This document discusses satellite communication architecture and access. It describes the key components of satellite communication systems including the satellite subsystems of sensors, transponders and transmitters and the earth station subsystems of ground stations, amplifiers, receivers and displays. It also discusses the different types of communication links between satellites and ground stations including uplinks, downlinks, crosslinks and intersatellite links. Multiple access techniques for satellite communications like FDMA, TDMA and CDMA are also summarized.

1. UNIT - III
SATELLITE ACCESS
PRESENTED BY,
R-THANDAIAH PRABU M.E.,

2. Communication
Architecture
• Arrangement or configuration of different
subsystems communicating from the Earth to
the Satellite.
• Architecture depends on three main
objectives:
• Mission Objectives
• Data Rates
• Earth-Satellite Link

3. Satellite Missions

4. Communication Subsystems
Satellite link:
• Sensor
• Transponder
• Transmitter
Earth link:
• Ground Station
• Amplifier and
Receiver
• Demodulation
• Display

5. Communication Links
• Ground Station to Satellite
• Uplink
• Downlink
• Satellite to Satellite
•
•
•
•
Crosslink
Intersatellite Link
Forward Link
Return Link

6. Store and Forward
• Has an altitude under 1000 km.
• Receives and stores information from a
group of ground stations.
• Has a small link access time to download
the information.
• Has a low-cost launch, wider antenna
bandwidth, minimum stabilization, and
covers polar areas.

7. Low Altitude
• Has an altitude higher than 1000 km.
• Connects via satellite crosslink.
• Has greater lifetime than other
satellites
• Depends on the inclination angle for
coverage pattern.
• Has complex dynamic controls, link
acquisition, and high link quality.
• The information moves for different
paths.

8. Criteria for the
Communication Architecture
• Orbit
• The satellite coverage depends on
the altitude and inclination.
• The transmitter and receiver power
depends on the altitude of the
satellite.
• The satellite orbit dictates whether
crosslink or intersatellite link is
required.

9. Satellite Orbits

10. • RF Spectrum
• The radio frequency is chosen for the
communication.
• The selection of the frequency changes
the size, mass, and complexity.
• The frequency is allocated depending on
the mission objectives.
• Data Rate
• The size of the transmitter depends on
the amount of data.
• The information can be compressed in
the satellite.

11. • Duty Factor
• Is the time needed to communicate between a
ground station and satellite.
• Is a function of the mission and the time the
satellite takes to orbit around the Earth.
• Is low when the ground station serves
different satellites.
• Link Availability
• Is defined as the time the link is available for
the user divided by the total time the satellite
covers all the area.
• Depends on the reliability of the equipment.

12. • Link Access Time
• Is the maximum time to access the
satellite information.
• Depends on the selection of the
orbit.
• Threat
• Depends on the perturbations due to
the Moon, Sun, atmosphere, and
weather.
• Depends on the noise created by
human beings.

13. Broadband Communications
 Telephone : 80- 8000 Hz
 Speech : 300 – 3400 Hz termed the speech base band
4 kHz / 64 kbps
 video : 5 – 7 MHz, 64 kbps to 10’s of Mbps
 TV : Compressed Video 18 Mbps, Uncompressed a few
hundred Mbps to a few Gbps
 High speed data, Internet and Web services
BISDN, ATM, IP : 155 Mbps
 Higher Frequency Bands : Ka band – 20 to 30 GHz

14. Digital & Analogue Television Transmission
TV may be relayed via satellite using either digital or analogue
transmission techniques.
• An analogue TV satellite feed might use up an entire 36MHz
transponder channel.
• Newer digital TV standards allow multiple feeds to TDM on a single
transponder.
• Except for North America and Japan, the Digital Video Broadcasting
– Satellite (DVB-S) standard is used.
– The Motion Pictures Experts Group (MPEG) standards are used
for compression of the video and audio bit streams.
• The newer Ku-band satellites produce sufficient power (say, 50W
per channel) that reception is possible with a small dish.
• This has led to broadcasting to the home via direct broadcast
satellites (DBS).

15. DVB-S
DVB-S is the oldest (1994) of the standards proposed by Digital Video
Broadcasting (DVB) project, a chiefly European consortium.
• DVB is better known in Australia for DVB-T (for Terrestrial), having
adopted it as our new digital free-to-air TV standard..
• Although primarily aimed at video broadcasting, DVB-S is suitable
in a wide range of broadband satellite communications applications.
• At the physical layer, the following operations are performed at
the transmitter:
Scrambling
RS Outer Code
Interleave Conv.
Inner Code Pulse
Shaping QPSK

16. • We understand the last two blocks: It is used for shaping
• Scrambling helps to ensure a balance of 1s and 0s.
• The coding blocks apply advanced forward error correcting
(FEC) codes to achieve a coding gain.
• The interleaver reorders the sequence of bits to guard
against error bursts.

