
– Internet of Things
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Signal Propagation Modes
• Ground‐wave propagation
• Sky‐wave propagation
• Line‐of‐sight propagation
77
Propagation Modes
78
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Ground Wave Propagation
• Follows contour of the earth
• Can propagate considerable distances
• Frequencies up to 2 MHz
• Example
– AM radio
79
Sky Wave Propagation
• Signal reflected from ionized layer of atmosphere
back down to earth
• Signal can travel a number of hops, back and forth
between ionosphere and earth’s surface
• Reflection effect caused by refraction
• Examples
– Amateur radio
– CB radio
– AM radio at night
80
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Line‐of‐Sight Propagation
• Transmitting and receiving antennas must be within line
of sight
– Satellite communication – signal above 30 MHz not reflected by
ionosphere
– Ground communication – antennas within effective line of site
due to refraction
• Refraction – bending of microwaves by the atmosphere
– Velocity of electromagnetic wave is a function of the density of
the medium
– When wave changes medium, speed changes
– Wave bends at the boundary between mediums
81
Line‐of‐Sight Equations
• Optical line of sight
• Effective, or radio, line of sight
• d = distance between antenna and horizon (km)
• h = antenna height (m)
• K = adjustment factor to account for refraction, rule
of thumb K = 4/3
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Optical and Radio Horizons
83
Line‐of‐Sight Equations
• Maximum distance between two antennas for
LOS propagation:
• h1 = height of first antenna
• h2 = height of second antenna
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Basic Propagation Mechanisms
1. Free‐space propagation
2. Transmission
– Through a medium
– Refraction occurs at boundaries
3. Reflections
– Waves impinge upon surfaces that are large compared to the
signal wavelength
4. Diffraction
– Secondary waves behind objects with sharp edges
5. Scattering
– Interactions between small objects or rough surfaces
85
Attenuation
• Strength of signal falls off with distance over
transmission medium
• Attenuation design factors for unguided media:
– Received signal must have sufficient strength so that
circuitry in the receiver can interpret the signal
– Signal must maintain a level sufficiently higher than noise
to be received without error
– Attenuation is greater at higher frequencies, causing
distortion
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Free Space Loss
• Free space loss, ideal isotropic antenna
• Pt = signal power at transmitting antenna
• Pr = signal power at receiving antenna
• λ = carrier wavelength
• d = propagation distance between antennas
• c = speed of light 3 ×108 m/s)
where d and λ are in the same units (e.g., meters)
87
Free Space Loss
• Recall we defined a decibel loss via the equation
LdB = ‐10 Log10 (Pout/Pin) = 10 Log10 (Pin/Pout)
• We can recast the free space loss equation then as:
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Free Space Loss
89
Path Loss Exponent in practical
systems
• Practical systems – reflections, scattering, etc.
• Beyond a certain distance, received power
decreases logarithmically with distance
– Based on many measurement studies
90
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Path Loss Exponent in practical
systems
91
Models Derived from Empirical
Measurements
• Need to design systems based on empirical data applied to
a particular environment
– To determine power levels, tower heights, height of mobile
antennas
• Okumura developed a model, later refined by Hata
– Detailed measurement and analysis of the Tokyo area
– Among the best accuracy in a wide variety of situations
• Predicts path loss for typical environments
–
–
–
–
–
Urban
Small, medium sized city
Large city
Suburban
Rural
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Categories of Noise
•
•
•
•
Thermal Noise
Intermodulation noise
Crosstalk
Impulse Noise
93
Thermal Noise
• Thermal noise due to agitation of electrons
• Present in all electronic devices and
transmission media
• Cannot be eliminated
• Function of temperature
• Particularly significant for satellite
communication
94
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Noise Terminology
• Intermodulation noise – occurs if signals with
different frequencies share the same medium
– Interference caused by a signal produced at a frequency
that is the sum or difference of original frequencies
• Crosstalk – unwanted coupling between signal paths
• Impulse noise – irregular pulses or noise spikes
– Short duration and of relatively high amplitude
– Caused by external electromagnetic disturbances, or faults
and flaws in the communications system
95
EXPRESSION Eb/N0
• Ratio of signal energy per bit to noise power density
per Hertz
• The bit error rate (i.e., bit error probability) for digital
data is a function of Eb/N0
– Given a value for Eb/N0 to achieve a desired error rate,
parameters of this formula can be selected
– As bit rate R increases, transmitted signal power must
increase to maintain required Eb/N0
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5.4 GENERAL SHAPE OF BER VERSUS Eb/N0 CURVES
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Other Impairments
• Atmospheric absorption – water vapor and
oxygen contribute to attenuation
• Multipath – obstacles reflect signals so that
multiple copies with varying delays are
received
• Refraction – bending of radio waves as they
propagate through the atmosphere
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Classifications of Transmission Media
• Transmission Medium
– Physical path between transmitter and receiver
• Guided Media
– Waves are guided along a solid medium
– E.g., copper twisted pair, copper coaxial cable, optical fiber
• Unguided Media
– Provides means of transmission but does not guide
