Satellite and Broadcast Systems

Transcription

1 Department of Computer Science Institute for System Architecture, Chair for Computer Networks Satellite and Broadcast Systems Mobile Communication and Mobile Computing Prof. Dr. Alexander Schill

2 Satellite System Inter-Satellite Link Uplink Downlink Ground Station User Footprint GSM,... Internet 2

3 Satellite systems: Basics (1) satellites describe elliptical or circular orbit around the earth distance to the earth remains constant: G ( ) 2 R / r m r ω 2 FZ F = m g = = (1) F F G Z m R r g ω f - Appeal of the Earth - Centrifugal force - Mass of the satellite - Earth radius, 6.370km - Distance of the satellite to the Earth s center - Ground acceleration, g = 9,81 m/s 2 - Angular frequency: ω = 2 π f, T = 1/ f = 2 π / ω - Cycle frequency of the satellite 3

4 Satellite systems: Basics (2) (1) resolved to r yields: r = 3 gr 2 ( 2π f ) 2 (2) that means, the distance of a satellite to the earth's surface depends only on its cycle duration (special case T = 24h -> r = km -> synchronous distance from the surface of the earth = r - R = km) 20 velocity [x1000km/h] Cycle duration [h] 12 4 Synchronous distance km x 10 6 m 4

5 Satellite system classes GEO (Geostationary Earth Orbit) approx km LEO (Low Earth Orbit) approx km MEO (Medium Earth Orbit) ~ km (and around km) Van-Allen-belts km km (partially no satellite use possible) 5

6 Geostationary Satellite systems Principle: Uplink Satellite Downlink Base for Inmarsat constant position to the Earth, 3 satellites cover complete earth (without polar caps), satellites move synchronously to the earth simple solution, long life time of the satellites: ~ 15 years large distance (36000 km), therefore high signal propagation delay low data rates, large transmission power required problems: on the other side of the 60th degree of latitude reception problems (elevation) because of high transmission power unfavorable for mobile telephones signal propagation delay too high (250 ms roundtrip) 6

7 LEO Systems non-stationary satellites (LEO - Low Earth Orbit) distance to the earth ~ km shorter signal delay times (5-10 ms), lower transmission power of the mobile stations sufficient however more satellites necessary (> 50), frequent handover between satellites, approximately every 10 min. shorter lifetime of the satellites because of atmospheric friction (5-8 years) examples: Iridium, Teledesic, Globalstar 7

8 MEO Systems ~ km, lower number of satellites necessary : ~12 slow movement: handover between satellites is hardly necessary cycle duration: 6h Problems: signal propagation delay: 70 to 80 ms higher transmission power necessary special antennas necessary 8

9 Comparison of satellite-based systems Satellite-based system GEO MEO LEO Distance, km r-r = km r-r= km r-r= km Cycle duration, T 24 h 6 h min Signal propagation delay, t Transmission power, W 0.25 s ms 10 ms Usage examples Numerous systems, approx. 2000: Sputnik (1957) Intelsat1-3(1965, 67, 69) Inmarsat-A (1982) Inmarsat-5 (2013): 64 kbit/s ICO: 10+2; 4,8kBit/s; ~ km Iridium (Satellites;data rate;height): 66+6;2,4-4,8kBit/s;~780km Globalstar: 48+4;9,6-144kBit/s;~1400km Teledesic: 288;2-64MBit/s;~700km Orbcomm (1. commercial LEOservice worldwide): 35;57,6kBit/s Life time, years

10 Global Positioning System, GPS 24 satellites on the 6 orbits (20200 km, time of circulation = 12h) 5 earth stations (Hawaii, Ascension Island, Diego Garcia, Kwajalein, Colorado Springs) Accuracy: up to 1 m (normally approx. 10 m) Functionality principle: Triangulation GPS-receiver calculates distance to the satellite based on Time of Arrival of the received signals distances to at least three satellites enable the calculation of position, a fourth satellite can be used for determination of elevation over zero 10