17. Data and Telephone Signal Multiple Access
Satellites may carry TDM VF (telephone) signalling similar to
Conventional terrestrial microwave radio links.
• In the 1970s, satellites were the chief means of trunking international
phone calls.
• In recent years, intercontinental optical fibre has been increasingly
used because of its lower delays and other advantages.
• Multiple access can be provided through by FDMA, TDMA and/or
CDMA.
• Moreover, different antennas on a satellite may have different
footprints, allowing further frequency re-use.
• Very small aperture terminals (VSATs) have become popular with
the rise of Ku-band satellites— only need a small dish.
• This allows low-cost, dedicated voice and data for corporate users.
• Satellite is still the most effective broadband internet option for
many outback users: several satellites service Australia.

18. DVB-S and DVB-RCS for Internet
DVB-S can be and is employed to provide satellite
internet services.
• DVB-S allows for a ‘1-way’ satellite internet service.
• The newer (1999) DVB-RCS (for Return Channel Satellite)
Allows 2-way satellite internet service.
• Multi-Frequency (MF-)TDMA is used for the return channel:
a combination of FDMA and TDMA.
• A common signaling channel is used to provide control and
synchronization.

19. Personal Communications via Satellite
Voice communication via GEO satellites has lost favor in part because
of the long round-trip delay involved: around 0.25 s.
• LEO and MEO satellites are much closer to earth lower delays.
• Also, they require less uplink power.
• This has led several companies to attempt to provide global satellite
mobile phone coverage.
• Three operators are noteworthy: Iridium, Globalstar and ICO.
• All started up in the 1990s.
• Continuous global coverage requires many satellites.
• Iridium launched 66 satellites; Globalstar 48.
• While these satellite operators were starting up, terrestrial mobile
networks, and international roaming agreements, boomed.
• This left the satellite operators only a narrow niche but with expensive
space-segment debts all filed for bankruptcy.

20. Multiplexing
• Multiplex channels (k)
in four dimensions
•
•
•
•
space (s)
time (t)
frequency (f)
code (c)
• Goal: multiple use
of a shared medium
channels ki
k1 k2 k3 k4 k5 k6
c
t
c
s1
f
c
s3
t
s2
f
t
f

21. Multiplexing
• Multiplexing means breaking up a higher speed
circuit into several slower circuits.
• The main advantage of multiplexing is cost;
multiplexing is cheaper because fewer
network circuits are needed.
• There are four categories of multiplexing:
• Frequency division multiplexing (FDM)
• Time division multiplexing (TDM)
• Statistical time division multiplexing
(STDM)
• Wavelength division multiplexing (WDM)

22. Frequency Division Multiplexing
(FDM)
• FDM works by making a number of smaller
channels from a larger frequency band. FDM is
sometimes referred to as dividing the circuit
“horizontally”.
• In order to prevent interference between channels,
unused frequency bands called guardbands are
used to separate the channels. Because of the
guardbands, there is some wasted capacity on an
FDM circuit.
• CATV uses FDM. FDM was also commonly used to
multiplex telephone signals before digital
transmission became common and is still used on
some older transmission lines.

23. Frequency Division Multiplexing

24. Time Division Multiplexing
• TDM allows multiple channels to be used by allowing
the channels to send data by taking turns. TDM is
sometimes referred to as dividing the circuit
“vertically”
• Figure in next slide shows an example of 4 terminals
sharing a circuit, with each terminal sending one
character at a time.
• With TDM, time on the circuit is shared equally with
each channel getting a specified time slot, whether or
not it has any data to send.
• TDM is more efficient than FDM, since TDM doesn’t
use guardbands, so the entire capacity can be divided
up between the data channels.

25. Time Division Multiplexing

26. Multiple Access Issues
• Spread Spectrum Techniques: Military and
commercial use of spread-spectrum.
Direct-Sequence PN, Frequency-Hop and
CDMA systems.
Principles of Multiple Access
Communications
• Multiplexing & Multiple Access FDMA, TDMA
• Concepts of Random Access: ALOHA, CSMA
Multiple Access Techniques:
• FDMA, TDMA, CDMA. DAMA and
Bandwidth-on-Demand (BoD).
• TDMA Networks: Time Slots, Preambles,
Suitability for DAMA and BoD.

