MODULE 2MODULE 2

Satellite Access MethodsSatellite Access Methods

AJAL.A.JAJAL.A.J Assistant Professor –Dept of ECE, Assistant Professor –Dept of ECE,

Federal Institute of Science And Technology (FISAT) Federal Institute of Science And Technology (FISAT) TMTM   MAIL: MAIL: ec2reach@gmail.comec2reach@gmail.com

The Earth is divided up into the northern The Earth is divided up into the northern hemisphere and the southern hemisphere:hemisphere and the southern hemisphere:

Equator

Northern hemisphere

Southern hemisphere

The Earth is tilted on an axisThe Earth is tilted on an axis

North pole

South pole

The Earth is kept in orbit by the force of…The Earth is kept in orbit by the force of…

GravityGravity

…and by the fact that is is moving at a high velocity

The Earth spins on its axis

While the Earth is spinning the side that faces the sun is in -------

Day and nightDay and night

The Earth orbits the sun every…The Earth orbits the sun every…

……year (365 year (365 11//44 days)days)

The Earth orbits the SunThe Earth orbits the Sun

Because of this spin the sun rises in Because of this spin the sun rises in the ______ and sets in the ______the ______ and sets in the ______

Gravity also keeps the moon in orbit around Gravity also keeps the moon in orbit around the Earth. The moon orbits the Earth every…the Earth. The moon orbits the Earth every…

……days month (28 days month (28 ))

Audio Spectrum

Noise floor

Peak power

Analog Signaling

Digital Signaling

(Coder-Decoder)

Example - PCM

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.)

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)

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

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

Electromagnetic Spectrum

Communication frequencies

• Microwave band terminology– L band 800 MHz - 2 GHz– S band 2-3 GHz– C band 3-6 GHz– X band 7-9 GHz– Ku band 10-17 GHz– Ka band 18-22 GHz

• Satellite up links and down links can operate in different frequency bands:

• The up-link is a highly directional, point to point link • The down-link can have a footprint providing coverage for a

substantial area "spot beam“.

Band Up-Link (Ghz)

Down-link (Ghz)

ISSUES

C 4 6 Interference with ground links.

Ku 11 14 Attenuation due to rain

Ka 20 30 High Equipment cost

Early satellite communications

• Used C band in the range 3.7-4.2 GHz

• Could interfere with terrestrial communications

• Beamwidth is narrower with higher frequencies

More recent communications

• Greater use made of Ku band

• Use is now being made of Ka band

Rain fade

• Above 10 GHz rain and other disturbances can have a severe effect on reception

• This can be countered by using larger receiver dishes so moderate rain will have less effect

• In severe rainstorms reception can be lost

• In some countries sandstorms can also be a problem

Ku band assignments

• © copyright 1996 MLE INC.

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

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

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

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)

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

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.

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

Multiplexing

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

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

Frequency-division Multiplexing

FDMA Channel Allocation

Frequency 1 User 1

Frequency 2 User 2

Base Station

Frequency n User n

… …

Mobile Stations

Time-division Multiplexing

TDMA Frame Illustration for Multiple Users

Time 1

Time 2

Time n

……

Base Station

User 1

User 2

User n

…

Mobile Stations

CDMA (Code Division Multiple Access)

Time

Frequency

Use

r n

Code

Use

r 1

Use

r 2

...

42

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

43

OFDM (Orthogonal Frequency Division Multiplexing)

Conventional multicarrier modulation used in FDMA

Orthogonal multicarrier modulation used in OFDM (normally a single user)

Frequency

Frequency

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

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

dish dish

uplink station downlink station

satellitetransponder

22,300 miles

Satellite Transmission Process

47

Satellite Navigation

http://en.wikipedia.org/wiki/File:ConstellationGPS.gif

Universiteit Utrecht

WORKING

150 MHz

200 MHz

Universiteit Utrecht

WORKING

150 MHz

150 MHz

Universiteit Utrecht

WORKING

• The receiver only knows that the satellite is neither approaching or departing

• So the ship must be on a line perpendicular to the orbit of the satellite

• However, farther from the orbit, the frequency transition is less

• A calculation will tell the receiver how far, but not which side

Local Area Augmentation System (LAAS)

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

Why Satellites remain in OrbitsWhy Satellites remain in Orbits

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

Fiber vs Satellite

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

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

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)

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

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

NGSO - Non Geostationary OrbitsNGSO - Non Geostationary Orbits

Orbit should avoid Orbit should avoid Van Allen radiation Van Allen radiation belts:belts:• Region of charged Region of charged

particles that can particles that can cause damage to cause damage to satellitesatellite

• Occur at Occur at ~2000-4000 km and ~2000-4000 km and ~13000-25000 km~13000-25000 km

Satellite OrbitsSatellite Orbits

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

LEO - Low Earth Orbits

• Circular or inclined orbit with < 1400 km altitude– Satellite travels across sky from horizon to horizon in

5 - 15 minutes => needs handoff– Earth stations must track satellite or have Omni

directional antennas– Large constellation of satellites is needed for

continuous communication (66 satellites needed to cover earth)

– Requires complex architecture– Requires tracking at ground

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

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

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

HEO - Highly Elliptical Orbits

• HEOs (i = 63.4°) are suitable to provide coverage at high latitudes (including North Pole in the northern hemisphere)

• Depending on selected orbit (e.g. Molniya, Tundra, etc.) two or three satellites are sufficient for continuous time coverage of the service area.

