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AEE Telecom and Electronics Core · Chapter 9

Fibre Optic and Satellite Communication

What to remember

  • Light stays in the core by total internal reflection, which needs n1 > n2. Numerical aperture NA = √(n1² − n2²) and acceptance angle θa = sin⁻¹(NA) (launching from air).
  • Single-mode fibre has V below 2.405, a core of about 9 µm and no modal dispersion. Attenuation is lowest near 1550 nm; dispersion is lowest near 1310 nm.
  • A geostationary satellite is about 35,786 km above the equator and has a period equal to the Earth's rotation. Uplink frequency is higher than downlink (C band: 6 GHz up, 4 GHz down).

Fibre structure and light guidance

An optical fibre has a core (higher refractive index n1), a cladding (lower index n2) and a protective coating or jacket. Light travelling from a dense to a rare medium above the critical angle θc = sin⁻¹ (n2 / n1) is totally reflected.

  • Acceptance angle: θa = sin⁻¹ (NA), where NA = √(n1² − n2²).
  • Relative index difference: Δ = (n1 − n2) / n1. NA ≈ n1 √(2Δ).
  • Normalized frequency (V number): V = (2π a / λ) × NA, where a is the core radius. A step-index fibre is single mode if V < 2.405. For a multimode step-index fibre the number of modes is about V² / 2.
Fibre typeCoreKey points
Step-index multimode50 or 62.5 µmLarge modal dispersion; low bit rate; short links
Graded-index multimode50 or 62.5 µmIndex falls smoothly from the centre; rays refocus; much lower modal dispersion
Single-modeabout 9 µmOnly one mode; very high bandwidth; long-haul, trunk and access networks

Cladding diameter is standard at 125 µm for most telecom fibres.

Loss and dispersion

  • Attenuation: α (dB/km) = (10 / L) log10 (Pin / Pout). Causes: absorption, scattering (Rayleigh scattering varies as 1/λ⁴), and bending (macro- and micro-bends).
  • Low-loss windows: about 850 nm, 1310 nm and 1550 nm. The 1550 nm window has the lowest loss, about 0.2 dB/km in good silica fibre.
  • Dispersion spreads pulses and limits bit rate:
  • Modal (intermodal) dispersion: occurs only in multimode fibre.
  • Chromatic dispersion: material plus waveguide; zero near 1310 nm in standard single-mode fibre.
  • Polarization mode dispersion (PMD): matters at very high bit rates.
  • Higher dispersion means a lower bandwidth-distance product.

Sources, detectors and components

ItemNotes
LEDLow cost, wide spectral width, incoherent; used for short links with multimode fibre
Laser diodeNarrow spectral width, coherent, high power; used with single-mode fibre for long links
PIN photodiodeSimple, no gain; low noise at moderate speed
APD (avalanche photodiode)Internal gain; more sensitive; needs a high bias voltage
EDFAErbium-doped fibre amplifier; amplifies the 1550 nm band; replaces regenerators
Connectors and splicesFusion splicing gives lower loss than mechanical splices
Couplers, WDMJoin or split signals by wavelength

WDM: several wavelengths on one fibre. DWDM uses very close wavelengths and gives very high capacity. Optical fibre is immune to electromagnetic interference, is light, is secure and has very wide bandwidth.

Link power budget: the link margin = (transmit power − receiver sensitivity) − (fibre loss + connector loss + splice loss), all in dB or dBm.

Satellite communication basics

A satellite orbits under gravity. Kepler's third law: T² is proportional to a³, where a is the semi-major axis.

OrbitHeightNotes
LEOa few hundred to about 2000 kmLow delay; fast moving; needs many satellites; Doppler shift
MEOabout 2000 km to just below GEONavigation satellites (GPS-type constellations)
GEOabout 35,786 km above the equatorPeriod about 24 h; appears fixed; three satellites cover most of the Earth
HEO (Molniya)Highly ellipticalUsed for high-latitude coverage

Geostationary orbit is circular, equatorial, with the same direction of rotation as the Earth. Radius from Earth's centre is about 42,164 km. Round-trip signal delay is about a quarter of a second. A GEO link suffers from long delay and poor coverage near the poles. Sun-transit outage and eclipses also occur.

