Last reviewed 16 Sept 2026 · 9 min read
Optical fibre communication
Block diagram
Information → electrical transmitter → optical source (LED/laser) → optical fibre (with repeaters/optical amplifiers) → optical detector (photodiode) → electrical receiver → output.
Advantages of optical fibres
- Enormous bandwidth — very high data rates.
- Low attenuation — long repeater spacing.
- Immunity to electromagnetic interference and lightning (dielectric) — suitable near power lines and in electrically noisy environments.
- No crosstalk; secure (difficult to tap).
- Small size and light weight.
- Electrical isolation, no spark hazard.
- Raw material (silica) abundant.
Limitations: fragile; splicing and connectors require precision; bending losses; cost of terminal equipment.
Structure
| Layer | Function |
|---|---|
| Core | Central glass (silica) or plastic region carrying light — higher refractive index |
| Cladding | Surrounds the core — lower refractive index — confines light by total internal reflection |
| Buffer / jacket | Protective coating |
Light propagation
FormulaOptical fibre relations
Total internal reflection (TIR) occurs when light in the core strikes the core–cladding interface at an angle greater than the critical angle:
Numerical aperture:
= acceptance angle (maximum half-angle of light entering the fibre from a medium of index , usually air = 1)
Relative refractive index difference: →
V-number (normalised frequency):
= core radius. Single-mode operation when V < 2.405; number of modes in step-index multimode fibre ≈ .
Types of fibres
| Type | Features | Use |
|---|---|---|
| Step-index multimode | Uniform core index, larger core (commonly 50–100 µm or more) | Short links; high modal dispersion |
| Graded-index multimode | Core index decreases gradually from centre — light paths curve, reducing modal dispersion | LANs, medium distances |
| Single-mode (step-index) | Very small core (commonly about 8–10 µm); one mode | Long-distance, high-bit-rate telecom |
Losses and dispersion
- Attenuation (dB/km) caused by absorption (impurities, OH ions), scattering (Rayleigh scattering ∝ ), bending losses (macro and micro bends), splice/connector losses.
- Low-loss wavelength windows: about 850 nm, 1310 nm and 1550 nm (lowest loss in silica fibres, roughly 0.2 dB/km).
- Dispersion (pulse broadening, limits bit rate): modal (intermodal) — multimode fibres; chromatic (material + waveguide) — due to source spectral width; polarisation mode dispersion.
- Optical amplifiers (e.g. erbium-doped fibre amplifiers, EDFA, near 1550 nm) extend reach.
Sources and detectors
| Component | Type | Features |
|---|---|---|
| Source | LED | Incoherent, broad spectrum, cheap, low power — multimode short links |
| Source | Laser diode | Coherent, narrow spectrum, high power and speed — single-mode long-haul links |
| Detector | PIN photodiode | Simple, low noise |
| Detector | Avalanche photodiode (APD) | Internal gain, higher sensitivity; needs high bias voltage |
Joints
- Splices (permanent) — fusion splicing (lowest loss), mechanical splicing.
- Connectors (demountable) — SC, LC, FC, ST types.
- OTDR (optical time domain reflectometer) locates faults and measures losses.
Applications
- Telecommunication backbones, FTTH (fibre to the home), data centres, undersea cables, cable TV, medical endoscopy.
- Fibre optic sensors in civil engineering — fibre Bragg grating (FBG) and distributed sensing (Brillouin/Raman) for strain, temperature and crack monitoring in bridges, tunnels, dams, pipelines and piles.
Wireless communication
Electromagnetic spectrum (radio bands)
| Band | Frequency | Typical uses |
|---|---|---|
| VLF | 3–30 kHz | Submarine communication, navigation |
| LF | 30–300 kHz | Long-wave radio, navigation |
| MF | 300 kHz–3 MHz | AM broadcasting (medium wave) |
| HF | 3–30 MHz | Short-wave radio, amateur radio |
| VHF | 30–300 MHz | FM radio, TV, aircraft, marine |
| UHF | 300 MHz–3 GHz | TV, mobile phones, Wi-Fi 2.4 GHz, GPS |
| SHF | 3–30 GHz | Satellite, radar, microwave links, Wi-Fi 5 GHz |
| EHF | 30–300 GHz | Millimetre-wave 5G, radar, radio astronomy |
, m/s.
