Satellite Communications System

Maria Leonora Guico

Tcom 126 Lecture 13

Capacity Allocation

Multiplexing

Transponders

Applications

Capacity Allocation Strategies

Frequency division multiple access (FDMA)

Time division multiple access (TDMA)

Code division multiple access (CDMA)

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

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

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

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

Satellite

Satellite payload are several repeaters, known as transponders

mounted on platform called bus containing solar cells for

power, telemetry and control systems, and small jets to

slightly adjust orbit (station-keeping)

Transponder Types Signals are received at one freq, amplified and moved to

another frequency for retransmission

Transponder is basically a linear amplifier and frequency translator

Usable with analog and digital signals)

Transponder Block Diagram (C-band)

Regenerative Repeater

Used for purely digital signals

Demodulate signal to baseband, decode the digital

information, then remodulate the signal on a carrier with a

different frequency

Allows the satellite repeater to regenerate the signal while

removing accumulated noise and distortion

Frequency Allocation

Earth station

Baseband in

FDM or

PCM/TDM

Modulator

(FM, PSK or

QAM)

Mixer Band pass

filter

(BPF)

High Power

Amplifier

(HPA)

Generator

Up-Converter

Tel

Data Video

To

Satellite

transponder

AN EARTH STATION TRANSMITTER

Low noise

Amplifier

(LNA)

Demodulator

(FM, PSK

or QAM

Mixer Band pass

filter

(BPF)

Generator

Down-Converter

Baseband out

(FDM or

PCM/TDM) Tel

Data Video

From satellite

transponder

AN EARTH STATION RECEIVER

Earth station transmitter

Baseband in

FDM or

PCM/TDM

Modulator

(FM, PSK or

QAM)

Mixer Band pass

filter

(BPF)

High Power

Amplifier

(HPA)

Generator

Up-Converter

Tel

Data Video

To

Satellite

transponder

AN EARTH STATION TRANSMITTER

- Intermediate freq (IF) modulator converts the input baseband signals

to either an FM, a PSK or a QAM modulated intermediate frequency.

- The up converter converts the IF to an appropriate RF carrier freq.

- The High Power Amplifier (HPA) provides the adequate input

sensitivity and output power to propagate the signal to the satellite

transponder.

Earth station receiver

Low noise

Amplifier

(LNA)

Demodulator

(FM, PSK

or QAM

Mixer Band pass

filter

(BPF)

Generator

Down-Converter

Baseband out

(FDM or

PCM/TDM) Tel

Data

Video

From satellite

transponder AN EARTH STATION RECEIVER

- LNA which is highly sensitive and low-noise device amplifiers

the received signal.

- The RF to IF down-converter is a mixer and bans pass filter

combination, which converts the received RF signal to an

intermediate frequency (IF)

Transmission Modes

Beam switching – output from a transponder can be switched

to any one of several antennas (applicable to digital signals,

can use TDM)

Cross-links (operate at 58-62 GHz range)

Transmit signals directly between satellites

Allow signals to be transmitted to areas outside the coverage

area of one satellite, w/o using an extra earth station in

between

Application of satellite communication

Some of the application s of satellite communications are:

Digital audio broadcasting

Television distribution

Serving remote areas

Point-to-multipoint communications

Remote monitoring and control

Vehicle tracking

Mobile communications

Maritime and air navigation

Video teleconferencing

Applications of Geostationary

Satellites

Television and radio broadcasting

Telephony

Data transmission

Television and Radio Broadcasting

Carry network program feeds

Transmit pay-TV and other cable-only channels to CATV

company head-ends

Transmit remote news feeds

Radio broadcasts are carried, usually as subcarriers on

the same transponders

Typically uses analog FM modulation

Direct-to-home TV system transmits digital video ( 3 to

7.5 Mb/s data transmission rate)

Telephony via Satellite

Not ideal because of time delay

Used for remote areas where it’s too costly to install cable or

microwave relays

Important for ships at sea and interoceanic communications

where connection is via Inmarsat satellites

Uses FDMA-FM and SCPC

Data via Satellite Large data users can lease a transponder or a part of a transponder

for a point-to-point or point-to-multipoint link

Data network with a central hub communicating with many remote sites

Remote sites use very small aperture satellites (VSATs) which are low-cost installations with small antennas (1.2 to 2.4 m) and low-power transmitters

Hub uses relatively expensive earth station with large antenna (5 to 7 m) and powerful transmitter

Typical applications are inventory control, point-of-sale, and banking

Mobile Telephone Systems Using

Geostationary Satellites Global coverage achieved with only 3 GEO satellites

INMARSAT (International Maritime Satellite Organization) Original mandate was to provide voice and data service to ships

at sea; services have expanded to include land and aeronautical mobile communication

Refer to http://www.alphatelecom.ru/inmarsat/engindex.htm

for the different Inmarsat standards and corresponding services

INMARSAT Total of eleven GEO

satellites controlled from the HQ in London via ground stations located around the globe.

