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, GHz Bandwidth, MHz
L S C Ku Ka
1.5 1.9 4 11 20
1.6 2.2 6 14 30
15 70 500 500 3500
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SatCom
SATellite COMmunnications Global business cross border cross continent Important technology for every country know local regulations.
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SatCom Pros and Cons
PROS
Bypass or support terrestrial network Ideal for broadcast Ideal for failover Support low and high data rates Fast to deploy Mobile
CONS
- Limited space - Very expensive earth terminals - Failure on satellite cannot be repaired - Big latency
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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 Satellites A weather satellite called the Geostationary Operational Environmental Satellite observes atmospheric conditions over a large area to help scientists study and forecast the weather Communication Satellites A communications satellite, such as the Tracking and Data Relay Satellite (TDRS) shown here, relays radio, television, and other signals between different points in space and on Earth Navigation Satellite
A navigation satellite, like this Global Positioning System (GPS) satellite, sends signals that operators of aircraft, ships, and land vehicles and people on foot can use to determine their location
Observing Satellite An Earth observing satellite surveys our planet's resources. This satellite, Aqua, helps scientists study ocean evaporation and other aspects of the movement 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 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 from SPECIFIC zone. All other signals outside zone will not be 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, downconverts 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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Waves
Electromagnetic 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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Waves
Waves 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 its 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 increases 2. 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
Uplink 5.9 6.4GHz Downlink 3.7-4.2GHz
X band 7/8GHz
- Uplink 7.9 8.4GHz - Downlink 7.25-7.75GHz
Ku band 11/14GHz
Uplink 1414.5GHz Downlink 10.9512.75GHz
Ka band 20/30GHz
- UL 29.5-30 or 27-30.5GHz - 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 W x 2 = 3dB So 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 means that for every original bit there is extra FEC encoder bits Rate 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 its 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 its 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.
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