Satellite Communications

Transcription

1 Signal processing elements Propagation and radio communications Engineering Satellite Communications Marc Van Droogenbroeck Department of Electrical Engineering and Computer Science University of Liège November 2012 Marc Van Droogenbroeck Satellite Communications

2 Signal processing elements Propagation and radio communications Engineering Outline 1 Signal processing elements What do we transmit? Information! Source coding Modulation Multiplexing 2 Propagation and radio communications Background Radiowave propagation Examples of antennas 3 Engineering Noise Link budget Marc Van Droogenbroeck Satellite Communications

3 Signal processing elements Propagation and radio communications Engineering What do we transmit? Information! Source coding Modulation Multiplexing Outline 1 Signal processing elements What do we transmit? Information! Source coding Modulation Multiplexing 2 Propagation and radio communications Background Radiowave propagation Examples of antennas 3 Engineering Noise Link budget Marc Van Droogenbroeck Satellite Communications

4 Signal processing elements Propagation and radio communications Engineering Main types of satellite What do we transmit? Information! Source coding Modulation Multiplexing Astronomical satellites: used for observation of distant planets, galaxies, and other outer space objects. Navigational satellites [GPS, Galileo]: they use radio time signals transmitted to enable mobile receivers on the ground to determine their exact location. Earth observation satellites: used for environmental monitoring, meteorology, map making. Miniaturized satellites: satellites of unusually low masses and small sizes. Communications satellites: stationed in space for the purpose of telecommunications. Modern communications satellites typically use geosynchronous orbits, or Low Earth orbits (LEO). Marc Van Droogenbroeck Satellite Communications

5 Signal processing elements Propagation and radio communications Engineering Analog and digital signals What do we transmit? Information! Source coding Modulation Multiplexing Marc Van Droogenbroeck Satellite Communications

6 Characterization Analog signal Digital signal Bandwidth [Hz] Bit rate [bit/s] Signal to Noise ratio (S/N) Bit error rate (BER) Bandwidth of the underlying channel [Hz] Going digital because: possibility to regenerate a digital signal better bandwidth usage analog PAL television signal: bandwidth of 8 [MHz] digital television, PAL quality 5[Mb/s] With a 64-QAM modulation, whose spectral eciency is 6b/s per Hz. 8[MHz] allows for 48[Mb/s]. Therefore, there is room for 10 digital television instead of one analog channel.

7 Data streams Types of data Control data Measurements Remote sensing Localization data Broadcasting Payload Characteristics Must be very reliable Accurate signals with constant monitoring High volume of downstream data Accurate time reference (synchronization) Television channels Unicast communication for mobile ground station

8 Data encapsulation: OSI model

9 Elements of a communication channel

11 Information theory and channel capacity Theorem [Shannon] The capacity of an Additive White Gaussian Noise (AWGN) channel is ( C = W log S ) N (1) where W is the bandwidth occupied by the signal, S is the signal power and N = WN 0 is the Gaussian noise variance. Let R be the bit rate and E b the energy per bit (in joules), we have S = E b R, and ( C = W log E ) b R (2) N 0 W The ratio R W is the spectral eciency expressed in [b/s] per [Hz].

12 Bit/packet error rate Assume a packet of size N and let P e be the probability error on one bit. The probability for the packet to be correct is Therefore the packet error rate is (1 P e ) N (3) P P = 1 (1 P e ) N (4) For large packets and small P e, this becomes P P 1 (1 NP e ) = N P e (5) Example With N = 10 5 bits and a bit error rate of P e = 10 7, P P 10 2.

13 Forward Error Coding A simplistic example of Forward Error Coding (FEC) is to transmit each data bit 3 times, known as a (3,1) repetition code.

14 Other forward error codes Hamming code ReedSolomon code Turbo code,...

16 Modulation Signal processing elements Propagation and radio communications Engineering What do we transmit? Information! Source coding Modulation Multiplexing A cosine cos(2πf c t) is the carrier. s(t) is the modulated signal s(t) = A(t)cos(2πf (t)t + φ(t)) (6) A(t): Amplitude Modulation (AM) f (t): Frequency Modulation (FM) φ(t): Phase Modulation (PM) Marc Van Droogenbroeck Satellite Communications

17 Demodulation

18 Basic digital modulation techniques

19 Quadrature modulation It is possible to use both a cosine and a sine with a unique bandwidth. s(t) = s I (t)cos(2πf c t) s Q (t)sin(2πf c t) (7)

20 Quadrature demodulation s(t) = s I (t)cos(2πf c t) s Q (t)sin(2πf c t) is the modulated signal. We want to recover s I (t) Step 1: multiply by cos(2πf c t) s(t) cos(2πf c t) = s I (t)cos 2 (2πf c t) s Q (t)sin(2πf c t)cos(2πf c t) = 1 2 s I (t) s I (t)cos(2π(2f c )t) 1 2 s Q(t)sin(2π(2f c )t) Step 2: lter to keep the baseband signal 1 2 s I (t) Steps 3 and 4: multiply by sin(2πf c t) and low-pass lter to get s Q (t)

