lecture-01 - ankara yıldırım beyazıt university · –1000‐fold increase in data trafficby...
TRANSCRIPT
2/20/2018
1
1
Evolution of terrestrial networking: circuit‐switched to packet‐switched
2
CxP 70022‐01, C3I Interoperability Standards Book, Vol.1, pg. 15
Why packetized communications?
2/20/2018
2
3
CxP 70022‐01, C3I Interoperability Standards Book, Vol.1, pg. 15
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)
4
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
2/20/2018
3
5
• 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
Wireless comes of age
6
• 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
Cellular Telephony
2/20/2018
4
7
• Profound
• Shrinks the world
• Always on
• Always connected
• Changes the way people communicate– Social networking
• Converged global wireless network
Wireless impact
8
Some milestones in wireless communications
2/20/2018
5
9
• Growth– 11 million users in 1990
– Over 7 billion today
• Mobile devices– Convenient
– Location aware
– Only economical form of communications in some places
Global cellular network(s)
10
• 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
Global cellular network(s)
2/20/2018
6
11
• 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
Mobile device revolution
12
• 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
Mobile device revolution
2/20/2018
7
13
• 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
Future Trends
14
• 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
Future Trends
2/20/2018
8
15
• 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.
Future Trends
16
What is 5G?
(eMBB)
(Massive M2M) Low‐latency, ultra‐reliable (LLUR)
2/20/2018
9
17
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
18
• 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
The trouble with wireless
2/20/2018
10
19
• 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)
Characteristics of wireless data transmission
20
• 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
Characteristics of wireless data transmission
2/20/2018
11
21
• 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
Focus of this course
22
Electromagnetic spectrum of telecommunications
2/20/2018
12
23
• 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
Political difficulties
24
Signals & Decibels
2/20/2018
13
25
• 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
Electromagnetic Signals
26
• 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
Signals: Time domain concepts
2/20/2018
14
27
Signals: Analog and Digital waveforms
28
• 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
Signals: Time‐domain concepts
2/20/2018
15
29
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
30
• 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
Signals: Sine Wave Parameters
2/20/2018
16
31s(t) = A sin (2πft + ϕ)
Signals: Sine Wave Parameters
32
• 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
Signals: Frequency Domain Concepts
2/20/2018
17
33
Signals: Addition of frequency Components(T = 1/f)
34
• 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
Signals: Frequency Domain Concepts
Frequency Components of Square Wave
2/20/2018
18
35
Signals: Acoustic spectrum of speech and music
36
• 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
Signals: Relationship between Data Rate and Bandwidth
2/20/2018
19
37
Signals: Attenuation of Digital Signals
38
• 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
Data Communication Terms
2/20/2018
20
39
• Analog• Video
• Audio
• Digital• Text
• Integers
Examples of Analog and Digital Data
40
• 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
Analog Signals
2/20/2018
21
41
• 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
Digital Signals
42
• 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
Reasons for Choosing Data and Signal Combinations
2/20/2018
22
43
• 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
Analog Transmission
44
• 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
Digital Transmission
2/20/2018
23
45
• 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
Wireless Systems focus: Maximize channel capacity
46
Effects of Noise on a Digital Signal
2/20/2018
24
47
• 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
Concepts Related to Channel Capacity
48
• For binary signals (two voltage levels)
– C = 2B
• With multilevel signaling
– C = 2B log2 M
• M = number of discrete signal or voltage levels
Nyquist Bandwidth
2/20/2018
25
49
• 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
Signal‐to‐Noise Ratio: Most important wireless communications metric
50
• Equation:
• Represents theoreticalmaximum 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
Shannon Capacity Formula
2/20/2018
26
51
• Spectrum of a channel between 3 MHz and 4 MHz ; SNRdB = 24 dB
• Using Shannon’s formula
Example of Nyquist and Shannon Formulations
52
• How many signaling levels are required?
Example of Nyquist and Shannon Formulations
2/20/2018
27
53
• 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
Classifications of Transmission Media
54
• Transmission and reception are achieved by means of an antenna
• Configurations for wireless transmission
– Directional
– Omnidirectional
Unguided Media
2/20/2018
28
55
Electromagnetic spectrum of telecommunications
56
• 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
General Frequency Ranges
2/20/2018
29
57
• 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
Terrestrial Microwave
58
• 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
Satellite Microwave
2/20/2018
30
59
• 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
Broadcast Radio
60
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!