electromagnetic signals
– Usually referred to as wireless transmission
– E.g., atmosphere, outer space
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Unguided Media
• Transmission and reception are achieved by
means of an antenna
• Configurations for wireless transmission
– Directional
– Omnidirectional
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Attenuation
• Strength of signal falls off with distance over
transmission medium
• Attenuation design factors for unguided media:
– Received signal must have sufficient strength so that
circuitry in the receiver can interpret the signal
– Signal must maintain a level sufficiently higher than noise
to be received without error
– Attenuation is greater at higher frequencies, causing
distortion
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Electromagnetic spectrum of telecommunications
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General Frequency Ranges
• Microwave frequency range
–
–
–
–
1 GHz to 40 GHz
Directional beams possible
Suitable for point‐to‐point transmission
Used for satellite communications
• Radio frequency range
– 30 MHz to 1 GHz
– Suitable for omnidirectional applications
• Infrared frequency range
– Roughly, 3x1011 to 2x1014 Hz
– Useful in local point‐to‐point multipoint applications
within confined areas
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Terrestrial Microwave
• Description of common microwave antenna
–
–
–
–
Parabolic "dish", 3 m in diameter
Fixed rigidly and focuses a narrow beam
Achieves line‐of‐sight transmission to receiving antenna
Located at substantial heights above ground level
• Applications
– Long haul telecommunications service
– Short point‐to‐point links between buildings
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Satellite Microwave
• Description of communication satellite
– Microwave relay station
– Used to link two or more ground‐based microwave
transmitter/receivers
– Receives transmissions on one frequency band (uplink),
amplifies or repeats the signal, and transmits it on another
frequency (downlink)
• Applications
– Television distribution
– Long‐distance telephone transmission
– Private business networks
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Broadcast Radio
• Description of broadcast radio antennas
– Omnidirectional
– Antennas not required to be dish‐shaped
– Antennas need not be rigidly mounted to a precise
alignment
• Applications
– Broadcast radio
• VHF and part of the UHF band; 30 MHZ to 1GHz
• Covers FM radio and UHF and VHF television
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Ground Wave Propagation
• Follows contour of the earth
• Can propagate considerable distances
• Frequencies up to 2 MHz
• Example
– AM radio
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Sky Wave Propagation
• Signal reflected from ionized layer of atmosphere
back down to earth
• Signal can travel a number of hops, back and forth
between ionosphere and earth’s surface
• Examples
– Amateur radio
– CB radio
– AM radio at night
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Line‐of‐Sight Propagation
• Transmitting and receiving antennas must be within line
of sight
– Satellite communication – signal above 30 MHz not reflected by
ionosphere
– Ground communication – antennas within effective line of site
due to refraction
• Refraction – bending of microwaves by the atmosphere
– Velocity of electromagnetic wave is a function of the density of
the medium
– When wave changes medium, speed changes
– Wave bends at the boundary between mediums
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Line‐of‐Sight Equations
• Optical line of sight
• Effective, or radio, line of sight
• d = distance between antenna and horizon (km)
• h = antenna height (m)
• K = adjustment factor to account for refraction, rule
of thumb K = 4/3
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Optical and Radio Horizons
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Line‐of‐Sight Equations
• Maximum distance between two antennas for
LOS propagation:
• h1 = height of first antenna
• h2 = height of second antenna
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Basic Propagation Mechanisms
1. Free‐space propagation
2. Transmission
– Through a medium
– Refraction occurs at boundaries
3. Reflections
– Waves impinge upon surfaces that are large compared to the
signal wavelength
4. Diffraction
– Secondary waves behind objects with sharp edges
5. Scattering
– Interactions between small objects or rough surfaces
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Reflection, Scattering and Diffraction
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Line‐of‐sight (LOS) wireless transmission impairments
•
•
•
•
•
•
Attenuation and attenuation distortion
Free space loss
Noise
Atmospheric absorption
Multipath
Refraction
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Attenuation
• Strength of signal falls off with distance over
transmission medium (signal spreads out)
• Attenuation factors for unguided media:
– Received signal must have sufficient strength so that
circuitry in the receiver can interpret the signal
– Signal must maintain a level sufficiently higher than noise
to be received without error
– Attenuation is greater at higher frequencies, causing
distortion
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Factors influencing effective LOS wireless communications
– [Economics] Cost of transmission facility; higher frequency
and greater bandwidth as a rule of thumb are more
expensive
– Pointing accuracy requirements (think satellites)
– Path loss and attenuation
– Reflection (high), diffraction, scattering at higher
frequencies
• E.g., infrared, laser, directed light (don’t go through walls)
– Laser communications (Lasercomm)
• Atmospheric attenuation and absorption
• Potential health hazards of high‐energy RF waves (e.g., gamma rays)