27. Capacity Allocation Strategies
• Frequency division multiple access
(FDMA)
• Time division multiple access (TDMA)
• Code division multiple access (CDMA)

28. Frequency Division
Multiplex (FDM)
• Separation of the whole spectrum into smaller
frequency bands
• A channel gets a certain band of the spectrum for the
whole time
+ no dynamic coordination necessary
k1 k2 k3 k4 k5
+ works also for analog signals
– waste of bandwidth if traffic
c
is distributed unevenly
– inflexible
• Example:
broadcast radio
t
k6
f

29. Frequency-Division Multiplexing
• Alternative uses of channels in point-to-point
configuration
•
•
•
•
•
•
•
1200 voice-frequency (VF) voice channels
One 50-Mbps data stream
16 channels of 1.544 Mbps each
400 channels of 64 kbps each
600 channels of 40 kbps each
One analog video signal
Six to nine digital video signals

30. Typical Satellite Transponder Frequency Plan for the
Downlink Channels

31. Frequency-Division Multiple
Access
• Factors which limit the number of
subchannels provided within a satellite
channel via FDMA
• Thermal noise
• Intermodulation noise
• Crosstalk

32. Forms of FDMA
• Fixed-assignment multiple access (FAMA)
• The assignment of capacity is distributed in a fixed
manner among multiple stations
• Demand may fluctuate
• Results in the significant underuse of capacity
• Demand-assignment multiple access (DAMA)
• Capacity assignment is changed as needed to respond
optimally to demand changes among the multiple
stations

33. FAMA-FDMA
• FAMA – logical links between stations are
preassigned
• FAMA – multiple stations access the
satellite by using different frequency bands
• Uses considerable bandwidth

34. DAMA-FDMA
• Single channel per carrier (SCPC) – bandwidth
divided into individual VF channels
• Attractive for remote areas with few user stations near
each site
• Suffers from inefficiency of fixed assignment
• DAMA – set of subchannels in a channel is treated
as a pool of available links
• For full-duplex between two earth stations, a pair of
subchannels is dynamically assigned on demand
• Demand assignment performed in a distributed fashion
by earth station using CSC

35. Time Division Multiplex
(TDM)
• A channel gets the whole spectrum for a
certain amount of time
+ only one carrier in the medium at any time
+ throughput high even
k1 k2 k3
for many users
– precise synchronization
c
necessary
• Example: Ethernet
t
k4
k5
k6
f

36. Reasons for Increasing Use of
TDM Techniques
• Cost of digital components continues to
drop
• Advantages of digital components
• Use of error correction
• Increased efficiency of TDM
• Lack of intermodulation noise

37. Example of TDMA Frame Format

38. TDMA Operation
• Fixed-assignments multiple access (FAMA): This
assignment of capacity within the overall satellite
channel is distributed in a fixed manner among
multiple stations.
• Transmission in the form of repetitive sequence of
frames
• Each frame is divided into a number of time slots
• Each slot is dedicated to a particular transmitter
• Earth stations take turns using uplink channel
• Sends data in assigned time slot
• Satellite repeats incoming transmissions
• Broadcast to all stations
• Stations must know which slot to use for

39. FAMA-TDMA Operation
• Transmission in the form of repetitive sequence of
frames
• Each frame is divided into a number of time slots
• Each slot is dedicated to a particular transmitter
• Earth stations take turns using uplink channel
• Sends data in assigned time slot
• Satellite repeats incoming transmissions
• Broadcast to all stations
• Stations must know which slot to use for
transmission and which to use for reception

40. FAMA-TDMA Uplink

41. FAMA-TDMA Downlink

42. Time and Frequency
Division Multiplex
• Combination of both methods
• A channel gets a certain frequency band for some
time
+ protection against frequency selective interference
+ protection against tapping
k1 k2 k3 k4
+ adaptive
c
– precise coordination required
k5
k6
f
• Example: GSM
t

43. Code Division Multiplex
(CDM)
• Each channel has a unique
code
• All channels use the same
spectrum at the same time
+ bandwidth efficient
+ no coordination or
synchronization
+ hard to tap
+ almost impossible to jam
– lower user data rates
– more complex signal
regeneration
• Example: UMTS
• Spread spectrum
k1
k2
k3
k4
k5
k6
c
f
t

44. Code Division Multiple
Access (CDMA)
• used in several wireless broadcast
channels (cellular, satellite, etc) standards
• unique “code” assigned to each user; i.e.,
code set partitioning
• all users share same frequency, but each
user has own “chipping” sequence (i.e.,
code) to encode data
• encoded signal = (original data) X (chipping
sequence)
• decoding: inner-product of encoded signal
and chipping sequence
• allows multiple users to “coexist” and
transmit simultaneously with minimal
interference (if codes are “orthogonal”)