2/20/2018
31
61
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.
62
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.
2/20/2018
32
63
Decibels Review
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.
When dB’s are absolute values and not ratios:
The use of dBm, dBμ, dBw, etc. is really an abbreviation
64
Antenna Fundamentals
2/20/2018
33
65
• 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
Antenna Fundamentals
66
• 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
Antenna Radiation Patterns
2/20/2018
34
67
Antenna Radiation Patterns
Also termed isotropic radiation
68
• 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
Types of Antennas
2/20/2018
35
69
Simple Antennas
70
Radiation Pattern in Three Dimensions
2/20/2018
36
71
Parabolic Reflective Antennas
72
• 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
Antenna Gain
2/20/2018
37
73
• 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
Antenna Gain
74
• 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)
Spectrum Considerations for Signal Transmission
2/20/2018
38
75
• 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
Spectrum Considerations for Signal Transmission
76
• 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
Spectrum Considerations for Signal Transmission
2/20/2018
39
77
• Ground‐wave propagation
• Sky‐wave propagation
• Line‐of‐sight propagation
Signal Propagation Modes
78
Propagation Modes
2/20/2018
40
79
• Follows contour of the earth
• Can propagate considerable distances
• Frequencies up to 2 MHz
• Example
– AM radio
Ground Wave Propagation
80
• 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
Sky Wave Propagation
2/20/2018
41
81
• 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
Line‐of‐Sight Propagation
82
• 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
Line‐of‐Sight Equations
2/20/2018
42
83
Optical and Radio Horizons
84
• Maximum distance between two antennas for LOS propagation:
• h1 = height of first antenna
• h2 = height of second antenna
Line‐of‐Sight Equations
2/20/2018
43
85
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
Basic Propagation Mechanisms
86
• 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
Attenuation
2/20/2018
44
87
• 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)
Free Space Loss
88
• We can recast the free space loss equation then as:
Free Space Loss
• Recall we defined a decibel loss via the equation
LdB = ‐10 Log10 (Pout/Pin) = 10 Log10 (Pin/Pout)
2/20/2018
45
89
Free Space Loss
90
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
2/20/2018
46
91
Path Loss Exponent in practical systems
92
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
2/20/2018
47
93
Categories of Noise
• Thermal Noise
• Intermodulation noise
• Crosstalk
• Impulse Noise
94
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
2/20/2018
48
95
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
96
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
2/20/2018
49
97
5.4 GENERAL SHAPE OF BER VERSUS Eb/N0 CURVES
98
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
2/20/2018
50
99
• 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
Classifications of Transmission Media
100
• Transmission and reception are achieved by means of an antenna
• Configurations for wireless transmission
– Directional
– Omnidirectional
Unguided Media
2/20/2018
51
101
• 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
Attenuation
102
Electromagnetic spectrum of telecommunications
2/20/2018
52
103
• 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
General Frequency Ranges
104
• 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
Terrestrial Microwave
2/20/2018
53
105
• 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
Satellite Microwave
106
• 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
Broadcast Radio
2/20/2018
54
107
• Follows contour of the earth
• Can propagate considerable distances
• Frequencies up to 2 MHz
• Example
– AM radio
Ground Wave Propagation
108
• 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
Sky Wave Propagation
2/20/2018
55
109
• 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
Line‐of‐Sight Propagation
110
• 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
Line‐of‐Sight Equations
2/20/2018
56
111
Optical and Radio Horizons
112
• Maximum distance between two antennas for LOS propagation:
• h1 = height of first antenna
• h2 = height of second antenna
Line‐of‐Sight Equations
2/20/2018
57
113
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
Basic Propagation Mechanisms
114
Reflection, Scattering and Diffraction
2/20/2018
58
115
• Attenuation and attenuation distortion
• Free space loss
• Noise
• Atmospheric absorption
• Multipath
• Refraction
Line‐of‐sight (LOS) wireless transmission impairments
116
• 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
Attenuation
2/20/2018
59
117
– [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
Factors influencing effective LOS wireless communications
118
Electromagnetic spectrum of telecommunications
2/20/2018
60
119
• 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
The Effects of Multipath Propagation
120
The Effects of Multipath Propagation
2/20/2018
61
121
The Effects of Multipath Propagation
122
The Effects of Multipath Propagation
2/20/2018
62
123
• Large‐scale fading
– Signal variations over large distances
– Path loss LdB as we have seen already
– Shadowing
• Statistical variations
– Rayleigh fading
– Ricean fading
Types of Fading
124
• 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
Types of Fading: Fast or Slow
2/20/2018
63
125
• 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
Types of Fading: Flat and Frequency Selective Fading
126
Types of Fading: Flat and Frequency Selective Fading
