ajal acs chap2 rev
DESCRIPTION
CHAPTER 2SATELLITE ACCESS SYSTEMTRANSCRIPT
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MODULE 2Satellite Access Methods
AJAL.A.J Assistant Professor –Dept of ECE,
Federal Institute of Science And Technology (FISAT) TM MAIL: [email protected]
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Audio Spectrum
Noise floor
Peak power
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Analog Signaling
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Digital Signaling
(Coder-Decoder)
Example - PCM
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Reasons for Choosing Data and Signal Combinations
• 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,
computational resources and switching equipment
• Digital data, analog signal– Transmission media will only propagate analog signals– Examples include optical fiber and POTS (3 kHz
bandwidth limited)
• Analog data, analog signal– Analog data easily converted to an analog signal via
some form of modulation (AM, FM, etc.)
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Unguided Media• Transmission and reception are achieved by
means of an antenna (rcvr + xmtr)
• Configurations for wireless transmission– Directional (infers gain)– Omnidirectional – Polarization (vertical, horizontal, circular)
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A Simplified Wireless Communications System – Unguided Media
Information received
(Voice/Data)
Information to be
transmitted (Voice/Data)
Coding Modulator Transmitter
Decoding Demodulator Receiver
Antenna
AntennaCarrier
Carrier
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Modulation Terms adding data to a radio frequency signal
Baseband – modulation techniques that do not use a sinusoidal carrier but encodes information directly as the amplitude, width of position of a pulse. PAM – pulse amplitude modulation PWM – pulse width modulation
Bandpass – modulation techniques that encode information as the amplitude, frequency or phase of a sinusoidal carrier. FSK – frequency shift keying, PSK – phase shift keying, AM, FM
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Electromagnetic Spectrum
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Characteristics of some Frequencies
• Microwave frequency range– 1 GHz to 40 GHz– Directional beams possible (small)– Suitable for point-to-point transmission– Used for satellite communications
• VHF/UHF Radio frequency range– 30 MHz to 1 GHz (no atmospheric propagation, LOS)– Suitable for omnidirectional applications
• Infrared frequency range– Roughly 3x1011 to 2x1014 Hz– Useful in local point-to-point multipoint applications within
confined areas
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Terrestrial Microwave• Description of common microwave antenna
– Parabolic "dish", 3 m in diameter
– Fixed rigidly which focuses a narrow beam
– Achieves a line-of-sight (LOS) transmission path to the receiving antenna
– Located at substantial heights above ground level
• Applications– Long haul telecommunications service (many repeaters)
– Short point-to-point links between buildings
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Satellite Microwave
• 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 (e.g., Direct TV)– Long-distance telephone transmission– Private business networks
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Broadcast Radio
• Description of broadcast radio antennas– Omnidirectional (HF-vertical polarization, VHF/UHF-
horizontal polarization)– 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• Below 30 MHz transmission (AM radio) is subjected to
propagation effects so not reliable for point-to-point communications (MUF or max usable freq)
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Network Architectures and Protocols
Systematic Signaling Steps for Information Exchange
Open Systems Interconnections (OSI) Transmission Control Protocol (TCP) Internet Protocol (IP)
Internet Protocol Version 4 (IPv4) Internet Protocol Version 6 (IPv6) – essentially
larger MAC addressing space for the influx of IP based devices
Mobile IP
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Ad Hoc Network (peer to peer)
Versus an infrastructure network (centralized) with its AP(Access Points) which is your WiFi/Hotspot/typical wireless network normally used to access the Internet.
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Multiplexing• Capacity of transmission medium usually
exceeds capacity required for transmission of a single signal
• Multiplexing - carrying multiple signals on a single medium– More efficient use of transmission medium
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Multiplexing
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Reasons for Widespread Use of Multiplexing
• Cost per kbps of transmission facility declines with an increase in the data rate (economy of scale)
• Effective cost of transmission and receiving equipment declines with increased data rate(cost per bit)
• Most individual data communication devices with their associated applications require relatively modest data rate support
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Multiplexing Techniques• Frequency-division multiplexing (FDM)
– Takes advantage of the fact that the useful bandwidth of the medium exceeds the required bandwidth of a given signal
– Requires guard bands
• Time-division multiplexing (TDM)– Takes advantage of the fact that the achievable bit rate of the
medium exceeds the required data rate of a digital signal– Requires accurate clock
• Code-division multiple access(CDMA)– Use of orthogonal codes to separate users who are all using
the same band of frequencies
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Frequency-division Multiplexing
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FDMA Channel Allocation
Frequency 1 User 1
Frequency 2 User 2
Base Station
Frequency n User n
… …
Mobile Stations
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Time-division Multiplexing
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TDMA Frame Illustration for Multiple Users
Time 1
Time 2
Time n
……
Base Station
User 1
User 2
User n
…
Mobile Stations
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CDMA (Code Division Multiple Access)
Time
Frequency
Use
r n
Code
Use
r 1
Use
r 2
...
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Transmitted and Received Signals in a CDMA System
Information bits
Code at transmitting end
Transmitted signal
Received signal
Code at receiving end
Decoded signal at the receiver
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OFDM (Orthogonal Frequency Division Multiplexing)
Conventional multicarrier modulation used in FDMA
Orthogonal multicarrier modulation used in OFDM (normally a single user)
Frequency
Frequency
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Satellite Microwave Transmission
• a microwave relay station in space
• can relay signals over long distances
• geostationary satellites – remain above the equator at a height of
22,300 miles (geosynchronous orbit)– travel around the earth in exactly the time the
earth takes to rotate
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Satellite Transmission Links
• earth stations communicate by sending signals to the satellite on an uplink
• the satellite then repeats those signals on a downlink
• the broadcast nature of the downlink makes it attractive for services such as the distribution of television programming
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dish dish
uplink station downlink station
satellitetransponder
22,300 miles
Satellite Transmission Process
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Satellite Transmission Applications
• television distribution– a network provides programming from a
central location– direct broadcast satellite (DBS)
• long-distance telephone transmission– high-usage international trunks
• private business networks
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Principal Satellite Transmission Bands
• C band: 4(downlink) - 6(uplink) GHz– the first to be designated
• Ku band: 12(downlink) -14(uplink) GHz– rain interference is the major problem
• Ka band: 19(downlink) - 29(uplink) GHz– equipment needed to use the band is still very
expensive
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Fiber vs Satellite
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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 earth’s center – Elliptical with one foci at earth’s center
• Orbit around earth in different planes– Equatorial orbit above earth’s 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)
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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 station’s antenna (>0o)– 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
GEO : Geosynchronous equatorial orbit
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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
LEO : Low earth orbit
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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
MEO : Medium Earth Orbit
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Satellite Systems
GEO
M EO
LEO
GEO (22,300 mi., equatorial) high bandwidth, power,
latency
MEO high bandwidth, power,
latency
LEO (400 mi.) low power, latency
more satellites
small footprint
V-SAT (Very Small Aperture
Terminal)
private WAN
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Geostationary Orbit
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GPS Satellite Constellation
• Global Positioning System• Operated by USAF• 28 satellites• 6 orbital planes at a height of 20,200 km• Positioned so a minimum of 5 satellites are visible at all times• Receiver measures distance to satellite
USAF - United States Air Force
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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
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THANKS FOR YOUR PATIENCE