Satellite Systems

History Basics Localization

Handover Routing Systems

•Global Coverage without wiring costs

•Independent of population density

•Chiefly for broadcast TV

•Useful addition to exisiting services – e.g. with UMTS

History of satellite communication

1945 - Arthur C. Clarke “Extra Terrestrial Relays“1957 - first satellite USSR’s SPUTNIK1960- first reflecting communication satellite ECHO1962 – Telstar launched, an important step1963 - first geostationary satellite SYNCOM1965 - first commercial geostationary satellite “Early

Bird” (INTELSAT I 68 kg): 240 duplex telephone channels or 1 TV channel, 1.5 years lifetime

1967-69 – Intelsat II, III; 1200 phone channels 1976 - three MARISAT satellites for maritime

communication; 40W power, 1.2 m antenna

History (Contd)

1982 first mobile satellite telephone system INMARSAT-A

1988 first satellite system for mobile phones and data communication INMARSAT-C; 600 bps, interface to X.25

1993 INMARSAT-M - first digital satellite telephone system; still very heavy equipment

1998 global satellite systems for small mobile phones – Iridium & Globalstar

Currently about 200 geo satellites.

Applications

Traditional Weather, radio and TV broadcastmilitary satellites – espionage, warning systemnavigation and localization (GPS)

Telecommunication – ‘cable in the sky’global telephone connections & mobilesbackbone for global networksremote/rural areasextend cellular systems (AMPS, GSM UMTS),

need low orbit satellites.

Satellite Functions

Transponder Receive on one frequency, repeat on

another frequency (transparent transponder)

May amplify or regenerate (regenerative transponder)

Inter satellite routing Error correction is essential

base stationor gateway

Classical satellite systems

Inter Satellite Link (ISL)

Mobile User Link (MUL) Gateway Link

(GWL)

footprint

small cells (spotbeams)

User data

PSTNISDN GSM

GWL

MUL

PSTN: Public Switched Telephone Network

Satellite NetworksSatellite Networks

SATELLITE RECEPTION

Footprint – area on earth’s surface where signal can be received

LOS (Line of Sight) to the satellite necessary for connection

Attenuation depends on distance, elevation, frequency of carrier and atmosphere

High elevation means less absorption due to rain, fog, atmosphere and buildings; at least 10 degrees needed.

Signal Loss Calculation (qualitative only)

Attenuation or power loss is determined by

gain of sending/receiving antennae

distance between sender and receiver

Carrier frequency This affects data rates

achievableOnly 10 bps may be achievablewith GEOs, compared to 10Kbps at 100 km, 2GHz carrier

24

c

frL

L: Lossf: carrier frequencyr: distancec: speed of light

Atmospheric attenuationExample: 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

Satellites - features

GEO: geostationary, ~ 36000 km from the earth

LEO (Low Earth Orbit): 500 - 1500 kmMEO (Medium Earth Orbit) or ICO

(Intermediate Circular Orbit): 6000 - 20000 kmHEO (Highly Elliptical Orbit) elliptical orbitsMicrowave, line of sight; GHz rangeUplink and downlink – different frequencies

Satellite orbit altitudes

Orbits II

earth

km

35768

10000

1000

LEO (Globalstar,

Irdium)

HEO

inner and outer VanAllen belts

MEO (ICO)

GEO (Inmarsat)

Inner & outer Van-Allen-Belts: ionized particles2000 - 6000 km, 15000 - 30000 km altitude

Table 17.1 Satellite frequency bandsTable 17.1 Satellite frequency bands

Band Downlink,

GHzUplink,

GHzBandwidth,

MHz

L 1.5 1.6 15

S 1.9 2.2 70

C 4 6 500

Ku 11 14 500

Ka 20 30 3500

Satellites in geosynchronous orbitTelephony, broadcast TV, Internet backbone

Geostationary satellites

35,786 km, equatorial (inclination 0°), 15 yrs life 24 hr period, synchronous to earth rotation fix antenna positions, no adjusting necessary large footprint (up to 34% of earth), limited frequency

reuse; 3 satellites are enough to cover bad elevations in areas with latitude above 60° high transmit power 10KW, high latency (0.25 s) not for global coverage for small mobile phones and

data transmission, suitable for radio & TV

MEOs – used for GPS

18000 km altitude

24 to cover the earth

6 hrs to orbit

GPS based on ‘triangulation’ – need distance from 4 points

Used widely by all sorts of users

LEO – global telephony

Polar orbits, 500-2000 km5-8 years lifetime90-120 min to orbit20000 – 25000 km/hr8000 km diameter footprintSystem of satellites = network of switches

