Wireless & Mobile Networks - 6. Satellite Communications

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Satellite Communications

Agenda

o Satellite Communication System

o Satellite Parameters and Configurations

• Satellite Orbits

• Frequency Bands

• Transmission Impairments

• Satellite Network Configurations

o Capacity Allocation

o Global Positioning System (GPS)

1Wireless and Mobile Networks © Imad J. Eid

In a satellite communications system, two or more stations on or near the

earth (earth stations) communicate via one or more satellites that serve

as relay stations in space.

A transmission from an earth station to the satellite is referred to as

uplink, whereas transmissions from the satellite to the earth station are

downlink.

The component in the satellite that takes an uplink signal and converts it

to a downlink signal is called a transponder.

Wireless and Mobile Networks 2

Satellite Communication System

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Ways of categorizing communications satellites:

o Coverage area

• Global, regional, national

o Service type

• Fixed service satellite (FSS)

• Broadcast service satellite (BSS)

• Mobile service satellite (MSS)

o General usage

• Commercial, military, amateur, experimental



Differences between satellite-based and terrestrial wireless communications

o The area of coverage of a satellite system far exceeds that of a terrestrial

system.

• A geostationary satellite antenna is visible to about one-fourth of the earth's

surface.

o Spacecraft power and allocated bandwidth are limited resources that call for

careful tradeoffs in earth station/satellite design parameters.

o Conditions between communicating satellites are more time invariant.

o Broadcast, multicast, and point-to-point applications are readily

accommodated.

o For a geostationary satellite, there is an earth-satellite-earth propagation

delay of about one-fourth of a second.



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Circular or elliptical orbit

o Circular - with center at the earth’s center

o Elliptical - with one foci at the earth’s center

Orbit around earth in different planes

o Equatorial orbit - above earth’s equator

o Polar orbit - passes over both poles

o Other orbits referred to as inclined orbits

Altitude of satellites

o Geostationary Orbit (GEO)

o Medium Earth Orbit (MEO)

o Low Earth Orbit (LEO)


Satellite Orbits

Elevation Angle (θ)The angle from the horizontal (a line tangent to the surface of the

earth at the antenna location) to the center of the main beam of

the antenna when the antenna is pointed directly at the

satellite

Elevation angle=0 Maximum Satellite coverage

Elevation angle must be >0o, because:

o Buildings, trees, and other terrestrial objects block the line

of sight

o Atmospheric attenuation is greater at low elevation angles

o Electrical noise generated by the earth's heat near its

surface adversely affects reception

Current design practice:

o For downlink: use elevation angle from 5° to 20°depending on frequency.

o For uplink: recommended elevation angle is 5°.


Geometry Terms

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Coverage Angle ( β)o A measure of the portion of the earth's surface visible to the satellite.

o Defines a circle on the earth's surface centered on the point directly below the satellite

The coverage of a satellite is typically expressed as the diameter of the

area covered which is:

2 βR

Where, R = earth's radius, 6370 km

The round-trip delay

is in the range:

Where, c = speed of light


Geometry Terms

c

dt

c

h max22

Satellite parameters as a function of orbit height


Satellite Orbits

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Proposed by the science fiction author Arthur C. Clarke, in a 1945

article in Wireless World

If a satellite is in a circular orbit, at 35,863 km above the earth's

surface, and in the equatorial plane, it will rotate at exactly the same

angular speed as the earth and will remain above the same spot on the

equator as the earth rotates.

Advantages of the GEO orbit

o No frequency change problems

• No Doppler effect

• As the satellite is stationary relative to earth

o Tracking of the satellite is simplified

o Wide coverage area

• Communicate with roughly a fourth of the earth

• Three satellites separated by 120o cover most of the inhabited portions of the entire

earth excluding only the areas near the north and south poles.


Geostationary Satellites

Disadvantages of the GEO orbit:

o Weak signal after traveling over 35,000 km

o Polar regions are poorly served

o Propagation delay is substantial

o Waste of spectrum for

point-to-point applications

The delay of communication between two locations on earth directly under

the satellite is: (2 X 35,863)/300,000 =~ 0.24 s.


GEO Orbit

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Circular/slightly elliptical orbit at altitude in the range of 500 to 1500 km

Often in polar orbit.

Orbit period ranges from 1.5 to 2 hours

Diameter of coverage is about 6000 km

Round-trip signal propagation delay is less than 20 ms

Satellite visibility time is up to 20 min

System must cope with large Doppler shifts

Practical use of this system requires the multiple

orbital planes be used, each with multiple

satellites in orbit

Communication between two earth stations

typically involve handing off the signal

from one satellite to another.


LEO Satellite

LEO vs. GEO Satellites:

o Reduced propagation delay

o A received LEO signal is much stronger than a GEO signal of the same

transmitted power.

o LEO coverage can be better localized so that spectrum can be better

conserved.

