Satellite Communication, Slides of Mobile Computing

Download Satellite Communication and more Slides Mobile Computing in PDF only on Docsity! Satellite Systems ◼ Introduction ◼ History ◼ Basics ◼ Orbit Types ◼ Routing ◼ Localization ◼ Handover Department of IT, RCOEM 1 Introduction ◼ What is a satellite ? ❑ A satellite is any object that orbits or revolves around another object. For example, the moon is a satellite of the earth & the earth is a satellite of the sun. Department of IT, RCOEM 2 Introduction ◼ Disadvantages of satellite communication ❑ Launching satellites into orbit is costly. ❑ Satellite bandwidth is gradually becoming used up. ❑ There is a larger propagation delay in satellite communication than in terrestrial communication. Department of IT, RCOEM 5 Introduction ◼ A satellite communication system uses satellites to relay radio transmissions between earth terminals ◼ The two types of communications satellites are PASSIVE and ACTIVE ❑ A passive satellite only reflects received radio signals back to earth ❑ An active satellite acts as a REPEATER; it amplifies signals received and then retransmits them back to earth. This increases signal strength at the receiving terminal to a higher level than would be available from a passive satellite. ◼ A typical operational link involves an active satellite and two or more earth terminals. One station transmits to the satellite on a frequency called the UP-LINK frequency. The satellite then amplifies the signal, converts it to the DOWN-LINK frequency, and transmits it back to earth. The signal is next picked up by the receiving terminal. Department of IT, RCOEM 6 Typical Satellite Systems _—_—_—"2 Inter Satellite Link (ISL) Mobile User Link (MUL) "-" “.& Gateway Link small cells (spotbeams) footprint PSTN: Public Switched User data Telephone Network Department of IT; RCOEM 7 Basics ◼ Inclination angle: angle between the equatorial plane and the plane described by the satellite orbit. An inclination angle of 0 degrees means that the satellite is exactly above the equator ◼ Elevation angle: angle between center of the satellite beam and the plane tangential to the earth’s surface ◼ Footprint : The area on earth where the signal of the satellite can be received ◼ Perigee : For non-circular orbits, the closest point to the earth ◼ Apogee : For non-circular orbits, the point farthest from earth ◼ Line of apsides : The line joining the perigee and apogee through the center of the earth. ◼ Ascending node: The point where the orbit crosses the equatorial plane going from south to north. ◼ Descending node: The point where the orbit crosses the equatorial plane going from north to south. ◼ Line of nodes: The line joining the ascending and descending nodes through the center of the earth ◼ Prograde orbit: An orbit in which the satellite moves in the same direction as the earths rotation. ◼ Retrograde orbit: An orbit in which the satellite moves in a direction counter to the earth's rotation. Department of IT, RCOEM 10 Inclination Angle plane of satellite orbit satellite orbit perigee inclination 6 equatorial plane Department of IT; RCOEM 11 Inclination Angle ◼ A satellite orbiting in any plane not identical with the equatorial plane is in an INCLINED ORBIT Department of IT, RCOEM 12 Elevation Angle of a Satellite ▪ Elevation angle effects the satellites coverage area. ▪ However, because of environmental factors like objects blocking the transmission, atmospheric attenuation, and the earth electrical background noise, there is a minimum elevation angle of earth stations. Department of IT, RCOEM 15 Atmospheric Attenuation Depending on the elevation, the signal has to penetrate a smaller or larger percentage of the atmosphere. An elevation less than 10 degrees is considered useless for communication Department of IT, RCOEM 16 The Van Allen radiation Belts ◼ The Van Allen radiation belts consists of ionized particles and are at heights of about 2,000-6,000 km ( inner belt) and about 15,000 – 30,000 km ( outer belt) respectively and make satellite communication very difficult in these orbits ◼ These regions are held captive by the magnetic influence of the earth ◼ Radiation is concentrated and closest to the earth at the poles (aurora) ◼ Satellite orbits are designed to spent as little time as possible in these belts or completely avoid them ◼ Satellites that travel in and around the belts may be damaged Department of IT, RCOEM 17 GEO ◼ Orbits at a height of 36,768 km, GEO satellites appear fix in sky; they have a period of 24 hours and an inclination of 0 degree ◼ Advantages ❑ Three GEO satellites are enough to cover the entire earth; handover generally not required due to large footprints ❑ Senders ad Receivers use fixed antennas positions, no adjusting is needed ❑ Ideal for TV and radio broadcasting ❑ Lifetime about 12 to 15 years hence very cost effective ❑ Do not exhibit Doppler shift ( change in the apparent frequency of operations to and from satellite, caused by the relative motion of the satellite and earth station ) because relative movement is zero ❑ The frequency bands of a geosynchronous satellite can be reused by different methods for increasing the channel capacities ❑ The most common orbit used for communications satellites. Department of IT, RCOEM 20 GEO Three GEO satellites covering the entire Earth Department of IT, RCOEM 21 GEO ◼ Disadvantages ❑ Due to low elevation, Northern or Southern regions have problems receiving these satellites; larger antennas required ❑ Shading of the signals in cities due to high buildings