• FCC transmit power limitations
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Electromagnetic spectrum of telecommunications
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The Effects of Multipath Propagation
• Reflection, diffraction, and scattering
• Multiple copies of a signal may arrive at different
phases
– If phases add destructively, the signal level relative to
noise declines, making detection more difficult
• Intersymbol interference (ISI)
– One or more delayed copies of a pulse may arrive at the
same time as the primary pulse for a subsequent bit
• Rapid signal fluctuations
– Over a few centimeters
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The Effects of Multipath Propagation
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The Effects of Multipath Propagation
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The Effects of Multipath Propagation
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Types of Fading
• Large‐scale fading
– Signal variations over large distances
– Path loss LdB as we have seen already
– Shadowing
• Statistical variations
– Rayleigh fading
– Ricean fading
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Types of Fading: Fast or Slow
• Doppler Spread
– Frequency fluctuations caused by movement
– Coherence time Tc characterizes Doppler shift
• How long a channel remains the same
– Coherence time Tc >> Tb bit time  slow fading
• The channel does not change during the bit time
– Otherwise fast fading
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Types of Fading: Flat and Frequency Selective Fading
• Multipath fading
– Multiple signals arrive at the receiver
– Coherence bandwidth Bc characterizes multipath
• Bandwidth over which the channel response remains
relatively constant
• Related to delay spread, the spread in time of the
arrivals of multipath signals
– Signal bandwidth Bs is proportional to the bit rate
– If Bc >> Bs, then flat fading
• The signal bandwidth fits well within the channel
bandwidth
– Otherwise, frequency selective fading
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Types of Fading: Flat and Frequency Selective Fading
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Channel Correction Mechanisms: Brief Overview
•
•
•
•
•
•
•
Forward error correction
Adaptive equalization
Adaptive modulation and coding
Diversity techniques and MIMO
OFDM
Spread spectrum
Bandwidth expansion
127
Forward Error Correction
• Transmitter adds error‐correcting code to data block
– Code is a function of the data bits
• Receiver calculates error‐correcting code from
incoming data bits
– If calculated code matches incoming code, no error
occurred
– If error‐correcting codes don’t match, receiver attempts to
determine bits in error and correct
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Forward Error Correction
5.15 Forward Error Correction Process
129
Adaptive Equalization
• Can be applied to transmissions that carry analog or
digital information
– Analog voice or video
– Digital data, digitized voice or video
• Used to combat intersymbol interference (from?)
• Involves gathering dispersed symbol energy back into
its original time interval
• Techniques
– Lumped analog circuits
– Sophisticated digital signal processing algorithms
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Diversity Techniques
• Diversity is based on the fact that individual channels
experience independent fading events
• Space diversity – techniques involving physical
transmission path, spacing antennas
• Frequency diversity – techniques where the signal is
spread out over a larger frequency bandwidth or
carried on multiple frequency carriers
• Time diversity – techniques aimed at spreading the
data out over time
• Use of diversity
– Selection diversity – select the best signal
– Combining diversity – combine the signals
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Multiple Input / Multiple Output (MIMO) Antennas
• Use antenna arrays for
–
–
–
–
Diversity – different signals from different antennas
Multiple streams – parallel data streams
Beamforming – directional antennas
Multi‐user MIMO – directional beams to multiple
simultaneous users
• Modern systems
– 4 × 4 (4 transmitter and 4 reciever antennas)
– 8×8
• Two dimensional arrays of 64 antennas
• Future: Massive MIMO with many more antennas
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Multiple Input / Multiple Output (MIMO) Antennas
This is a good
summary slide
for MIMO
133
Adaptive modulation and coding (AMC)
• The modulation process formats the signal to
best transmit bits
– To overcome noise
– To transmit as many bits as possible
• Coding detects and corrects errors
• AMC adapts to channel conditions
– 100’s of times per second
– Measures channel conditions
– Sends messages between transmitter and receiver to
coordinate changes
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Bandwidth expansion
• A signal can only provide a limited bps/Hz
– More bandwidth is needed
• Carrier aggregation
– Combine multiple channels
• Example: Fourth‐generation LTE combines third‐generation carriers
• Frequency reuse
– Limit propagation range to an area
– Use the same frequencies again when sufficiently far away
– Use large coverage areas (macro cells) and smaller coverage areas
(outdoor picocells or relays and indoor femtocells)
• Millimeter wave (mmWave)
– Higher carrier frequencies have more bandwidth available
– 30 to 300 GHz bands with millimeter wavelengths
– Yet these are expensive to use and have problems with obstructions
135
Bandwidth expansion: Carrier Aggregation
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Key Data Transmission Terms
137
Encoding and Modulation Techniques
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Signal Encoding Criteria
• What determines how successful a receiver will be in
interpreting an incoming signal?