Frequency bands:

BandUplink / Downlink
Cabout 6 GHz / 4 GHz (rain-resistant)
Kuabout 14 GHz / 12 GHz (small dishes, DTH)
Kaabout 30 GHz / 20 GHz (high rain fade)

Uplink uses the higher frequency to keep earth-station antenna and amplifier size manageable and to reduce satellite power needs.

Key terms: a transponder receives, shifts frequency and amplifies the signal. EIRP = Pt + Gt (in dB units). G/T is the receiver figure of merit. Free-space path loss FSPL (dB) = 32.44 + 20 log10 d (km) + 20 log10 f (MHz). VSAT is a small earth terminal; DTH is direct-to-home TV. Multiple access methods are FDMA, TDMA and CDMA. Navigation: a receiver needs signals from at least four satellites for position (three coordinates) and time. India's regional system is NavIC.

Fibre system design and testing

  • Basic link: electrical signal, driver, source (LED or laser), fibre with splices and connectors, optical detector, receiver amplifier, decision circuit.
  • Rise-time budget: the total rise time of the system is √(tTx² + tfibre² + tRx²). A common rule is that it must be below about 70% of the bit period for NRZ data.
  • Bandwidth-length product (for example in MHz·km) tells how far a given bit rate can go. It is far higher for single-mode than for multimode fibre.
  • OTDR (optical time-domain reflectometer): sends a pulse and reads the back-scattered light. It shows fibre length, splice loss, connector loss and the position of faults.
  • Optical power meter and source: measure end-to-end loss.
  • Cables: loose-tube cables protect fibres from stress in outdoor ducts; tight-buffer cables are used indoors. Strength members (steel or aramid) bear the pulling load.
  • Non-linear effects appear at high power and in DWDM: stimulated Raman and Brillouin scattering, and four-wave mixing.

Satellite subsystems and launch

  • Satellite parts: payload (antennas and transponders), bus (structure, power, thermal control, attitude control, propulsion, telemetry and command).
  • Power: solar panels charge batteries; batteries supply power during eclipse.
  • Attitude control: spin stabilization or three-axis stabilization keeps antennas pointed at the Earth.
  • Station keeping: small thrusters correct the drift caused by the Sun, the Moon and the Earth's shape.
  • Launch: a satellite is first put into a transfer orbit and then, with an apogee motor, moved to its final circular orbit. India launches with its PSLV and GSLV rockets.
  • Earth station: a large dish, high-power amplifier, low-noise amplifier (LNA) and up- and down-converters. The LNA sets the receiver noise temperature.
  • Rain fade matters above about 10 GHz and is handled with extra power margin.

Worked examples

  • 1. NA: n1 = 1.48, n2 = 1.46. NA = √(2.1904 − 2.1316) = √0.0588 ≈ 0.2425; θa = sin⁻¹ 0.2425 ≈ 14°.
  • 2. Critical angle: sin⁻¹ (1.46 / 1.48) ≈ 80.6°.
  • 3. Loss: 10 mW in, 0.1 mW out over 20 km: loss = 10 log 100 = 20 dB; 1 dB/km.
  • 4. V number: a = 4.5 µm, λ = 1.3 µm, NA = 0.1: V = (2π × 4.5 / 1.3) × 0.1 ≈ 2.17. This is below 2.405, so single mode.
  • 5. Link margin: Tx = −3 dBm; Rx sensitivity = −33 dBm; 30 km at 0.5 dB/km = 15 dB; two connectors at 1 dB = 2 dB. Margin = 30 − 17 = 13 dB.
  • 6. FSPL: d = 36,000 km, f = 4000 MHz: 32.44 + 91.13 + 72.04 ≈ 195.6 dB.

Exam traps

  • Critical angle is for light going from the core to the cladding; acceptance angle is for light entering from air.
  • Lowest attenuation is at 1550 nm; zero dispersion (standard fibre) is at 1310 nm. Do not swap them.
  • Single mode means V < 2.405, not V > 2.405.
  • Graded index is a multimode fibre; it is not single mode.
  • APD has gain; PIN does not.
  • LED goes with multimode fibre; the laser diode goes with single mode.
  • Uplink is the higher frequency (6 GHz), downlink the lower (4 GHz) in C band.
  • GEO is about 35,786 km above the surface, but about 42,164 km from the Earth's centre.
  • Ka band has more rain fade than C band.
  • Navigation needs four satellites, not three, because the receiver clock is unknown.

