Fundamentals on satellite communica0ons

Transcription

1 Fundamentals on satellite communica0ons Visi0ng course: Satellite Communica.ons Dalhousie University, 30 May 3 June 2016 Lecturer: Claudio Sacchi, PhD, University of Trento, Department of Informa0on Engineering and Computer Science (DISI), sacchi@disi.unitn.it

2 Outline Satellite system components; Satellite orbits; Satellite payloads; Satellite placorms (summary); Frequency allocahon and radio regulahons. 2/36

3 Satellite system components Space segment and ground segment The space-segment is made by the inorbit satellite and the control sta0on; The control sta0on (located on Earth) monitors the orbit parameters (Telemetry and Tracking) and drives the satellite engines (Control); The ground segment consists of the Earth Sta0ons, equipped with communica0ons hardware able at transmiung/receiving informa0onbearing signals (analog or digital). 3/36

4 Satellite system components Space segment and ground segment (pictorial example) Home-satellite receiver 4/36

5 Satellite system components Earth sta.ons Earth sta0on can be defined as every installa0on, located on Earth, that is able at transmiung/receiving informa0on to/from the satellite ; The home-satellite receiver for TV broadcas0ng and the satellite telephone are small Earth sta0ons; The big data centers of Satellite TV or Satellite Internet providers are big Earth sta0ons; Some Earth sta0ons can only receive informa0on (like e.g. TV receiver), other Earth sta0ons can transmit and receive informa0on (e.g. the data center and the satellite telephone); Small earth sta0ons can be directly connected to the satellite (VSAT, satellite telephones), bigger base sta0ons are generally connected to personal terminals by wireless or wired terrestrial networks. 5/36

6 Satellite system components Basic scheme of an Earth sta.on Bi-direc0onal Earth sta0on (transmits and receives informa0on); This very generic scheme does not encompass baseband sec0ons, just RF sec0ons. 6/36

7 Satellite system components The satellite The satellite is substan0ally an in-orbit flying object with some communica0on equipment on-board; The satellite is made by two parts: the payload and the pla`orm; The payload is the sec0on of the satellite containing all the hardware necessary to transmit physical carriers (antennas, RF front-ends, modulators and demodulators, etc.); The pla`orm contains all the subsystems required to operate the payload and to allow the satellite to fly. 7/36

8 Satellite orbits Generic (Keplerian) orbit (1) The orbit of communica0on satellites are defined on the basis of Kepler s laws; The satellite orbits are generically ellipses, defined in the orbital plane as follows: ( ) r = p 1+ ecos θ θ 0 p = H 2 µm 2 µ = m 3 sec 2 e = ρ 0 p < 1 H satellite angular momentum (Nm) m satellite mass (Kg) 8/36

9 Satellite orbits Generic (Keplerian) orbit (2) The parameter e is called: eccentricity; It is possible to rewrite the ellip0cal orbit in the following manner: r = a( 1 e 2 ) ( 1+ ecosν ) a = p ( 1 e 2 ) ν = θ θ 0 The parameter a is called: major axis. For e=0, the ellipse becomes a circumference (circular orbit); It is possible to compute both the velocity of the satellite and the orbital period: V = µ ( 2 r 1 a) T = 2π a 3 µ 9/36

10 Satellite orbits Circular orbits parameters In the table below, al0tude, radius, period and velocity for some examples of circular orbits are reported: Low al0tude satellites have lower period than high al0tude satellites and travel faster in the Sky! 10/36

11 Satellite orbits Geosynchronous (GSO) and Sun- synchronous (SSO) orbits A geosynchronous orbit is an orbit around the Earth with an orbital period of one sidereal day (approximately 23 hours 56 minutes and 4 seconds); A typical example of GSO is the geosta0onary orbit: circular orbit with zero inclina0on w.r.t. the Equator. The GEO satellite appears like a fixed point in the Sky; A Sun-synchronous orbit is a geocentric orbit that combines al0tude and inclina0on in such a way that the satellite passes over any given point of the planet's surface at the same local solar 0me; SSO can place a satellite in constant sunlight and is useful for imaging, spy, and weather satellites. 11/36

12 Satellite orbits Satellite footprint The satellite footprint is the ground area where satellite transponders offer coverage; The footprint enlarges for higher satellite al0tude and larger antenna diameter; 12/36

13 Satellite orbits Examples of real satellite orbits: circular geosta.onary orbit The geosta0onary orbit is circular, with zero degree inclina0on (w.r.t. Equator) and is geosynchronous. The orbital height is 35,768 Km. Geosta0onary satellites are big and cover large areas of the Earth Globe; Few GEO satellites (max. 3) can provide global coverage. 13/36

