--- title: "FHMA principle and near-far effect in CDMA" subject: "Wireless Communication" chapter: 7 key: "semester_7/wireless_communication:fhma-near-far-cdma" source: https://www.bctengineer.com/q/semester_7/wireless_communication/fhma-near-far-cdma site: BCT Engineering --- # FHMA principle and near-far effect in CDMA Wireless Communication · Chapter 7: Multiple Access in Wireless Communication - Asked in: 2071 Magh, 3+4 marks - Video: [23 11 Near far problem 826)](https://www.youtube.com/watch?v=d_UI90RPAAo) — VideosCoursera - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/wireless_communication/fhma-near-far-cdma - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-7 ## Question Explain the principle of FHMA. What do you mean by near-far effect in CDMA? How is it solved? Explain. ## Short answer FHMA spreads the signal by hopping the carrier frequency pseudo-randomly according to a PN code; the transmitter mixes data (e.g., FSK) with a hopping synthesizer output, and the receiver de-hops with the synchronized code. The near-far effect in CDMA is when a nearby mobile's much stronger signal swamps a distant one, reducing capacity. It is solved primarily by fast power control (open-loop, closed-loop, outer-loop), and aided by hybrid techniques like TCDMA and DS/FHMA. ## Answer #### Principle of FHMA Frequency Hopping Multiple Access is a spread spectrum method in which the **carrier frequency of each user hops pseudo-randomly** among many narrow channels inside a wide band. Each user follows its own PN hopping pattern; the receiver uses the same, synchronised pattern to de-hop the signal. ```text Transmitter data->[FSK modulator]->(x)->[BPF]->[PA]-> antenna ^ [Frequency synthesizer] ^ [PN code generator] Receiver antenna->[BPF]->(x)->[IF filter]->[FSK demod]-> data ^ [Frequency synthesizer] ^ [PN gen] (synchronised to Tx) ``` - At any instant the signal is narrowband; over time it occupies the whole band. - Slow FH: several symbols per hop (GSM); fast FH: several hops per symbol. - Collisions between users are rare and are corrected by coding and interleaving. #### Near-far effect in CDMA In the CDMA uplink all mobiles share the same frequency. Path loss varies a lot with distance, so a **near** mobile arrives at the base station with much more power than a **far** mobile (often 60–80 dB difference without control). The receiver sees other users as noise, so the strong near signal swamps the weak far signal and the far user's call fails. Capacity drops sharply. #### How it is solved 1. **Power control** – the main solution: every mobile's power is adjusted so all signals reach the base station at about the same level. - **Open-loop**: mobile estimates path loss from the received forward signal and sets its power (fast, rough). - **Closed-loop**: base station measures received Eb/N0 and sends power up/down bits (IS-95: 800 bps, ±1 dB steps). - **Outer-loop**: base station changes the target Eb/N0 to keep frame error rate at the desired level. 2. **Hybrid DS/FHMA** – near and far users seldom share a hop frequency. 3. **TCDMA** – only one user per cell transmits at a time. 4. **Multi-user detection / successive interference cancellation** – strong signals are detected and subtracted first. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What describes FHMA? A. Each user's carrier hops pseudo-randomly among narrow channels by its own PN pattern B. All users use one fixed carrier C. Users are separated by time slots D. Users are separated by beams Answer: A. Each user hops over the wide band with its own pattern; the receiver de-hops with a synchronised pattern. (Key point: FHMA principle) 2. Without control, the near-far power difference at the base station can be about: A. 1-2 dB B. 60-80 dB C. 0 dB D. Exactly 1 dB Answer: B. Path loss differences between near and far mobiles can be tens of dB (60-80 dB). (Key point: Near-far magnitude) 3. What is open-loop power control? A. Base station sends up/down bits B. Base station changes target Eb/N0 from frame error rate C. Mobile estimates path loss from received forward signal and sets its power (fast, rough) D. Fixed maximum power Answer: C. Open loop is mobile-based; closed loop uses base commands; outer loop adjusts the target. (Key point: Open-loop control) 4. What does outer-loop power control adjust? A. The hop rate B. The Walsh code C. The slot length D. The target Eb/N0 to keep frame error rate at the desired level Answer: D. Outer loop maintains the FER by changing the Eb/N0 target. (Key point: Outer-loop control) 5. How does successive interference cancellation help? A. Strong signals are detected first and subtracted B. Weak signals are amplified C. Users are given slots D. Codes are shortened Answer: A. Removing strong users first uncovers the weak far user. (Key point: Interference cancellation) ## Related in Wireless Communication - [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 - [Multiple access and near-far effect mitigation](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Amultiple-access-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 - [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 ## 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