Unguided Media

1
Unguided Media
2
Data Transmission
3
Lesson Objectives

• By the end of this lesson, you should be able to
• List the impairments found in unguided
  transmission media
• Describe the impairments found in unguided
  transmission media
• List three major design considerations for
  antennas and describe their relationship
• Describe the characteristics of terrestrial
  microwave, satellite, and radio wireless
  transmission
• Distinguish between LEO, MEO, and GEO satellite
  systems
• Describe what is meant by VSAT

4
Unguided Media

• Unguided media no medium to control or contain
  signals therefore, no boundaries
• Unguided media air, atmosphere
• Types of unguided media systems
• Microwave
• Satellite
• Radio

5
Transmission Impairments

• Analog signal impairments result in random
  modifications that degrade signal quality, and
  can cause errors
• Digital signal impairments result in bit errors
• Types of unguided media impairments
• Free-space loss
• Absorption
• Atmospheric absorption
• Multipath
• Refraction
• Noise/Interference

6
Free Space Loss

• Unguided media has no boundaries, so signals
  can disperse more widely and more easily
• Signal dispersion directly relates to distance
• Free-space loss path loss
• Primary cause of signal loss

7
Free-Space Loss Characteristics

• The higher the frequency, the greater the
  free-space loss
• Compensate with
• Higher gain antennas
• Higher transmitter power
• Shorter spans
• Directional antennas

8
Absorption

• Waves can be absorbed by objects buildings,
  trees, hills
• Organic materials absorb more than inorganic
• Pine needles especially effective in absorbing
  radio frequency emissions (800 MHz range)
• At 2.4 GHz, loss 0.35 dB/meter of loss
• Compensate with
• Higher gain antennas
• Higher transmitter power levels
• Shorter spacing between transmitter and receiver
  i.e., shorter spans
• Spans with fewer objects in the transmission paths

9
Atmospheric Absorption

• Atmospheric conditions absorb waves
• Water vapor, oxygen greatest contributors
• Peak attenuation _at_ 22 GHz due to water vapor
  less below 15 GHz
• Peak attenuation _at_ 60 GHz due to oxygen less
  below 30 GHz
• Rain, fog major impediments
• Heavy rain 0.5 dB/mile of loss (_at_ 5.8 GHz)
• Fog 0.07 dB/mile of loss (_at_ 5.8 GHz)
• Compensate with
• Lower frequencies
• Shorter spans

10
Multipath Fading
Reflected path
Reflected path
Compensate for multipath fading by careful site
selection
11
Multipath Fading

• Waves reflect off of objects buildings,
  vehicles, water, etc.
• Some reflected waves travel to intended
  destination
• One direct signal, multiple indirect signals,
  and
• Waves arrive with different delays result
  phase differences
• Waves can either contribute to, or detract from,
  direct signal
• Also known as Rayleigh fading

12
Fresnel Zones
2
1
D1
D2
F1
? wavelength
13
Fresnel Zones So What?

• F1 1st Fresnel Zone.
• Every point where distance is exactly ½
  wavelength longer than direct path
• F2 2nd Fresnel Zone.
• Every point where distance is exactly 1
  wavelength longer than direct path
• Reflections from
• Odd Fresnel Zone reduces signal level at
  receiver
• Even Fresnel Zone increases signal level at
  receiver

14
Refraction

• Waves are bent as they pass through atmosphere
• Signal speed increases with altitude
• Somewhat predictable, but weather conditions can
  cause aberrations in tendencies

15
Noise

• Noise unwanted electromagnetic energy inserted
  in the signals somewhere between transmission and
  reception
• Types of Noise
• Thermal Noise
• Cochannel interference
• Intermodulation noise

16
Thermal Noise

• As with guided media, thermal noise is
  unavoidable
• Arise from the thermal activity of devices and
  media
• Impact increases as signal strength decreases

17
Spectrum Reuse

• Wireless spectrum is limited a major limitation
  to wireless systems
• Two fundamental solution sets
• Space division carve up geography into smaller
  coverage areas

• Multiple access share spectrum
• among several users

While increasing the number of possible users,
these solutions also can lead to other types of
noise/interference
18
Cochannel Interference

• Occurs when more than 1 transmitter in wireless
  system is on same frequency
• Caused by frequency assignments with too little
  geographic dispersion
• By-product of basic tenet of cellular systems
  frequency reuse
• Managed or reduced by
• Reducing power levels
• Maintaining geographic dispersion
• Types of antennas
• Management of cochannel interference is the
  number 1 limiting factor
• in maximizing capacity of a wireless system

19
Cochannel Interference

• Use reduced gain antennas
• Decrease power output
• Use downtilt antennas
• Reduce height of towers

20
Intermodulation Interference

• Occurs whenever signals of different frequencies
  share the same medium
• When two frequencies share the same medium,
  supplemental frequencies are produced (harmonics)
• a ß, a ß
• Could interfere with deliberate signals at these
  resultant frequencies
• Degree of noise is a function of power output
• Occurs when there is some nonlinearity in system
• Can be managed through compensating circuits

21
Unguided Transmission

• Key is the antenna
• Role of antenna conversion between electrical
  signals and airborne signals
• Transmission antenna gets electrical signals,
  and radiates airborne energy into the medium
  i.e., air
• Reception antenna receives airborne waves from
  the surrounding medium and converts them to
  electrical signals
• Every wireless system MUST have antennas
• Antenna design is related to three major
  considerations
• Frequency to be transmitted
• Direction of transmission
• Power needed for transmission

22
Antenna Relationship to Frequency
?
Speed of light

• Wavelength (?)

