MODULE 2Satellite Access Methods

AJAL.A.J Assistant Professor –Dept of ECE,

Federal Institute of Science And Technology (FISAT) TM  MAIL: ec2reach@gmail.com

Audio Spectrum

Noise floor

Peak power

Analog Signaling

Digital Signaling

(Coder-Decoder)

Example - PCM

Reasons for Choosing Data and Signal Combinations

• Digital data, digital signal– Equipment for encoding is less expensive than digital-

to-analog equipment

• Analog data, digital signal– Conversion permits use of modern digital transmission,

computational resources and switching equipment

• Digital data, analog signal– Transmission media will only propagate analog signals– Examples include optical fiber and POTS (3 kHz

bandwidth limited)

• Analog data, analog signal– Analog data easily converted to an analog signal via

some form of modulation (AM, FM, etc.)

Unguided Media• Transmission and reception are achieved by

means of an antenna (rcvr + xmtr)

• Configurations for wireless transmission– Directional (infers gain)– Omnidirectional – Polarization (vertical, horizontal, circular)

A Simplified Wireless Communications System – Unguided Media

Information received

(Voice/Data)

Information to be

transmitted (Voice/Data)

Coding Modulator Transmitter

Decoding Demodulator Receiver

Antenna

AntennaCarrier

Carrier

Modulation Terms adding data to a radio frequency signal

Baseband – modulation techniques that do not use a sinusoidal carrier but encodes information directly as the amplitude, width of position of a pulse. PAM – pulse amplitude modulation PWM – pulse width modulation

Bandpass – modulation techniques that encode information as the amplitude, frequency or phase of a sinusoidal carrier. FSK – frequency shift keying, PSK – phase shift keying, AM, FM

Electromagnetic Spectrum

Characteristics of some Frequencies

• Microwave frequency range– 1 GHz to 40 GHz– Directional beams possible (small)– Suitable for point-to-point transmission– Used for satellite communications

• VHF/UHF Radio frequency range– 30 MHz to 1 GHz (no atmospheric propagation, LOS)– Suitable for omnidirectional applications

• Infrared frequency range– Roughly 3x1011 to 2x1014 Hz– Useful in local point-to-point multipoint applications within

confined areas

Terrestrial Microwave• Description of common microwave antenna

– Parabolic "dish", 3 m in diameter

– Fixed rigidly which focuses a narrow beam

– Achieves a line-of-sight (LOS) transmission path to the receiving antenna

– Located at substantial heights above ground level

• Applications– Long haul telecommunications service (many repeaters)

– Short point-to-point links between buildings

Satellite Microwave

• Description of communication satellite– Microwave relay station– Used to link two or more ground-based microwave

transmitter/receivers– Receives transmissions on one frequency band (uplink),

amplifies or repeats the signal and transmits it on another frequency (downlink)

• Applications– Television distribution (e.g., Direct TV)– Long-distance telephone transmission– Private business networks

Broadcast Radio

• Description of broadcast radio antennas– Omnidirectional (HF-vertical polarization, VHF/UHF-

horizontal polarization)– Antennas not required to be dish-shaped– Antennas need not be rigidly mounted to a precise

alignment

• Applications– Broadcast radio

• VHF and part of the UHF band; 30 MHz to 1GHz• Covers FM radio and UHF and VHF television• Below 30 MHz transmission (AM radio) is subjected to

propagation effects so not reliable for point-to-point communications (MUF or max usable freq)

Network Architectures and Protocols

Systematic Signaling Steps for Information Exchange

Open Systems Interconnections (OSI) Transmission Control Protocol (TCP) Internet Protocol (IP)

Internet Protocol Version 4 (IPv4) Internet Protocol Version 6 (IPv6) – essentially

larger MAC addressing space for the influx of IP based devices

Mobile IP

Ad Hoc Network (peer to peer)

Versus an infrastructure network (centralized) with its AP(Access Points) which is your WiFi/Hotspot/typical wireless network normally used to access the Internet.

Multiplexing• Capacity of transmission medium usually

exceeds capacity required for transmission of a single signal

• Multiplexing - carrying multiple signals on a single medium– More efficient use of transmission medium

Multiplexing

Reasons for Widespread Use of Multiplexing

• Cost per kbps of transmission facility declines with an increase in the data rate (economy of scale)

• Effective cost of transmission and receiving equipment declines with increased data rate(cost per bit)

• Most individual data communication devices with their associated applications require relatively modest data rate support

Multiplexing Techniques• Frequency-division multiplexing (FDM)

– Takes advantage of the fact that the useful bandwidth of the medium exceeds the required bandwidth of a given signal

– Requires guard bands

• Time-division multiplexing (TDM)– Takes advantage of the fact that the achievable bit rate of the

medium exceeds the required data rate of a digital signal– Requires accurate clock

• Code-division multiple access(CDMA)– Use of orthogonal codes to separate users who are all using

the same band of frequencies

Frequency-division Multiplexing

FDMA Channel Allocation

Frequency 1 User 1

Frequency 2 User 2

Base Station

Frequency n User n

… …

Mobile Stations

Time-division Multiplexing

TDMA Frame Illustration for Multiple Users

Time 1

Time 2

Time n

……

Base Station

User 1

User 2

User n

…

Mobile Stations

CDMA (Code Division Multiple Access)

Time

Frequency

Use

r n

Code

Use

r 1

Use

r 2

...

