Fundamentals of Microwave

1
Fundamentals of Microwave Satellite Technologies

• ICS 620
• Fall 2003
• Week 10

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Historical Perspective

• Founded during WWII
• Used for long-haul telecommunications
• Displaced by fiber optic networks
• Still viable for right-of-way bypass and
  geographic obstruction avoidance

3
Microwave Spectrum

• Range is approximately 1 GHz to 40 GHz
• Total of all usable frequencies under 1 GHz gives
  a reference on the capacity of in the microwave
  range

4
Microwave Impairments

• Equipment, antenna, and waveguide failures
• Fading and distortion from multipath reflections
• Absorption from rain, fog, and other atmospheric
  conditions
• Interference from other frequencies

5
Microwave Engineering Considerations

• Free space atmospheric attenuation
• Reflections
• Diffractions
• Rain attenuation

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Microwave Engineering Considerations-contd

• Skin affect
• Line of Sight (LOS)
• Fading
• Range
• Interference

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Free Space Atmospheric Attenuation

• Free space atmospheric attenuation is defined
  by the loss the signal undergoes traveling
  through the atmosphere. Changes in air density
  and absorption by atmospheric particles.

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Reflections

• Reflections can occur as the microwave signal
  traverses a body of water or fog bank cause
  multipath conditions

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Diffraction

• Diffraction is the result of variations in the
  terrain the signal crosses

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Rain Attenuation

• Raindrop absorption or scattering of the
  microwave signal can cause signal loss in
  transmissions.

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Skin Affect

• Skin Affect is the concept that high frequency
  energy travels only on the outside skin of a
  conductor and does not penetrate into it any
  great distance. Skin Affect determines the
  properties of microwave signals.

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Line of SightFresnel Zone Clearance

• Fresnel Zone Clearance is the minimum clearance
  over obstacles that the signal needs to be sent
  over. Reflection or path bending will occur if
  the clearance is not sufficient.

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LOS FZC-contd
Fresnel Zone
D2
D1
D1 X D2 F x D
72.2
secret formula
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Microwave Fading
Normal Signal
Reflective Path
Caused by multi-path reflections and heavy rains
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Range

• The distance a signal travels and its increase in
  frequency are inversely proportional
• Repeaters extend range
• Back-to-back antennas
• reflectors

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Range-contd

• High frequencies are repeated/received at or
  below one mile
• Lower frequencies can travel up to 100 miles but
  25-30 miles is the typical placement for repeaters

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Interference

• Adjacent Channel Interference
• digital not greatly affected
• Overreach
• caused by signal feeding past a repeater to the
  receiving antenna at the next station in the
  route. Eliminated by zigzag path alignment or
  alternate frequency use between adjacent stations

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Components of a Microwave System

• Digital Modem
• Radio Frequency (RF) Unit
• Antenna

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Digital Modem

• The digital modem modulates the information
  signal (intermediate frequency or IF).

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RF Unit

• IF is fed to the RF unit which is mounted as
  close physically to the antenna as possible
  (direct connect is optimal).

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Antenna

• The antenna is a passive device that radiates the
  modulated signal. It is fed by direct connect of
  the RF unit, coaxial cable, or waveguides at
  higher frequencies.

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Waveguides
Waveguides are hollow channels of low-loss
material used to direct the signal from the RF
unit to the antenna.
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Modulation Methods

• Primarily modulated today with digital FM or AM
  signals
• Digital signal remains quiet until failure
  threshold bit error rate renders it unusable

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Bit Error Rate (BER)

• The BER is a performance measure of microwave
  signaling throughput
• 10 or one error per million transmitted bits of
  information
• Data fail over is at 10 voice traffic can
  withstand this error rate

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-3
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Diversity

• Space Diversity
• Frequency Diversity
• Hot Standby
• PRI

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Space Diversity
Normal Signal
Faded Signal
Transmitter
Receiver
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Space Diversity-contd

• Space Diversity protects against multi-path
  fading by automatic switch over to another
  antenna place below the primary antenna. This is
  done at the BER failure point or signal strength
  attenuation point to the secondary antenna that
  is receiving the transmitted signal at a stronger
  power rating.

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Frequency Diversity
RCVR Frequency 1
Active XTMR Frequency 1
RCVR Frequency 2
Protect XTMR Frequency 2
Receiver
Transmitter
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Frequency Diversity-contd

• Frequency Diversity uses separate frequencies
  (dual transmit and receive systems) it monitors
  primary for fail over and switches to standby.
  Interference usually affects only one range of
  frequencies. Not allowed in non-carrier
  applications because of spectrum scarcity.

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Hot Standby
Active RCVR 1
System XTMR Primary 1
Standby RCVR 2
System XTMR Standby 2
failure switch
Receiver
Transmitter
Hot standby is designed for equipment failure
only
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PRI
System Transmission Facilities
System Receiver Facilities
Connect to PRI interface PSTN
Connect to PRI interface PSTN
Receiver
Transmitter
To PSTN
To PSTN
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Availability Formula
Percent Availability equals
1 (outage hours/8760 hours per year)
Private microwaves have 99.99 availability
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Microwave Path Analysis

• Transmitter output power
• Antenna gain
• proportional to the physical characteristics of
  the antenna (diameter)
• Free space gain
• Antenna alignment factor
• Unfaded received signal level

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Microwave Radio Applications
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Satellite Communications
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Satellite-Related Terms

