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Dar es Salaam institute of Technology (DIT)

IPT TRAINING

Satellite Communication

Ally, J

jumannea@gmail.com

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SATELLITE COMMUNICATION

SYSTEMS

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Introduction to satellite communication

Communication satellites bring the world to you anywhere and any time…..

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What exactly is a satellite?

• The word satellite originated from the Latin word “Satellit”- meaning an attendant, one who is constantly hovering around & attending to a “master” or big man.

• For our own purposes however a satellite is simply any body that moves around another (usually much larger) one in a mathematically predictable path called an orbit.

• A communication satellite is a microwave repeater station in space that is used for telecommunication , radio and television signals.

• The first man made satellite with radio transmitter was in 1957.. There are about 750 satellite in the space, most of them are used for

communication.

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How do Satellites Work?

* Two Stations on Earth want to communicate through radio broadcast but are too far away to use conventional means.The two stations can use a satellite as a relay station for their communication.

* One Earth Station transmits the signals to the satellite. Uplink frequency is the frequency at which Ground Station is communicating with Satellite.

* The satellite Transponder converts the signal and sends it down to the second earth station. This frequency is called a Downlink.

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Applications

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Satellite frequency band

Band Downlink, GHz Uplink, GHzBandwidth,

MHz

L 1.5 1.6 15

S 1.9 2.2 70

C 4 6 500

Ku 11 14 500

Ka 20 30 3500

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SatCom

� SATelliteCOMmunnications

� Global business –cross border crosscontinent

� Importanttechnology for everycountry – know localregulations.

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SatCom Pros and Cons

PROS CONS� Bypass or support - Limited space

terrestrial network � Ideal for broadcast - Very expensive earth � Ideal for failover terminals� Support low and high - Failure on satellite

data rates cannot be repaired � Fast to deploy - Big latency � Mobile

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Where are they used?

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How it all started?� Arthur Clarke envisioned Extra Terrestrial

Relays in 1945. From 1945-1957 mostly theoretical work on SatCom.

� In 1957 Russia launched first satellite –Sputnik

� In 1965-1980 Major means of phone trunking in US [In 1964 Intelsat formed, 965 First commercial satellite, 1969 first GLOBAL satellite coverage ]

� Fiber takes over voice – expansion of VSATs� Expansion of SatTV and VSATs

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Types of Satellite

� Weather SatellitesA weather satellite called the GeostationaryOperational Environmental Satellite observesatmospheric conditions over a large area tohelp scientists study and forecast the weather� Communication SatellitesA communications satellite, such as the Trackingand Data Relay Satellite (TDRS) shown here,relays radio, television, and other signalsbetween different points in space and on Earth� Navigation SatelliteA navigation satellite, like this GlobalPositioning System (GPS) satellite, sends signalsthat operators of aircraft, ships, and landvehicles and people on foot can use to determinetheir location� Observing SatelliteAn Earth observing satellite surveys our planet'sresources. This satellite, Aqua, helps scientistsstudy ocean evaporation and other aspects of themovement and distribution of Earth's water

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How does it looks like?

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Where are they?

� High above earth is ideal place� How to keep them there – use gravity!

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Geosynchronous Orbit (GEO)� 35800 km above equator� Revolve with same speed as Earth� Stays in same position relative to surface of Earth� Covers up to ¼ of Earth surface� Have 24 hour view of covered area

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Low Earth Orbit (LEO)� Between 250 and 1500 km above Earth� Short period of visibility over particular area up to 20

minutes� Mainly used for observation, science and military use� Smaller coverage area

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Medium Earth Orbit (MEO)

� Between 8,000 and 10,000 km above Earth� Visible from 2 to 8 hours from Earth� Larger coverage area than LEO� Mostly used for GPS navigation

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SatCom today� 850 ACTIVE satellites in orbit today – many more

not active

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Satellite Position� We need to know where our satellite is� Satellite operator will provide you with its position [

usually deg E or deg W ] – longitude

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Satellite Operator Map

http://www.intelsat.com/network/satellite/

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Co-location� Multiple satellites share same longitude� Considering distance between Earth and Satellite it

looks like they are in same position while in reality they are separated

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How we bring them up� Launch vehicles are usually rockets� Once Space shuttle was used for launch� Space centers [China, USA, Russia, Kazahstan]

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Footprints� Each satellite can see certain part of Earth surface –

but it can also be “cross-eyed” and see multiple spots at same time

� Global view [well actually just about 40% of Earth Surface]

� Hemi [hemisphere view: North, South, East, West]

� Spot [On continent, on zone within continent]

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Global Footprint

One half [actually 40%] of Earth surface

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Hemi Footprint

One HEMIshpere covered [E/W or N/S]

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Spot beam

Highly directional antenna focusing on one particular spot

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Uplink and Downlink

Satellite will send signal to and receive signal fromSPECIFIC zone. All other signals outside zone will notbe received.

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What they actually do?� Payload of the satellite actually performs work.� In communications satellites they are called

TRANSPONDERS� Transponder is actually amplifying REPEATER� There are multiple repeaters on each satellite

(usually from 20 to 40)

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Shift and Convert/OBP (On Board Processor)

� Most of satellite transponders receive signal on one frequency [i.e 14Ghz] , amplifies signal with low noise amplifier, down-converts signal to lower frequency and then amplifies to high power level for transmission down to Earth.

