EE4N L01-02 CCC Overview

EE4NEE4N

Satellite and Cellular RadioSatellite and Cellular Radio

Dr Costas Constantinou

Mobile communicationMobile communication

• Two aspects of mobility:

– user mobility: users communicate (wireless) “anytime, anywhere, with anyone”

– device portability: devices can be connected anytime, anywhere to “the” network

• Wireless vs. mobile Examples stationary computer notebook in a hotel wireless LANs in historic buildings Personal Digital Assistant (PDA)

• The demand for mobile communication creates the need for integration of wireless networks into existing fixed networks:

– local area networks: standardization of IEEE 802.11

– internet: Mobile IP extension of the internet protocol IP

– wide area networks: e.g., internetworking of GSM and ISDN, VoIP over WLAN and POTS

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Mobile devicesMobile devices

performanceperformance

Pager

• receive only

• tiny displays

• simple text

messages

Mobile phones

• voice, data

• simple graphical displays

PDA

• graphical displays

• character recognition

• simplified WWW

Smartphone

• tiny keyboard

• simple versions

of standard applications

Laptop/Notebook

• fully functional

• standard applications

Sensors,

embedded

controllers

www.scatterweb.net

No clear separation between device types possible

(e.g. smart phones, embedded PCs, …)3

Historical overview of wireless Historical overview of wireless

systemssystemscellular phones satellites

wireless LANcordless

phones

1992:

GSM

1994:

DCS 1800

2001:

UMTS/3G

1987:

CT1+

1982:

Inmarsat-A

1992:

Inmarsat-B

Inmarsat-M

1998:

Iridium

1989:

CT 2

1991:

DECT 199x:

proprietary

1997:

IEEE 802.11

1999:

802.11b, Bluetooth

1988:

Inmarsat-C

analog

digital

1991:

D-AMPS

1991:

CDMA

1981:

NMT 450

1986:

NMT 900

1980:

CT0

1984:

CT1

1983:

AMPS

1993:

PDC

4G – fourth generation: when and how?

… rather an incremental deployment!

2000:

GPRS2000:

IEEE 802.11a

200?:

Fourth Generation

(Internet based)

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Cellular radioCellular radio

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0102030405060708090

source: www.gsmworld.com

Total number of cellular subscribers in the world (estimated January 2010): 4,650,000,000

Cellular radio system overviewCellular radio system overview

• Focus on global system for mobile communications (GSM) public land mobile network (PLMN)

• Extension to public switched telephone network (PSTN) –both use signalling system 7 (SS7)

• GSM can be thought of as the “wireless local loop” of PSTN, but with two major differences:– Wide area mobility of subscribers needs to be managed

(mobility management)

– The radio link between a subscriber terminal is not permanent and is highly unreliable (radio resource management)

• Radio propagation issues:– signal dependence is close to d–4, not d–2

– many users lead to interference limited operation

• Spectrum scarcity– not all frequencies are suitable; too low a frequency makes antennas too big and

thus handsets not mobile; too high a frequency makes path losses too severe

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Spectrum and cellular coverageSpectrum and cellular coverage

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User 1

User 2

User n

…

Time

Frequency

Frequency division multiple

access (FDMA)

User

1

User

2

User

n

…

Time

Frequency

Time division multiple

access (TDMA)U

se

r 1

Time

Frequency

User

2

User

n

Code

...

Code division multiple

access (CDMA)

Space division multiple access

BS

Service area

(Zone)

BS

Service area

(Zone)

User 2

User 1

The cellular conceptThe cellular concept

• Limited frequencies

• Low power transmitters

• Solution:– Spatial reuse of frequencies

– Use FDMA (1G), TDMA (2G), or CDMA (3G) to separate users in each cell

• FDMA: simple to implement, but wastes system capacity due to need to employ guard bands between channels

• TDMA: efficient, but requires precise timing to implement

• CDMA: very efficient, but requires complex decoding of multiplexed signals, and accurate power control of interfering transmissions

– Frequency reuse patterns in cells• Cell tessellation – idealised cells are hexagonal (in reality they

are irregular)

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The cellular concept (cont.)The cellular concept (cont.)

