--- title: "OFC block diagram vs microwave link" subject: "Propagation and Antenna" chapter: 6 key: "semester_6/propagation_and_antenna:ofc-block-diagram-vs-microwave" source: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/ofc-block-diagram-vs-microwave site: BCT Engineering --- # OFC block diagram vs microwave link Propagation and Antenna · Chapter 6: Optical Fibres - Asked in: 2078 Chaitra, 3+4 marks - Video: [OFC | UNIT 1 | Overview of Optical Fiber Communication |Elements of Optical link-Transmission Window](https://www.youtube.com/watch?v=ZXtiMJDXLiY) — SSIT.GANDHINAGAR - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/propagation_and_antenna/ofc-block-diagram-vs-microwave - Whole chapter: https://www.bctengineer.com/md/semester_6/propagation_and_antenna/chapter-6 ## Question Explain the block diagram of optical communication system. State advantages and applications of optical fiber compared to microwave communication link. ## Short answer An optical communication system consists of electrical Tx, optical source (LED/laser), fibre channel (TIR), optical detector (PIN/APD), and electrical Rx. Compared to microwave, fibre offers Tb/s bandwidth, immunity to rain and interference, no licence, and high security. Applications include backbone networks, FTTH, and data centres. ## Answer #### Block diagram of an optical communication system ```text +--------+ +----------+ +---------+ | Info |-->| Electr. |-->| Optical | | source | | Tx/driver| | source | +--------+ +----------+ +----+----+ | coupler ==== optical fibre, splices, ==== ==== repeaters / EDFA ==== | +--------+ +----------+ +----v----+ | Dest. |<--| Electr. |<--| Optical | | | | Rx/decode| | detector| +--------+ +----------+ +---------+ ``` - **Electrical transmitter:** encodes the message and drives the source (intensity modulation). - **Optical source:** LED or laser diode converts current to light at 850/1310/1550 nm. - **Fibre channel:** guides light by TIR; connectors, splices and repeaters/optical amplifiers along the route. - **Optical detector:** PIN or avalanche photodiode converts light to current. - **Electrical receiver:** amplifies, equalises and regenerates the signal for the user. #### Advantages of optical fibre over a microwave link | Point | Optical fibre | Microwave link | |---|---|---| | Bandwidth | Tb/s | Hundreds of Mb/s to a few Gb/s | | Repeater spacing | 80–100 km (more with EDFA) | 40–60 km, needs LOS towers | | Weather | Unaffected | Rain fade, multipath, ducting | | Interference | None | Co-channel interference | | Spectrum licence | Not needed | Needed | | Security | High | Lower (open air) | #### Applications - Long-haul trunks, national backbones and submarine cables - Mobile (3G/4G/5G) tower backhaul and FTTH broadband - LANs and data centres - Cable TV distribution - Power utilities (OPGW on transmission lines), railways, military and sensing ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does the optical source (LED or laser diode) do in the optical communication system? A. Converts light into current B. Amplifies the received signal C. Guides light by TIR D. Converts the electrical drive current into light Answer: D. The transmitter's source turns the electrical signal into intensity-modulated light at 850/1310/1550 nm. (Key point: Optical source) 2. Which devices act as the optical detector at the receiver? A. PIN or avalanche photodiode B. LED or laser diode C. EDFA or repeater D. Coupler or splice Answer: A. The detector converts light back to current; PIN and APD photodiodes are used. (Key point: Detector) 3. How does repeater spacing of fibre compare with a microwave link? A. Fibre: 40-60 km, shorter than microwave's 80-100 km B. Fibre: 80-100 km, much longer than microwave's 40-60 km line-of-sight hops C. Both need repeaters every 40-60 km D. Fibre needs repeaters every 1-5 km Answer: B. Fibre's low loss allows 80-100 km spacing (more with EDFA), while microwave needs LOS towers every 40-60 km. (Key point: Repeater spacing) 4. Which advantage does fibre have over a microwave link regarding weather and spectrum? A. It suffers more rain fade but needs no licence B. It is unaffected by weather but needs a spectrum licence C. It is unaffected by rain fade and needs no spectrum licence D. It suffers multipath and ducting like microwave Answer: C. Light in glass is free from rain fade, multipath and interference, and no spectrum licence is required. 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