Week-4.pptx

CHARACTERISTICS OF SATELLITE COMMUNICATION SYSTEM

WEEK 4

Information

 Recommended Study Material:

 Book-1: Electronic Communication Systems, By George

Kennedy Book-2: Satellite Communications, By D.C Agarwal

 Lecture Notes

CHARACTERISTICS OF SATELLITE COMMUNICATION SYSTEM

Topics:

 General and Technical Characteristics of a

Satellite Communication System

 Advantages of Satellite Communication Active and Passive Satellites

Advent of Digital Satellite Communication

Communication Satellite

 A Communication Satellite can be looked

upon as a large microwave repeater

 It contains several transponders which

listens to some portion of spectrum, amplifies the incoming signal and

broadcasts it in another frequency to

Satellite Microwave

Transmission

 Satellites can relay signals over a long

distance

 Geostationary Satellites

 Remain above the equator at a height of

about 22300 miles (geosynchronous orbits)

 Travel around the earth in exactly the same

Space Segment

 Satellite Launching Phase

 Transfer Orbit Phase

 Deployment

 Operation

 TT&C - Tracking Telemetry and Command

Station

 SSC - Satellite Control Center, a.k.a.:

 OCC - Operations Control Center  SCF - Satellite Control Facility

Ground Segment

 Collection of facilities, Users and Applications

 Earth Station = Satellite Communication Station

Satellite Uplink and

Downlink

 Downlink

 The link from a satellite down to one or more

ground stations or receivers

 Uplink

 The link from a ground station up to a satellite.

 Some companies sell uplink and downlink

services to

 television stations, corporations, and to other

telecommunication carriers.

 A company can specialize in providing uplinks,

Source: Cryptome [Cryptome.org]

 When using a satellite for long distance communications, the satellite acts as a repeater.  An earth station transmits the

signal up to the satellite (uplink), which in turn

retransmits it to the receiving earth station (downlink).

 Different frequencies are used for uplink/downlink.

Satellite Transmission Links

 Earth stations Communicate by sending

signals to the satellite on an uplink

 The satellite then repeats those signals

on a downlink

 The broadcast nature of downlink makes

Direct to User Services

Satellite Signals

 Used to transmit signals and data over

long distances

 Weather forecasting

 Television broadcasting

 Internet communication

Satellite Transmission Bands

Frequency Band Downlink Uplink

C

Ku

Ka

Types of Satellite Orbits

 Based on the inclination, i, over the equatorial

plane:

 Equatorial Orbits above Earth’s equator (i=0°)  Polar Orbits pass over both poles (i=90°)

 Other orbits called inclined orbits (0°<i<90°)

 Based on Eccentricity

Types of Satellite based

Networks

 Based on the Satellite Altitude

 GEO – Geostationary Orbits

 36000 Km = 22300 Miles, equatorial, High latency

 MEO – Medium Earth Orbits

 High bandwidth, High power, High latency

 LEO – Low Earth Orbits

 Low power, Low latency, More Satellites, Small

Footprint

 VSAT

 Very Small Aperture Satellites

Source: Federation of American Scientists [www.fas.org]

Satellite Orbits

 Geosynchronous Orbit (GEO): 36,000 km above Earth,

includes commercial and military communications

satellites, satellites providing early warning of ballistic

missile launch.

 Medium Earth Orbit (MEO): from 5000 to 15000 km, they include navigation satellites (GPS, Galileo, Glonass).

Advantages of Satellite

Communication

 Can reach over large geographical area  Flexible (if transparent transponders)  Easy to install new circuits

 Circuit costs independent of distance  Broadcast possibilities

 Temporary applications (restoration)  Niche applications

 Mobile applications (especially "fill-in")  Terrestrial network "by-pass"

 Provision of service to remote or underdeveloped

areas

 User has control over own network

Advantages of Satellite

Communication

Transmitter

Base band signal Input >> Encoder >> Modulator >> Upconverter >> HPA

Receiver

Base band signal Output >> Decoder >> De-Modulator >> Down converter >> LNA

Antenn a

Disadvantages of Satellite Communication

 Large up front capital costs (space

segment and launch)

 Terrestrial break even distance

expanding (now approx. size of Europe)

 Interference and propagation delay

When to use Satellites

 When the unique features of satellite communications

make it attractive

 When the costs are lower than terrestrial routing  When it is the only solution

 Examples:

 Communications to ships and aircraft (especially safety

communications)

 TV services - contribution links, direct to cable head, direct

to home

 Data services - private networks  Overload traffic

 Delaying terrestrial investments  1 for N diversity

When to use Terrestrial

 PSTN - satellite is becoming increasingly

uneconomic for most trunk telephony routes

 but, there are still good reasons to use

satellites for telephony such as: thin routes,

diversity, very long distance traffic and remote locations.