22 Signal processing elements Propagation and radio communications Engineering Multiplexing: combining several sources What do we transmit? Information! Source coding Modulation Multiplexing Frequency Division Multiplexing (FDM) Time Division Multiplexing (TDM) Code Division Multiplexing (CDM) Space Division Multiplexing + combinations! Marc Van Droogenbroeck Satellite Communications

23 Frequency Division Multiplexing (FDM)

24 Demultiplexing

25 Signal processing elements Propagation and radio communications Engineering Time Division Multiplexing (TDM) What do we transmit? Information! Source coding Modulation Multiplexing Marc Van Droogenbroeck Satellite Communications

26 Spread spectrum for Code Division Mulplexing Principle of spread spectrum: multiply a digital signal with a faster pseudo-random sequence. At the receiver, the pseudo-random sequence is generated and used to despread the signal.

27 Code Division Multiple Access

28 Summary

29 Signal processing elements Propagation and radio communications Engineering Background Radiowave propagation Examples of antennas Outline 1 Signal processing elements What do we transmit? Information! Source coding Modulation Multiplexing 2 Propagation and radio communications Background Radiowave propagation Examples of antennas 3 Engineering Noise Link budget Marc Van Droogenbroeck Satellite Communications

30 Signal processing elements Propagation and radio communications Engineering Background Radiowave propagation Examples of antennas Satellite link denition Marc Van Droogenbroeck Satellite Communications

31 Frequency bands But it is better to designate the carrier frequency directly

32 Regulatory bodies International Telecommunications Union (ITU): Radiocommunications Sector (ITU-R) service regions organizes WARC (World Administrative Radio Conference) - worldwide allocation of frequencies Regional body: European Conference of Postal and Telecommunications Administrations (CEPT)

33 Extract of the allocation plan/radio spectrum (by the ITU)

34 Orbits

36 Signal processing elements Propagation and radio communications Engineering Background Radiowave propagation Examples of antennas Radiowave propagation Important parameters: channel characteristics attenuation (distance) wave polarization rain mitigation antenna design power budget Marc Van Droogenbroeck Satellite Communications

37 Inverse square law of radiation The power ux density (or power density) S, over the surface of a sphere of radius r a from the point P, is given by P t S a = 4πra 2 [ ] W m 2 (8)

38 Eective Isotropic Radiated Power [EIRP] Denition The Eective Isotropic Radiated Power (EIRP) of a transmitter is the power that the transmitter appears to have if the transmitter were an isotropic radiator (if the antenna radiated equally in all directions). Therefore, at the receiver, P t = P E G E (9)

39 Eective area Denition The eective area of an antenna is the ratio of the available power to the power ux density (Poynting vector): A e = P R S e (10) Theorem The eective area of an antenna is related to its gain by the following formula A e = G E λ 2 4π (11)

40 Friis relationship We have P R = S e,r A e,r ( ) PE G E = A 4πd 2 e,r = ( PE G E 4πd 2 )( ) λ 2 4π ( ) λ 2 G R = P E G E G R 4πd Free space path loss L FS = ( λ 4πd Friis's relationship ) 2 ε = P E PR = ( 4πd λ ) 2 1 GE GR

41 Decibel as a common power unit x 10log 10 (x)[db] (12) P [dbm] = 10log 10 P [mw ] 1[mW ] (13) x [W ] 10log 10 (x)[dbw ] 1[W ] 0[dBW ] 2[W ] 3[dBW ] 0,5[W ] 3[dBW ] 5[W ] 7[dBW ] 10 n [W ] 10 n [dbw ]

42 Are high frequencies less adequate? In [db], Friis's relationship becomes ε = log f [MHz] + 20log d [km] G E [db] G R [db] The attenuation (loss) increases with f. So?! but A e = G E λ 2 4π (14) ε = ( 4πd λ ) 2 1 G E G R = ( 4πd λ ) 2 λ 2 4πA E λ 2 4πA R (15) = λ 2 d 2 A E A R = c 2 d 2 f 2 A E A R (16) It all depends on the antenna gains.

43 Are high frequencies less adequate? In [db], Friis's relationship becomes ε = log f [MHz] + 20log d [km] G E [db] G R [db] The attenuation (loss) increases with f. So?! but A e = G E λ 2 4π (14) ε = ( 4πd λ ) 2 1 G E G R = ( 4πd λ ) 2 λ 2 4πA E λ 2 4πA R (15) = λ 2 d 2 A E A R = c 2 d 2 f 2 A E A R (16) It all depends on the antenna gains.