Wave propagation
| Mode | Frequencies | Mechanism |
|---|---|---|
| Ground (surface) wave | Low frequencies (up to about 2–3 MHz) | Travels along the earth's surface; attenuation increases with frequency |
| Sky wave (ionospheric) | About 3–30 MHz (HF) | Reflected (refracted) by the ionosphere — long-distance short-wave communication; depends on time of day and season |
| Space wave (line of sight) | Above about 30 MHz (VHF and above) | Direct and ground-reflected waves; range limited by earth's curvature and antenna heights |
| Tropospheric scatter | UHF/SHF | Scattering in the troposphere beyond horizon |
| Satellite | Microwave | Via satellites |
FormulaLine-of-sight range
Radio horizon distance (with standard atmospheric refraction, 4/3 earth radius):
(Optical horizon: km.)
Antennas
- Convert guided electrical signals to radiated waves and vice versa; reciprocal (same properties in transmission and reception).
- Types: half-wave dipole (length λ/2), monopole (λ/4 over ground plane), Yagi-Uda (directional TV antenna), parabolic dish (high gain microwave/satellite), patch (mobile devices), helical, arrays.
- Parameters: gain, directivity, radiation pattern, beamwidth, polarisation, impedance, bandwidth.
Cellular concept
- Service area divided into cells (hexagonal model), each served by a base station.
- Frequency reuse — the same frequencies reused in cells far enough apart to limit co-channel interference; cluster size N; reuse distance .
- Handoff (handover) — transferring an ongoing call when a user moves between cells.
- Cell splitting and sectoring increase capacity.
Generations of mobile communication
| Generation | Key technology | Main features |
|---|---|---|
| 1G | Analog (AMPS, FDMA) | Voice only |
| 2G | Digital — GSM (TDMA), CDMA | Digital voice, SMS, basic data (GPRS/EDGE) |
| 3G | UMTS/WCDMA, CDMA2000 | Mobile internet, video calling |
| 4G | LTE — OFDMA, all-IP network | High-speed broadband, VoLTE |
| 5G | New Radio — OFDM, massive MIMO, mmWave | Very high speed, low latency, massive IoT connectivity |
Satellite communication
- Uplink (earth → satellite) and downlink (satellite → earth) at different frequencies; satellite transponders receive, amplify, frequency-convert and retransmit.
- Geostationary orbit (GEO) — about 35 786 km above the equator; orbital period equal to earth's rotation — satellite appears fixed (TV broadcasting, VSAT); round-trip delay about 0.24 s (earth–satellite–earth).
- Low earth orbit (LEO) (a few hundred to about 2000 km) — low delay, needs constellations (satellite internet, earth observation); MEO — navigation satellites.
- Frequency bands: C, Ku, Ka bands.
Navigation (GNSS)
- GPS (USA), GLONASS (Russia), Galileo (EU), BeiDou (China) and India's regional system NavIC (IRNSS) developed by ISRO.
- Position computed by trilateration from signals of at least four satellites (three coordinates + receiver clock bias).
- DGPS/RTK techniques give centimetre-level accuracy for surveying and construction layout (see GIS, GPS & Remote Sensing in Surveying).
Short-range wireless and IoT
| Technology | Features |
|---|---|
| Wi-Fi (IEEE 802.11) | Wireless LAN in 2.4 GHz and 5 GHz (and 6 GHz) bands |
| Bluetooth (IEEE 802.15.1 origins) | Short-range personal area networks |
| Zigbee (IEEE 802.15.4) | Low-power mesh sensor networks |
| RFID / NFC | Identification and tracking — materials, assets, toll tags (FASTag) |
| LPWAN — LoRa, NB-IoT | Long range, low power — remote sensors |
| IoT | Networked sensors and actuators — smart buildings, water networks, structural and environmental monitoring |