Satellite Communications: link budget

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Satellite Communications: link budget

  1. 1. Satellite Communications: Link Bugdet Francisco J. Escribano October 19, 2016 Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 1 / 63
  2. 2. Table of contents 1 Motivation 2 Received Power 3 Attenuation 4 Noise Antenna Receiver 5 Carrier-to-Noise Power Ratio 6 Intermodulation 7 Interferences 8 Signal-to-noise ratio 9 Final remarks 10 Conclusions Uplink Downlink 11 References Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 2 / 63
  3. 3. Motivation Motivation Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 3 / 63
  4. 4. Motivation Satellite link Objective of the satellite link: ◮ Deliver services with the best quality and reliability, under strict cost constraints. Design target: ◮ Accurately analyze the main factors involved, such as system character- istics and propagation model. Figure 1: System model. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 4 / 63
  5. 5. Motivation Important side factors Side factors to be taken into account: ◮ Satellite size and weight. ◮ Functions developed. ◮ Assigned frequencies. ◮ Terrestrial Stations dimensions. ◮ Medium access techniques. Main references: [4–9, 11, 12]. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 5 / 63
  6. 6. Received Power Received Power Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 6 / 63
  7. 7. Received Power Power radiated by an antenna Radiated power intensity. ◮ Power radiated by the antenna per solid angle. ◮ Total radiated power is PT . ◮ In general, U (θ, φ) = dP(θ,φ) dΩ W·sr−1 . ◮ If the power is radiated isotropically, U (θ, φ) = PT 4π W·sr−1 . Figure 2: Directional antenna radiation pattern. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 7 / 63
  8. 8. Received Power Antenna gain The gain is defined as the ratio of the actual radiated intensity to the intensity of the isotropic equivalent. ◮ Gain as a function of the direction, G (θ, φ) = U(θ,φ) PT /4π ◮ Maximal gain, GMAX = UMAX PT /4π . ◮ Gain of an antena: G(dBi) = 10 log10 (GMAX). ◮ dBi means we are taking as comparison the istropic case. Figure 3: Directional antenna radiation pattern. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 8 / 63
  9. 9. Received Power Radiation pattern The power captured by an antenna depends on its radiation pattern. ◮ 3dB beam width, for a parabolic reflector, θ3dB ≈ 70 · λ D o (degrees). ◮ λ is the wavelength, D the diameter of the paraboloid. Figure 4: Beam width. Figure 5: Gain versus angle. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 9 / 63
  10. 10. Received Power Power density Equivalent Isotropic Radiated Power (EIRP). ◮ EIRP (θ, φ) = PT · G (θ, φ). Power irradiated per unit area (power density flux). Figure 6: Isotropic antenna. Φ = PT 4πd2 . Figure 7: Directional antenna. Φ (θ, φ) = PT 4πd2 · G (θ, φ). d is the distance to the antenna. Φ (θ, φ) = EIRP(θ,φ) 4πd2 . Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 10 / 63
  11. 11. Received Power Received power Aperture area ARe = πR2 = πD2 4 . Effective area AReff = η · ARe. ◮ η: efficiency (ratio of the effec- tively captured power to total incident power). Figure 8: Paraboloid with diameter D. PR = Φ · AReff. GR = 4π λ2 · AReff = η · πD λ 2 . Received power for an EIRP radiated from a distance d: PR = EIRP 4πd2 · AReff = EIRP 4πd2 GR λ2 4π = EIRP·GR (4πd λ ) 2 . Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 11 / 63
  12. 12. Attenuation Attenuation Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 12 / 63
  13. 13. Attenuation Free space losses Medium: homogeneous, istropic and no obstacles. ◮ lfs = 4πd λ 2 , d distance from the emitter. ◮ Lfs(dB) = 92.44 + 20 log f + 20 log d, f in GHz, d in Km. Figure 9: Free space losses at different frequencies. These are the minimum possible losses in a link. For a geostationary sat (d ≈ 35786 Km over the Equator), they are around 200 dB. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 13 / 63
  14. 14. Attenuation Pointing losses Near the maximal radiation direction, for a deviation of θ ≤ θ3dB, the antenna gain can be approximated as: ◮ G(dB) ≈ GMAX − 12 · θ θ3dB 2 . LptTx ≈ 12 · θeTx θ3dBTx 2 LptRx ≈ 12 · θeRx θ3dBRx 2 Figure 10: Pointing mismatch. When considering the satellite antenna coverage, it is necessary to adjust −3 dB at the coverage edge. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 14 / 63