� [Band Pass Filter, Low Noise Amp, Mixer, Band Pass Filter, High Power Amp]

� On board processors (OBP)� Combine INPUT signal from multiple transponders and

demodulated to packets, routed, switched, modulated back to one Radio Frequency signal.

� Only on some very modern satellites

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Very Small Aperture Terminal (VSAT)

Antennas

Five most common types of antennas� Symmetric – single reflector

� Symmetric – dual reflector Cassegrain

� Symmetric – dual reflector Gregorian

� Offset – single reflector

� Offset – dual reflector Gregorian

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Antenna

� Symmetric – single reflector in prime focus

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Antenna

� Symmetrical - Dual reflector Cassegrain

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Antenna� Symmetric – dual reflector Gregorian

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Antenna� Offset

� Offset – dual reflector

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Isotropic Antenna� Isotropic antenna spreads signal evenly across

the sphere that surrounds it. � It has 0dB gain.

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Directional Antenna� Directional antenna focuses the power in one

direction. � Dish antennas for VSAT usually have 35dB gain.

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WavesElectromagnetic waves are� Made of oscillating electric and magnetic fields� Travel trough air and vacuum space� Bounce of metals� Travel at speed of light 300000 km/s

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WavesWaves are defined by� Amplitude [strength of the wave at its peak]� Unit: V, W or dBW

� Frequency [wave length]� Unit: Hz – we use Ghz

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Polarization� Microwaves are launched by a feedhorn which is

section on wave guide open on one end.� Wave is launched in plane and this is called

linear polarization.

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Polarization

� Two signals can be launched in same time–in opposite polarizations.

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Circular Polarization

� If the microwave is launched with electric field spinning around exactly once per microwave cycle we call this circular polarization.

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Linear Polarization vs. Circular Polarization

� Linear polarization feed horn cheaper than CP

� CP - no need to adjust feed!

� Less prone to rain fade

� Both used in C, Ka and Ku bands

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Side lobe� Directional antenna has the most gain along it’s

main axis. � Away from it, the gain falls, but can raise again

to form side lobes.

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Side lobe

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Sidelobes Side-Effect� Sidelobes can affect adjacent satellite or

terrestrial communications

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Size and Gain Correlation� As you increase the size of antenna two things happen:

1. The gain increases2. The beam width decreases

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Size and Pointing� The smaller the dish – the wider the beam is� Small dish is easier to point than larger one

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Dish size / Band / Beam width

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Latency

� Time for signal to travel from one earth station to another

� (35000+35000)km/300000km/s = 0.23s

� This cannot be shorter unless you find new speed of light!

� Implication of latency – delay [voice]

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Frequency Bands

C band 4/6GHz X band 7/8GHz� Uplink 5.9 – 6.4GHz - Uplink 7.9 – 8.4GHz� Downlink 3.7-4.2GHz - Downlink 7.25-7.75GHz

Ku band 11/14GHz Ka band 20/30GHz� Uplink 14–14.5GHz - UL 29.5-30 or 27-30.5GHz � Downlink 10.95–12.75GHz - DL 19.7-20.2 or 18.3-202.2GHz

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C Band� First band used for SatCom

� 500MHz bandwidth

� Not affected (much) by rain

� Large satellite footprints

� Band shared with microwaves – interference!

� Large dishes needed

� High power transmitter needed

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Ku band� Small dishes

� Low transmit power

� More downlink bandwidth available – 2Ghz

� Less interference than C band

� Rain sensitive – must add extra margin

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X Band and Ka Band� X band military use only

� Ka band is new – lots of bandwidth available

� Small spot footprints allowing re-usage of bandwidth

� Very small dishes and low transmit power

� Seriously affected by rain [more than Ku]

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Units you should know

Watt = W� milliWatt = mW = 1/1000W� kiloWatt = kW = 1000W

Hertz = Hz� kiloHertz = kHz = 1000 Hz� MegaHertz = MHz = 1,000,000Hz� GigaHertz= GHz = 1,000,000,000 Hz� deciBells = dB

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dB

� Convenient multiplier for large and small numbers

� They let you add numbers instead of multiply� Technically dB=10 log [Power1/Power2]

� x 10 = + 10 dB� x 2 = + 3 db� x 1 = 0 dB� x 0.5 = -3 dB� x 0.1 = -10dB

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dB example

� 10 W = 2 x 5 Wx 2 = 3dBSo 10 Watts is 3dB more than 5 W

� 3 db loss in cable that has input signal of 1W. How much comes out?3db loss = -3dB-3dB = 0.5 so output = 0.5x1W = 0.5W

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dB example

� 1 W = 1000mW = 0dBW = 30 dBmW

� 10mw = 10dBmW

� 10x10mw= 10dBmW + 10dBmW

� 10x10x10mW = 10+10+10 dBmW

� 2W =?

