ipt satellite communication
TRANSCRIPT
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Dar es Salaam institute of Technology (DIT)
IPT TRAINING
Satellite Communication
Ally, J
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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.