One-liners

  • 1. Fibre guidance relies on total internal reflection.
  • 2. Core has the higher refractive index.
  • 3. NA = √(n1² − n2²).
  • 4. Standard cladding diameter is 125 µm.
  • 5. 1550 nm has the lowest attenuation.
  • 6. EDFA amplifies near 1550 nm.
  • 7. Rayleigh scattering varies as 1/λ⁴.
  • 8. A GEO satellite appears fixed to an observer on Earth.
  • 9. C-band downlink is about 4 GHz.
  • 10. A transponder shifts frequency and amplifies.
  • 11. Kepler's third law links period and orbit size.
  • 12. NavIC is India's regional navigation system.

Practice questions

  1. Light is guided in an optical fibre by

    1. polarization
    2. diffraction
    3. total internal reflection
    4. Doppler shift
    Answer

    C. total internal reflection

    Light is reflected totally at the core-cladding boundary when n1 > n2 and the angle exceeds the critical angle.

  2. In a fibre, the core refractive index compared with the cladding is

    1. equal
    2. zero
    3. lower
    4. higher
    Answer

    D. higher

    Total internal reflection needs n1 > n2.

  3. Which wavelength window gives the lowest attenuation in silica fibre?

    1. 850 nm
    2. 1310 nm
    3. 1550 nm
    4. 650 nm
    Answer

    C. 1550 nm

    Loss is lowest, about 0.2 dB/km, near 1550 nm.

  4. The wavelength of zero chromatic dispersion in a standard single-mode fibre is near

    1. 980 nm
    2. 1310 nm
    3. 1550 nm
    4. 850 nm
    Answer

    B. 1310 nm

    Standard fibre has zero dispersion near 1310 nm.

  5. Which fibre has no modal dispersion?

    1. Single-mode fibre
    2. Plastic multimode fibre
    3. Step-index multimode fibre
    4. Graded-index multimode fibre
    Answer

    A. Single-mode fibre

    Only one mode exists, so there is no modal dispersion.

  6. The usual cladding diameter of telecom fibre is

    1. 125 µm
    2. 9 µm
    3. 250 mm
    4. 50 µm
    Answer

    A. 125 µm

    Standard glass fibres have a 125 µm cladding.

  7. An EDFA is used to amplify optical signals near

    1. 850 nm
    2. 1310 nm only
    3. 1550 nm
    4. 550 nm
    Answer

    C. 1550 nm

    Erbium-doped fibre amplifies in the 1550 nm band.

  8. A photodetector with internal gain is the

    1. LED
    2. PIN photodiode
    3. Zener diode
    4. avalanche photodiode
    Answer

    D. avalanche photodiode

    APDs multiply carriers by avalanche action.

  9. A geostationary satellite orbits at about what height above the equator?

    1. 20,200 km
    2. 35,786 km
    3. 3,578 km
    4. 357,860 km
    Answer

    B. 35,786 km

    The standard GEO altitude is about 35,786 km.

  10. The period of a geostationary satellite is about

    1. 90 minutes
    2. 12 hours
    3. 48 hours
    4. 24 hours (one sidereal day)
    Answer

    D. 24 hours (one sidereal day)

    It matches the Earth's rotation.

  11. In C-band satellite communication the uplink frequency is about

    1. 12 GHz
    2. 6 GHz
    3. 4 GHz
    4. 30 GHz
    Answer

    B. 6 GHz

    C band is 6 GHz up and 4 GHz down.

  12. The device in a satellite that receives, shifts frequency and amplifies is the

    1. rectifier
    2. isolator
    3. transponder
    4. modem
    Answer

    C. transponder

    A transponder receives the uplink and sends the downlink at a different frequency.

  13. A light source suited to long-distance single-mode links is the

    1. laser diode
    2. LED
    3. incandescent lamp
    4. photo-transistor
    Answer

    A. laser diode

    Laser diodes give narrow spectral width and high power.