11 Principle: TOA (Time of Arrival) / TDOA (Time Difference of Arrival) Distance d, Signal Delay T Mobile Object synchronized clocks measurement of signal delay based on speed of light between satellite and receiver, for instance T = 70 ms hence calculation of distance: d = T c = 0, s m/s = 2, m = km calculation of spheres around each satellite the position is on the intersection point of three spheres 11

12 Principles satellites send a signal composed of three components 50 times per second: identification component: PRC (Pseudo Random Code), provides satellite recognition and status information position component: exact position of satellite time component: timestamp, when signal is transmitted the time offset measured by the receiver is corresponding to the Time of Arrival, from TOA the distance is calculated for measurement of TOA of signals very accurate clocks are required the exact position of the satellites must be known 12

13 Sources of errors Clocks highly accurate atom clocks in the satellites simple clocks in the receivers are calibrated via measurement of a fourth satellite Satellite position satellite orbits are relatively stable and known in detail deviations are transmitted as correction factor to the satellites using the PRC Miscellaneous error sources atmospheric faults multi-path propagation 13

14 Differential GPS (DGPS) use of a stationary receiver as reference position of this receiver is exactly known the stationary receiver carries out position determination and calculates correction factor from the actually obtained position on the base of deviations correction factor is delivered to the mobile receiver 14

15 DGPS accuracy grades Accuracy under 10cm: professional applications, for instance in meteorology or machine control systems etc. Accuracy under 1m: geography, general control of machines, traffic control systems, agriculture Accuracy under 10m: Agriculture / forestry, railway (wagon search service), car navigation (private/commercial) 15

16 EU Project Galileo A system that both competes with and complements the GPS system ITS (Intelligent Transport System) based on a constellation of 30 MEO-satellites ( replacement) ground stations providing information concerning the positioning of users applicable in many sectors, high positioning accuracy (approx. 50 cm): transport (vehicle location, route searching, speed control, etc.) social services (e.g. aid for the disabled or elderly) GIS (geographical information systems) 16

17 Galileo Costs: 3,5 Billion Euro Main partners: Germany, Italy, France and United Kingdom Operational pilot since 2015, full operation by

18 Pager systems: Overview Eurosignal (predecessor of current systems) 4 different audio signals using 4 diverse call numbers are assigned to each user. Semantics must be agreed. 85 senders in the 87 MHz-area (ultra short waves) called person location must be approximately known: 3 area codes: North 0509, Middle 0279, South 0709 Cityruf (currently up and running) additional to 4 audio- or respectively optical signals, transmission of short numerical (15 digitals) or alpha-numerical messages (80 characters), receiving station is smaller than with Eurosignal Better reachability than with GSM due to lower frequency range (e.g. emergency calls for doctors) ERMES (European Radio Message System) ETSI-Standard for pan-european radio service, uses 169 MHz-band with 60 Mio. Addresses (but cancelled due to interference with TV channels) 18

19 TETRA (Terrestrial Trunked Radio) Multicast / broadcast network; especially suitable for emergency teams (fire, ambulance etc.); very reliable; closed (non-public) infrastructure frequencies: , MHz Uplink; , MHz Downlink; bandwidth of each channel: 25 khz Services: Voice + (V+D)- Service: Speech and, channeloriented, uni-, multi- and broadcast possible Packet Optimized (PDO)- Service: packet-oriented, point-to-point and point-to-multipoint communication carrier services with data rate up to 28,8 kbit/s unprotected; 9,6 kbit/s error-protected confirmed and/or non-confirmed Group Call (however this is also possible with GSM today: up to 16 participants) listening to other calls is possible (so called open-channel mode ) fast dialing: approx. 300 ms ( push to talk ), GSM: several seconds cost-efficient, especially for limited user quantity 19