– Signal‐to‐noise ratio (or better Eb/N0)
– Data rate
– Bandwidth
• An increase in data rate increases bit error rate
• An increase in SNR decreases bit error rate
• An increase in bandwidth allows an increase in data
rate
Importantly, another factor can be utilized to improve performance
and that is the encoding scheme
139
Factors Used to Compare Encoding Schemes
• Signal spectrum
– With lack of high‐frequency components, less bandwidth
required
• Clocking
– Ease of determining beginning and end of each bit position
• Signal interference and noise immunity
– Certain codes exhibit superior performance in the
presence of noise (usually expressed in terms of a BER)
• Cost and complexity
– The higher the signal rate to achieve a given data rate, the
greater the cost
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Basic Encoding Techniques
• Digital data to analog signal
– Amplitude‐shift keying (ASK)
• Amplitude difference of carrier frequency
– Frequency‐shift keying (FSK)
• Frequency difference near carrier frequency
– Phase‐shift keying (PSK)
• Phase of carrier signal shifted
141
Modulation of Analog Signals for Digital Data
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Phase‐Shift Keying (PSK)
• Two‐level PSK (BPSK)
– Uses two phases to represent binary digits
143
Phase‐Shift Keying (PSK)
• Differential PSK (DPSK)
– Phase shift with reference to previous bit
• Binary 0 – signal burst of same phase as previous signal
burst
• Binary 1 – signal burst of opposite phase to previous
signal burst
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Differential Phase‐Shift Keying
145
Quadrature Phase‐Shift Keying (PSK)
• Four‐level PSK (QPSK)
– Each element represents more than one bit
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QPSK Constellation Diagram
147
QPSK and OQPSK Modulators
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Phase‐Shift Keying (PSK)
• Multilevel PSK
– Using multiple phase angles with each angle
having more than one amplitude, multiple signal
elements can be achieved
•
•
•
•
D = modulation rate, baud or symbols/sec
R = data rate, bps
M = number of different signal elements = 2L
L = number of bits per signal element
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Performance
• Bandwidth of modulated signal (BT)
– ASK, PSK BT = (1+r)R
– FSK
BT = 2Δf+(1+r)R
• R = bit rate
• 0 < r < 1; related to how signal is filtered
• Δf = f2 – fc = fc ‐ f1
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Performance
• Bandwidth of modulated signal (BT)
– MPSK
– MFSK
• L = number of bits encoded per signal element
• M = number of different signal elements
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Bit Error rate (BER)
• Performance must be assessed in the presence
of noise
• “Bit error probability” is probably a clearer term
– BER is not a rate in bits/sec, but rather a probability
– Commonly plotted on a log scale in the y‐axis and
Eb/N0 in dB on the x‐axis
– As Eb/N0 increases, BER drops
• Curves to the lower left have better
performance
– Lower BER at the same Eb/N0
– Lower Eb/N0 for the same BER
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Theoretical Bit Error Rate for Various Encoding Schemes
153
Theoretical Bit Error Rate for Multilevel FSK, PSK, and QAM
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Quadrature Amplitude Modulation
• QAM is a combination of ASK and PSK
– Two different signals sent simultaneously on the
same carrier frequency
155
QAM Modulator
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16‐QAM Constellation Diagram
157
158
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Antenna Supplemental Material [1]
[1] Michael
Borsuk
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•The ionosphere is composed of various “layers”: D (44‐55 miles up), E (65‐75 miles up), F1
(90‐120 miles), and F2 (200 miles).
•The D layer is good at absorbing AM Broadcast frequencies (0.5 – 1.7 MHz) during the day,
but it disappears at night.... the E and F layers bounce the waves back to the earth during
nighttime hours. Many AM stations shut down or reduce power at night to avoid
interference.
•Communication between most two points on Earth is possible at most times of the day
with low attenuation at some frequency in the Short Wave band: 3 – 30 MHz. Most short
wave licenses have multiple frequencies assigned to take advantage of time of day and
161
other variations.
But wait, isn’t EM propagation subject to the 1/R2 rule?*
•In general, free space electromagnetic waves spread out in three dimensions and are
attenuated by the square of the distance from the source. The most general condition
is “from an isotropic source” or radiation equally in all directions—a situation that is
not usually the case, but radio waves not affected by anything else do always observe
the inverse square law as does even “beams” of radio in free space (when not affected
by other factors).
•Ionospheric reflection and absorption are special but very important cases.
•How waves are launched and received by antennas, the interactions with the Earth,
obstacles, and the Ionosphere must all be taken into account.
•In free space from an isotropic antenna energy travels outward equally in straight
lines with this power density, S:
Siso(R) = Prad/4R2 watts/square meter
Prad is the total power radiated and R is the distance from the source.
Under these special conditions, radio is attenuated as per the inverse square law. That
is, the energy is spread out as the area of its coverage increases.
*Some of the background for this and the following slides are taken from Foundations of Electrical Engineering by J. R.
Cogdell, 1996 Prentice Hall, pages 594 – 608.
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FACTORS AFFECTING RADIO WAVE PROPAGATION
Polarization. Electromagnetic waves consist of electric and magnetic waves that at
every point in wave have directions. While the E and M field vectors are perpendicular
to each other, they have a specific orientation at each point in space. Polarization is
important because most antennas produce and receive only one polarization. Circular
polarization is a special case and produces a signal receivable well by both “horizontal”
and vertical” polarized antennas. Having the correct polarization is often critical for
radio communications as cross polarization losses can be over 10 dB.
Surface Waves. Lower frequencies can follow the curvature of the Earth. Losses may
be due to resistive losses in the soil in addition to the 1/R2 loss. Surface waves do not
have to be line of sight but bend with the Earth.
Frequency Dependence. In general, lower frequencies (below 50 MHz) propagate via
surface waves and/or ionosphere reflections and can not be particularly polarity
sensitive.
Frequencies above 50 MHz, while occasionally subject to spectacular (or annoying)
“skip” conditions, require strict antenna polarization and propagate line of sight—that
is, they do not curve in general with the Earth or over obstacles such as mountains.
They do penetrate buildings via reflections.
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Radio Wave Phenomena. There are many. Here are some:
•Reflection can occur from the Earth’s surface, bodies of water, building, and at low
frequencies, the ionosphere. Radar is based on this phenomena. Multipath distortion is
when a signal interferes with itself because of reflections.
