UNIT - III SATELLITE ACCESS

PRESENTED BY,R-THANDAIAH PRABU M.E.,

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

Satellite Missions

Communication Subsystems

Satellite link:• Sensor• Transponder• Transmitter

Earth link:• Ground Station• Amplifier and

Receiver• Demodulation• Display

Communication Links

• Ground Station to Satellite• Uplink• Downlink

• Satellite to Satellite• Crosslink• Intersatellite Link• Forward Link• Return Link

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.

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.

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.

Satellite Orbits

• 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.

• 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.

• 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.

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

Digital & Analogue Television Transmission

TV may be relayed via satellite using either digital or analoguetransmission techniques.

• An analogue TV satellite feed might use up an entire 36MHztransponder channel.

• Newer digital TV standards allow multiple feeds to TDM on a singletransponder.

• Except for North America and Japan, the Digital Video Broadcasting– Satellite (DVB-S) standard is used.– The Motion Pictures Experts Group (MPEG) standards are usedfor compression of the video and audio bit streams.

• The newer Ku-band satellites produce sufficient power (say, 50Wper channel) that reception is possible with a small dish.

• This has led to broadcasting to the home via direct broadcastsatellites (DBS).

DVB-SDVB-S is the oldest (1994) of the standards proposed by Digital VideoBroadcasting (DVB) project, a chiefly European consortium.

• DVB is better known in Australia for DVB-T (for Terrestrial), havingadopted it as our new digital free-to-air TV standard..

• Although primarily aimed at video broadcasting, DVB-S is suitablein a wide range of broadband satellite communications applications.

• At the physical layer, the following operations are performed atthe transmitter:ScramblingRS Outer CodeInterleave Conv.Inner Code PulseShaping QPSK

• 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.

Data and Telephone Signal Multiple Access

Satellites may carry TDM VF (telephone) signalling similar toConventional terrestrial microwave radio links.

• In the 1970s, satellites were the chief means of trunking internationalphone calls.

• In recent years, intercontinental optical fibre has been increasinglyused because of its lower delays and other advantages.

• Multiple access can be provided through by FDMA, TDMA and/orCDMA.

• Moreover, different antennas on a satellite may have differentfootprints, allowing further frequency re-use.

• Very small aperture terminals (VSATs) have become popular withthe 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 formany outback users: several satellites service Australia.

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.

Personal Communications via Satellite

Voice communication via GEO satellites has lost favor in part becauseof 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 satellitemobile 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 mobilenetworks, and international roaming agreements, boomed.

• This left the satellite operators only a narrow niche but with expensivespace-segment debts all filed for bankruptcy.

• Multiplex channels (k) in four dimensions

• space (s)• time (t)• frequency (f)• code (c)

• Goal: multiple use of a shared medium

• Important: guard spaces needed!

• Example: radio broadcast

Multiplexing

s2

s3

s1f

tc

k2 k3 k4 k5 k6k1

f

tc

f

tc

channels ki

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)

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.

Frequency Division Multiplexing

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.

Time Division Multiplexing

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.

Capacity Allocation Strategies

• Frequency division multiple access (FDMA)

• Time division multiple access (TDMA)

• Code division multiple access (CDMA)

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+ works also for analog signals– waste of bandwidth if traffic

is distributed unevenly– inflexible

• Example:broadcast radio

k2 k3 k4 k5 k6k1

f

t

c

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

Typical Satellite Transponder Frequency Plan for the Downlink Channels

Frequency-Division Multiple Access

• Factors which limit the number of subchannels provided within a satellite channel via FDMA• Thermal noise• Intermodulation noise• Crosstalk

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

FAMA-FDMA

• FAMA – logical links between stations are preassigned

• FAMA – multiple stations access the satellite by using different frequency bands

• Uses considerable bandwidth

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

f

t

c

k2 k3 k4 k5 k6k1

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 for many users

– precise synchronization necessary

• Example: Ethernet

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

Example of TDMA Frame Format

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 transmission and which to use for reception

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

FAMA-TDMA Uplink

FAMA-TDMA Downlink

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+ adaptive – precise coordination required

• Example: GSM f

t

c

k2 k3 k4 k5 k6k1

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

k2 k3 k4 k5 k6k1

f

t

c

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”)