14 Satellite orbits Examples of real satellite orbits: circular low-earth orbit (LEO) The low-earth orbit is circular, with 90 inclina0on (w.r.t. Equator). The orbital period is 1.5 hours. The orbital height amounts to some hundreds of Km; Being the orbital height very low, the LEO satellite footprint will be much smaller than GEO one LEO satellite can provide the global coverage. 14/36

15 Satellite orbits Examples of real satellite orbits: circular medium earth orbit (MEO) MEO orbits are placed between LEO and GEO orbits at around 10,000 Km height. MEO orbits are circular, generally inclined with respect to the Equator; MEO satellite can provide global coverage. Such kind of orbits are used for GPS, GNSS, GLONASS and other geolocaliza0on systems. 15/36

16 Satellite orbits Ellip.cal orbits They have originally used by early Soviet satellites (Sputnik, MOLNYA, TUNDRA); In ellip0cal orbits, the velocity of the satellite is not constant (it is minimum at the apogee); An inclina0on of 64 with respect to the equatorial plane, would ensure 8 hours coverage at the apogee. Therefore 3 satellites are enough to provide 24-hours con0nuous transmission (at the apogee). 16/36

17 Satellite orbits Propaga.on.me and Doppler shir (1) Be R the distance between Earth and satellite, the signal propaga0on 0me is simply given by: τ = R c c = m / sec. The movement of satellite with respect to Earth (at velocity V) causes a Doppler effect on the transmired radio-frequency. In par0cular, the Doppler shis can be computed as follows: Δf d = V cosφ( f 0 c)[hz] Angle between the direchon of the point considered and the velocity of the satellite 17/36

18 Satellite orbits Propaga.on.me and Doppler shir (2) A very important parameter which will affect the performance of the automa0c frequency control of the coherent receiver is the Doppler varia0on rate: d ( Δf ) d dt = V sinφ ( dφ dt) ( f 0 c)[hz] On equatorial circular orbit, the maximum Doppler shis (when satellite appears or disappears at the horizon -> fi=0) can be es0mated: ( Δf ) d max f 0 m R m R = number of revolu0ons per day of the satellite 18/36

19 Satellite orbits Propaga.on.me and Doppler shir (3) Let s consider some very important prac0cal numbers (maybe not exact, but useful to understand); Propaga0on delay for a LEO satellite (200 Km): 0.67 msec., for a MEO satellite (11,000 Km): 36.7 msec., and for a GEO satellite (35,786 Km): 120 msec. Maximum Doppler shis for a LEO satellite (200 Km) transmiung at 2GHz (m R =15.21): KHz, for a MEO satellite (11,000 Km) transmiung at 2 GHz (m R =3): 9.2 KHz and for geosynchronous GEO satellite (m R =0): 0 Hz (theore0cally). 19/36

20 Satellite payloads Func.ons of the payload To capture the carriers transmired in a given frequency band and with a given polariza0on by the earth sta0ons of the network; To limit the interference coming from unwanted signals; To amplify the received carriers while limi0ng noise and distor0on as much as possible (no0ce: the level of received carriers is of the order of few tens of pw); To change the frequency of the carriers received on the uplinks to that of the downlinks; To provide the power required in a given frequency band at the interface with the transmiung antenna (from tens to hundreds of W) and to radiate the carriers in a given frequency band and with a given polariza0on. 20/36

21 Satellite payloads Payload classifica.on Satellite payloads are classified in non-regenera0ve and regenera0ve: Non-regenera.ve payloads work as transparent relays: they receive the uplink signal from the Earth transmirer, change its frequency from uplink to downlink and, finally, retransmit the signal to the Earth receiver; Regenera.ve payloads demodulate the uplink signal, process the demodulated baseband signal (regenerahon), and, finally modulate and transmit the regenerated signal to the Earth receiver using the downlink frequencies. 21/36

22 Satellite payloads Characteriza.on of the payload Independently from the previous classifica0on (transparent or regenera0ve), the characteris0c parameters of a satellite payload can be listed as follows: TransmiUng and receiving frequency bands; EffecHve isotropic radiated power (EIRP) or power flux achieved in given region; The power flux density required at the satellite receiving antenna to cope with performance requirements; Figure of merit (G/T) of the receiving system in a given region; The nonlinear characterishcs; The reliability a]er N years for a specified number of channels. 22/36