  15. 15. Attenuation Atmospheric propagation impairments Propagation impairment Physical cause Prime importance Attenation and sky noise in- crease Atmospheric gases, clouds, rain Fequencies above around 10 GHz Signal depolarization Rain, ice crystals Dual-polarization systems at C and Ku bands (depends on sys- tem configuration) Refraction, atmospheric multi- path Atmospheric gasses Communication and tracking at low elevation angles Signal scintillations Tropospheric and ionospheric re- fractivity fluctuations Tropospheric at frequencies above 10 GHz and low ele- vation angles; ionospheric at frequencies below 10 GHz Reflection multipath, blockage Earth’s surface, objetcts on sur- face Mobile satellite services Propagation delays, variations Troposphere, ionosphere Precise timing and location sys- tems; time division multiple ac- cess (TDMA) systems Intersystem interference Ducting, scatter, diffraction Mainly C band; rain scatter may be significant at higher frequen- cies Table 1: Propagation concerns for Satellite Communication Systems. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 15 / 63
  16. 16. Attenuation Attenuation due to rain The attenuation due to rain is calculated according to ITU-R.PN618 as ◮ ARAIN(dB) = γR · LE . ◮ γR(dB/Km) is the specific attenuation due to rain. ◮ LE (Km) is the effective link length accross the rain. The specific attenuation depends on the quantity of precipitations and on the frequency. ◮ These data are based on statistical estimations recorded throughout the different geographical regions. Figure 11: Rainy day in Switzerland. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 16 / 63
  17. 17. Attenuation γR, LE calculations I Rain height (Km) as a function of latitude: ◮ hR = 4 , 0 < lat < 36o 4 − 0.075 · (lat − 36o ) , lat > 36o . Trajectory accross the rain (Km) as a function of the elevation: ◮ LS = hR −hS sin(El) when El > 5o . Rain inhomogeneity factor: ◮ r0.01 = 90 90+4LS cos(El) . The effective length (Km) results: ◮ LE = LS · r0.01. Figure 12: Rainy atmosphere model. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 17 / 63
  18. 18. Attenuation γR, LE calculations II Determine R0.01(mm/h), precipitations exceded 0.01% of the time dur- ing an average year. ◮ This is done with the help of recorded precipitation maps. Figure 13: Rain profile in America. Figure 14: Rain profile in Afroeurope. Figure 15: Rain profile in Asia. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 18 / 63
  19. 19. Attenuation γR, LE calculations III Once R0.01 is calculated, one possibility is to calculate γR with the help of a so-called nomogram. Figure 16: Nomogram. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 19 / 63
  20. 20. Attenuation γR, LE calculations IV Other possibility to calculate γR as reported in ITU-R.P838: γR = k · (R0.01)α k = 4.21 · 10−5 · f 2.42 , 2.9 < f (GHz) < 54 4.09 · 10−2 · f 0.699 , 54 < f (GHz) < 180 . α = 1.41 · f −0.0779 , 8.5 < f (GHz) < 25 2.63 · f −0.272 , 25 < f (GHz) < 164 . The attenuation exceeded 0.01% of the time during an average year would be: ◮ A0.01(dB) = γR · LE . For percentages p other than 0.01%, the attenuation would be calcu- lated as: ◮ Ap = A0.01 · 0.12 · p−(0.546+0.043 log10(p)) Please note how all these calculations rely partly on theoretical results, partly on experimental data and partly on heuristics. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 20 / 63
  21. 21. Attenuation Attenuation by atmospheric gases We follow recommendation ITU-R.P676. ◮ The expressions listed below are simplifications. These losses are mainly related to oxigen and water vapor. ◮ Their effects are less important than those of the rain, except for specific frequencies. γo (dB/Km) = 7.1 f 2+0.36 + 4.5 (f −57)2+0.98 · f 2 · 10−3 . γw (dB/Km) = 0.067 + 3 (f −22.3)2+7.3 · ρw · f 2 · 10−4 . ρw = 10 gr/m3 . ho (Km) = 6. hw (Km) = 2.2 + 3 (f −22.3)2+3 . AG (dB) = γo·ho·e − hS ho +γw ·hw sin(El) . hS in Km, f in GHz. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 21 / 63