� 2W = 30dBmw + 3dBmW

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dB simply adds up

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Modulation� Modulation is all about how information is

encoded onto satellite signals and how errors are prevented.

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Modulation

� Can be Analog or Digital

� Analog modulation can be:� Amplitude modulation [AM]� Frequency modulation [FM]

� Digital modulation� Used for satellite communications� Signal is digitized first then manipulated� ASK and/or FSK and/or PSK

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BPSK Modulation

� Binary Phase Shifting Keying� A zero data bit is represented by reversing

carrier phase

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QPSK Modulation� Quadrature Phase Shifting Keying� Data bits arranged in groups� 00, 01, 10, 11� Each group has one of four phase states� 0, 90, 180, 270� Very efficient coding!

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8PSK / 16PSK Modulation

� 8-art Phase Shifting Keying� 8 groups 000,001,010,011,100,101,110,111� 16 groups 0000,0001,…..

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Symbols and Bits

� Bit is unit of DATA

� Symbol is unit of MODULATION

� Bit rate means how many bits per second go trough link [bits per second bps]

� Symbol rate means how many modulations change state [symbols per second sps]

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Symbols and Bits

� BPSK – each symbol has two states and represents one bit [0 or 1]� Bit rate = 1 x symbol rate� 512kbps = 512 ksps

� QPSK – each symbol has 4 states and represents 2 bits [00, 01, 10, 11]� Bit rate = 2 x symbol rate� 512 kbps = 256 ksps

� 16PSK – 16 states and 4 bits� 512 kbps = ?

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Typical Bit Rates

� Full transponder = 36 – 72 Mbps

� VSAT outbound [HUB] = 5-36 Mbps

� VSAT [terminal] = 64-512 kbps

� Video = 2-4 Mbps

� Voice = 4.8 kbps

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Forward Error Correction (FEC)� Rate ½ means that for every original bit� there is extra FEC encoder bits� Rate ¾ means that for every 3 original� bits 4th is coming from FEC encoder

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Bit Error Rate (BER)

� BER is defined as ratio of uncorrected bits to originally transmitted

� 10-6 rate means that for every million bits transmitted there will be ONE error

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TCP and BER

� TCP/IP traffic is sensitive to BER as packets will be dropped even for one error. Multiple drops results in speed reduction

� So if BER is high link is still on but it is SLOOOOOOW

� Solution – reduce BER

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Access Methods� How earth stations access satellites and

efficiently share spectrum

� SCPC

� TDM/TDMA

� FDMA

� CDMA

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Single Carrier Per channel (SCPC)� Single carrier per channel or Point to Point is the

simplest possible way to connect two earth stations together, but it is not most efficient.

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SCPC – Multiple Remotes

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SCPC� Each VSAT has it’s own dedicated pair of frequency

channels [one inbound one outbound].� Because it build links between points it is called

Point to Point� Flexible – you can add link between any two points

on the network.� Allows incremental use of satellite transponder

spectrum so network growth is flexible.� Each link requires modem at each earth station.� NO BANDWIDTH SHARING between stations –

every link must be sized to support maximum data rate needed, so most of the time bandwidth is not fully used.

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Shared Spectrum

� Costs are really high so it’s better to share spectrum

� TDMA� VSAT tuned to same frequency� VSATs take turn in using the channel� On broadcast stream data packets addressed to

specific VSATs

� FDMA� Each VSAT tuned to different frequency� Channels assigned on per need basis

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TDM

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TDMA

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TDM/TDMA

� TDM used for broadcasting and outbound traffic

� TDMA is used for inbound traffic and VSAT transmit in burst [pulses]. GPS location important because of time-slots.

� TDM/TDMA network are star topology

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Mesh TDMA

� No hub – all VSATs talk to all other VSATs� Single hop [least delay]

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CDMA� Also called spread spectrum� All VSAT continuously transmit and data is scrambled

with unique code for each VSAT.� Receiving VSAT use same code to decode� All other signal is considered as noise

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Comparison of Techniques

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Link Terminology� Uplink = to the satellite� Downlink = from the satellite� Outbound = from hub to remote station� Inbound = from remote station to the hub

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Link Budget Anatomy

� Start from hub and work way back to remote VSAT station

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Typical Satellite Link

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Tools [Site Survey]� To perform site survey you need first to get GPS

coordinates of location where VSAT will be installed� GPS or Thuraya phone� Once you have GPS coordinates use sites that can help in

establishing Azimuth/Elevation (Az/El) for VSAT� www.satlex.de� Use compass to find out where antenna will be

pointing to� Use compass with elevation to determine if you have

any obstacles in front of VSAT� Check out where modem will be installed� Measure distance for IFL (Interfacility Link Cable)� Check for obstacles� Check for weather!

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GPS Coordinates� For this site (DIT) they are: (6 deg 48 min

South) and (39 deg 16 min East)

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Calculate Az/El

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Check on map if possible

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Most Important Information� Satellite name e.g. INTELSAT902

� Satellite ID e.g. IS902

� Satellite Position (degree E/deg W) – LONG.

� Antenna type (OFFSET?)

� Antenna Diameter

� OFFSET angle

� Make and model

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Thanks!

Technology changes but communication lasts.