•Depolarization occurs when a wave loses its polarization—usually due to a reflection. This
can be good or bad depending on the antennas used.
•Refraction is bending of a wave towards the direction of denser material. Since the
atmosphere is denser at lower levels, refraction extends the “radio horizon” slightly beyond
pure line of sight.
•Diffraction occurs when waves spread into a shadow region behind an obstacle. This is a
well known wave phenomena (think ocean waves) that allows radio signals to go around
and into buildings.
•Scattering occurs when radio waves bounce off multiple small objects. Ocean waves or
even dust particles in the air effect propagation, depending on the wavelength. Often
scattering as well as attenuation due to matter, such as rain or atmospheric gases, can
occur for high microwave frequencies.
•Doppler Shift is a change of frequency that occurs due to the source, reflecting object, or
receiver moving. This can allow the police to measure your speed or make aircraft
communications on microwave frequencies impossible.
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ANTENNAS
VLA New Mexico, Keith Stanley
165
An antenna is a structure that couples between a
guided and a free electromagnetic wave.
from Foundations of Electrical Engineering, J. R. Cogdell
•Antennas are really just unshielded transmission lines optimized for coupling to free
space. Optimization includes size, shape, height, and other physical and electrical
characteristics such as physical or artificial ground (counterpoise).
•Transmitting and Receiving antennas can be identical in design but often physical
details are different (e.g. thicker components to minimize ohmic losses when used for
transmitting).
•Frequency of Signal, Polarization, Gain, Radiation (or acceptance) Patterns, Drive
Impedance are specification factors.
•Different types used for fixed, mobile, portable, use on towers or buildings, etc.
determines physical size.
•Radiating elements get very inefficient when sized less than an appreciable fraction
of a wavelength (e.g. less than ¼ wavelength in size).
•Other issues: affixed to device, removable, possible to locate remotely from
equipment, proximity to other antennas (pattern interaction, front end overload*).
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*Front end overload
What is it?
Front end overload is where a receiver is not capable of ignoring strong signals which are outside of the
specific frequency range it is designed to receive but usually in the same general part of the spectrum.
Although, for example, a wireless LAN should not pick up anything outside of its specific channel, a
strong signal on the nearby cordless telephone frequencies may be able to overload the receiver
circuitry.
What are the effects of front end overload?
This type of interference usually causes a reduction of sensitivity of the receiver (called “desense”),
although in extreme cases intermodulation distortion can occur or even the receiving device can be
rendered inoperative until action is taken regarding the offending transmitter.
Who's fault is it?
As the equipment suffering the problem should be able to reject nearby signals, it is the fault of that
equipment itself. Usually it is due to the first amplifier stage within the receiver being driven into non‐
linearity by the strong nearby signal. Often but not always, low end cost equipment will be more likely to
suffer from this problem.
What can I do about it?
With regard to your wireless LAN access point, cordless phones, and even microwave oven—all within
the general 2.4 GHz band—it is best to locate these units some distance apart. (Of course, if you notice
anything due to the microwave, it is probably leaking energy and should be replaced for safety reasons.)
It might be best to do some frequency management of your own and avoid purchasing a 2.4 GHz
cordless phone, for example, if you have a IEEE 802.11 wireless LAN.
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THE BASIC MATHEMATICS OF ANTENNAS
1. How well an antenna couples energy into electromagnetic waves is
called the Radiation Efficiency, η (Greek letter eta).
η = Prad/Pin
2. The GAIN of an antenna describes how well it can focus the energy
rather than radiate (or accept) radio waves isotropically*. Some parabolic
reflector antennas (such as the ones at the VLA) can have gains well
over 30 dB.
G = η S(R)/Siso(R)
S(R) is the Power Density, usually in units of microwatts per square meter,
μw/m2. This is the usual measure of the electromagnet field strengths,
especially with respect to radio.
Siso(R) is the field strength produced by substituting (or calculating) what
would be radiated from a perfect isotropic antenna. Some antennas are
spec’ed as compared to a dipole antenna, in which case Sd(R) is used.
*Isotropic = independent (equal) in all directions, as radiation from a point source.
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THE FIELD POWER DENSITY, S(R)
•The field power density is the measure of how much signal is “out there.”
The higher S, the more signal an antenna used for receiving can couple from
the field. You still have to take into account the result of the transmission
line loss, noise and interference, and the performance of the receiver
circuitry.
•Since EM fields in free space obey the inverse square law, combining this
with the previous two equations gives the well known relationship
describing the field strength produced by an antenna at any distance, R.
S(R) = PinG/4πR2
•Field strength meters are quite common for a wide range of frequencies,
and calibrated test antennas (even quiet antenna ranges are available) to
test antenna performance.
•Pin is not the whole story since the drive impedance, Zin, of the antenna
determines how well the feed‐line can couple energy into it.
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Transmitting Antenna Example:
An antenna used for a microwave point to point link has a gain of 22 dB. We need
to have a minimum power density of 2 μw/m2 at 10 miles away.
10 miles = 16.1 Km
22 dB = a ratio of 158, so
Pin = 4πR2S(R)/G
= 41.1 watts.
You would need to supply
The antenna with 41.1 watts.
At right: the “Meet Me” room in this
new office building terminates
facilities from many different telco's,
said to be the most interconnections
anywhere in the world.