  22. 22. Attenuation Attenuation due to clouds and fog We follow recommendation ITU-R.PN840. The procedure is similar to the one proposed for rain attenuation. ACLOUD(dB) = L·Kl sin(El) Figure 17: Specific attenuation by water droplets, Kl . Figure 18: Normalized total columnar content of cloud liquid water, L. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 22 / 63
  23. 23. Attenuation Attenuation due to cross-polarization / angle of arrival The rain also induces losses as it affects signal polarization. Recommendation ITU-R.PN618 handles this issue. ◮ Depends on frequency, elevation angle, polarization angle, exceeded value for a given probability. Refraction in the atmosphere (ITU-R.P834) allows calculation of the apparent elevation angle to account for losses due to mismatched angle of arrival: Figure 19: Signal angular deviation. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 23 / 63
  24. 24. Noise Noise Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 24 / 63
  25. 25. Noise Noise in Communications Systems How to characterise noise in communications? How could we cope with it? Figure 20: Cases of signal to noise balance. Figure 21: Signal vs noise. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 25 / 63
  26. 26. Noise Gaussian Noise Different sources of noise add contributions to the overall system noise. ◮ There are other sources to take into account wrt conventional systems. Figure 22: Galactic noise. Figure 23: Solar noise. Figure 24: Storm noise. Additive white Gaussian noise is a convenient and realistic model. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 26 / 63
  27. 27. Noise Noise modelling: pdf Gaussian noise and its probability density function (pdf). ◮ Zero mean and power σ2 . Figure 25: Noise pdf. Figure 26: Noise. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 27 / 63
  28. 28. Noise Noise modelling: spectrum White Gaussian noise model: constant power spectral density (p). ◮ It has infinite power: Rn (0) = ∞. ◮ Makes sense within a limited band. Figure 27: Noise autocorrelation. Figure 28: Noise psd. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 28 / 63
  29. 29. Noise Thermal noise It is one of the main sources of noise. ◮ Associated to the random movements of electrons. ◮ It is mainly flat when f < 1013 Hz. Figure 29: Thermal noise psd. Figure 30: Noise equivalent circuit. E ν2 n =< ν2 n >= Rn (0) = 4kTBRideal N = <ν2 n>/4 Rideal = kTB Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 29 / 63
  30. 30. Noise Transmitter & receiver TX: signal is far larger than noise → limited problem. RX: signal and noise have similar values → the problems are located here. Figure 31: Transmitter and receiver typical models. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 30 / 63
  31. 31. Noise Noise factor/figure It is a way to measure the noise added by a device. F = (S/N)|in (S/N)|out = Sin/Nin G·Sin/G·(Nin+Na) = 1 + Na Nin Na: noise added by the device. G: device gain. F: is the so-called noise factor. NF(dB) = 10 · log10 (F) is the noise figure. This quantity is relative to the input noise level. ◮ The standard reference is kT0, with T0 = 290o K. kT0 = −204dBW/Hz Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 31 / 63
  32. 32. Noise Noise temperature The device can be modelled as an additional noise source. Na = (F − 1) · Nin kTd B = (F − 1)kT0B Td = (F − 1)T0 Figure 32: Noise temperature setup. Nout = G · (Nin + Na) = G · k · (Ts + Td) · B Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 32 / 63
  33. 33. Noise Noise factor for an attenuator Noise at the output is equal to the noise at the input. Signal is attenuated! If all the components have the same temperature: Figure 33: Attenuator model. kTg B = GKTSB + GNLi NLi = 1−G G kTSB = kTLB TL = 1−G G TS TL = (L − 1)TS → F = L Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 33 / 63
  34. 34. Noise Noise in cascaded systems Equivalent noise factor at the input for N cascaded systems. ◮ Leftmost system, 1; rightmost system N. Feq = F1 + F2−1 G1 + F3−1 G1G2 + · · · + FN −1 G1G2G3···GN−1 Equivalent noise temperature at the input. Teq = T1 + T2 G1 + T3 G1G2 + · · · + TN G1G2G3···GN−1 The two first elements in the cascaded system are the main contributors to the resultant noise!!! Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 34 / 63
