---
title: "Near-far problem in CDMA"
subject: "Data Communication"
chapter: 6
key: "semester_5/data_communication:cdma-near-far"
source: https://www.bctengineer.com/q/semester_5/data_communication/cdma-near-far
site: BCT Engineering
---

# Near-far problem in CDMA

Data Communication · Chapter 6: Multiplexing and Spreading

- Asked in: 2078 Kartik, 2+1 marks
- Video: [Lecture 33: Near-Far Problem in CDMA](https://www.youtube.com/watch?v=d2lrnUxI_4M) — NOC15 July-Sep EC05
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/data_communication/cdma-near-far
- Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-6

## Question

Explain the "near-far problem" in CDMA. How can it be solved?

## Short answer

The near-far problem occurs when a nearby mobile's signal overwhelms a far mobile's signal at the base station because codes are not perfectly orthogonal. It is solved by fast closed-loop power control, where the base station commands mobiles to adjust transmit power so all signals arrive at equal strength.

## Answer

#### Near-far problem
In CDMA all users transmit in the same band at the same time, separated only by codes. The base station receives the sum of all signals. A mobile **near** the base station arrives with high power, while a mobile **far** away arrives weak. Since codes are not perfectly orthogonal, the strong signal's residual correlation produces interference that is larger than the weak signal itself, so the far user's signal cannot be decoded and capacity falls.

```text
 Mobile A (near) ~~~~ strong ~~~~> [ BS ]
 Mobile B (far)  ~~ weak ~~~~~~~~>
 BS sees: A >> B -> B is drowned out
```

#### Solution
- **Power control** (the main solution): the base station measures the received power of each mobile and commands it to raise or lower its transmit power so that all signals arrive at about the same level. In IS-95 this is done about 800 times per second (closed-loop), plus open-loop control by the mobile.
- Using codes with low cross-correlation and multiuser detection (interference cancellation) also helps.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why does the near-far problem occur?
   A. Near mobile arrives strong and, with imperfect orthogonality, drowns the weak far one
   B. Far mobiles use more power
   C. Different frequencies
   D. Time slots overlap
   Answer: A. Residual correlation of the strong signal exceeds the weak one. (Key point: Cause)

2. What is the main solution?
   A. Power control so signals arrive at similar levels
   B. More bandwidth
   C. Slot allocation
   D. Higher power for near
   Answer: A. The base station commands power changes. (Key point: Main solution)

3. About how often does IS-95 do closed-loop power control?
   A. 8 times/s
   B. 80 times/s
   C. 800 times/s
   D. 8000 times/s
   Answer: C. About 800 times per second. (Key point: IS-95 rate)

## Related in other subjects

- [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
- [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
- [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