  14. A fibre has n1 = 1.5 and n2 = 1.2. The numerical aperture is

    1. 0.3
    2. 1.35
    3. 0.9
    4. 0.45
    Answer

    C. 0.9

    NA = √(2.25 − 1.44) = √0.81 = 0.9.

  15. For n1 = 1.5 and n2 = 1.2 (NA = 0.9), the critical angle at the core-cladding boundary is

    1. 53.1°
    2. 90°
    3. 60°
    4. 36.9°
    Answer

    A. 53.1°

    sin θc = 1.2/1.5 = 0.8, so θc ≈ 53.1°.

  16. A fibre loses 20 dB over 40 km. The attenuation per km is

    1. 2 dB/km
    2. 0.05 dB/km
    3. 5 dB/km
    4. 0.5 dB/km
    Answer

    D. 0.5 dB/km

    20/40 = 0.5 dB/km.

  17. Input power is 1 mW and output is 0.1 mW. The loss in dB is

    1. 0.1 dB
    2. 10 dB
    3. 20 dB
    4. 1 dB
    Answer

    B. 10 dB

    10 log10 (1/0.1) = 10 dB.

  18. Input power is 100 mW and loss is 30 dB. The output power is

    1. 1 mW
    2. 3 mW
    3. 10 mW
    4. 0.1 mW
    Answer

    D. 0.1 mW

    30 dB is a factor 1000; 100/1000 = 0.1 mW.

  19. For a step-index fibre of core radius 5 µm, λ = 1 µm and NA = 0.2, the V number is closest to

    1. 6.28
    2. 12.6
    3. 3.14
    4. 1.0
    Answer

    A. 6.28

    V = 2π × 5/1 × 0.2 = 6.28.

  20. A step-index multimode fibre has V = 20. The approximate number of modes is

    1. 400
    2. 200
    3. 40
    4. 20
    Answer

    B. 200

    Modes ≈ V²/2 = 400/2 = 200.

  21. Free-space path loss at 36,000 km and 4000 MHz is closest to

    1. 100 dB
    2. 240 dB
    3. 195.6 dB
    4. 145 dB
    Answer

    C. 195.6 dB

    32.44 + 91.13 + 72.04 ≈ 195.6 dB.

  22. A fibre link has Tx power 0 dBm, Rx sensitivity −30 dBm, fibre loss 0.4 dB/km over 50 km and 4 dB of connector loss. The margin is

    1. 26 dB
    2. 6 dB
    3. 16 dB
    4. 10 dB
    Answer

    B. 6 dB

    Total loss = 20 + 4 = 24 dB; margin = 30 − 24 = 6 dB.

  23. Free-space path loss rises by how many dB when the distance is doubled?

    1. 12 dB
    2. 20 dB
    3. 3 dB
    4. 6 dB
    Answer

    D. 6 dB

    20 log10 2 ≈ 6 dB.

  24. The minimum number of satellites needed to fix a position (three coordinates and time) is

    1. 4
    2. 2
    3. 3
    4. 1
    Answer

    A. 4

    Four unknowns (x, y, z and clock error) need four signals.

  25. According to Kepler's third law, the square of the orbital period is proportional to

    1. the semi-major axis
    2. the inverse of the radius
    3. the cube of the semi-major axis
    4. the square of the semi-major axis
    Answer

    C. the cube of the semi-major axis

    T² ∝ a³.

  26. Which satellite band is most affected by rain fade?

    1. VHF
    2. C band
    3. Ka band
    4. L band
    Answer

    C. Ka band

    Attenuation due to rain rises with frequency, so Ka band suffers the most.

  27. Which test instrument finds the location of a break in a fibre?

    1. Oscilloscope
    2. OTDR
    3. Megger
    4. Spectrum analyser
    Answer

    B. OTDR

    The optical time-domain reflectometer locates faults from backscatter.

  28. Rayleigh scattering loss varies with wavelength as

    1. λ
    2. 1/λ
    3. λ²
    4. 1/λ⁴
    Answer

    D. 1/λ⁴

    Rayleigh scattering is proportional to 1/λ⁴.