LEO is proposed for communicating with mobile terminals

LEO Categories

o Little LEOs

• Frequencies below 1 GHz

• Bandwidth is < 5MHz

• Data rates up to 10 kbps

• Aimed at tracking, monitoring, and low-rate messaging


LEO Satellite

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o Example on Little LEOs

• Orbcomm:

Used by businesses to track trailers, railcars, heavy equipment, and other remote

and mobile assets.

Used also to monitor remote utility meters and oil and gas storage tanks, wells,

and pipelines.

o 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

• E.g.: Globalstar

Uses CDMA for mobile users

48 satellites and 8 spares


LEO Satellite

Circular orbit at an altitude in the range of 8,000 to 18,000 km

Orbit period ranges from 5 to10 hours

Diameter of coverage is 12,000 to 15,000 km

Round trip signal propagation delay is typically less than 150 ms

Satellite visibility time is a few hours (2- 8 hours)

Fewer handoffs than LEOs

Used for digital voice, data, positioning,

and messaging


MEO Satellite

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More bandwidth is available in the higher-frequency bands

The higher the frequency, the greater the effect of transmission

impairments.

The uplink band is always of higher frequency. Why?


Frequency Bands for Satellite Communications

The performance of a satellite link depends on three factors:

o Distance between earth station antenna and satellite antenna

o For downlink, terrestrial distance between earth station antenna and

the “aim point” of the satellite.

o Atmospheric attenuation


Transmission Impairments

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Free Space Loss:

o Proportional to distance2

o Proportional to frequency2


Distance and Free Space Loss

2

2

2

244

c

fdd

P

P

r

t

The center point of coverage area (aim point) will receive the highest

radiated power, and the power drops off as the earth-station moves

away from the center point in any direction.


Satellite Footprint

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The primary causes of atmospheric attenuation are oxygen, which is of

course always present, and water (fog and rain)

The smaller the elevation angle, the more of the atmosphere that the

signal must travel through.

The higher the frequency, the greater

the atmospheric attenuation


Atmospheric Attenuation

Point-to-Point Link: between two distant ground-based antennas.

Broadcast Link: communications between one ground-based transmitter

and a number of ground-based receivers.


Satellite Communication Configurations

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VSAT Configuration: two-way communication among earth stations, with

one central hub and many remote stations.

VSAT (very small aperture terminal) system.

o A number of subscriber stations are equipped with low-cost VSAT antennas


Satellite Communication Configurations

Typically, a single satellite will handle a large bandwidth (e.g., 500 MHz)

Divide it into a number of channels of smaller bandwidth (e.g., 40 MHz)

o The entire channel may be dedicated to a single user or application (such as TV

broadcasting or a single 50-Mbps digital data stream)

o The channel may be shared by many users (channel allocation technique is

needed)

Channel Allocation Strategies

o Frequency Division Multiple Access (FDMA)

o Time Division Multiple Access (TDMA)

o Code Division Multiple Access (CDMA)


Capacity Allocation

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Global Positioning System

A space-based satellite navigation system that provides location and

time information in all weather conditions, anywhere on or near the Earth

where there is an unobstructed line of sight to four or more GPS

satellites.

Used by military, civil and commercial users around the world.

GPS was created and realized by the U.S. Department of defense (DoD)

It is maintained by the U.S. government and is freely accessible to

anyone with a GPS receiver.

6 planes each of at least 4 satellites.

It became fully operational in 1995.

MEO orbit, altitude ~= 20,200 km


The system is known as NAVSTAR: Navigation System with Timing And

Ranging

Other Systems:

o The Russian Global Navigation Satellite System (GLONASS)

o European Union Galileo positioning system

o Chinese Compass navigation system

o Indian Regional Navigational Satellite System

All satellites broadcast at the same two frequencies:

o L1 signal at 1.57542 GHz (C/A Code - Coarse/Acquisition Code available for civilian use)

o L2 signal at 1.2276 GHz (P Code - Precise Code used by the military )

The satellite network uses a BPSK-DSSS-CDMA technique

o Using PN-sequences (Gold Sequences) that is different for each satellite.



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Objectives:

o Time, Position (3D, includes altitude), and Velocity.

The satellites simultaneously broadcast beacon messages (called

navigation messages) contain the satellite’s time and location information.

A GPS receiver measures time of arrival to the satellites, and then uses

“triangulation” to determine its position

By knowing how far one is from three satellites one can ideally find their

3D coordinates

To correct for clock errors one needs to receive four satellites




Coordinates

Expressed in terms of Latitude (north-south position, reference is Equator) and Longitude (west-east, reference is the Prime Meridian)

Three common formats:

Example: Latitude : 31°57'41.46"N or: 31° 57.691'N or: 31.961517°

Longitude : 35°11'4.32"E or: 35° 11.072'E or: 35.184533°

Elevation : 785 m

Symbols

° Degrees

' Minutes

" Seconds

Coordinates Formats

DDD° MM' SS.S" Degrees, Minutes and Seconds

DDD° MM.MMM' Degrees and Decimal Minutes

DDD.DDDDD° Decimal Degrees

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Wireless & Mobile Networks - 6. Satellite Communications