Reference Calif State University
Rooftop microwave antennas at One Wilshire in
Los Angeles.
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RECEIVING ANTENNA MATH
We have an intuitive feeling that bigger antennas work better. It is true, and assuming that
a receiving antenna is correctly oriented towards the transmitting antenna and that the
polarization of the two antennas are the same, then:
Pav = Aeff S(R) watts
Where Pav is the available power that the antenna receives, and Aeff is the Effective Area of
the antenna—a measure of its size. Understanding the details of Aeff is a little difficult, but
the concept is really simple: the antenna works better if there is more metal. It’s kind of
thinking that a bigger ladle scoops up more soup, but the devil is in the details. For us it’s
good enough to realize that the higher the gain, the bigger Aeff must be (and must have
been). With that in mind, we get:
Aeff = λ2Gr/4π square meters
Where λ is the wavelength of your signal. The more gain, the bigger the effective area. FYI,
the above formula comes from Maxwell’s equations applied to conservation of energy. You
can’t be more basic than that. The formula says that the effective area goes up with gain
and as the square with wavelength. That means that antennas for higher frequencies don’t
work very well unless they have huge gains. That’s one reason microwaves systems use
dishes. Since higher gains are as the result of directionality, omnidirectional microwave
systems (such as wireless LANs) have very low range.
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Receiving Antenna Example:
Let’s look at the receiving end of the link from the
top of One Wilshire from the previous example.
Let’s say the receiving antenna is identical, and the
frequency is 5020 MHz, in the 5005 – 5060 MHz
FIXED SERVICE (point to point microwave) band in
the US.
λ = 300/5020 = 5.97 cm
G = 158
S(R) = 2 μw/m2
Aeff = λ2Gr/4π = 4.50 x 10‐2 m2
Pav = Aeff S(R) = .0901 μw
Actually we might be interested in the voltage the
antenna couples to the transmission
line. Let’s say the receiver which is attached to a very short feed‐line and is mounted
right behind the antenna so that there is no transmission line losses. We know that
P = V2/R. Therefore, V = square root of PxR. So for a 50 ohm feed‐line, V = 2.12 mv,
which is a pretty strong signal. This is a good link.
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LET’S PUT IT ALL TOGETHER – THE ENTIRE HOP
•If we combine the formulas from the last two examples, we get:
•Prec = PinGtGr/(4πR/λ)2
•Where Gt and Gr are the gains of the transmitting and receiving antennas respectively. It makes
sense that the link doesn’t care (for this formula) which antenna has more gain; it’s the product
of the gains that matters*. So the numerator is the power from the transmitter antenna times
the total gain.
•The denominator of the above formula is called the space loss and is often given in dB. Notice
that the formula calculates the distance (to be applied to the inverse square rule as we would
expect) as R/λ or how many wavelengths apart the two antennas are.
•For the examples we just did with the antennas 10 miles apart and a frequency of 5020 MHz,
R/λ = 2.69 x 105 or 269,000 wavelengths.
The space loss is (4πR/λ)2 = 1.143 x 1013
130.6 dB
•The signal loses 130.6 dB in its hop between the two antennas. Good thing we have a total gain
of 44 dB, a factor of 25,000. If we used isotropic antennas, we would have needed over 1
million watts for this link instead of just 41.1 watts. Again, think of your wireless LAN, where
both the AP and your laptop might be using omnidirectional antennas.
•For all it’s worth, your cell phone antenna probably has a gain of 2 dB or less.
*The respective gains do matter if you are concerned with interfering signals.
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LOTS OF KINDS OF ANTENNAS – YOU SHOULD KNOW A LITTLE
ABOUT EACH
Reflector Type: The real antenna is a small approximately close to 0 dB gain
antenna—called the feed—at the focus of (usually) a parabolic reflector. These
antennas are usually described in terms of the diameter of the parabola (e.g.
“a 3 meter dish”).
It’s not all that hard to calculate the ideal gain of such antennas; it’s roughly 2π
times the area of the antenna dish (given in number of wavelengths at the
operating frequency). This formula assumes that about half the energy
impinging on the dish is actually reflected into the feed due to roughness in the
dish and assuming that the feed is at the correct position at the focus of the
dish.
So for our antenna of the earlier examples, if you assume a feed gain of 0 dB:
G = 22 dB = 158 times = ½ π2 (D/λ)2
If you plug in the numbers, you find that the diameter of the dish must have
been 33.8 cm, about the size of home TV satellite dishes.
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Typical Studio Transmitter 950 MHz
Link antenna – WSUM, Madison
WI.
G = (approximately) 2π x The Area of the dish in square wavelengths
G = π2 (D/λ)2/2 (answer as a ratio)
G = 20 log10B + 20 log10F + 7.5 dB
The 2nd formula simplifies the calculation with B the diameter of the dish in
feet and F the operating frequency and gives the answer in dB.
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LOW GAIN ANTENNAS
Quarter wavelength long “whip” antennas have a very modest
gain, only 0.85 dBi but are practical. They have the advantage of
radiating horizontally. That’s where the gain comes from since
no energy goes straight up or down. The simplest whip is ¼
wavelength long, but somewhat higher gains can be achieved
by increasing its length. 5/8 wavelength is a common size.
