--- title: "Wireless technologies and their error rates" subject: "Telecommunications" chapter: 2 key: "semester_8/telecommunications:wireless-technologies-error-rates" source: https://www.bctengineer.com/q/semester_8/telecommunications/wireless-technologies-error-rates site: BCT Engineering --- # Wireless technologies and their error rates Telecommunications · Chapter 2: Transmission Media - Asked in: 2081 Bhadra, 4+4 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_8/telecommunications/wireless-technologies-error-rates - Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-2 ## Question Describe the working principles of wireless communication technologies, including high-frequency radio, and microwave transmission. What are the typical error rates associated with these wireless technologies? ## Short answer HF radio uses ionospheric sky-wave refraction for long distance, with BER about 10^-2 to 10^-4. Terrestrial microwave uses line-of-sight directional dishes with repeaters every 40-60 km, giving BER about 10^-6 to 10^-9; satellite microwave gives about 10^-6 to 10^-8. ## Answer #### High-frequency (HF) radio (3–30 MHz) ```text ionosphere (F, E layers) /\ /\ / \ refract / \ [Tx] __/ \________/ \__ [Rx] sky wave hops (1000s of km) ``` - Long-distance communication by **sky-wave propagation**: the wave is refracted back to earth by the **ionosphere**, possibly in several hops, covering thousands of km without repeaters; ground wave is used for shorter ranges. - Simple equipment (transmitter, antenna such as dipole/rhombic, receiver); used for maritime, military, aviation, amateur and emergency links, and earlier for international telephony and remote districts in Nepal. - Limitations: the ionosphere changes with day/night, season and sunspot cycle, so the operating frequency must be changed to stay below the maximum usable frequency (MUF); **fading** due to multipath, noise and interference; narrow bandwidth (a few kHz) and low data rates. #### Microwave transmission (1–40 GHz) ```text [Dish])))) ──── line of sight ──── ((((([Dish] Tower A 40-60 km hop Tower B/repeater ``` - **Terrestrial line-of-sight** links using highly directional parabolic antennas on towers; repeaters every 40–60 km (depending on tower height, earth curvature and Fresnel zone clearance). - Large bandwidth – carries hundreds to thousands of voice channels or high-rate digital streams (PDH/SDH); used for trunk networks and mobile backhaul. - **Satellite** microwave uses the satellite as a repeater in the sky (uplink/downlink, e.g. C and Ku bands). - Impairments: rain attenuation above ~10 GHz, multipath fading and obstruction; reduced by diversity (space/frequency) and fade margins. #### Typical error rates Error rate is given as the **bit error rate (BER)** = erroneous bits / total bits. Typical values (without heavy error correction): | Medium | Typical BER | |---|---| | HF radio (sky wave) | $10^{-2}$ to $10^{-4}$ (poor, fading-dependent) | | VHF/UHF radio, mobile | $10^{-3}$ to $10^{-5}$ before coding | | Terrestrial LOS microwave | $10^{-6}$ to $10^{-9}$ (better in clear weather) | | Satellite microwave | about $10^{-6}$ to $10^{-8}$ | | (Optical fibre, for comparison) | $10^{-9}$ to $10^{-12}$ | HF needs ARQ and forward error correction for reliable data, while microwave links are designed for a BER of around $10^{-6}$ or better for most of the time. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does HF radio (3-30 MHz) achieve long-distance communication? A. By line-of-sight between towers every 50 km B. By guiding the wave along a fibre C. By reflection from a satellite D. By refraction from the ionosphere in one or more hops Answer: D. Sky-wave propagation is refracted back by the ionosphere, covering thousands of km without repeaters. (Key point: HF sky wave) 2. Why are terrestrial microwave repeaters placed every 40-60 km? A. Because the signal is line-of-sight, limited by tower height, earth curvature and Fresnel clearance B. Because microwaves are absorbed by the ionosphere C. Because microwave bandwidth is only a few kHz D. Because dipole antennas have short range Answer: A. Microwave links need a clear line of sight with directional dishes, which limits hop length. (Key point: Microwave link) 3. Why must the HF frequency be changed with time of day and season? A. Because HF antennas change shape B. Because the ionosphere's condition changes (MUF varies) C. Because rain attenuates HF strongly D. Because HF has huge bandwidth Answer: B. Ionospheric changes with day/night, season and sunspots alter the usable frequency. (Key point: HF limitation) 4. Which BER ordering is correct for typical media (worst to best)? A. Optical fibre, microwave, HF radio B. HF radio, terrestrial microwave, optical fibre C. Microwave, HF radio, optical fibre D. HF radio, optical fibre, microwave Answer: B. HF is 10^-2 to 10^-4, LOS microwave 10^-6 to 10^-9, and fibre 10^-9 to 10^-12. 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