Satellite Communications

Lecture 26

Overview Requirement of Satellite Communication Satellite UpLink and DownLink Types of Satellites Satellite Foot Print (Coverage Area) Satellite Transmission Bands UpLink and DownLink Frequencies Signal Propagation Delay Transponder Effect of Rain on Satellite Communication Microwave Communication (Why) Satellite System Elements Losses Capacity Allocation Strategies

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Why Satellite Communication?

The Earth is a sphere & the microwave frequencies travel in straight line but to connect two regions very far away on the two side of the sphere, the link requires lot of repeaters because of Earth’s curvature.

A single satellite can do the magic linking the continents with one repeater.

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Motivation to use Satellites

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Communication Satellite A Communication Satellite can be looked

upon as a large microwave repeater It contains several transponders which

listens to some portion of spectrum, amplifies the incoming signal and broadcasts it in another frequency to avoid interference with incoming signals.

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Satellite It is a repeater which receives signal from

Earth at one frequency, amplify it & transmit it back to Earth at other frequency.

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EARTH STATION There are two earth station in a simple

Satellite communication link. One transmits the signal to satellite called transmitting Earth station.

The other receives the signal from satellite called receiving Earth Station.

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UPLINK & DOWN LINK The communication link from

Transmitting earth station to satellite is called Up-link.

The communication link from satellite To receiving earth station is called Down-link.

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Altitudes of Orbits Above the Earth

There are 3 common types of satellite based on altitude, i.e. GEO, MEO & LEO

Orbit Altitude Missions possibles

Low-Earth orbit LEO 250 to 1,500 km

Earth observation, meteorology,

telecommunications (constellations)

Medium-Earth orbit MEO

10,000 to 30,000 km Telecommunications

(constellations), positioning, science

Geostationary Earth orbit GEO

35,786 km Telecommunications, positioning, science

Elliptical orbit Between 800 and

27,000 km Telecommunications

Hyperbolic orbit Up to several million

km Interplanetary

missions 10

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Types of Satellite Orbits Based on the inclination, i, over the

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

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

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Types of Satellite based Networks

Based on the Satellite Altitude GEO – Geostationary Orbits

36000 Km = 22300 Miles, equatorial, High latency MEO – Medium Earth Orbits

High bandwidth, High power, High latency LEO – Low Earth Orbits

Low power, Low latency, More Satellites, Small Footprint

VSAT Very Small Aperture Satellites

Private WANs13

Satellite Orbits Geosynchronous Orbit (GEO): Geosynchronous Orbit (GEO):

36,000 km above Earth, 36,000 km above Earth, includes commercial and includes commercial and military communications military communications satellites, satellites providing satellites, satellites providing early warning of ballistic early warning of ballistic missile launch.missile launch.

Medium Earth Orbit (MEO): Medium Earth Orbit (MEO): from 5000 to 15000 km, they from 5000 to 15000 km, they include navigation satellites include navigation satellites (GPS, Galileo, Glonass).(GPS, Galileo, Glonass).

Low Earth Orbit (LEO): from Low Earth Orbit (LEO): from 500 to 1000 km above Earth, 500 to 1000 km above Earth, includes military intelligence includes military intelligence satellites, weather satellites.satellites, weather satellites.

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Satellite Orbits

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GEO - Geostationary Orbit In the equatorial plane Orbital Period = 23 h 56 m 4.091 s = 1 sidereal day* Satellite appears to be stationary over any

point on equator: Earth Rotates at same speed as Satellite Radius of Orbit r = Orbital Height + Radius of

Earth Avg. Radius of Earth = 6378.14 Km

3 Satellites can cover the earth (120° apart)16

NGSO - Non Geostationary Orbits

Orbit should avoid Van Allen radiation belts: Region of charged particles that can cause damage

to satellite Occur at

~2000-4000 km and ~13000-25000 km

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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 18

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)

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Satellite Orbits

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Why Satellites Remain in Orbits

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Orbital Period The time taken by a satellite to complete one

rotation in its orbit is called its period. The GEO satellite takes 23 hrs & 56 minutes

& 4.1 Seconds to complete its rotation which is approximately equal to the period of rotation of earth around its axis. This is why it appears to be stationary by the observer on Earth moving with the same speed as that of satellite. So one GEO stationary satellite can serve a ground user round the clock.

