Satellite Systems
•Global Coverage without wiring costs
•Independent of population density
•Chiefly for broadcast TV
•Useful addition to exisiting services – e.g. with UMTS
History Handover
Basics Routing
Localization Systems
History of satellite communication
1945 - Arthur C. Clarke “Extra Terrestrial Relays“
1957 - first satellite USSR‟s SPUTNIK
1960- first reflecting communication satellite ECHO
1962 – Telstar launched, an important step
1963 - first geostationary satellite SYNCOM
1965 - 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 broadcast
military satellites – espionage, warning system
navigation and localization (GPS)
Telecommunication – „cable in the sky‟
global telephone connections & mobiles
backbone for global networks
remote/rural areas
extend 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
Classical satellite systems
Inter Satellite Link
(ISL)
Mobile User
Link (MUL) MUL
Gateway Link
(GWL) GWL
small cells
(spotbeams)
base station
or gateway
footprint
ISDN PSTN GSM
PSTN: Public Switched User data
Telephone Network
Satellite 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
4 r f
2
gain of sending/receiving
antennae L
distance between sender
and receiver c
Carrier frequency
This affects data rates
achievable L: Loss
f: carrier frequency
Only 10 bps may be achievable
r: distance
with GEOs, compared to 10 c: speed of light
Kbps at 100 km, 2GHz carrier
Atmospheric attenuation
Attenuation of
the signal in % Example: satellite systems at 4-6 GHz
50
40 rain absorption
30
fog absorption
e
20
10
atmospheric
absorption
5° 10° 20° 30° 40° 50°
elevation of the satellite
Satellites - features
GEO: geostationary, ~ 36000 km from the
earth
LEO (Low Earth Orbit): 500 - 1500 km
MEO (Medium Earth Orbit) or ICO
(Intermediate Circular Orbit): 6000 - 20000 km
HEO (Highly Elliptical Orbit) elliptical orbits
Microwave, line of sight; GHz range
Uplink and downlink – different frequencies
Satellite orbit altitudes
Orbits II
GEO (Inmarsat)
HEO MEO (ICO)
LEO inner and outer Van
(Globalstar, Allen belts
Irdium)
earth
1000
10000
35768
km
Inner & outer Van-Allen-Belts: ionized particles
2000 - 6000 km, 15000 - 30000 km altitude
Table 17.1 Satellite frequency bands
Downlink, Uplink, Bandwidth,
Band
GHz GHz 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 orbit
Telephony, 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 km
5-8 years lifetime
90-120 min to orbit
20000 – 25000 km/hr
8000 km diameter footprint
System of satellites = network of switches
Little Leos - < 1GHz, low data rate messaging
Big Leos (1-3 GHz) – Globalstar, Iridium
Broadband 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 - present
66 satellites, 6 orbits, altitude
750 km.
Originally for global voice, data,
fax, paging, navigation
Spectrum - 1.6 G, ISL 23 G
66 x 48 spot beams or cells
2000 cells to cover the earth
240 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 Mbps
Complex 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 ISL
Best to forward connections or data packets within the satellite
network as long as possible
Only 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 data
HLR (Home Location Register): static user data
VLR (Visitor Location Register): (last known)
location of the mobile station
SUMR (Satellite User Mapping Register):
satellite assigned to a mobile station
positions of all satellites
Localisation of Mobiles
Registration of mobile stations
Mobile‟s signal received by several satellites,
reported to gateway(s)
Localization of the mobile station is via the
satellite‟s position
requesting user data from HLR
updating VLR and SUMR
Calling a mobile station
localization using HLR/VLR similar to GSM
connection setup using SUMR & the appropriate
satellite
Handover in satellite systems
More complex, due to motion of satellites
Intra satellite handover
handover from one spot beam to another
mobile station still in the footprint of the
satellite, but in another cell
Inter satellite handover
handover from one satellite to another
satellite
mobile station leaves the footprint of one
satellite
Handover (Contd.)
Gateway handover
Handover from one gateway to another
mobile station still in the footprint of a satellite,
but satellite moves away from the current gateway
Inter 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 systems
Iridium Globalstar ICO Teledesic
# satellites 66 + 6 48 + 4 10 + 2 288
altitude 780 1414 10390 ca. 700
(km)
coverage global 70° latitude global global
min. 8° 20° 20° 40°
elevation
frequencies 1.6 MS 1.6 MS 2 MS 19
[GHz 29.2 2.5 MS 2.2 MS 28.8
(circa)] 19.5 5.1 5.2 62 ISL
23.3 ISL 6.9 7
access FDMA/TDMA CDMA FDMA/TDMA FDMA/TDMA
method
ISL yes no no yes
bit rate 2.4 kbit/s 9.6 kbit/s 4.8 kbit/s 64 Mbit/s
2/64 Mbit/s
# channels 4000 2700 4500 2500
Lifetime 5-8 7.5 12 10
[years]
cost 4.4 B$ 2.9 B$ 4.5 B$ 9 B$
estimation