  35. 35. Noise System noise temperature We consider the typical first stages of a receiver in Satellite Communi- cations. ◮ We have the antenna, the attenuation determined by the transmission line linking the antenna to the receiver, together with all the connectors involved. Figure 34: RX model. Tsys = Tant + Teq = Tant + TL + LTR Tsys = Tant + (L − 1)T0 + L(F − 1)T0 Tsys = Tant + (LF − 1)T0 Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 35 / 63
  36. 36. Noise Antenna Antenna temperature The antenna acts as a lens whose contribution depends on its direction. Figure 35: Earth station. Tant = Tsky + TEarth + TRAIN Figure 36: Satellite. Tant = TEarth ≈ 290oK Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 36 / 63
  37. 37. Noise Antenna Contributions to antenna temperature Figure 37: Clear sky temperature. Tsky + TEarth Rain temperature: TRAIN = Tab 1 − 1 ARAIN where Tab ≈ 275oK Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 37 / 63
  38. 38. Noise Receiver Linear components, bandlimited To characterise the noise at the receiver, we need to evaluate how linear systems respond to additive white Gaussian noise. Figure 38: Linear Time-invariant System. Input/output: Gaussian noise GY (f ) = GX (f ) · |H(f )|2 Figure 39: Bandpass filtering. Bandpass filtering reduces noise: one of the first stages at RX. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 38 / 63
  39. 39. Noise Receiver Narrowband noise at the mixer Figure 40: Mixer & noise. Figure 41: Equivalent baseband noise. The mixer is a nonlinear device. All this gives reason of the effects of linear/nonlinear systems on input noise. ◮ Total noise at the input is given by the system tem- perature (cascaded systems: filters and mixers have corresponding noise factors). ◮ Then the noise power is calculated taking into account the band limitations. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 39 / 63
  40. 40. Carrier-to-Noise Power Ratio Carrier-to-Noise Power Ratio Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 40 / 63
  41. 41. Carrier-to-Noise Power Ratio Evaluation of the link quality The quality of a satellite link is measured as a function of its carrier power to the noise power, depending on the type of system. ◮ For digital systems: C N0 (Hz) = PR k·Tsys = EIRP·GR /Ltot k·Tsys = EIRP·GR k·Ltot·Tsys C N0 (dBHz) = EIRP (dBW) − Ltot (dB) + GR Tsys (dB/K) + 228.6 ◮ For analog systems: C N (dB) = C N0 (dBHz) − 10 · log10 (BN (Hz)) Ltot comprises all the losses and attenuation effects (these, in fact, act as margins for a given disponibility). BN is the noise bandwidth (in most of the cases, we will take it as equal to the signal bandwidth). Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 41 / 63
  42. 42. Carrier-to-Noise Power Ratio Uplink & Downlink Figure 42: Uplink setup. C N0 U = EIRP|TS − L|U + GR Tsys sat + 228.6 C N0 D = EIRP|sat − L|D + GR Tsys TS + 228.6 Figure 43: Downlink setup. EIRP is a characteristic of the TX. G/T is a figure of merit of the RX. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 42 / 63
  43. 43. Carrier-to-Noise Power Ratio Total C N0 Figure 44: Complete link balance. Signal and noise: C|T = PT |TS · GT |TS · GR |sat ·Gsat · GT |sat GR |TS L|U · L|D , N0|T = N0|U · GT |sat ·Gsat · GR |TS L|D + N0|D . Rearranging: C N0 T −1 = C N0 U −1 + C N0 D −1 (C/N0)|T given above is calculated in natural units (Hz). Note that the lowest term dominates the final carrier-to-noise ratio. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 43 / 63
  44. 44. Intermodulation Intermodulation Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 44 / 63
  45. 45. Intermodulation Sources of nonlinear effecs Intermodulation is mainly due to the presence of nonlinear distortion in the devices. Principles in [10]. ◮ The main source of nonlinearity is the power amplifier (PA) in the satellite: high power travelling-wave tube (TWT) PAs. ◮ PAs are more efficient near the saturation region → need for a trade-off. ◮ One solution is to drive the PA below the saturation region: input/output backoff Effect of input backoff: C N0 U = C N0 Usatur. − BOi Effect of output backoff: C N0 D = C N0 Dsatur. − BOo Figure 45: Amplifier I/O curve. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 45 / 63