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Orbital PeriodSatellite System

Orbital Height (Km)

Orbital Velocity (Km/Sec)

Orbital Period(H M S)

Intelsat (GEO) 35,786 3.0747 23 56 4.1

New ICO (MEO) 10,255 4.8954 5 55 48.4

Iridium (LEO) 1,469 7.1272 1 55 17.8

Notice as altitude decreases, the velocity must be increased to minimize the gravitational effect.

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Coverage Area of Satellite The Earth surface covered by satellite radiations is

called FOOT PRINT. The coverage area is inversely proportional to frequency. The foot print will be large if the frequency of down link is low.

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GEO satellite Coverage One GEO can cover 1/3 of earth surface

so the earth is divided in 3 regions. AOR (Atlantic Ocean Region) POR (Pacific Ocean region) IOR (Indian Ocean region)

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FREQUENCIES for Satellite Communication

Letter Designation Frequency range USE

L band 1 to 2 GHz Satellite phone, GPS

S band 2 to 4 GHz Satellite phone

C band 4 to 8 GHz TV transmissionX band 8 to 12 GHz

Ku band 12 to 18 GHz TV transmission, Communication

K band 18 to 26.5 GHz

Ka band 26.5 to 40 GHz Satellite InternetQ band 30 to 50 GHz Experimental

U band 40 to 60 GHz Experimental27

Satellite Transmission Bands

Frequency Band Downlink Uplink

C 3,700-4,200 MHz 5,925-6,425 MHz

Ku 11.7-12.2 GHz 14.0-14.5 GHz

Ka 17.7-21.2 GHz 27.5-31.0 GHz

The C band is the most frequently used. The Ka and Ku bands are reserved exclusively for satellite communication but are subject to rain attenuation

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FREQUENCIES For Uplink & Down link

Uplink uses higher frequency than the down link.

Frequency of satellite is always specified as

UPLINK frequency/ Down link Frequency

e.g. C band 6/4 GHz Ku band14/11 GHz Ka band30/20 GHz 29

Signal Propagation DELAY Using c= 3*10 ^ 8 m/s & time= distance(altitude)/

speed Uplink delay from earth station to Satellite. Round trip delay 4* uplink delay. All other delays in signal coding, compression, &

processing on Satellite & earth Station are neglected.orbit Average altitude

of OrbitUplink Delay Round trip delay

LEO 800 Km 2.7 ms 10.8 ms

MEO 10,355 Km 34.5 ms 138 ms

GEO 35,786 Km 119.3 ms 480 ms = ½ Second

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Round trip delay of GEO signal

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Transponder The BW of satellite is divided into channels allowing

many earth stations to use this BW. The Electronics to support each channel is called Transponder.

Each transponder consist of band pass filter to select particular channel, down converter & output amplifier.

Each satellite can have a large number of active & spare transponders typically 12 to 44 active transponder in each satellite.

The total BW of transponder is usually 36,54 or 72 MHz.

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Transponder

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Effect of rain on signal Rain heavily effects the wireless communication

above 10 GHz. So Ku band & Ka band will be effected by rain &

specially above 20 GHz the Ka Band link can fail during heavy rain fall.

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Why fup is always Higher than fdown?

The beam of higher frequency is narrow & that of lower is broad. As the earth station has to target the signal to a small point (satellite) in space so it does it by using narrow beam produced by higher frequency. While the Satellite has to cover a large area on earth to provide services to many Earth station so it does it by using broad beam produced by lower frequency.

As the rain effects higher frequencies more than lower one so they need to be boosted up more to overcome the propagation losses. The Energy can be given to signal much more easily on earth than on satellite because the satellite has limited power resources like solar cells & batteries so we use higher frequencies on Earth & amplify them with enough power supply resources we have on Earth 35

Up & down frequencies of C band

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Satellite Signals Base band signals can be analog or

digital. But the final signal transmitted towards the satellite or from the satellite is always analog as it is an RF signal.

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Increasing bandwidth of Satellite

Multiplexing Use of Audio & video compression in

telephone & TV signals. Frequency Reuse Same frequency but orthogonal

polarization. Increasing angular spacing of two

satellites in same orbit using similar frequencies. 38

Life of Satellite A satellite has two life. Design life: it is the predicted life of the

electronic systems working in satellite. Maneuver life: It is the life during which full

maneuver capabilities exist in satellite to change its position. It depends on fuel tank storage capacity & it is usually less than design life.