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1

3

2

1

3

2

1

3

2

1

3

2

1

3

2

1

3

2

1

3

2

1

3

2

Cluster of cells

Number of cells in a cluster is:

c = i2 + ij +j2

Here c = 3 (i = 1 & j = 1)

Repeating pattern of the

basic group of cells4

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1

2

3

6

5

A 7-cluster cell

(i = 1 & j = 2)

Frequency reuse distance, Ru: the closest distance

between the centres of two cells using the same

frequency (in different clusters)

Show that, Ru = (3c)½R, where R is the radius of the

circumscribed circle to (or in this case the length of

the side of) a hexagonal cell

The cellular concept (cont.)The cellular concept (cont.)

• Reality is much more complex– Sector antennas

– Different size cells• Macrocells (radius between

1 km and 30 km)

• Microcells (radius between 100 m and 1 km)

• Picocells (radius between 5m and 100 m)

• Femtocells (cover one room only)

– Matching number of channels in each cell to traffic demand (especially for motoways)

• Cell splitting

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The GSM system (overview)The GSM system (overview)

• Architecture of the GSM system

• GSM is a PLMN (Public Land Mobile Network)– several providers setup mobile networks following the GSM

standard within each country

– subsystems• RSS (radio subsystem): covers all radio aspects

• NSS (network and switching subsystem): call forwarding, handover, switching

• OSS (operation subsystem): management of the network

– components• MS (mobile station)

• BS (base station)

• MSC (mobile switching center)

• LR (location register)

• …

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The ingredients of GSMThe ingredients of GSM

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Mobile phones, PDAs, etc: The visible

but smallest part of the network

?rm=show_details&modell=1380

?rm=show_details&modell=1255


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Antennas: Still visible – cause many public controversies


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Base Station

infrastructure (you

may see this if you

look hard)

Cabling

Microwave

point-to-point

links


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Switching units

Databases

Management

Monitoring

Not visible, but comprise the

major part of the network

(also from an investment

point of view)

GSM: overviewGSM: overview

fixed network

BSC

BSC

MSC MSC

GMSC

OMC, EIR,

AUC

VLR

HLR

NSS

with OSS

RSS

VLR

BTS

BTS

BTSBTS

BTS

MS

MS

MS

Satellite communication Satellite communication

applicationsapplications

• Traditionally

– weather satellites

– radio and TV broadcast satellites

– military satellites

– satellites for navigation and localization (e.g., GPS)

• Telecommunication

– global telephone connections*

– backbone for global networks*

– connections for communication in remote places or

underdeveloped areas

– global mobile communication

* Largely superseded by long-haul fibre optic networks

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Satellite communicationsSatellite communications

• Advantages

– Large coverage

capability, 3 x GEO

satellite give whole

earth coverage

– Cost insensitive to

distance on earth’s

surface

– Broadcast capability,

allows monopoly

supplier

• Disadvantages

– High initial cost

– Hard to maintain,

spacecraft can repair

but at high cost

– Hard to put into orbit

– Hard to compete

against fibre/wireless

revolution, allows free

market in supply

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Earth station

Uplink (UL) & Downlink (DL)

footprint

PSTN for

country x

Control station (telemetry, tracking

& command (TT&C))

Earth station

PSTN for

country x

Uplink (UL) & Downlink (DL)

Space

segment

Ground

segment

Satellite global mobile Satellite global mobile

communicationscommunications

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base station

or gateway

Inter Satellite Link

(ISL)Mobile User

Link (MUL) Gateway Link

(GWL)

footprint

small cells

(spotbeams)

User data

PSTNISDN GSM

GWL

MUL


• Satellite systems basic terminology:– 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

• typically separated frequencies for uplink and downlink

• transponder used for sending/receiving and shifting of frequencies– transparent transponder: only shift of frequencies

– regenerative transponder: additionally signal regeneration

– The satellite communications jargon is enormous – see European Space Agency web site http://gaia.esac.esa.int/gpdb/glossary.txt for a flavour

• Satellite systems categorisation:– Coverage area

• Global, regional, national

– Service type• Fixed service satellite (FSS)

• Broadcast service satellite (BSS)

• Mobile service satellite (MSS)

– General usage• Commercial, military, experimental

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http://gaia.esac.esa.int/gpdb/glossary.txt

Satellite systemsSatellite systems

• Frequency bands used: VHF, L band, S band, C band,

Ku band, Ka band

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Satellite systems (cont.)Satellite systems (cont.)