Frequency
Frequency Wavelength
AM Radio 530 kHz 1,853 feet Cell Phones 900
MHz 12 inches Satellite TV 11.7 GHz 1 inch
Antenna length should be proportional to
wavelength for optimal transmission
23
Impact of Direction on Antenna Design
Omnidirectional
24
Microwave

• First used by military in WWII
• Successful application led to civilian use
  substitute for coaxial cable in late 1940s
• Generally operates at 1 GHz 50 GHz
• Vulnerable to reflections, absorption, frequency
  reuse
• Highly directional beam
• Affected by weather
• Requires line-of-sight free of obstructions
• Distance between Systems also dependent upon
  frequencies
• 2, 4, 6 GHz system towers could range 45 miles
  with LOS restrictions, closer to 35 miles
• 18, 23, 45 GHz systems range 1 5 miles

25
Microwave Antennas

• Highly Directional
• Most common form is a parabolic reflector
• Dish 6 10 in diameter
• Radome loss 0.5 1 dB

Radome
Reflective Dish
Feed Horn
Coax cable or wave guide
26
Microwave Pros and Cons

• Cost savings
• Portability
• Reconfiguration flexibility
• Bandwidth

• Requires line-of-sight
• Susceptible to natural environmental conditions
• Regulatory licensing requirements
• Potential community environmental restrictions

27
Satellite

• 1947 Arthur Clarke (2001 A Space Odyssey)
  presented a paper suggesting the use of
  satellites for communications
• 1963 NASA launched 1st experimental satellite
• 1965 1st commercial satellite
• 2003 space clutter gt250 communications
  satellites, total satellites exceed 700 plus
  250,000 pieces of debris
• Satellite microwave repeater/relay station
• Receives transmissions on uplink, retransmits
  them on downlink

28
Satellite Effectiveness
Point-to-multipoint communications e.g., USA Today
Economic benefit increases as of locations
increase
Footprint
29
Satellite Characteristics

• Key component transponder
• Accepts signal from earth
• Shifts signal to another frequency
• Amplifies signal and
• Rebroadcasts signal to earth
• Distance has impact on system
• Requires significant power
• Amount of delay is measurable and significant
  factor
• Uplink always at a higher frequency than downlink

30
Classes of Satellites
Three main classes of satellites
MEO
GEO
LEO
31
GEO Satellites

• Geosynchronous earth orbit
• 22,300 miles above earth
• Requires the most power
• Adds greatest delay 0.25 sec/leg
• Position is constant relative to earth same
  rotational speed as the earth
• Provides largest footprint of all satellites
• Three satellites can cover earth
• Applications One way broadcasts, international TV

32
MEO Satellites

• Middle earth orbit
• Orbit 6,200 9,400 miles above earth
• Delay reduced to 0.05 per leg
• Smaller footprint requires 10-15 to cover earth
• Applications regional use due to footprint and
  speed, such as mobile voice, low-speed data
• Most rapidly growing application GPS

33
LEO Satellites

• Low earth orbit
• Closest to earth 400 1,000 miles above earth
• Least amount of delay 0.025 seconds/leg
• Least amount of power required can be directed
  into users handheld device
• Smallest footprint requires approximately 60 to
  cover earth
• Functionality is new due to speed and small
  footprint switching capability was needed and
  the system is very complex
• Jitter is a significant issue
• Applications mobile voice, low-speed data,
  high-speed data

34
VSAT

• Very Small Aperture Terminal
• Characterized by very small antenna (0.6 meters
  or less)
• Low cost, easy and quick installation
• Applications
• Vehicle tracking systems
• Broadband Internet access (Hughes DirecPC
  provides downlinks _at_ 2 Mbps)
• Business video

35
Satellites Pros and Cons

• Access to remote areas
• Covers large geographies
• Insensitive to topology
• Insensitive to distance-related costs
• High bandwidth
• Economic value increases with number of locations

• High initial cost
• Propagation delay
• Vulnerable to environmental interference
• Licensing requirements
• Vulnerable to space clutter
• Low security requires encryption

36
Radio

• Microwave
• Antennas are less directional, ranging to full
  omnidirectional
• Common frequency range 3 KHz 300 GHz
• Most significant application mobile telephony

37
Radio Pros and Cons

• Less sensitive to environmental attenuation
• Cost savings
• Portability
• Reconfiguration flexibility
• Bandwidth

• Requires line-of-sight
• Regulatory licensing requirements
• Potential community environmental restrictions
• Vulnerable to multipath interference

38
What Weve Covered

• List the impairments found in unguided
  transmission media
• Describe the impairments found in unguided
  transmission media
• List three major design considerations for
  antennas and describe their relationship
• Describe the characteristics of terrestrial
  microwave, satellite, and radio wireless
  transmission
• Distinguish between LEO, MEO, and GEO satellite
  systems
• Describe what is meant by VSAT