and the low elevation further away from the equator limit the transmission quality ❑ The transmit power needed is relatively high causing problems for battery operated devices ❑ High latency ( 0.25 s one way) ❑ Due to larger footprint, either frequencies cannot be reused or the GEO satellite needs special antennas focusing on a smaller footprint ❑ Transferring a GEO into orbit is very expensive ❑ Cannot be used for small mobile phones Department of IT, RCOEM 22 Equinox Department of IT, RCOEM 25 NGSO ◼ Early ventures with satellite communications used satellites in Non-geostationary low earth orbits due to the technical limitations of the launch vehicles in placing satellites in higher orbits ◼ With the advancement of launch vehicles and satellite technologies, once the Geo stationary orbit was achieved, majority of the satellites for telecommunications started using GSO due to its many advantages ◼ During 1990s the interest in NGSOs were rekindled due to several advantages of NGSO in providing global personal communications in spite of its many disadvantages Department of IT, RCOEM 26 NGSO ◼ Advantages ❑ Less booster power required to put a satellite in lower orbit ❑ Less space loss for signal propagation at lower altitudes leading to lower on board power requirement ❑ Less delay in transmission path- reduced problem of echo in voice communications ❑ Suitability for providing service at higher latitudes ❑ Lower cost to build and launch satellites ❑ Use of VHF and UHF frequency bands permits low cost antennas for hand held terminals Department of IT, RCOEM 27 NGSO ◼ Depending of orbit height ❑ Low Earth Orbit (LEO) ❑ Medium Earth Orbit (MEO)/ Intermediate Circular Orbit (ICO) ❑ Highly Elliptical Orbit (HEO) Department of IT, RCOEM 30 LEO Orbit LEO Polar Orbit Department of IT; RCOEM 31 LEO ◼ Operates at a distance of about 500 to 1,500 km. ◼ Ensures high elevation for every spot on earth to provide a high quality communication link ◼ Short period, typically – 95 to 120 minutes ◼ Visibility of the satellite around 10 minutes ◼ A special type of LEO is the Polar Orbit. This is a LEO with a high inclination angle (close to 90degrees). This means the satellite travels over the poles. ◼ Further classification ❑ Little LEO (Low Bandwidth, 100 bit/sec) ❑ Big LEO (High Bandwidth, 1000 bit/sec) ❑ Broadband LEO (Mbits/sec) ◼ Examples: ❑ Iridium (start 1998, 66 satellites) ❑ Globalstar (start 1999, 48 satellites) Department of IT, RCOEM 32 MEO ◼ Orbits at 5000 – 12000 km above earth surface ◼ Can be positioned between LEO’s & GEO’s ◼ Example: ICO (Intermediate Circular Orbit, INMARSAT) start 2000 ◼ Advantages ❑ Less number of satellites needed than LEO ❑ Move more slowly relative to earth’s rotation allowing for simpler system design ❑ Period of about six hours ❑ Depending on inclination, few handovers required for large population coverage Department of IT, RCOEM 35 MEO ◼ Disadvantages ❑ Higher latency, 70 – 80 ms ❑ Higher transmit power needed ❑ Special antennas needed for small footprints Department of IT, RCOEM 36 HEO ◼ Also known as Molniya orbits, named after a series of Soviet/Russian Molniya (Russian: "Lightning") communications satellites which have been using these types of orbits since 1960’s Department of IT, RCOEM 37 HEO ◼ For continuous coverage over the entire planet at all times, such as the Department of Defense's Global Positioning System (GPS), we require a constellation of satellites with orbits that are both different in location and time. In this way, there is a satellite over every part of the Earth at any given time. In the case of the GPS system, there are three or more satellites covering any location on the planet. Department of IT, RCOEM 40 HEO ◼ Highly elliptical orbits are mainly perturbed by the Earth ’ s oblateness (flattened at poles) and by gravitational attraction of the Sun and Moon. ◼ HEOs are popular orbits for Earth magentospheric measurements and astronomical observatories. A Molniya orbit is not suitable for manned spacecraft because it repeatedly crosses the high-energy Van Allen belt. Department of IT, RCOEM 41 Orbit Types MEO 5000 – 12,000 km LEO 500 - 1,500km HEO 500 -30,000 km GEO 35,786 km Department of IT, RCOEM 42 Localization of mobile station ◼ Mechanisms similar to terrestrial cellular networks; however here the base station also moves ◼ 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 ◼ Registration of mobile stations ❑ Mobile station sends signal which several satellites receive ❑ Receiving satellites report the event to the gateway ❑ Gateway determine the location of the user via the location of the satellites ❑ User data is requested from the HLR, VLR and SUMR are updated ◼ Calling a mobile station ❑ Call forwarded to a gateway which localizes the MS using HLR/VLR ❑ connection setup with the appropriate satellite using SUMR Department of IT, RCOEM 45 Handover in satellite systems ◼ Several additional situations for handover in satellite systems compared to cellular terrestrial mobile phone networks caused by the movement of the 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 or the satellite moves away ◼ Can also takes place if satellites support ISLs ❑ Gateway handover ◼ Handover from one gateway to another ◼ mobile station still in the footprint of a satellite, but gateway leaves the footprint ❑ Inter system handover ◼ Handover from the satellite network to a terrestrial cellular network ◼ mobile station can reach a terrestrial network again which might be cheaper, has a lower latency, etc. Department of IT, RCOEM 46