• The satellite communication payload is typically a

transponder – a schematic of a transparent transponder

is shown here:

– Noise-limited operation

– Problem with FDMA: inter-modulation products

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LNA D/C14GHz LNA

1GHzF

requency D

MU

X

Fre

quency M

UX

HPAU/CEqualiser

1GHz 11GHz

11GHz

Satellite orbitsSatellite orbits

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

– Elliptical with one of its 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

• Four different types of satellite orbits can be identified depending on the shape and diameter of the orbit:– GEO: geostationary orbit, ca. 36000 km above earth surface

– LEO (Low Earth Orbit): ca. 500 - 1500 km

– MEO (Medium Earth Orbit) or ICO (Intermediate Circular Orbit): ca. 6000 - 20000 km

– HEO (Highly Elliptical Orbit) elliptical orbits

– N.B. The earth’s atmosphere stops at around 30 km

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Satellite orbits (cont.)Satellite orbits (cont.)

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earth

km

35768

10000

1000

LEO

(Globalstar,

Iridium)

HEO

inner and outer Van

Allen belts

MEO (ICO)

GEO (Inmarsat)

Van-Allen Belts:

ionized particles

200 – 1000 km and

23000 – 30000 km

above earth surface

and over the equator


• Satellites in circular orbits– Gravitational attractive force Fg = mg(R/r)²

– Centripetal force Fc = mr²• m: mass of the satellite

• R: radius of the earth (R = 6371 km)

• r: distance of satellite to the center of the earth

• g: acceleration of gravity (g = 9.81 m/s²)

• : angular velocity ( = 2f, f : rotation frequency)

– Stable orbit• Fg = Fc

– Geosynchronous orbit• f = 1/T; T = 24 hours, gives, h = r – R = 35786 km

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32

2

)2( f

gRr


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inclination d

d

satellite orbit

perigee

(closest to earth)

plane of satellite orbit

equatorial plane

apogee

(furthest from earth)


minimum elevation angle:

reasons why e > 0 at earth station antenna:

• 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 reception28

Elevation:

angle e between center of satellite beam

and surface

e


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Example: satellite systems at 4-6 GHz

elevation of the satellite

5° 10° 20° 30° 40° 50°

Attenuation of

the signal in %

10

20

30

40

50

rain absorption

fog absorption

atmospheric

absorption

e


• Geosynchronous orbit:– Circular orbits around the earth having a period of 24 hours

• Geostationary orbit (GEO):– A geosynchronous orbit with 0 degree inclination

– Situated in equatorial plane & complete rotation exactly one day –satellite is synchronous to earth rotation

– Advantages of GEO orbit • Tracking of the satellite is simplified

• High coverage area (large footprint)

– Disadvantages of GEO orbit• Bad elevations in areas with latitude above 60° due to fixed position above the

equator; polar regions are poorly served

• Weak signal after traveling over 35,000 km; high transmit powers required

• Signal sending delay is substantial; of order 250 ms

• Frequency reuse difficult

• Not useful for global coverage for small mobile phones and data transmission, typically used for radio and TV transmission

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• Polar orbits (PO)– Orbits with an inclination of 90 degrees.

– Polar orbits are useful for satellites that carry out mapping and/or surveillance operations because as the planet rotates the spacecraft has access to virtually every point on the earth’s surface

• Circular inclined orbits– Orbits with varying inclination angle.

– If they are low or medium earth orbits they will have a period of a few hours. This means that they move around the earth as it rotates to overfly large areas of the earth. Clusters of them are used for earth coverage constellations such as GPS navigation (in MEO) and mobile communications (in LEO)

• Molniya orbits– Highly eccentric earth orbits with periods of approximately 12 hours

– The orbital inclination is chosen so the rate of perigee precession is zero, thus both apogee and perigee can be maintained over fixed latitudes. This condition occurs at inclinations of 63.4 degrees and 116.6 degrees. For these orbits the perigee is typically placed in the southern hemisphere, so the satellite remains above the northern hemisphere near apogee for approximately 11 hours per orbit. This orientation can provide good ground coverage at high northern latitudes

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EE4N L01-02 CCC Overview

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