Satellite Communications
Outline
 Applications of Satellite
 Architecture of Satellite Communication System
 Satellite Network Segments
 Operational Frequency Bands
 Problems in Satellite Communication
 Multiple Access Techniques
 Representative Networks
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Satellite
 Communication satellites are stationed at 36,000 km
above the surface of the earth, in geostationary orbit.
 Because the signal has to travel nearly 36,000 km in
each direction, the signal received by the satellite as well
as the remote is very weak.
 As soon as the signal is received, it has to be amplified
before further processing.
 The main attraction of satellite communication is that it
provides communication facilities to any part on the
earth—satellites are insensitive to the distance.
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Applications of Satellites
 Astronomy
 Space exploration
 Communication
 Global Positioning System (GPS)
 Navigation
 Remote sensing
 Weather monitoring
 Surveillance
 Search and rescue operations
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Architecture of a Satellite
Communication System
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Topology and Network Segments
 The architecture of a satellite communication system
consists of two segments:
 Space segment
 Ground segment
 Communication satellites operate in two configurations:
(a) mesh; and
(b) star.
 In mesh configuration, a remote station can communicate
directly with another remote station. In star configuration,
two remote stations communicate via a central station or
hub
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Space Segment
 The space segment consists of the satellite, which
has three main systems:
(a) Fuel system;
(b) Satellite and telemetry control system; and
(c) Transponders.
 A) The fuel system is responsible for making the
satellite run for years. It has solar panels, which
generate the necessary energy for the operation of
the satellite.
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Space Segment
 B) The satellite and telemetry control system is used for
sending commands to the satellite as well as for sending
the status of onboard systems to the ground stations.
 C) The transponder is the communication system, which
acts as a relay in the sky. The transponder receives the
signals from the ground stations, amplifies them, and
then sends them back to the ground stations. The
reception and transmission are done at two different
frequencies. The transponder needs to do the necessary
frequency translation.
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Ground Segment
 The ground segment consists of a number of Earth
stations. In a star configuration network, there will be
a central station called the hub and a number of
remote stations.
 Each remote station will have a very small aperture
terminal (VSAT), an antenna of about 0.5 meter to
1.5 meters.
 Along with the antenna there will be an outdoor unit
(ODU) and an indoor unit (IDU).
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Ground Segment
 The outdoor unit (ODU) contains the radio hardware
to receive the signal and amplify it.
 The radio signal is sent to an indoor unit (IDU) that
demodulates the signal and carries out the
necessary baseband processing. IDU is connected
to an end systems, such as a PC, LAN, or PBX.
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Ground Segment
 The central station consists of a large antenna (4.5
meters to 11 meters) along with all associated electronics
to handle a large number of VSATs.
 The central station also will have a Network Control
Center (NCC) that does all the management functions,
such as configuring the remote stations, keeping a
database of the remote stations, monitoring the health of
the remotes, traffic analysis.
 The NCC's main responsibility is to assign the necessary
channels to various remotes based on the requirement
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Operational Frequency Bands
 The three widely used frequency bands in satellite
communication systems are
 C band,
 Ku band, and
 Ka band.
 The higher the frequency, the smaller will be the
antenna size. However, the effect of rain is greater at
higher frequencies.
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Operational Frequency Bands
 C band:
 Uplink frequency band:
6 GHz
 Downlink frequency band: 4 GHz
 Ku band:
 Uplink frequency band:
14 GHz
 Downlink frequency band: 11/12 GHz
 Ka band:
 Uplink frequency band:
30 GHz
 Downlink frequency band: 20 GHz
 Ka band is used for broadcasting applications.
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Operational Frequency Bands
 Because the frequency of operation is higher in the Ku band,
the antenna size will be much smaller as compared to C band
antennas.
 However, the effect of rain is greater in Ku band than in C
band.
 For many years, only C band was used for satellite
communication. With advances in radio components such as
amplifiers, filters, modems, and so on, the effect of rain on Ku
band can be nullified by necessary amplification. Presently, Ku
band is used extensively for communication.
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Problems in Satellite Communication
 Propagation delay: In a star network, the total delay
from one VSAT to another VSAT is nearly 0.5
seconds if the VSAT has to communicate via the
hub.
 This type of delay is not acceptable particularly for
voice communication, because it results in echo and
talker overlap. Special protocols need to be
designed for data communication networks that use
satellites.
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Problems in Satellite Communication
 If the VSAT communicates directly with another
VSAT, the propagation delay is nearly 0.25 seconds.
We will discuss multiple access techniques that
facilitate direct communication from one VSAT to
another VSAT.
 The TCP/IP protocol stack used in computer
communication will not perform well on satellite
networks. The stack is suitably modified to overcome
the problems due to propagation delay.
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Problems in Satellite Communication
 Low bandwidth: As compared to the terrestrial
media, particularly the optical fiber, the bandwidth
supported by satellites is much less.
 Though present satellites provide much more
bandwidth than the satellites of the 1970s and
1980s, the bandwidth is nowhere comparable to the
optical fiber bandwidth.
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Problems in Satellite Communication
 Noise: Satellite channels are affected by rain,
atmospheric disturbances, etc. As a result, the
performance of satellite links is generally poor as
compared to terrestrial links.
 If data is received with errors, the data has to be
retransmitted by the sender. To reduce
retransmissions, forward error correcting (FEC)
codes are implemented.
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Multiple Access Techniques
 Two Multiple Access Techniques are available:
 DAMA-SCPC

Demand Assigned Multiple Access – Single Channel
Per Carrier
 TDM-SCPC

Time Division Multiplex – Single Channel Per Carrier
 TDMA
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DAMA-SCPC
 A channel is assigned to a remote only when the
remote has data to transmit.
 The channel assignment is done by one station that
acts as the network control center (NCC).
 Once the channel is assigned, the remote can
directly transmit data to another remote (as in a
mesh configuration).
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DAMA-SCPC Architecture
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TDM Control and TDMA Request Frame
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Call Setup Procedure
 The remote sends a request in its slot of the TDMA request
channel indicating the address of the called remote.
 The network control center sends the control information in
the TDM slot assigned to the remote indicating the carrier
assigned.
 Using the modem for the assigned carrier, the remote sends
its data to the other remote.
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Call Setup Procedure (cont’d)
 Once the data transfer is complete, the remote sends the
request for disconnection in the TDMA request channel.
 The network control center sends the command to the
remote to free the modem corresponding to the carrier
assigned earlier.
 The carrier assigned to the remote is now available in the
pool of carriers that can be assigned to the other remotes
based on demand.
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TDM-SCPC
 In time division multiplex–single channel per carrier
(TDM-SCPC), every remote broadcasts its data in
TDM mode.
 Each remote is assigned a carrier frequency
permanently, and so each remote will have one
modulator.
 However, each remote will have a bank of
demodulators to demodulate the data received from
other remotes.
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TDM-SCPC (cont’d)
 Every remote will listen to transmissions from other
remotes and decode the data meant for it based on
the address.
 The attractive feature of this configuration is that
there is no need for a network control center. Also,
there is no need for call setup.
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TDM-SCPC Architecture
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TDM Frame Format
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TDMA
 In TDMA, all the remotes use the same frequency for
transmission.
 At each remote there will be a burst modem.
 Each remote will transmit its data as a burst in the
TDMA time slot assigned to it.
 The time slot allocation is done by the network
control center.
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TDMA
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TDMA Frame Format
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Representative Network
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Representative Network
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Reference
 Principles of Digital Communication Systems and
Computer Networks by Dr. K.V. Prasad
 Chapter 13
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