  46. 46. Intermodulation Multicarrier case Figure 46: Multicarrier setup. Model for n equal carriers. C N IM ≈ 10.532 − 0.09 · n + 1.7−4 · n2 + 0.82 · BOi dB Figure 47: Typical IM curves. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 46 / 63
  47. 47. Intermodulation Total C N0 Figure 48: C N0 curves. The final carrier-to-noise ratio would be: C N0 T −1 = C N0 U −1 + C N0 D −1 + C N0 IM −1 Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 47 / 63
  48. 48. Interferences Interferences Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 48 / 63
  49. 49. Interferences Global effect of interferences Figure 49: Interferences between two systems. The interferences are mainly caused by the secondary lobes in the radiation pattern of the an- tennas. The effect is modeled as an ad- ditional carrier-to-noise contribu- tion. C I T −1 = C I U −1 + C I D −1 C N T −1 = C N U −1 + C N D −1 + C N IM −1 + C I T −1 Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 49 / 63
  50. 50. Interferences Limiting interferences In order to limit the amount of interference, the following diagram pattern has been proposed. ◮ Frequency range, from 2 to 30GHz. Antennas with D < 100 · λ G (θ) = 29 − 25 · log (θ) , 1o < θ < 48o −10, 48o ≤ θ < 180o Antennas with D > 100 · λ G (θ) = 52 − 10 · log D λ − 25 · log (θ) , 100·λ D < θ < 48o −10 − 10 · log D λ , 48o ≤ θ < 180o Figure 50: Beam width. Figure 51: Gain versus angle. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 50 / 63
  51. 51. Interferences Estimating interferences Under the hypothesis of equal frequencies, equal polarization and link symmetry, the interference of B on A, in the uplink, is: EIRPI = EIRPmax − GTI,max + GTI (θB→A) The corresponding carrier-to-interference ratio would thus be: C I U = EIRPU − EIRPI − GRU + GRI (θA→B ) Similar expression holds for the downlink, consider- ing the corresponding TX and RX involved. Figure 52: Interferences setup. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 51 / 63
  52. 52. Signal-to-noise ratio Signal-to-noise ratio Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 52 / 63
  53. 53. Signal-to-noise ratio SNR equivalences To calculate final performance, we have to derive Es /N0 or Eb/N0. Given that C/N is the signal-to-noise ratio in the channel, we will use the following equivalences: C N0 = Es N0 · Rs C N = Es N0 · Rs BN Rs is the symbol rate. BN is the noise bandwidth, though in some cases the channel total bandwidth could be considered. ◮ If we use an RRC matched filter pair, then BN = Rs. ◮ If nothing is specified, we would use nominal values, where BN = B (occupied bandwidth, e.g. for PSK B = 2Rs). Recall that, at the output of the matched filter+sampler, S N = 2Es N0 . Final Eb/N0 value will have to take into account FEC and modulation schemes. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 53 / 63
  54. 54. Final remarks Final remarks Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 54 / 63
  55. 55. Final remarks GEO/MEO/LEO The principles derived in the previous sections apply to any satellite configuration (GEO/MEO/LEO). ◮ Nevertheless, the implicit context is TX/RX to and from GEO satellites, so that a static situation is assumed. ◮ For MEO/LEO, all is valid for the same type of emitting/receiving an- tennas and under the assumption of perfect acquisition and tracking. Figure 53: GEO/LEO/MEO orbit configuration and associate transmission delays [1]. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 55 / 63
  56. 56. Final remarks LEO/MEO I In case LEO/MEO systems use dish antennas, the calculations for the link budget are essentially the same. The main differences are associated with: ◮ Lower TX/RX delay. ◮ Need for signal acquisition and tracking (acquisi- tion is performed using beacons). ◮ Lower distance → lower free-space losses. ◮ Higher throughputs are possible. ◮ Satellites are not always visible: limited time avail- ability. ◮ Different kind of applications: GNSS, Earth Ob- servation, mesh networks. Figure 54: Antenna positioner [2]. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 56 / 63