At the end of useful GEO satellite life , it is raised to graveyard orbit.

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Uses of Satellite Military communication Telecommunication Satellite phone. VSAT (very small aperture terminal) Cable TV DBS (Direct Broadcast Satellite) TV (with Dish Antenna) GPS (Global Positioning System) Satellite Internet Weather forecasting Photography GIS (geographical Information System) X-Ray & infrared view of universe Navigation Deep Space exploration & many more 40

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Satellite Missions

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Satellite Microwave Transmission

Satellites can relay signals over a long distance

Geostationary Satellites Remain above the equator at a height of

about 22300 miles (geosynchronous orbits) Travel around the earth in exactly the same

time, the earth takes to rotate

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Why Do Microwaves are Used for Satellite Communications?

High enough frequency to carry the information, long enough wavelength to penetrate the atmosphere. They do not rapidly disperse in the atmosphere so the

power does not need to be very high to reach a distant point.

They can be focused by a suitable dish to increase the reception of low power signals.

They can be modulated to carry the signal and are resistant to interference.

They travel in straight enough lines to be able to aim them. They are easy to produce and easy to detect.

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Satellite System Elements

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Space Segment Satellite Launching Phase Transfer Orbit Phase Deployment Operation

TT&C - Tracking Telemetry and Command Station

SSC - Satellite Control Center, a.k.a.: OCC - Operations Control Center SCF - Satellite Control Facility

Retirement Phase 46

Ground Segment Collection of facilities, Users and Applications

Earth Station = Satellite Communication Station (Fixed or Mobile)

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Satellite Uplink and Downlink

Downlink The link from a satellite down to one or more ground

stations or receivers

Uplink The link from a ground station up to a satellite.

Some companies sell uplink and downlink services to

television stations, corporations, and to other telecommunication carriers.

A company can specialize in providing uplinks, downlinks, or both. 48

Why Two Frequencies for Uplink and Downlink in Satellite Com

The reason the uplink and downlink frequencies are different in satellites is because otherwise the satellite's transmitter and receiver would interfere with one another. The signals have to operate on different frequencies.

But If you could send a signal, then wait, the receiver could

be protected from the transmitted signal on the same frequency, but with high speed, continuous transmission, the receiver cannot be turned off while the transmitter is transmitting. (an example of something that transmits and receives on the same frequency is pulsed RADAR, where the transmitter sends out a pulse, and then the echo is picked up by the receiver) 49

Why Uplink Frequency is Higher than Downlink Frequency

The signals have to cross the atmosphere which presents a great deal of attenuation. The higher the frequency, the more is the signal loss and more power is needed for reliable transmission.

A satellite is a light-weight device which cannot support high-power transmitters on it. So, it transmits at a lower frequency (higher the frequency, higher is the transmitter power to accommodate losses) as compared to the stationary earth station which can afford to use very high-power transmitters. This is compensated by using highly sensitive receiver circuits on the earth station which is in the line-of-sight (LOS) of the satellite. 50

Uplink/Downlink Mobile/Satellite

A mobile is a portable device which cannot afford high-power transmission as it has a small battery with limited power. The 'free space path loss' comes to play. The higher the transmitting frequency, the higher is the loss. Since a mobile station (cellphone) cannot afford to transmit at high power to compensate for this loss, it must transmit on a lower frequency as a lower frequency presents lesser free space path loss. Therefore, mobile-to-base station (uplink) frequencies are lower than base station-to-mobile(downlink) frequencies.

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Satellite Uplink and Downlink

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Satellite Communication When using a satellite for long When using a satellite for long

distance communications, the distance communications, the satellite acts as a repeater.satellite acts as a repeater.

An earth station transmits the An earth station transmits the signal up to the satellite signal up to the satellite (uplink), which in turn (uplink), which in turn retransmits it to the receiving retransmits it to the receiving earth station (downlink).earth station (downlink).

Different frequencies are used Different frequencies are used for uplink/downlink.for uplink/downlink.