Each satellite has: hemispherical beam

and spot beams Maximum EIRP of 48

dBW in spot beams Power and BW

dynamically allocated L-band (1.5/1.6 GHz)

• Inmarsat mini-M (operate on land and coastal waters) has typical portable transceiver, including antenna the size of a laptop computer

INMARSAT/2

MSAT (Mobile Satellite) Joint Canadian and US project that uses one GEO satellite to

provide coverage for North and Central America, the Caribbean and Hawaii (via spot beam), and surrounding coastal waters

About 10 times more powerful than those used by Inmarsat

EIRP of at least 57.3 dBW in coverage area

Mobile terminals use reasonably compact roof-mounted antenna; portable terminals about the size of notebook computer with lid-mounted antenna

Only one ground station with an 11 m dish antenna

LEO Satellites

Preferred when satellites are used with portable or mobile equipment (lower transmitter power and antenna gain)

Requires many satellites (40 to 70) and a complex network

Main problems with LEOs are: 1. Position in space is not fixed wrt ground station 2. Tendency to disappear below the horizon – use a constellation of

satellites to address problem 3. Doppler effect (transmitted freqs are shifted higher as satellite

approaches a point on the ground and lower as the satellite recedes) – requires careful receiver design

LEO Satellites/2

Most complex and expensive wireless communication

systems

One system (Iridium) went bankrupt after going into service

As of April 2001, two LEO wireless telephony systems

(Iridium and Globalstar) were operating; both were in

financial difficulty

Iridium

66 Iridium LEO satellites provide 100% Global Coverage except

Hungary, Poland, N. Korea and N. Sri Lanka

Satellites are cross-linked

Uses digital modulation, FDMA/TDMA

Globalstar

Globalstar constellation consists of 48 LEO satellites, with an additional four satellites in orbit as spares, and operates at an altitude of 876 miles (1414 km) in space

CDMA; utilizes soft handoff techniques

Designed to provide high quality satellite-based services to a broad range of users, including: · Voice calling · Short Messaging Service (SMS) · Roaming · Positioning · Facsimile · Data transmission

http://www.alphatelecom.ru/globalstar/engindex.htm

Little LEOs

ORBCOMM

Operational in 1988; 35 LEO satellites as of Feb 2001; used for

SMS, email, and vehicle tracking

LEO One

Designed to use 48 satellites for paging and SMS

E-Sat

Designed to use 6 satellites using CDMA for remote meter

reading

MEOs

More satellites are needed than for GEO (on the order of 6

to 20 for real time communication) but fewer than for LEO

Delay and propagation loss lesser than for GEO

Main advantage over LEOs is financial

ICO

One MEO satellite launched in June 2001, referred to as "F2," which currently provides data gathering services for an agency of the U.S. government

Next-generation mobile satellite services (MSS) operator

Building a hybrid system to offer satellite and terrestrial wireless services throughout the US

ICO’s network is being designed to provide wireless voice, data, video, and/or Internet service throughout the United States on mobile and portable devices.

Ellipso Uses an interesting combination of elliptical and circular orbits.

Designed to initially include 6 satellites, later increasing to 10, in a circular orbit about 8000 km above the equator for worldwide coverage and to be complemented by 8 active satellites (plus two spares) in inclined elliptical orbits ( 520-7800 km)

CDMA technology; voice communication using portable and mobile terminals

Advantages/disadvantages of satellite

system Advantages of a satellite system include:

It can access to wide geographical area

Wide bandwidth

High reliability

Distance sensitive cost

Independent of terrestrial infrastructure

Disadvantages of satellite system

High initial cost

It has propagation delay

System performance

Transponder

Gain, Gsat

HPA LNA

Uplink Downlink

Gr Gt

Lp Lp

Pr

Gr Gt

Lf

Pt

Po Pin

Earth station transmitter Earth station receiver

HPA – high power amplifier

Po - HPA output power

Lf - feeder loss

Gt - transmit antenna gain

Lp - path loss

Gr - receive antenna gain

LNA – low noise amplifier

Pt - total radiated power, Pt = Po - Lf

EIRP - Effective Isotropic Radiated Power

EIRP = Pt * Gt

Link-Power Budget Formula

Link-power budget calculations take into account all the gains and losses from the transmitter, through the medium to the receiver in a telecommunication system. Also taken into the account are the attenuation of the transmitted signal due to propagation and the loss or gain due to the antenna.

The decibel equation for the received power is: [PR] = [EIRP] + [GR] - [LOSSES]

Where:

[PR] = received power in dBW

[EIRP] = equivalent isotropic radiated power in dBW

[GR] = receiver antenna gain in dB

[LOSSES] = total link loss in dB

Link-Power Budget Formula Variables

Link-Power Budget Formula for the received power [PR]:

[PR] = [EIRP] + [GR] - [LOSSES]

The equivalent isotropic radiated power [EIRP] is:

[EIRP] = [PS] + [G] dBW, where:

[PS] is the transmit power in dBW and [G] is the transmitting antenna gain in dB.

[GR] is the receiver antenna gain in dB.

[LOSSES] = [FSL] + [RFL] + [AML] + [AA] + [PL], where:

[FSL] = free-space spreading loss in dB = PT/PR (in watts)

[RFL] = receiver feeder loss in dB

[AML] = antenna misalignment loss in dB

[AA] = atmospheric absorption loss in dB

[PL] = polarisation mismatch loss in dB

The major source of loss in any ground-satellite link is the free-space spreading loss.