 Land mobile/personal communications - in

urban areas of developed countries new

terrestrial infrastructure is likely to dominate (e.g. GSM, etc.)

 but, satellite can provide fill-in as terrestrial

networks are implemented, also provide similar services in rural areas and underdeveloped

Frequency Bands Allocated to

the FSS

 Frequency bands are allocated to different services at

World Radio-communication Conferences (WRCs).

 Allocations are set out in Article S5 of the ITU Radio

Regulations.

 It is important to note that (with a few exceptions) bands

are generally allocated to more than one radio services.

 _CONSTRAINTS

 Bands have traditionally been divided into “commercial" and

"government/military" bands, although this is not reflected in the Radio Regulations and is becoming less clear-cut as

Earth’s atmosphere

Satellite Communications

 Alternating vertical and

horizontal polarisation is widely used on satellite communications

 This reduces interference

between programs on the same frequency band

transmitted from adjacent satellites (One uses vertical, the next horizontal, and so on)

 Allows for reduced angular

separation between the satellites.

Design of the Satellite Link

Active and Passive satellites

 Active satellites:

 The satellite receives a fraction of power from transmitting station  Received is amplified by active electronic means usually in

conjunction with frequency shifting

 Power level of receive power by receiving system determined by

spacecraft transmitter.

 Passive Satellite

A large reflector such as spherical balloon

Ground station transmits power at the reflector

Receiving station receives a fraction of power intercepted and radiated by reflector

Active and Passive Satellites

Active Satellite Passive Satellite

Greater Communication capability due to use of

directional antennas at higher altitude.

Incapable of competing with active systems at high altitudes.

Modern satellites are active

satellite with long life electronic equipment

Advent of Digital Satellite Communication

 The advantages of digital signal

transmission

 The ease and efficiency of multiplexing  Convenient packet switching

 Insensitivity of digital circuit to retransmission

of noise

 Low error rate and fidelity through error

detection and correction.

 Flexibility of digital hardware implementation  Microprocessor, miniprocessors, digital

Advent of Digital Satellite Communication

 FDM-FM-FDMA is good and stable, used

in analog systems can employ on limited earth station

 QPSK-TDMA can accommodate large

number of earth station with small loss in transponder capacity.

Capacity Allocation

 FDMA

 FAMA-FDMA

 DAMA-FDMA

 TDMA

FDMA

 Satellite frequency is already broken into

bands, and is broken in to smaller

channels in Frequency Division Multiple Access (FDMA).

 Overall bandwidth within a frequency

FDMA (cont.)

 The number of sub-channels is limited by

three factors:

 Thermal noise (too weak a signal will be

effected by background noise).

 Intermodulation noise (too strong a signal

will cause noise).

 Crosstalk (cause by excessive frequency

FDMA (cont.)

 FDMA can be performed in two ways:

 Fixed-assignment multiple access (FAMA):

The sub-channel assignments are of a fixed allotment. Ideal for broadcast

satellite communication.

 Demand-assignment multiple access

(DAMA): The sub-channel allotment

TDMA

 TDMA (Time Division Multiple Access)

breaks a transmission into multiple time slots, each one dedicated to a different transmitter.

 TDMA is increasingly becoming more

widespread in satellite communication.

 TDMA uses the same techniques (FAMA

TDMA (cont.)

 Advantages of TDMA over FDMA.

 Digital equipment used in time division

multiplexing is increasingly becoming cheaper.

 There are advantages in digital

transmission techniques. Ex: error correction.

 Lack of intermodulation noise means

increased efficiency.

Efficiency Techniques

 Demand Assignment