25

Transmitted and Received Signals in a CDMA System

Information bits

Code at transmitting end

Transmitted signal

Received signal

Code at receiving end

Decoded signal at the receiver

26

OFDM (Orthogonal Frequency Division Multiplexing)

Conventional multicarrier modulation used in FDMA

Orthogonal multicarrier modulation used in OFDM (normally a single user)

Frequency

Frequency

Satellite Microwave Transmission

• a microwave relay station in space

• can relay signals over long distances

• geostationary satellites – remain above the equator at a height of

22,300 miles (geosynchronous orbit)– travel around the earth in exactly the time the

earth takes to rotate

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 the downlink makes it attractive for services such as the distribution of television programming

dish dish

uplink station downlink station

satellitetransponder

22,300 miles

Satellite Transmission Process

Satellite Transmission Applications

• television distribution– a network provides programming from a

central location– direct broadcast satellite (DBS)

• long-distance telephone transmission– high-usage international trunks

• private business networks

Principal Satellite Transmission Bands

• C band: 4(downlink) - 6(uplink) GHz– the first to be designated

• Ku band: 12(downlink) -14(uplink) GHz– rain interference is the major problem

• Ka band: 19(downlink) - 29(uplink) GHz– equipment needed to use the band is still very

expensive

Fiber vs Satellite

Satellite-Related Terms

• Earth Stations – antenna systems on or near earth

• Uplink – transmission from an earth station to a satellite

• Downlink – transmission from a satellite to an earth station

• Transponder – electronics in the satellite that convert uplink signals to downlink signals

Ways to CategorizeCommunications Satellites

• Coverage area– Global, regional, national

• Service type– Fixed service satellite (FSS)– Broadcast service satellite (BSS)– Mobile service satellite (MSS)

• General usage– Commercial, military, amateur, experimental

Classification of Satellite Orbits

• Circular or elliptical orbit– Circular with center at earth’s center – Elliptical with one foci at earth’s center

• Orbit around earth in different planes– Equatorial orbit above earth’s equator– Polar orbit passes over both poles– Other orbits referred to as inclined orbits

• Altitude of satellites– Geostationary orbit (GEO)– Medium earth orbit (MEO)– Low earth orbit (LEO)

Geometry Terms

• Elevation angle - the angle from the horizontal to the point on the center of the main beam of the antenna when the antenna is pointed directly at the satellite

• Minimum elevation angle

• Coverage angle - the measure of the portion of the earth's surface visible to the satellite

Minimum Elevation Angle

• Reasons affecting minimum elevation angle of earth station’s antenna (>0o)– Buildings, trees, and other terrestrial objects block

the line of sight– Atmospheric attenuation is greater at low elevation

angles– Electrical noise generated by the earth's heat near

its surface adversely affects reception

GEO Orbit

• Advantages of the the GEO orbit – No problem with frequency changes– Tracking of the satellite is simplified– High coverage area

• Disadvantages of the GEO orbit– Weak signal after traveling over 35,000 km– Polar regions are poorly served– Signal sending delay is substantial

GEO : Geosynchronous equatorial orbit

LEO Satellite Characteristics

• Circular/slightly elliptical orbit under 2000 km• Orbit period ranges from 1.5 to 2 hours• Diameter of coverage is about 8000 km• Round-trip signal propagation delay less than 20 ms• Maximum satellite visible time up to 20 min• System must cope with large Doppler shifts• Atmospheric drag results in orbital deterioration

LEO : Low earth orbit

LEO Categories

• Little LEOs– Frequencies below 1 GHz – 5MHz of bandwidth – Data rates up to 10 kbps– Aimed at paging, tracking, and low-rate messaging

• Big LEOs– Frequencies above 1 GHz – Support data rates up to a few megabits per sec– Offer same services as little LEOs in addition to voice and

positioning services

MEO Satellite Characteristics

• Circular orbit at an altitude in the range of 5000 to 12,000 km

• Orbit period of 6 hours• Diameter of coverage is 10,000 to 15,000 km• Round trip signal propagation delay less than 50 ms• Maximum satellite visible time is a few hours

MEO : Medium Earth Orbit

Satellite Systems

GEO

M EO

LEO

GEO (22,300 mi., equatorial) high bandwidth, power,

latency

MEO high bandwidth, power,

latency

LEO (400 mi.) low power, latency

more satellites

small footprint

V-SAT (Very Small Aperture

Terminal)

private WAN

Geostationary Orbit

GPS Satellite Constellation

• Global Positioning System• Operated by USAF• 28 satellites• 6 orbital planes at a height of 20,200 km• Positioned so a minimum of 5 satellites are visible at all times• Receiver measures distance to satellite

USAF - United States Air Force

Frequency Bands Available for Satellite Communications

Satellite Link Performance Factors

• Distance between earth station antenna and satellite antenna

• For downlink, terrestrial distance between earth station antenna and “aim point” of satellite– Displayed as a satellite footprint (Figure 9.6)

• Atmospheric attenuation– Affected by oxygen, water, angle of elevation, and higher

frequencies

Satellite Footprint

Satellite Network Configurations

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 using CSC

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

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