--- title: "Multiple access and near-far effect mitigation" subject: "Wireless Communication" chapter: 7 key: "semester_7/wireless_communication:multiple-access-near-far" source: https://www.bctengineer.com/q/semester_7/wireless_communication/multiple-access-near-far site: BCT Engineering --- # Multiple access and near-far effect mitigation Wireless Communication · Chapter 7: Multiple Access in Wireless Communication - Asked in: 2076 Bhadra, 5+4 marks - Video: [Near and Far terminals | MC | Mobile Computing | Lec-26 | Bhanu Priya](https://www.youtube.com/watch?v=72gSP273z0s) — Education 4u - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/wireless_communication/multiple-access-near-far - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-7 ## Question Explain the term multiple access. What do you mean by near-far effect in CDMA and state all possible near-far effect mitigation techniques. ## Short answer Multiple access lets many users share a limited spectrum by dividing frequency, time, code, or space. In CDMA the near-far effect occurs when a nearby mobile's strong signal swamps a far user's weak signal. Mitigation includes power control, hybrid DS/FH, TCDMA, multi-user detection, and low cross-correlation codes. ## Answer #### Multiple access Multiple access is the technique that lets **many users share a limited radio spectrum (a common channel) at the same time** without serious interference. The spectrum is divided in frequency, time, code or space, and each user gets a share. | Technique | Resource divided | Example | |---|---|---| | FDMA | Frequency bands | AMPS | | TDMA | Time slots on a carrier | GSM, IS-136 | | CDMA | Unique codes, same band and time | IS-95, WCDMA | | SDMA | Space (antenna beams) | Smart antennas | | PRMA | Packets (random access) | Data networks | Multiple access is combined with a duplexing method: FDD (separate frequencies for uplink/downlink) or TDD (separate time slots). Narrowband systems (FDMA, TDMA) give each user a small part of the band; wideband systems (CDMA) let all users use the whole band. ```text freq|##|##|##| FDMA time| | | | TDMA | | | | |slots CDMA: all users fill whole freq-time plane, separated by codes ``` #### Near-far effect in CDMA In the uplink, all mobiles transmit on the same frequency. A mobile **near** the base station is received with much higher power than a mobile **far** away (path loss differs by tens of dB). Because CDMA receivers treat other users as noise, the strong nearby signal raises the interference level so much that the weak far user cannot be despread. This is the near-far problem. #### Mitigation techniques 1. **Power control** (main method): each mobile adjusts power so that all arrive at the base station with nearly equal power. - Open-loop: mobile sets power from received forward signal strength. - Closed-loop: base station sends up/down commands (800 times/s in IS-95, 1 dB steps). - Outer-loop: adjusts target Eb/N0 based on frame error rate. 2. **Hybrid DS/FH (DS/FHMA)** – a near user rarely shares the same hop frequency. 3. **Time division CDMA (TCDMA)** – only one user per cell transmits in a slot. 4. **Multi-user detection / interference cancellation** – the receiver detects strong users first and subtracts them. 5. **Codes with low cross-correlation** and higher processing gain. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What does multiple access achieve? A. It lets many users share a limited radio spectrum at the same time without serious interference B. It gives every user a dedicated spectrum that no one else can use C. It increases the transmit power of each user D. It converts analog speech to digital Answer: A. Multiple access divides a common spectrum in frequency, time, code or space so many users can share it. (Key point: Meaning of multiple access) 2. Why does a near mobile prevent a far mobile from being despread in CDMA uplink? A. The near mobile uses a different carrier frequency B. The near mobile's signal arrives much stronger and, since other users act as noise, it swamps the weak far signal C. The far mobile uses a longer PN code D. The base station antenna cannot receive two users Answer: B. All users share one band; the strong nearby signal raises the interference level so the weak user cannot be recovered. (Key point: Cause of near-far effect) 3. Which is the main method for reducing the near-far effect? A. Increasing the cell size B. Using FDMA guard bands C. Reducing the processing gain D. Power control so all signals reach the base station with nearly equal power Answer: D. Each mobile adjusts its power so received powers at the base station are about equal. (Key point: Main mitigation) 4. In closed-loop power control in IS-95, what happens? A. The mobile sets its power only from the received forward signal strength B. The base station adjusts the target Eb/N0 from frame error rate C. The base station sends up/down commands, 800 times per second in 1 dB steps D. The mobile transmits at maximum power at all times Answer: C. Closed loop uses base-station up/down commands at 800/s with 1 dB steps; the others describe open-loop and outer-loop control. (Key point: Closed-loop power control) ## Related in Wireless Communication - [FHMA principle and near-far effect in CDMA](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Afhma-near-far-cdma) — Wireless Communication, chapter 7 - [SDMA and hybrid SS techniques against near-far](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Asdma-hybrid-near-far) — Wireless Communication, chapter 7 - [FHMA principle and hybrid techniques for near-far](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Afhma-hybrid-near-far) — Wireless Communication, chapter 7 - [Near-far effect and a hybrid SS technique](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Anear-far-hybrid-technique) — Wireless Communication, chapter 7 - [Spread spectrum multiple access; FDMA vs CDMA](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspread-spectrum-ma-fdma-cdma) — Wireless Communication, chapter 7 - [CDMA vs TDMA; FHSS multiple access](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Acdma-advantages-fhss-types) — Wireless Communication, chapter 7 ## Related in other subjects - [Near-far problem in CDMA](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Acdma-near-far) — Data Communication, chapter 6 - [RF safety practices and reactive/near/far fields](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Arf-safety-practices-near-far-field) — RF and Microwave Engineering, chapter 7 - [Multiple access protocols](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Amultiple-access-protocols) — Computer Networks and Security, chapter 3, asked 2× - [Multiple access in IEEE 802.5](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aieee-802-5-access) — Computer Networks and Security, chapter 3