--- title: "Short note: Interference in mobile communication" subject: "Wireless Communication" chapter: 2 key: "semester_7/wireless_communication:interference-mobile-communication" source: https://www.bctengineer.com/q/semester_7/wireless_communication/interference-mobile-communication site: BCT Engineering --- # Short note: Interference in mobile communication Wireless Communication · Chapter 2: Cellular Mobile Communication Concepts - Asked in: 2080 Baisakh, 4 marks - Video: [Interference and System Capacity- Fundamentals of Mobile Communication - Mobile Communication System](https://www.youtube.com/watch?v=LUrJLadLNOQ) — Ekeeda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/wireless_communication/interference-mobile-communication - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-2 ## Question Write a short note on interference in wireless/mobile communication. ## Short answer Interference limits cellular capacity and quality. Co‑channel interference from same‑frequency cells requires a minimum reuse distance (Q = √(3N)). Adjacent channel interference from nearby frequencies is worsened by the near‑far effect and is reduced by filtering and channel separation. Other types include multipath, multiple‑access, and intermodulation; mitigation uses power control, sectoring, and frequency planning. ## Answer Interference is the major factor limiting the capacity and quality of cellular systems. It causes cross-talk in voice calls, missed and dropped calls, and errors in data. Sources include other mobiles in the same cell, calls in neighbouring cells, other base stations on the same frequency, and non-cellular systems leaking into the band. #### 1. Co-channel interference (CCI) - Caused by cells that use the same frequency set under frequency reuse. - Cannot be reduced by increasing power; reduced by increasing the co-channel reuse ratio Q = D/R = √(3N). - S/I = (√(3N))ⁿ / i₀ for i₀ first-tier interferers; e.g. N = 7, n = 4 gives about 18.7 dB. #### 2. Adjacent channel interference (ACI) - Caused by signals on neighbouring frequencies leaking through imperfect receiver filters; worsened by the near–far effect. - Reduced by good filtering, keeping channels in a cell widely separated in frequency, and power control. #### 3. Other types - **Intersymbol interference** from multipath delay spread (digital systems). - **Multiple access interference** in CDMA from other users' codes. - **Out-of-band and intermodulation interference** from other radio systems. #### Methods to reduce interference - Proper cluster size and frequency planning - Sectoring with directional antennas and down-tilt - Power control (keep each mobile at the minimum needed power) - Good filters and channel assignment - Frequency hopping, equalisers and diversity Interference-limited design trades capacity (small N) against quality (high S/I). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Interference in cellular systems mainly: A. Increases the number of channels available B. Limits capacity and call quality C. Only affects data, never voice D. Is removed by frequency reuse Answer: B. Interference causes cross-talk, dropped calls and errors, limiting the system. (Key point: Effect of interference) 2. How is co-channel interference reduced? A. By raising transmit power B. By using a smaller cluster size C. By removing filters D. By increasing the co-channel reuse ratio Q = D/R Answer: D. Power increase raises interference equally; larger Q separates co-channel cells. (Key point: CCI reduction) 3. For N = 7 and n = 4 with 6 first-tier interferers, S/I is about: A. 18.7 dB B. 12 dB C. 30 dB D. 7.5 dB Answer: A. (sqrt21)^4/6 = 441/6 = 73.5 = 18.7 dB. (Key point: S/I example) 4. Adjacent channel interference is caused by and reduced by: A. Reuse of the same frequency; reduced by larger D/R B. Multipath delay; reduced by sectoring only C. Leakage through imperfect filters; reduced by better filtering, channel separation and power control D. Other users' codes; reduced by bigger clusters Answer: C. ACI is a filtering and channel-plan problem, unlike CCI. 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