20 Broadcast Systems C C C Information stream is optimized for expected access behavior of all consumers t C B B A B A Examples: - DVB, Digital Video Broadcast - DAB, Digital Audio Broadcast Individual access sample of diverse consumers can more or less deviate from expected access behavior 20

21 Digital Audio Broadcast, DAB Audio-transmission in CD-Quality Robust against interferences of multi-path-propagation Use of SFN (Single Frequency Network) i.e. all senders of some broadcast-program are working on the same frequency as a rule Frequencies: UHF,VHF, e.g MHz, MHz Modulation method: DQPSK (Differential Quadrature Phase Shift Keying) Optionally OFDM (Orthogonal Frequency Division Multiplexing) is used with several carrier frequencies inside a DAB-channel FEC (Forward Error Correction) mechanism for fault correction Up to 6 stereo-programs with 192 kbit/s in the same frequency band are transmittable Alternatively data can be transmitted with up to 1,5 Mbit/s Various problems concerning acceptance in practice; DAB is available, but its future is questionable 21

22 Digital Audio Broadcast, DAB Two Transport Mechanisms Main Service Channel (MSC):, Audio, Multimedia 2 Transport Modes: Stream Mode, Packet Mode Fast Information Channel (FIC): Transport of Fast Information Blocks (FIB, 32 Byte) control data for interpretation of in the MSC, can be also used for services such as Traffic Dispatches, Paging etc. Audio-encoding: PCM 48 khz & MPEG2-Audiocompression High transmission rates at high velocities, up to 300 km/h, according to distance from sender and error security class, e.g. for high-speed trains MOT (Multimedia Object Transfer) protocol for data transmission Cyclic repeating and caching of data blocks 22

23 Dynamic DAB channel reconfiguration Ensemble Configuration Audio KBit/s Audio KBit/s Audio KBit/s Audio KBit/s Audio KBit/s Audio KBit/s PAD PAD PAD PAD PAD PAD D1 D2 D3 D4 D5 D6 D7 D8 Temporarily changed Ensemble Configuration Audio KBit/s Audio KBit/s Audio KBit/s PAD Audio KBit/s Audio KBit/s Audio 7 96 KBit/s Audio 8 96 KBit/s PAD PAD D10 D11 PAD PAD PAD PAD D1 D2 D3 D4 D5 D6 D7 D8 23

24 DVB - Digital Video Broadcasting Goal: joint digital Television System for Europe Specifications: DVB-S (Satellite), DVB-T (Terrestric), DVB-C (Cable), DVB-H (Handheld) Frequency areas: 200, 550, 700 MHz Cell size: up to 60 km Used data rate: ~38,5 Mbit/s Velocity of mobile stations: up to 300 km/h Central Unit: combined DVB-Receiver-Decoder (set-top-box) can receive DVB- also via satellites, ADSL etc. some transmission systems offer a feedback channel for Video on Demand etc. DVB-S and DVB-T widely available in practice; DVB-C available in some areas; DVB-H being introduced step by step 24

25 DVB - Digital Video Broadcasting Different Quality Levels defined: SDTV (Standard Definition TV) EDTV (Enhanced DTV) HDTV (High DTV) transport: User : MPEG2-Container ( Transfer Unit) like DAB, Container doesn t define the type of data Service Information about MPEG2-Containercontent: NIT (Network Information Table): Information from a provider about offered services and optional data for the receiver SDT (Service Description Table): Description and parameters for each service in the MPEG2-stream EIT (Event Information Table): about actual transmission status TDT (Time and Date Table): e.g. updating and resynchronization of DVB-receiver 25

26 Possible contents of DVB/MPEG2-Container MPEG2/DVB-Container HDTV MPEG2/DVB-Container EDTV Single channel (High Definition TV) Several channels (Enhanced DTV) MPEG2/DVB-Container MPEG2/DVB-Container SDTV Several channels (Standard Definition TV) Multimedia (data broadcasting) 26

27 Some further readings GPS system: DVB consortium: 27