Often two quarter wavelength segments are cascaded to
increase the gain by 3 dB and focus the radiation pattern even
more. This sort of antenna is common on mobile cell antennas.
Note the inductor at the mid‐point of the antenna to couple
the two ¼ wavelength elements.
Two quarter length segments feed in the middle is a “dipole”
antenna. Dipoles are used by everything from FM broadcast
transmitters to calibrated test equipment. The gain of a dipole
is theoretically 2.14 dBi = 0 dBd.
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A VERY BIG WHIP ANTENNA
AM Broadcasts wavelengths are from 150
to 600 meters!
Often the entire tower is a 1/4th or 5/8
wavelength antenna, and sometimes a
number of such 40 – 200 meter towers
are constructed in a field to provide some
gain and directionality to avoid
interference with neighboring areas.
It is also not uncommon for much higher
frequency antennas to be mounted on the
¼ wavelength radiating towers.
Note the insulator at the bottom of this
200 meter high AM broadcast antenna
and the VHF, UHF, and microwave
antennas mounted at various heights.
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Left: A tunable dipole in which the element
lengths (rightmost part) can be mechanically
adjusted to quarter wavelengths at the
operating frequency.
Above: A Yaggi antenna (named for the
Japanese inventor) is a number dipoles
aligned to focus the signal in one direction.
Gains of up to 20 dB or more are possible in
one direction as 3 dB gain is achieved each
time the number of elements are doubled.
This one has circular polarization for
communication with satellites.
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Amateur Radio installation including a number of short wave Yaggi type antennas
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TV antennas are Yaggi’s made to work over a broad range of frequency. They have little gain but
a very wide frequency range. Notice that this TV antenna intended for use in Spain has vertical
polarization. TV stations in the United Stations require horizontal antennas.
One type of such antennas is the log periodic, a very common for commercial short wave
operation where one large antenna must cover many frequencies to allow world wide
communication. You see these at government and military installations.
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FM radio stations require both horizontal and vertical polarization to accommodate
listeners’ horizontally polarized rooftop antennas and vertical “whip” antenna
automobile installations.
A typical FM station will have a number of circular or diagonally polarized antennas on
one tower to provide some gain and lower the radiation pattern. Gains of 6 dB
requiring four such antennas or 9 dB with eight antennas are common.
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Notice the 5 antenna (little more than 6 dB gain) FM radio station in the background and the
many “whips”. Most of these are co‐linear types which are multiple quarter wavelength long to
provide gains in the 6 – 10 dB range or multiple vertically polarized dipoles on the same mast
for the same purpose. Or, as on the right, the dipoles can be placed to make the pattern more
omnidirectional.
The microwave dish is a link for the Boulder County Sheriff Department’s emergency services
communication center (the E911 AP) to a transmitter located in the green building. The
Sheriff’s Department maintains a network of such sites throughout the county.
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These two curves are typical antenna patterns from tower side mounted antennas
such as for cell/PCS sites, TV or FM radio stations. The curve is the field strength (in dB)
at a given distance, usually where the signal is just strong enough to provide good
coverage. For example, north might be up and the curve shows the field strength (in
microwatts/meter) for each direction from the transmitter at say 30 miles, the limits
that the station wants to be heard (and sell advertising for). Let’s hope that the
population center is west of the transmitter site. Cell phone site curves would look
very similar, but the contour would be for a much smaller range.
Here TV station KAUP is comparing their antenna patterns for their old analog
broadcasts versus their new digital/HDTV transmitter site.
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The curve on the left is for a 5.3 GHz 28
dBi gain High Performance Parabolic
Dish Wireless LAN Antenna for IEEE
802.11a wireless LANs. The dB scale
shows relative power in the horizontal
direction. The radiation off the back of
the dish is down 35 dB from the favored
direction.
Full specs for this antenna are at:
http://www.hyperlinktech.com/web/hg5328d.php
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DISTANCE TO THE HORIZON
The distance to the horizon (in kilometers) increases with the
height of the antenna above the ground (in meters) according to
the above approximation.
The maximum “line of sight” distance between two radio sites
requires that both antennas be elevated.
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CLASS EXAMPLE: MICROWAVE LINK FADE MARGIN CALULATION*
1. Free Space Loss
You will have more opportunities to do this in later courses, but we might as well take
a quick look at the procedure now. It will give you some exposure of how wireless
links are engineered while reinforcing your feel how real radio systems work.
Earlier in the lecture, we saw that the Free Space Loss was calculated as follows:
FSL = (4πR/λ)2
where R/λ is the number of wavelengths at the operating frequency between the
two antennas. Since FSL is usually given in dB, an additional conversion to that
measure is necessary.
A more common statement (with English units and the result in dB) for FSL is as
follows:
FSL = 36.6 + 20 log F + 20 log D
where F = Frequency in MHz, and D = Distance between the antennas in miles.
*Notes adapted from a paper given at the 2004 FCC Broadband Forum paper
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The “American” restatement of the Free Space Loss formula makes the calculation easy.
Notice that at any distance the 2.4 GHz IEEE 802.11g wireless LANs will be 8 dB stronger
than the 5.8 GHz IEEE 802.11a type. Note that doubling the distance has the same effect as
doubling the frequency. This formula uses MHz. Add 60 dB for GHz.