2/20/2018
64
127
• Forward error correction
• Adaptive equalization
• Adaptive modulation and coding
• Diversity techniques and MIMO
• OFDM
• Spread spectrum
• Bandwidth expansion
Channel Correction Mechanisms: Brief Overview
128
• 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
Forward Error Correction
2/20/2018
65
129
5.15 Forward Error Correction Process
Forward Error Correction
130
• 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
Adaptive Equalization
2/20/2018
66
131
• 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
Diversity Techniques
132
• 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
Multiple Input / Multiple Output (MIMO) Antennas
2/20/2018
67
133
Multiple Input / Multiple Output (MIMO) Antennas
This is a good summary slide for MIMO
134
• 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
Adaptive modulation and coding (AMC)
2/20/2018
68
135
• 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
Bandwidth expansion
136
Bandwidth expansion: Carrier Aggregation
2/20/2018
69
137
Key Data Transmission Terms
138
Encoding and Modulation Techniques
2/20/2018
70
139
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
140
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
2/20/2018
71
141
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
142
Modulation of Analog Signals for Digital Data
2/20/2018
72
143
Phase‐Shift Keying (PSK)
• Two‐level PSK (BPSK)
– Uses two phases to represent binary digits
144
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
2/20/2018
73
145
Differential Phase‐Shift Keying
146
Quadrature Phase‐Shift Keying (PSK)
• Four‐level PSK (QPSK)
– Each element represents more than one bit
2/20/2018
74
147
QPSK Constellation Diagram
148
QPSK and OQPSK Modulators
2/20/2018
75
149
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
150
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
2/20/2018
76
151
Performance
• Bandwidth of modulated signal (BT)
– MPSK
– MFSK
• L = number of bits encoded per signal element
• M = number of different signal elements
152
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
2/20/2018
77
153
Theoretical Bit Error Rate for Various Encoding Schemes
154
Theoretical Bit Error Rate for Multilevel FSK, PSK, and QAM
2/20/2018
78
155
Quadrature Amplitude Modulation
• QAM is a combination of ASK and PSK
– Two different signals sent simultaneously on the same carrier frequency
156
QAM Modulator
2/20/2018
79
157
16‐QAM Constellation Diagram
158
2/20/2018
80
159
160
Antenna Supplemental Material [1]
[1] Michael Borsuk
2/20/2018
81
161
•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 other variations.
162
But wait, isn’t EM propagation subject to the 1/R2 rule?*
•In general, free space electromagneticwaves 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/4R2 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.
2/20/2018
82
163
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.
164
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.
2/20/2018
83
165
ANTENNAS
VLA New Mexico, Keith Stanley
166
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*).
2/20/2018
84
167
*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.
168
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.
2/20/2018
85
169
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.
170
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 supplyThe 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 UniversityRooftop microwave antennas at One Wilshire in
Los Angeles.
2/20/2018
86
171
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.
172
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 transmissionline. 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.
2/20/2018
87
173
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.
174
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.
2/20/2018
88
175
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.
Typical Studio Transmitter 950 MHz Link antenna – WSUM, Madison WI.
176
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.
LOW GAIN ANTENNAS
2/20/2018
89
177
AM Broadcasts wavelengths are from 150 to 600 meters!
Often the entire tower is a 1/4th or 5/8wavelength 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.
A VERY BIG WHIP ANTENNA
178
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.
2/20/2018
90
179
Amateur Radio installation including a number of short wave Yaggi type antennas
180
TV antennas are Yaggi’smade 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.
2/20/2018
91
181
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.
182
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.
2/20/2018
92
183
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.
184
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
2/20/2018
93
185
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.
186
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 Losswas 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
2/20/2018
94
187
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.
188
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.
2/20/2018
95
189
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.
20%
60%
20%
190
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.
2/20/2018
96
191
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.
192
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.
2/20/2018
97
193
The receiver and transmitter will test the reliability of packet exchange and only provide the full data rate when the SNR is adequate.
194
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 dBmprovides 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.
2/20/2018
98
195
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.
196
Picture Credit: Royal Army, UK