  57. 57. Final remarks LEO/MEO II Gain calculations are different for other kind of an- tennas. ◮ GNSS RX antennas usually have 3dBi gain. ◮ Active GNSS RX antennas may add around 20dB LNA gain. Being in movement, Doppler effect has to be con- sidered and compensated. Misshaps in tracking and Doppler issues may re- quire additional link budget margins. Beware of the point-ahead effect. Figure 55: Passive GPS antenna [3]. Figure 56: Point-ahead angle [13]. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 57 / 63
  58. 58. Conclusions Conclusions Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 58 / 63
  59. 59. Conclusions Uplink Uplink The satellite antenna beamwidth provided to cover a specific service area determines the gain of the receiver antenna. To avoid large gain variations due to pointing mismatches, antennas with large D require tracking. Rain attenuates the received power, but it does not contribute sig- nificantly to the noise temperature (relatively high, around 290oK); a countermeasure could be increasing the transmitted power. The input power density reaching the satellite should be controlled to avoid intermodulation due to PA saturation. The orbital separation between geostationary satellites that operate in neighbour bands is low (a few degrees): it is important to have TS narrow beamwidth antennas with low secondary lobes. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 59 / 63
  60. 60. Conclusions Downlink Downlink The transmitted power is strictly limited. The antenna gain needs to be adjusted to the coverage area to avoid interferences and issues with band licensing, nationally owned rights and so forth. Rain attenuates the received power and, besides, may increase signifi- cantly the noise temperature of the TS. The ouput power density of the satellite should be controlled as well to avoid intermodulation due to PA saturation. Another important reason to limit the ouput power of the satellite downlink is the need to limit interferences. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 60 / 63
  61. 61. References References Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 61 / 63
  62. 62. References Bibliography I [1] url: http://www.harriscaprock.com/blog/high-throughput- satellite- communications- systems- meo-vs- leo-vs-geo. [2] url: http://orbit-cs.com. [3] url: http://www.globalsources.com/gsol/I/GPS-antenna/p/sm/1043313208.htm#1043313208. [4] Giovanni E. Corazza. Digital Satellite Communications. New York: Springer, 2007. [5] ITU-R.P676. “Attenuation by atmospheric gases”. In: International Telecommunication Union (). url: %7Bhttp://www.itu.int/rec/R-REC-P.676/en%7D. [6] ITU-R.P834. “Effects of tropospheric refraction on radiowave propagation”. In: International Telecommunication Union (). url: http://www.itu.int/rec/R-REC-P.834/en. [7] ITU-R.P838. “Specific attenuation model for rain for use in prediction methods”. In: International Telecommunication Union (). url: %7Bhttp://www.itu.int/rec/R-REC-P.838/en%7D. [8] ITU-R.PN618. “Propagation data and prediction methods required for the design of Earth-space telecommunication systems”. In: International Telecommunication Union (). url: %7Bhttps://www.itu. int/rec/R-REC-P.618/en%7D. [9] ITU-R.PN840. “Attenuation due to clouds and fog”. In: International Telecommunication Union (). url: http://www.itu.int/rec/R-REC-P.840/en. [10] ITU-R.SM.2021. “Production and mitigation of intermodulation products in the transmitter”. In: International Telecommunication Union (). url: http://www.itu.int/pub/R-REP-SM.2021. [11] Gérard Maral. Satellite Communications Systems: Systems, Techniques and Technology. Chichester: John Wiley & Sons, Inc., 1998. [12] Timothy Pratt. Satellite Communications. New York: John Wiley & Sons, Inc., 2003. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 62 / 63
  63. 63. References Bibliography II [13] Kim A. Winick. “Atmospheric turbulence-induced signal fades on optical heterodyne communication links”. In: Appl. Opt. 25.11 (June 1986), pp. 1817–1825. doi: 10.1364/AO.25.001817. url: http://ao. osa.org/abstract.cfm?URI=ao- 25-11-1817. Francisco J. Escribano Satellite Communications: Link Bugdet October 19, 2016 63 / 63

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