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Fundamentals of Microwave

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Title: Fundamentals of Microwave


1
Fundamentals of Microwave Satellite Technologies
  • ICS 620
  • Fall 2003
  • Week 10

2
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

6
Microwave Engineering Considerations-contd
  • Skin affect
  • Line of Sight (LOS)
  • Fading
  • Range
  • Interference

7
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.

8
Reflections
  • Reflections can occur as the microwave signal
    traverses a body of water or fog bank cause
    multipath conditions

9
Diffraction
  • Diffraction is the result of variations in the
    terrain the signal crosses

10
Rain Attenuation
  • Raindrop absorption or scattering of the
    microwave signal can cause signal loss in
    transmissions.

11
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.

12
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.

13
LOS FZC-contd
Fresnel Zone
D2
D1
D1 X D2 F x D
72.2
secret formula
14
Microwave Fading
Normal Signal
Reflective Path
Caused by multi-path reflections and heavy rains
15
Range
  • The distance a signal travels and its increase in
    frequency are inversely proportional
  • Repeaters extend range
  • Back-to-back antennas
  • reflectors

16
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

17
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

18
Components of a Microwave System
  • Digital Modem
  • Radio Frequency (RF) Unit
  • Antenna

19
Digital Modem
  • The digital modem modulates the information
    signal (intermediate frequency or IF).

20
RF Unit
  • IF is fed to the RF unit which is mounted as
    close physically to the antenna as possible
    (direct connect is optimal).

21
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.

22
Waveguides
Waveguides are hollow channels of low-loss
material used to direct the signal from the RF
unit to the antenna.
23
Modulation Methods
  • Primarily modulated today with digital FM or AM
    signals
  • Digital signal remains quiet until failure
    threshold bit error rate renders it unusable

24
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

-6
-3
25
Diversity
  • Space Diversity
  • Frequency Diversity
  • Hot Standby
  • PRI

26
Space Diversity
Normal Signal
Faded Signal
Transmitter
Receiver
27
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.

28
Frequency Diversity
RCVR Frequency 1
Active XTMR Frequency 1
RCVR Frequency 2
Protect XTMR Frequency 2
Receiver
Transmitter
29
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.

30
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
31
PRI
System Transmission Facilities
System Receiver Facilities
Connect to PRI interface PSTN
Connect to PRI interface PSTN
Receiver
Transmitter
To PSTN
To PSTN
32
Availability Formula
Percent Availability equals
1 (outage hours/8760 hours per year)
Private microwaves have 99.99 availability
33
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

34
Microwave Radio Applications
35
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36
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37
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38
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39
Satellite Communications
40
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

41
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

42
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

44
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

45
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

46
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

47
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

48
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

49
Frequency Bands Available for Satellite
Communications
50
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

51
Satellite Footprint
52
Satellite Network Configurations
53
Capacity Allocation Strategies
  • Frequency division multiple access (FDMA)
  • Time division multiple access (TDMA)
  • Code division multiple access (CDMA)

54
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

55
Frequency-Division Multiple Access
  • Factors which limit the number of subchannels
    provided within a satellite channel via FDMA
  • Thermal noise
  • Intermodulation noise
  • Crosstalk

56
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

57
FAMA-FDMA
  • FAMA logical links between stations are
    preassigned
  • FAMA multiple stations access the satellite by
    using different frequency bands
  • Uses considerable bandwidth

58
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

59
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

60
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

61
FAMA-TDMA Uplink
62
FAMA-TDMA Downlink
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