EEE440 Modern Communication Systems

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EEE440 Modern
Communication Systems
Satellite Systems
System Architecture
• Comprises a space segment, a ground
segment and a control segment
System Architecture
• Space segment – contains one or several active
and spare satellites organised in a constellation
• Ground segment – consists of all the traffic earth
stations.
• Control segment – consists of all ground
facilities for the control and monitoring of
satellites (named as Tracking, Telemetry and
Command (TTC) and also of the traffic, the
Network Operation Centre (NOC)
System Architecture
• The space segment contains a number of
subsystems
– The Structural Subsystems - provides the
mechanical base structure, shields the satellite from
extreme temperature changes and micro-meteorite
damage, and controls the satellite’s spin functions.
– The Telemetry Subsystems - monitors the on-board
equipment operations, transmits equipment operation
data to the earth control station, and receives the
earth control station’s commands to perform
equipment operation adjustments.
System Architecture
– The Power Subsystems - consists of solar
panels and backup batteries that generate
power when the satellite passes into the
earth’s shadow.
– The Thermal Control Subsystems - protect
electronic equipment from extreme
temperatures due to intense sunlight or the
lack of sun exposure on different sides of the
satellite’s body
System Architecture
• The Attitude and Orbit Controlled Control
Subsystems - consists of small rocket thrusters that
keep the satellite in the correct orbital position and keep
antennas positioning in the right directions.
• Communication payload- made up of transponders
capable of :
– Receiving uplinked radio signals from earth satellite transmission
stations (antennas).
– Amplifying received radio signals
– Sorting the input signals and directing the output signals through
input/output signal multiplexers to the proper downlink antennas
for retransmission to earth satellite receiving stations (antennas).
System Architecture
• The ground segment includes three classes of stations
• Interface stations known as gateways which interconnect the space
segment to a terrestrial network
• Service stations such as hub/feeder stations which collect/distribute
information from/to users via the space segment
• User terminals such as handsets, portables, mobile stations, VSAT
or USAT which allow direct access to the space segment
• The control segment – exchange information with the
TTC section on board and includes the NOC that
manages the traffic and resources onboard the satellite
System Design
• Three specific aspects
– Orbital configuration
– Coverage
– Air interface
System Design
• For telecom satellites, orbit is in general
circular and classified according to altitude
– LEO – 500 to 1700 km
– MEO – 5000 km to 10000 km and over 20000
km
– GEO – at 35800 km
System Design
• The GEO orbit has high delay (120 – 135 ms
Earth-satellite) and high free space losses.
• The earth (excluding the polar regions) can be
covered with just three satellites
• The LEO orbits has better delay performance (
20-40 ms) and lower free space losses.
• However, a large number of satellites is
necessary to provide full coverage
• MEO is in between LEO and GEO.
• Usually used for navigation and localisation
system such the GPS
System Design
System Design
• The choice of altitude, A has strong impact
on the constellation size, Ns
System Design
• The satellite is generally a simple repeater that
amplifies and re-directs the signal
• The carrier-to-noise ratio C/N0 is expressed by
C
1 GR 1
 PT GT
N0
L TE k
L = losses due to feeder, free space and atmosphere\
TE = system equivalent noise temperature
K = Boltzmann constant
Fixed Satellite Services
• The FSS make use of the following bands
– 6/4 GHz or C band : Around 6 GHz for the uplink and
around 4 GHz for the downlink
• Occupied by the oldest systems (Intelsat etc) and tend to be
saturated
– 8/7 GHz or X band: Around 8 GHz for the uplink and
around 7 GHz for the downlink
• Reserved by agreements btw administrations for government
use
– 14/12 GHz or Ku band: 14GHz UL and 12 GHz DL
• Current operational developments (eg Eutelsat..)
– 30/20 GHz or Ka band: 30 GHz UL and 20 GHz DL
• Little present use but of raising interest due to huge
bandwidth
Mobile Satellite Services
• The MSS make use of the following bands
– VHF ( 137-138 MHz DL and 148-150 MHz
UL) and UHF ( 400-401 MHz DL and 454-460
MHz UL ) are for non-GEO systems only
• 1.6 GHz UL and 1.5 GHz DL – mostly
used by geostationary systems such as
Inmarsat
• 2.2 GHz DL and 2 GHz UL – for satellite
component of IMT2000
Mobile Satellite Services
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