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2. Fresnel Zone
We know that at EM radiation at microwave frequencies travel in straight lines (called,
“line of sight”) and need a clear path: that is one without obstacles.
The Fresnel (please pronounce this correctly—it’s fra‐nell) zone is a calculation of how
wide a swath the radio signal path needs to be clear. This is because the signal is not
pencil thin; it spreads out. We wouldn’t want it as thin as a laser anyway since then
the aiming of the antennas would be too critical and subject to loss in wind or even if a
bird flew between them. But we are worried that there is something that will “reflect”
the signal along the path.
This is not purely a simple calculation as there are more than one Fresnel Zone, but
usually engineers calculate the “First Fresnel Zone” as an estimate.
The Fresnel zones at any point in the path are:
Fn = 72.1*SQRT((nd1d2)/fD)
where Fn = nth Fresnel zone radius in feet, d1 = distance from one end of path a
reflection (obstacle) point in miles, D = total length of path in miles, d2 = D ‐ d1, and f =
frequency in GHz. Notice that F gets smaller as the square root of F.
Usually engineers are happy if nothing penetrates the 1st Fresnel zone within 40% of
the beam at any point, or in other words less than 0.6F1 is clear of obstructions.
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20%
60%
20%
Fn = 72.1*SQRT((nd1d2)/fD). You don’t want anything penetrating the 1st Fresnel Zone
maximally within 0.6 F1. The site engineers ensure that the antennas are high enough and that
the path avoids obstacles, sometimes requiring a zig‐zag path around anything that might
penetrate the Fresnel Zone.
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3. Antenna Gain
We have already looked at this for dishes where G can be very big.
G = 20 log10B + 20 log10F + 7.5 dB
Where B is the diameter of the dish in feet and F is in GHz.* Other types of antennas vary
between a little more than 0 dBi (whips and dipoles) and maybe between 10 and 20 dB
(Yaggi’s).
4. Transmit Power and Receiver Sensitivity – System Operating Margin
These are equipment specifications. The transmit power in watts is what the transmit
applies to the feed‐line, not the signal actually delivered to the antenna which will be less.
The receiver sensitivity is a minimum signal level (sometimes given in dBm but more usually
given in microvolts applied to the input port of the receiver equipment), again after the
losses in the receiver transmission line. For most radio equipment, one to 10 microvolts are
required minimally.
5. Feed line losses
At microwave frequencies, the loss per meter can be very high. It’s always better to use
short lengths of cable, better cables, or even locate receivers and transmitters near the
antennas, sometimes on the towers themselves.
*In the Wilshire One Bldg example: B = 1 foot, F = 5.02 GHz. G was approx 22 dB.
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6. Multipath Interference
This is when some of the signal arrives at the receive antenna after being
reflected off the ground, from atmospheric reflections, or a passing aircraft. This
is usually destructive to the signal. Siting, antenna height, etc., and other design
schemes are employed to minimize these possibilities. Multipath is responsible
for your FM radio sounding poorly even when the signal is strong—as here in
Boulder because of reflections off the Flatirons—and accounts for “ghosts” on
analog TV reception.
7. Signal‐to‐Noise Radio
S/N (usually in dB). This is a more critical number than “Receiver Sensitivity” (as
in optical fiber calculations) since as you probably know various wireless systems
will test the “connection” during the initial session set‐up or even every second
or so for reliable packet reception and step down the data rate to provide for
error free performance.
This is why your IEEE 802.11g wireless lap top connection might show 54 Mbps
sometimes but 36 Mbps down to 11 Mbps or less if you stray too far from the
wireless Access Point.
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How much more signal is presented to the receiver than required is the System
Operating Margin, usually given in dB (just like for optical fibers). But you have
to take into account the minimum S/N ratio required as well.
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The receiver and transmitter will test the reliability of packet exchange and only provide
the full data rate when the SNR is adequate.
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The calculation of the SYSTEM OPERATING MARGIN (also called FADE MARGIN) puts all the
factors together.
Here two dishes each with 24 dB gain are 20 miles apart. A receiver sensitivity of ‐83 dBm
provides the minimally adequate S/N level. The actual installation has a transmitter power of 24
dBm or 1/4 watt, and the feed‐lines have losses of 1 dB each.
The FADE MARGIN is 23 dB, a factor of 200 times the minimum signal required.
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So why so high a “fade margin”?
1. Transmit power is pretty cheap, but gain in antennas is cheaper.
2. Shorter antenna spacing is very expensive as is making the towers
higher. These are to be avoided.
3. Most engineers agree that 20 dB is a minimum fade margin to take care
of real world factors, but some installations have margins of as low as 10
dB. It’s hard to get a microwave engineer to accept a design of less than
14 dB margin (or 25 times the power).
4. The real world factors include interference (from other transmitters or
even electrical equipment), antennas misaimed or worries about their
becoming out of aim or even shaking in the wind, atmospheric conditions
(skip or even dirt or rain) which can account for a daily variation of up to
+/- 6 dB, and ice on the antenna or water in the feed-line*. 14 dB margin
doesn’t seem so high if you consider these factors.
*It is not uncommon to find gallons of water leaking out of replaced air gap feed lines.
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Picture Credit: Royal Army, UK
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