23 Satellite payloads Figure of merit of a receiver The figure of merit of a radio receiver expresses the ra0o between RF front-end gains, receiver losses and the equivalent noise link temperature: ( ) G / T ˆ= G A G LNA Τ link L Rx K 1 G A = receiving antenna gain (in linear scale) G LNA = low-noise amplifier gain (in linear scale) L Rx = receiver losses (in linear scale) T link = equivalent noise link temperature (in K) 23/36

24 Satellite payloads Nonlinear characteriza.on of the payload (1) Payloads (and Earth sta0ons) embark high-gain power amplifiers: for superior performance, Travelling Wave Tube Amplifiers (TWTAs) are employed; 24/36

25 Satellite payloads Nonlinear characteriza.on of the payload (2) Such kind of amplifiers are definitely nonlinear: they involve amplitude distor.on and unwanted phase modula.on to the transmired signals; Input power values beyond the 1dB compression point are in principle prohibited. 25/36

26 Satellite payloads Reliability (and redundancy) In order to guarantee payload reliability, hardware redundancy is adopted; Substan0ally, the most fault-cri0cal elements of the payload, or even the en0re payload architecture, are duplicated in order to prevent unrecoverable in-orbit failures. An example of redundancy is shown below: 26/36

27 Satellite pla`orms (summary) Pla_orm components The satellite pla`orm is made by the following components: Metallic structure of the satellite body; Power supply (ba`eries and solar panels); Temperature control system; Orbit control system; Propulsion system; Telemetry, tracking and control (TT&C) equipment. 27/36

28 Frequency alloca0on and radio regula0ons Current alloca.on (source: ESA) 28/36

29 Frequency alloca0on and radio regula0ons Current alloca.on: frequencies vs. services 29/36

30 Frequency alloca0on and radio regula0ons Future alloca.ons: EHF bands The EHF bands (frequencies beyond Ka-band ), so far used for radars and radio-astronomy, will be allocated in the near-future for broadband satellite services: Uplink Downlink Q/V-band GHz GHz GHz GHz W-band GHz GHz A payload launched in the framework of ESA ALPHASAT mission is tes0ng the Q/V band propaga0on ( Aldo Paraboni payload, in memory of Prof. Aldo Paraboni, passed away in 2013, the pioneer of the use of mm-waves for satcoms) 30/36

31 Frequency alloca0on and radio regula0ons Recommenda.ons for spectrum usage The spectrum usage in satellite communica0ons is subjected to recommenda0ons that, really, are mandatory regula0on issues; As the link budget is power-constrained due to longdistance arenua0ons, the power resources must be carefully used and interference levels dras0cally reduced; The co-existence with exis0ng terrestrial radiocommunica0ons sharing satellite spectrum por0ons (e.g. UMTS, LTE, fixed services) must be guaranteed. 31/36

32 Frequency alloca0on and radio regula0ons ITU terrestrial/satellite interference model This model is conven0onally adopted by the Interna0onal Telecommunica0ons Union (ITU): 32/36

33 Frequency alloca0on and radio regula0ons EIRP limita.ons for earth sta.ons and space sta.ons ITU recommenda0ons (RR S.21.8-S21.13) fixed a maximum permissible EIRP produced by transmiung earth sta0ons: 40 dbw for f<=15 GHz and 64 dbw for f>15 GHz; The same recommenda0ons impose also specific limita0ons on power flux density produced by a Space Sta0on on the Earth s surface depending on frequency and arrival angles; In the next slide, the ITU-R table is shown. 33/36

34 Frequency alloca0on and radio regula0ons EIRP limita.ons for space sta.ons: the ITU tables Φ = EIRP 4π L 2 ( ) [W / m2 ] Power flux density at distance L 34/36

35 Frequency alloca0on and radio regula0ons Spectral masks for adjacent channel interference limita.on ITU fixed also some masks aimed at bounding the power spectral density of the transmired RF signal; The purpose is to keep the adjacent channel interference affec0ng channelized signals below an acceptable threshold: we should take into account that nonlinear amplifica0on can produce spectral regrowth of the modulated signal. NOTICE: this mask is considered by ITU for 6MHz DVB-T channel. However, masks for satellite DVB-S2 standard are very similar. 35/36

36 Frequency alloca0on and radio regula0ons INTELSAT masks for satellite system amplitude and frequency responses INTELSAT consor0um fixed in some recommenda0ons the shape of the amplitude and frequency linear responses of the satellite system: frequency response is expressed in terms of group delay. If linear components (filters) does not cope with these masks, equaliza0on must be adopted. 36/36