• Earth Stations antenna systems on or near earth
• Uplink transmission from an earth station to a
  satellite
• Downlink transmission from a satellite to an
  earth station
• Transponder electronics in the satellite that
  convert uplink signals to downlink signals

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Ways to CategorizeCommunications Satellites

• Coverage area
• Global, regional, national
• Service type
• Fixed service satellite (FSS)
• Broadcast service satellite (BSS)
• Mobile service satellite (MSS)
• General usage
• Commercial, military, amateur, experimental

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Classification of Satellite Orbits

• Circular or elliptical orbit
• Circular with center at earths center
• Elliptical with one foci at earths center
• Orbit around earth in different planes
• Equatorial orbit above earths equator
• Polar orbit passes over both poles
• Other orbits referred to as inclined orbits
• Altitude of satellites
• Geostationary orbit (GEO)
• Medium earth orbit (MEO)
• Low earth orbit (LEO)

43
Geometry Terms

• Elevation angle - the angle from the horizontal
  to the point on the center of the main beam of
  the antenna when the antenna is pointed directly
  at the satellite
• Minimum elevation angle
• Coverage angle - the measure of the portion of
  the earth's surface visible to the satellite

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Minimum Elevation Angle

• Reasons affecting minimum elevation angle of
  earth stations antenna (gt0o)
• Buildings, trees, and other terrestrial objects
  block the line of sight
• Atmospheric attenuation is greater at low
  elevation angles
• Electrical noise generated by the earth's heat
  near its surface adversely affects reception

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GEO Orbit

• Advantages of the the GEO orbit
• No problem with frequency changes
• Tracking of the satellite is simplified
• High coverage area
• Disadvantages of the GEO orbit
• Weak signal after traveling over 35,000 km
• Polar regions are poorly served
• Signal sending delay is substantial

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LEO Satellite Characteristics

• Circular/slightly elliptical orbit under 2000 km
• Orbit period ranges from 1.5 to 2 hours
• Diameter of coverage is about 8000 km
• Round-trip signal propagation delay less than 20
  ms
• Maximum satellite visible time up to 20 min
• System must cope with large Doppler shifts
• Atmospheric drag results in orbital deterioration

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LEO Categories

• Little LEOs
• Frequencies below 1 GHz
• 5MHz of bandwidth
• Data rates up to 10 kbps
• Aimed at paging, tracking, and low-rate messaging
• Big LEOs
• Frequencies above 1 GHz
• Support data rates up to a few megabits per sec
• Offer same services as little LEOs in addition to
  voice and positioning services

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MEO Satellite Characteristics

• Circular orbit at an altitude in the range of
  5000 to 12,000 km
• Orbit period of 6 hours
• Diameter of coverage is 10,000 to 15,000 km
• Round trip signal propagation delay less than 50
  ms
• Maximum satellite visible time is a few hours

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Frequency Bands Available for Satellite
Communications
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Satellite Link Performance Factors

• Distance between earth station antenna and
  satellite antenna
• For downlink, terrestrial distance between earth
  station antenna and aim point of satellite
• Displayed as a satellite footprint (Figure 9.6)
• Atmospheric attenuation
• Affected by oxygen, water, angle of elevation,
  and higher frequencies

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Satellite Footprint
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Satellite Network Configurations
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Capacity Allocation Strategies

• Frequency division multiple access (FDMA)
• Time division multiple access (TDMA)
• Code division multiple access (CDMA)

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Frequency-Division Multiplexing

• Alternative uses of channels in point-to-point
  configuration
• 1200 voice-frequency (VF) voice channels
• One 50-Mbps data stream
• 16 channels of 1.544 Mbps each
• 400 channels of 64 kbps each
• 600 channels of 40 kbps each
• One analog video signal
• Six to nine digital video signals

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Frequency-Division Multiple Access

• Factors which limit the number of subchannels
  provided within a satellite channel via FDMA
• Thermal noise
• Intermodulation noise
• Crosstalk

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Forms of FDMA

• Fixed-assignment multiple access (FAMA)
• The assignment of capacity is distributed in a
  fixed manner among multiple stations
• Demand may fluctuate
• Results in the significant underuse of capacity
• Demand-assignment multiple access (DAMA)
• Capacity assignment is changed as needed to
  respond optimally to demand changes among the
  multiple stations

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FAMA-FDMA

• FAMA logical links between stations are
  preassigned
• FAMA multiple stations access the satellite by
  using different frequency bands
• Uses considerable bandwidth

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DAMA-FDMA

• Single channel per carrier (SCPC) bandwidth
  divided into individual VF channels
• Attractive for remote areas with few user
  stations near each site
• Suffers from inefficiency of fixed assignment
• DAMA set of subchannels in a channel is treated
  as a pool of available links
• For full-duplex between two earth stations, a
  pair of subchannels is dynamically assigned on
  demand
• Demand assignment performed in a distributed
  fashion by earth station using CSC

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Reasons for Increasing Use of TDM Techniques

• Cost of digital components continues to drop
• Advantages of digital components
• Use of error correction
• Increased efficiency of TDM
• Lack of intermodulation noise

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FAMA-TDMA Operation

• Transmission in the form of repetitive sequence
  of frames
• Each frame is divided into a number of time slots
• Each slot is dedicated to a particular
  transmitter
• Earth stations take turns using uplink channel
• Sends data in assigned time slot
• Satellite repeats incoming transmissions
• Broadcast to all stations
• Stations must know which slot to use for
  transmission and which to use for reception

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FAMA-TDMA Uplink
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FAMA-TDMA Downlink