  29. Why is the satellite uplink at a higher frequency than the downlink?

    1. Lower downlink frequency has less path loss for the power-limited satellite
    2. Higher uplink avoids orbit drift
    3. Lower downlink removes rain fade completely
    4. Higher frequency lowers the signal delay
    Answer

    A. Lower downlink frequency has less path loss for the power-limited satellite

    The satellite has limited power, so its downlink uses the lower frequency with less loss; the earth station has ample power for the higher uplink.

  30. A WDM system sends several signals on one fibre by using different

    1. polarizations only
    2. wavelengths
    3. core diameters
    4. cladding materials
    Answer

    B. wavelengths

    Wavelength-division multiplexing uses separate optical wavelengths.

  31. Polarization mode dispersion is most significant at

    1. power cables
    2. audio frequencies
    3. low bit rates on multimode fibre
    4. very high bit rates on single-mode fibre
    Answer

    D. very high bit rates on single-mode fibre

    PMD limits long single-mode links at high data rates.

  32. A geostationary satellite orbit is

    1. polar and elliptical
    2. circular and polar
    3. circular and equatorial
    4. inclined at 45°
    Answer

    C. circular and equatorial

    A GEO satellite orbits above the equator in a circle.

  33. Consider: 1. A step-index single-mode fibre has V < 2.405. 2. Graded-index fibre has no modal dispersion at all.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Graded-index reduces but does not remove modal dispersion.

  34. Consider: 1. Fusion splices usually have lower loss than mechanical splices. 2. Fibre is affected by electromagnetic interference.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Fibre is dielectric and immune to EMI.

  35. Consider: 1. The LNA sets the noise temperature of an earth station receiver. 2. Satellite attitude is kept using the attitude control system.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both statements are standard.

  36. Consider: 1. Round-trip delay through a GEO satellite is about a quarter of a second. 2. LEO satellites appear fixed in the sky.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    LEO satellites move quickly across the sky.

  37. Consider: 1. Ku band is widely used for DTH television. 2. Rain fade is more serious at C band than at Ka band.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Rain fade rises with frequency, so Ka is worse than C.

  38. Consider: 1. Chromatic dispersion has material and waveguide components. 2. Modal dispersion occurs in single-mode fibre.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Modal dispersion is only present in multimode fibres.

  39. Consider: 1. Lower attenuation is available at 1550 nm than at 850 nm. 2. LEDs are preferred for long single-mode trunk links.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Laser diodes, not LEDs, are used for long single-mode links.

  40. Match the fibre type to its core size: 1. Single-mode 2. Multimode (common).

    1. 9 µm; 125 µm
    2. 125 µm; 9 µm
    3. 50 µm; 9 µm
    4. About 9 µm; 50 or 62.5 µm
    Answer

    D. About 9 µm; 50 or 62.5 µm

    Single-mode core is about 9 µm; common multimode cores are 50 or 62.5 µm.

  41. Match the satellite band to its common use: 1. C 2. Ku.

    1. DTH with small dishes; rain-resistant links
    2. Broadcast only; navigation only
    3. Rain-resistant links; DTH with small dishes
    4. Navigation only; radar only
    Answer

    C. Rain-resistant links; DTH with small dishes

    C is rain-resistant; Ku suits small DTH dishes.

  42. Match the satellite orbit to its feature: 1. LEO 2. GEO.

    1. High delay; many satellites
    2. Low delay, many satellites; fixed in the sky
    3. Fixed in the sky; low delay
    4. Polar; elliptical
    Answer

    B. Low delay, many satellites; fixed in the sky

    LEO has low delay and needs many satellites; GEO appears fixed.

  43. If the loss in a fibre cable is 0.25 dB/km, the loss over 80 km is

    1. 2 dB
    2. 20 dB
    3. 32 dB
    4. 0.31 dB
    Answer

    B. 20 dB

    0.25 × 80 = 20 dB.

  44. Consider: 1. Dispersion limits the bit rate of a fibre link. 2. Attenuation is measured in dB/km.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both statements are correct.

  45. Consider: 1. A PIN photodiode has internal avalanche gain. 2. An EDFA amplifies the optical signal without converting it to electrical form.

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    PIN has no gain; EDFA amplifies the light directly.

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