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Satellite Communications

Satellite Communications

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Satellite Communications

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  1. Satellite Communications Alison Griffiths – Room C203, Beacon Building

  2. MCCS • What we will look at • What are satellite phones • How do they operate • How do they integrate into the PSTN network • Who operates this phone network

  3. MCCS • Over the next few weeks we will look at mobile telecommunications • 2G, 2.5G, 3G, 4G • These are all designed to operate within a proximity of a transmitter / receiver (base stations) • Now consider what happens in areas where there are no base stations • This could be for financial reasons, as in there is just not enough use to justify the base station • Practical reasons as in it is impossible to install a base station, like the middle of the Pacific ocean • For these users an alternative needs to be offered • For a lot of people it unacceptable to be disconnected from the world for long periods of time • This is becoming increasingly true in the wired world

  4. MCCS • Areas where cellular communications are not possible Satellite phones are an option • Original theorised by Arthur C. Clarke in 1945 • Paper called “Extra Terrestrial Rays” • A number of private sector businesses are investing in this infrastructure • They are hoping to gain access to the large majority of the world who do not have access to Mobile telecommunications • They also want to gain the travelling business people • Although as we will discuss over the new few weeks any standard of telecommunications after 2G are supposed to be compatible outside of the host country there are still problems in reality • Local roaming agreements are needed with each of the countries which are not always in place • Different countries use different technologies

  5. MCCS - Companies • Only a few companies control the worldwide satellite market • Some of them are listed below • Iridium • Globalstar • ICO • Ellipse • Asian Cellular System (ACES) • Thuraya • Teledesic

  6. MCCS - Companies • There are three kinds of satellite used for these communications • LEO • Low Earth Orbit • 350 to 1400 Km above the Earth • MEO • Medium Earth Orbit • 6,000 to 20,000 Km above the Earth • Global Positioning System is a user of these satellites • GEO • Geostationary Earth Orbit • 36,000 Km above the equator

  7. MCCS - Satellites MEO GEO LEO

  8. MCCS - Companies • The closer the satellite is to the Earth the less area is covered by each one • GEO orbit • A minimum 3 satellites are needed to cover the planet • The satellite will remain in one location rotating with the Earth • The company ACES uses this technology • ACES actually only uses 1 satellite as they only offer services over Asia, not globally • Relatively a Large amount of power is needed to transmit • LEO • A large number of satellites are needed to cover the planet • Iridium, uses 66 satellites for global coverage + spares in case of problems

  9. MCCS – GEO • Using a GEO satellite • Means that if 2 people wish to communicate in the same area a ground station is not required • The signal can be received and transferred back to the recipient • Due to the distance involved with GEO • Delay may occur within the call • About 260 ms delay in what is said before it can be returned to the other party. • If both parties in the call are on satellite phones this will get longer and more noticeable Person 2 Person 1

  10. MCCS – Connecting • Using a LEO satellite • These satellites are closer to Earth • Each one will cover a small area • Referred to as the “footprint” • This has large advantages in the respect frequencies can be reused in different parts of the planet at the same time • This is the same basis that cellular communications operate upon • Due to the distance to satellite • The latency issue is less of a concern average 10 ms • This is similar delay as you would get on any land base PSTN • Less power is required to transmit to the satellite

  11. MCCS – Connecting • Two types of satellites exist for LEO designs • Repeaters (also known as “bent pipe”) • These just receive a signal and forward it to a ground station to be routed to the destination • Very simple operation and rely on the fact that a ground station is within transmission distance • This is seen in the following diagram

  12. MCCS – LEO - Repeater Person 1 Person 2

  13. MCCS – Connecting • Two types of satellites exist for LEO designs • Routers • These operate like a router in a computer network • The destination of a packet is inspected • This allows inter satellite handover or call routing • The removal of the transmission to the ground station removes a potential problem for the network. • Ground stations only need to be informed at the end for call charging purposes

  14. MCCS – LEO - Router Person 1 Person 2

  15. MCCS – Connecting • The way that each of the methods communicate varies with each of the companies • The satellites use a variety of standards to allow communication with the handsets • TDMA/FDMA and CDMA are predominant technologies • These will both be covered in a later lecture • Each of the companies are issued with a range of radio frequency • These are reserved and offered by the ITU • International Telecommunications Union • More about this organisation will be discussed in the GSM lectures

  16. MCCS – Frequencies • Operating frequencies • A number of frequencies are issued • Each of these frequency ranges are issued a letter to designate the frequency and the potential use • The use depends on the intention of the satellite • C • 4 GHz – 6 GHz • L • 390 MHz – 1.55 GHz • GPS System • Iridium • Ku • 12 – 18 GHz • Ka • 18 – 40 GHz • X • 5.2GHz – 10.9 GHz • V • 50 GHz – 75 GHz

  17. MCCS – Connecting • Ground stations • Depending on the technology affects the number of ground stations • Operations • Link satellites into the PSTN • Send/receive control signals • Control billing / access information PSTN

  18. MCCS – Connecting • Communications needed • Communicate with another satellite phone • Communication with another satellite phone in a different position • Communications with phone attached to the PSTN • This will include Mobile phones as these will be routed through the PSTN

  19. MCCS – Connecting • Technologies used for LEO with Inter satellite handover • Mean that it is POSSIBLE to hand only one ground station for all calls being handled • This would however mean • A single point of failure within the network • Routing PSTN calls further than required • Increased inter satellite communications • In the event of a single satellite failing all worldwide communications could be effected • Using multiple ground stations allows for • More control • Redundancy in the network • Reduced cost of connecting to the PSTN

  20. MCCS – Connecting • Single Ground station • Someone in China phoning a PSTN in China China PSTN Canada

  21. MCCS – Connecting • Multiple Ground stations • Someone in China phoning a PSTN in China PSTN China

  22. MCCS • Satellite phones • Need a clear line of sight to operate properly • The line of sight is towards the satellite • This is usually towards the horizon, where the best opportunity is available to receive a satellite signal

  23. MCCS - Handsets • Current generation if handsets look like cellular phones • An aerial must be used to get the best signal • Voice quality is not as good as the GSM network • The phones are slightly bulkier than the new slim line GSM devices • Battery life is lower than GSM devices • There are multiple standard at the moment for phones • Devices which work with one satellite group may not work with another •

  24. MCCS - Future • Future use of Satellite based phones will be seen • Most of the current companies are having financial problems • Iridium went bankrupt and was bought out in 1999/2000 • Other companies are scaling down planned growth of the systems • The IMT-2000 specifications for 3G communications does include a discussion on Satellite phones to make them compatible with cellular handsets • The intention is to attempt to allow a “standard” 3G compliant phone operate anywhere

  25. MCCS - Future IMT-2000 Vision

  26. MCCS - Future • IMT-2000 • Looking at using Satellite phones for areas where cellular communications will not be possible • These phones will offer increased voice quality and data rates • 144 Kbps actual rates will define once operators start to use this technology • Current Data Rates are about 8Kbps for satellite phones • Different sorts of Satellite communications can be used for high speed data transfer in the region of 2 to 45 Mbps • New technology starting 2005 is Icomera, which is providing wireless LAN access via satellite to trains in Europe •

  27. MCCS • Conclusion • Looked at satellite technology • Different types of satellites • Considerations • Future of satellite phone technology