Little Leos - < 1GHz, low data rate messagingBig Leos (1-3 GHz) – Globalstar, IridiumBroadband Leos (like fibre) - Teledesic

LEO systems

visibility ~ 10 - 40 minutes, period of 95-120 min global radio coverage possible, 50-200 satellites latency similar to terrestrial long distance: 5 - 10 ms smaller footprints (i.e. cells), better frequency reuse handover necessary from one satellite to another High elevation even in polar regions more complex systems due to moving satellites Need for routing

LEOS

ISL Inter Satellite Link

GWL – Gateway Link

UML – User Mobile Link

Iridium 1998 - present66 satellites, 6 orbits, altitude

750 km.Originally for global voice, data,

fax, paging, navigationSpectrum - 1.6 G, ISL 23 G66 x 48 spot beams or cells2000 cells to cover the earth240 channels of 41 KHz each, can support 253 440 users.

Applications – telephony ($7 per minute) and data 2.4 kbps (10 kbps under new ownership)Inter satellite links for routing 25 MbpsComplex software for call routing via ISL

Globalstar

48 Satellites, 6 orbits

Altitude of 1400 km

Relaying uses earth stations as well as satellites – ‘bent pipe’.

Ground stations can create stronger signals

Voice and data at 4.8 kbps

Teledesic – planned but never materialised

288 satellites, 12 polar orbits,1350 km

BB channels – Internet in the sky

8 satellites form a unit, earth stations are also used

Earth divided into several 10k’s cells, each assigned a time slot to transmit

User terminals to communicate directly

155 M/1.2G up/down links – Ka band

Routing between satellites, gateways, fixed networks: ISL or terrestrial?

Reduced number of gateways needed with ISLBest to forward connections or data packets within the satellite network as long as possibleOnly one uplink and one downlink per direction needed for the connection of two mobile phones

PROBLEMS - ISL

more complex focusing of antennas between satellites satellites need routing software high system complexity due to moving routers higher fuel consumption, shorter lifetime Iridium and Teledesic planned with ISL

Other systems use terrestrial gateways and also terrestrial networks

Localization of mobile stations

Mechanisms similar to GSM, except ‘base stations’ are satellites.

Gateways maintain registers with user dataHLR (Home Location Register): static user dataVLR (Visitor Location Register): (last known)

location of the mobile stationSUMR (Satellite User Mapping Register):

satellite assigned to a mobile stationpositions of all satellites

Localisation of Mobiles

Registration of mobile stationsMobile’s signal received by several satellites,

reported to gateway(s)Localization of the mobile station is via the

satellite’s positionrequesting user data from HLRupdating VLR and SUMR

Calling a mobile station localization using HLR/VLR similar to GSMconnection setup using SUMR & the appropriate

satellite

Handover in satellite systems

More complex, due to motion of satellitesIntra satellite handover

handover from one spot beam to anothermobile station still in the footprint of the

satellite, but in another cellInter satellite handover

handover from one satellite to another satellite

mobile station leaves the footprint of one satellite

Handover (Contd.)

Gateway handoverHandover from one gateway to anothermobile station still in the footprint of a satellite,

but satellite moves away from the current gatewayInter system handover

Handover from the satellite network to a terrestrial cellular network

mobile station can use a terrestrial network again which might be cheaper, have a lower latency.

Overview of LEO/MEO systemsIridium Globalstar ICO Teledesic

# satellites 66 + 6 48 + 4 10 + 2 288altitude(km)

780 1414 10390 ca. 700

coverage global 70° latitude global globalmin.elevation

8° 20° 20° 40°

frequencies[GHz(circa)]

1.6 MS29.2 19.5 23.3 ISL

1.6 MS

2.5 MS 5.1

6.9

2 MS

2.2 MS 5.2

7

19

28.8 62 ISL

accessmethod

FDMA/TDMA CDMA FDMA/TDMA FDMA/TDMA

ISL yes no no yesbit rate 2.4 kbit/s 9.6 kbit/s 4.8 kbit/s 64 Mbit/s

2/64 Mbit/s # channels 4000 2700 4500 2500Lifetime[years]

5-8 7.5 12 10

costestimation

4.4 B$ 2.9 B$ 4.5 B$ 9 B$