44 Practical case: VSAT in the Ku-band [1] Antenna gain: 48.93[dB] The free space path loss is, in [db], L FS = log f [MHz] + 20log d [km] = 205.1[dB] The received power is, in [db], P R = P E + G E + G R L FS (17) = = 97.24[dB] (18) In [W ], the received power is P R = = [W ] = 189[pW ] (19)

46 Signal processing elements Propagation and radio communications Engineering Background Radiowave propagation Examples of antennas Terrestrial antennas Marc Van Droogenbroeck Satellite Communications

47 Beamwidth and aperture

48 Ground station antenna

49 Parabolic (dish) antenna Radiation pattern

50 Deployable antenna

51 Horn antenna and waveguide feed

52 Yagi antenna

53 Patch array antenna

54 Phased array antenna

55 Radiowave propagation mechanisms

56 Earth atmosphere λ [m] = c f = [m/s] f [Hz]

57 Attenuation due to atmospheric gases Zenith attenuation due to atmospheric gases (source: ITU-R P.676-6)

58 Service Level Agreement (SLA)

59 Rain attenuation Total path rain attenuation as a function of frequency and elevation angle. Location: Washington, DC, Link Availability: 99%

60 Cloud attenuation Cloud attenuation as a function of frequency, for elevation angles from 5 to 30

61 Signal processing elements Propagation and radio communications Engineering Noise Link budget Outline 1 Signal processing elements What do we transmit? Information! Source coding Modulation Multiplexing 2 Propagation and radio communications Background Radiowave propagation Examples of antennas 3 Engineering Noise Link budget Marc Van Droogenbroeck Satellite Communications

62 Signal processing elements Propagation and radio communications Engineering Noise Link budget Noise The noise power P N is given by the Nyquist formula: where P N = k B T W (20) k B = 1, [J/K] is the constant of Boltzmann (=198[dBm/K/Hz] = =228.6[dBw/K/Hz]), T is the equivalent noise temperature of the noise source W is the noise bandwidth Marc Van Droogenbroeck Satellite Communications

63 Two-port device Denitions Noise Factor (F): Noise Figure (NF): F= ( S ) N ( in S ) N out > 1 (21) NF=10 log 10 F (22) Eective noise temperature T e (T 0 = 290[K]): T e = T 0 (F 1) (23)

64 Noise factor of cascaded stages F = F 1 + F 2 1 G 1 + F 3 1 G 1 G 2 + = F 1 + T e = T e1 + T e2 G 1 + T e3 G 1 G 2 + = T e1 + n i=2 n i=2 F i 1 i 1 j=1 G j T ei i 1 j=1 G j (24) (25)

65 Receiver front end Rule of thumb: highest gain and best noise gure rst. Then NF = NF 1 + NF 2 1 G 1 + NF 3 1 G 1 G 2 + NF 1 + NF 2 1 G 1 (26) T e = T e1 + T e2 G 1 + T e3 G 1 G 2 + T e1 + T e2 G 1 (27)

66 Calculation of noise budget [1] Low Noise Amplier: T LA = 290 ( ) = 438[K] Line. For a passive two-part, F = A. T Line = 290 ( ) = 289[K], G Line = 1 2 The eective noise temperature, including the antenna noise, is T e = t A + T e1 + T e2 G 1 + T e3 G 1 G 2 + (28) = }{{} = 509.3[K] (29)

67 Representative values for the increase in antenna temperature due to rain [1]

68 Signal processing elements Propagation and radio communications Engineering Noise Link budget Outline 1 Signal processing elements What do we transmit? Information! Source coding Modulation Multiplexing 2 Propagation and radio communications Background Radiowave propagation Examples of antennas 3 Engineering Noise Link budget Marc Van Droogenbroeck Satellite Communications

69 Signal processing elements Propagation and radio communications Engineering Noise Link budget Link budget: typical parameters for a communication satellite [1] Parameter Uplink Downlink Frequency 14.1[GHz] 12.1[GHz] Bandwidth 30[MHz] 30[MHz] Transmitter power [W ] [W ] Transmitter antenna gain 54[dBi] 36.9[dBi] Receiver antenna gain 37.9[dBi] 52.6[dBi] Receiver noise gure 8[dB] 3[dB] Receiver antenna temperature 290[K] 50[K] Free space path loss (30 elevation) 207.2[dB] 205.8[dB] Marc Van Droogenbroeck Satellite Communications

70 Signal processing elements Propagation and radio communications Engineering Noise Link budget For further reading L. Ippolite. Satellite Communications Systems Engineering: Atmospheric Eects, Satellite Link Design and System Performance. Wiley, J. Gibsons. The Communications Handbook. CRC Press, Wikipedia. Marc Van Droogenbroeck Satellite Communications