• All traffic must be periodically transferred from the “setting” satellite to the “rising” satellite (Satellite Handover)

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

Geostationary Orbit

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

Frequency Bands Available for Satellite Communications

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

Satellite Footprint

Satellite CommunicationsSatellite Communications Alternating vertical and Alternating vertical and

horizontal polarisation is horizontal polarisation is widely used on satellite widely used on satellite communicationscommunications

This reduces interference This reduces interference between programs on the between programs on the same frequency band same frequency band transmitted from adjacent transmitted from adjacent satellites (One uses vertical, satellites (One uses vertical, the next horizontal, and so the next horizontal, and so on)on)

Allows for reduced angular Allows for reduced angular separation between the separation between the satellites.satellites.

Satellite Network Configurations

Capacity Allocation Strategies

• Frequency division multiple access (FDMA)

• Time division multiple access (TDMA)

• Code division multiple access (CDMA)

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

Frequency-Division Multiple Access

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

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

FAMA-FDMA

• FAMA – logical links between stations are preassigned

• FAMA – multiple stations access the satellite by using different frequency bands

• Uses considerable bandwidth

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

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

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

FAMA-TDMA Uplink

FAMA-TDMA Downlink

Satellite SignalsSatellite Signals

►Used to transmit signals and data over Used to transmit signals and data over long distanceslong distances Weather forecastingWeather forecasting Television broadcastingTelevision broadcasting Internet communicationInternet communication Global Positioning SystemsGlobal Positioning Systems

Communication SatelliteCommunication Satellite

►A Communication Satellite can be A Communication Satellite can be looked upon as a large microwave looked upon as a large microwave repeaterrepeater

► It contains several transponders which It contains several transponders which listens to some portion of spectrum, listens to some portion of spectrum, amplifies the incoming signal and amplifies the incoming signal and broadcasts it in another frequency to broadcasts it in another frequency to avoid interference with incoming avoid interference with incoming signals.signals.

Types of Satellite OrbitsTypes of Satellite Orbits

► Based on the inclination, i, over the equatorial Based on the inclination, i, over the equatorial plane:plane: Equatorial Orbits above Earth’s equator (i=0°)Equatorial Orbits above Earth’s equator (i=0°) Polar Orbits pass over both poles (i=90°)Polar Orbits pass over both poles (i=90°) Other orbits called inclined orbits (0°<i<90°)Other orbits called inclined orbits (0°<i<90°)

► Based on EccentricityBased on Eccentricity Circular with centre at the earth’s centreCircular with centre at the earth’s centre Elliptical with one foci at earth’s centre Elliptical with one foci at earth’s centre

IntelsatIntelsat► INTELSAT is the original "Inter-governmental Satellite INTELSAT is the original "Inter-governmental Satellite

organization". It once owned and operated most of the World's organization". It once owned and operated most of the World's satellites used for international communications, and still satellites used for international communications, and still maintains a substantial fleet of satellites.maintains a substantial fleet of satellites.

► INTELSAT is moving towards "privatization", with increasing INTELSAT is moving towards "privatization", with increasing competition from commercial operators (e.g. Panamsat, Loral competition from commercial operators (e.g. Panamsat, Loral Skynet, etc.).Skynet, etc.).

► INTELSAT Timeline:INTELSAT Timeline:► Interim organization formed in 1964 by 11 countriesInterim organization formed in 1964 by 11 countries

► Permanent structure formed in 1973Permanent structure formed in 1973

► Commercial "spin-off", New Skies Satellites in 1998Commercial "spin-off", New Skies Satellites in 1998

► Full "privatization" by April 2001 Full "privatization" by April 2001 ► INTELSAT has 143 members.INTELSAT has 143 members.

Intelsat Structure

Advantages of Satellite Advantages of Satellite CommunicationCommunication

Can reach over large geographical areaCan reach over large geographical area Flexible (if transparent transponders) Flexible (if transparent transponders) Easy to install new circuits Easy to install new circuits Circuit costs independent of distance Circuit costs independent of distance Broadcast possibilities Broadcast possibilities Temporary applications (restoration) Temporary applications (restoration) Niche applications Niche applications Mobile applications (especially "fill-in") Mobile applications (especially "fill-in") Terrestrial network "by-pass" Terrestrial network "by-pass" Provision of service to remote or underdeveloped Provision of service to remote or underdeveloped

areas areas User has control over own network User has control over own network 1-for-N multipoint standby possibilities 1-for-N multipoint standby possibilities

Disadvantages of Satellite Disadvantages of Satellite CommunicationCommunication

Large up front capital costs (space Large up front capital costs (space segment and launch) segment and launch)

Terrestrial break even distance Terrestrial break even distance expanding (now approx. size of expanding (now approx. size of Europe) Europe)

Interference and propagation delay Interference and propagation delay Congestion of frequencies and orbits Congestion of frequencies and orbits

When to use SatellitesWhen to use Satellites

When the unique features of satellite communications When the unique features of satellite communications make it attractive make it attractive

When the costs are lower than terrestrial routing When the costs are lower than terrestrial routing When it is the only solution When it is the only solution Examples:Examples:

• Communications to ships and aircraft (especially safety Communications to ships and aircraft (especially safety communications) communications)

• TV services - contribution links, direct to cable head, direct to TV services - contribution links, direct to cable head, direct to homehome

• Data services - private networks Data services - private networks • Overload traffic Overload traffic • Delaying terrestrial investments Delaying terrestrial investments • 1 for N diversity 1 for N diversity • Special events Special events

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