Title: Fundamentals of Microwave
1Fundamentals of Microwave Satellite Technologies
- ICS 620
- Fall 2003
- Week 10
2Historical 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
3Microwave 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
4Microwave Impairments
- Equipment, antenna, and waveguide failures
- Fading and distortion from multipath reflections
- Absorption from rain, fog, and other atmospheric
conditions - Interference from other frequencies
5Microwave Engineering Considerations
- Free space atmospheric attenuation
- Reflections
- Diffractions
- Rain attenuation
6Microwave Engineering Considerations-contd
- Skin affect
- Line of Sight (LOS)
- Fading
- Range
- Interference
7Free 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.
8Reflections
- Reflections can occur as the microwave signal
traverses a body of water or fog bank cause
multipath conditions
9Diffraction
- Diffraction is the result of variations in the
terrain the signal crosses
10Rain Attenuation
- Raindrop absorption or scattering of the
microwave signal can cause signal loss in
transmissions.
11Skin 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.
12Line 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.
13LOS FZC-contd
Fresnel Zone
D2
D1
D1 X D2 F x D
72.2
secret formula
14Microwave Fading
Normal Signal
Reflective Path
Caused by multi-path reflections and heavy rains
15Range
- The distance a signal travels and its increase in
frequency are inversely proportional - Repeaters extend range
- Back-to-back antennas
- reflectors
16Range-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
17Interference
- 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
18Components of a Microwave System
- Digital Modem
- Radio Frequency (RF) Unit
- Antenna
19Digital Modem
- The digital modem modulates the information
signal (intermediate frequency or IF).
20RF Unit
- IF is fed to the RF unit which is mounted as
close physically to the antenna as possible
(direct connect is optimal).
21Antenna
- 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.
22Waveguides
Waveguides are hollow channels of low-loss
material used to direct the signal from the RF
unit to the antenna.
23Modulation Methods
- Primarily modulated today with digital FM or AM
signals - Digital signal remains quiet until failure
threshold bit error rate renders it unusable
24Bit 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
25Diversity
- Space Diversity
- Frequency Diversity
- Hot Standby
- PRI
26Space Diversity
Normal Signal
Faded Signal
Transmitter
Receiver
27Space 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.
28Frequency Diversity
RCVR Frequency 1
Active XTMR Frequency 1
RCVR Frequency 2
Protect XTMR Frequency 2
Receiver
Transmitter
29Frequency 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.
30Hot 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
31PRI
System Transmission Facilities
System Receiver Facilities
Connect to PRI interface PSTN
Connect to PRI interface PSTN
Receiver
Transmitter
To PSTN
To PSTN
32Availability Formula
Percent Availability equals
1 (outage hours/8760 hours per year)
Private microwaves have 99.99 availability
33Microwave 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
34Microwave Radio Applications
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39Satellite Communications
40Satellite-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
41Ways 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
42Classification 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)
43Geometry 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
44Minimum 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
45GEO 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
46LEO 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
47LEO 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
48MEO 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
49Frequency Bands Available for Satellite
Communications
50Satellite 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
51Satellite Footprint
52Satellite Network Configurations
53Capacity Allocation Strategies
- Frequency division multiple access (FDMA)
- Time division multiple access (TDMA)
- Code division multiple access (CDMA)
54Frequency-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
55Frequency-Division Multiple Access
- Factors which limit the number of subchannels
provided within a satellite channel via FDMA - Thermal noise
- Intermodulation noise
- Crosstalk
56Forms 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
57FAMA-FDMA
- FAMA logical links between stations are
preassigned - FAMA multiple stations access the satellite by
using different frequency bands - Uses considerable bandwidth
58DAMA-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
59Reasons 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
60FAMA-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
61FAMA-TDMA Uplink
62FAMA-TDMA Downlink