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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 downlink makes

it attractive for services such as the distribution of TV programs

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Direct to User Services

One way Service One way Service (Broadcasting)(Broadcasting)

Two way Service Two way Service (Communication)(Communication)

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Satellite Signals Used to transmit signals and data over

long distances Weather forecasting Television broadcasting Internet communication Global Positioning Systems

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Advantages of Satellite Communication

Can reach over large geographical area Flexible (if transparent transponders) Easy to install new circuits Circuit costs independent of distance Broadcast possibilities Temporary applications (restoration) Niche applications Mobile applications (especially "fill-in") Terrestrial network "by-pass" Provision of service to remote or underdeveloped areas User has control over own network 1-for-N multipoint standby possibilities

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Disadvantages of Satellite Communication

Large up front capital costs (space segment and launch)

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

Interference and propagation delay Congestion of frequencies and orbits

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When to use Satellites When the unique features of satellite communications

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

Communications to ships and aircraft (especially safety communications)

TV services - contribution links, direct to cable head, direct to home Data services - private networks Overload traffic Delaying terrestrial investments 1 for N diversity Special events 59

When to use Terrestrial PSTN - satellite is becoming increasingly uneconomic

for most trunk telephony routes but, there are still good reasons to use satellites for

telephony such as: thin routes, diversity, very long distance traffic and remote locations.

Land mobile/personal communications - in urban areas of developed countries new terrestrial infrastructure is likely to dominate (e.g. GSM, etc.)

but, satellite can provide fill-in as terrestrial networks are implemented, also provide similar services in rural areas and underdeveloped countries

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Frequency Bands Allocated to the FSS(Fixed service satellite)

Frequency bands are allocated to different services at World Radio-communication Conferences (WRCs).

Allocations are set out in Article S5 of the ITU Radio Regulations.

It is important to note that (with a few exceptions) bands are generally allocated to more than one radio services.

CONSTRAINTS Bands have traditionally been divided into “commercial"

and "government/military" bands, although this is not reflected in the Radio Regulations and is becoming less clear-cut as "commercial" operators move to utilize "government" bands.

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Earth’s atmosphere Source: All about GPS [www.kowoma.de]

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Atmospheric Losses Different types of atmospheric losses can

disturb radio wave transmission in satellite systems: Atmospheric absorption Atmospheric attenuation Traveling ionospheric disturbances

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Atmospheric Absorption Energy absorption by atmospheric

gases, which varies with the frequency of the radio waves.

Two absorption peaks are observed (for 90º elevation angle): 22.3 GHz from resonance

absorption in water vapour (H2O)

60 GHz from resonance absorption in oxygen (O2)

For other elevation angles: [AA] = [AA]90 cosec

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Atmospheric Attenuation Rain is the main cause of atmospheric attenuation (hail,

ice and snow have little effect on attenuation because of their low water content).

Total attenuation from rain can be determined by: A = L [dB] where [dB/km] is called the specific attenuation, and

can be calculated from specific attenuation coefficients in tabular form that can be found in a number of publications

where L [km] is the effective path length of the signal through the rain; note that this differs from the geometric path length due to fluctuations in the rain density.

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Traveling Ionospheric Disturbances

Traveling ionospheric disturbances are clouds of electrons in the ionosphere that provoke radio signal fluctuations which can only be determined on a statistical basis.

The disturbances of major concern are: Scintillation; Polarisation rotation.

Scintillations are variations in the amplitude, phase, polarisation, or angle of arrival of radio waves, caused by irregularities in the ionosphere which change over time.

The main effect of scintillations is fading of the signal.66

What is Polarisation? Polarisation is the property of electromagnetic

waves that describes the direction of the transverse electric field.

Since electromagnetic waves consist of an electric and a magnetic field vibrating at right angles to each other.

it is necessary to adopt a convention to determine the polarisation of the signal.

Conventionally, the magnetic field is ignored and the plane of the electric field is used.

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Types of Polarisation Linear Polarisation (horizontal or

vertical): the two orthogonal

components of the electric field are in phase;

The direction of the line in the plane depends on the relative amplitudes of the two components.

Circular Polarisation: The two components are

exactly 90º out of phase and have exactly the same amplitude.

Elliptical Polarisation: All other cases.Linear Polarisation Circular Polarisation

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Satellite Communications Alternating vertical and

horizontal polarisation is widely used on satellite communications

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

Allows for reduced angular separation between the satellites. 69

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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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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

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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 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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Summary

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Requirement of Satellite Communication Satellite UpLink and DownLink Types of Satellites Satellite Foot Print (Coverage Area) Satellite Transmission Bands UpLink and DownLink Frequencies Signal Propagation Delay Transponder Effect of Rain on Satellite Communication Microwave Communication (Why) Satellite System Elements Losses Capacity Allocation Strategies