---
title: "Short note: CDMA design considerations"
subject: "Wireless Communication"
chapter: 7
key: "semester_7/wireless_communication:cdma-design-considerations"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/cdma-design-considerations
site: BCT Engineering
---

# Short note: CDMA design considerations

Wireless Communication · Chapter 7: Multiple Access in Wireless Communication

- Asked in: 2080 Baisakh, 4 marks
- Video: [Lec 6 | CDMA | Code Division Multiple Access | Wireless Communication | Vaishali Kikan](https://www.youtube.com/watch?v=Mg1NnzBB4Wo) — Dopamine
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/wireless_communication/cdma-design-considerations
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-7

## Question

Write a short note on CDMA design considerations.

## Short answer

CDMA design considers spreading codes with low cross-correlation, processing gain G = W/R, fast power control against the near-far problem, and interference-limited capacity. Also needed are code acquisition and synchronization, RAKE receivers for multipath, soft handoff support, and enough bandwidth such as 1.25 MHz in IS-95.

## Answer

**CDMA design considerations** are the factors that decide the capacity and quality of a CDMA system, since all users share the same band at the same time.

1. **Spreading codes** – Codes must have sharp autocorrelation (for synchronisation and multipath resolution) and low cross-correlation (for low multiple-access interference). Walsh codes (orthogonal) and long/short PN codes (m-sequences, Gold codes) are used.
2. **Processing gain** – G = W/R (chip rate/bit rate). Higher G gives more interference rejection and more users; e.g. IS-95: 1.2288 Mcps / 9.6 kbps = 128 (21 dB).
3. **Power control** – Because of the **near–far problem**, a nearby mobile can swamp a distant one. Fast closed-loop and open-loop power control keep all signals arriving at equal power (IS-95: 800 updates/s).
4. **Interference-limited capacity** – Capacity N ≈ 1 + (W/R)/(Eb/N₀); so lowering required Eb/N₀ (good coding), voice activity (factor ≈ 2.5), sectorization and frequency reuse factor 1 increase capacity.
5. **Synchronisation and code acquisition** – Receivers must acquire and track the code phase within a fraction of a chip.
6. **Multipath handling** – RAKE receivers combine resolvable paths (separated by more than one chip).
7. **Soft handoff** – Mobile communicates with several base stations together; needs network support.
8. **Bandwidth** – Wide band needed; must suit spectrum available (1.25 MHz in IS-95).

## Check yourself

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

1. What properties should CDMA codes have?
   A. Sharp autocorrelation and low cross-correlation
   B. High cross-correlation
   C. Flat autocorrelation
   D. Identical codes for all users
   Answer: A. Sharp autocorrelation helps sync and multipath; low cross-correlation reduces MAI. (Key point: Spreading codes)

2. IS-95 has 1.2288 Mcps and 9.6 kbps. What is its processing gain?
   A. 128 (about 21 dB)
   B. 64 (18 dB)
   C. 1280
   D. 12.8
   Answer: A. 1.2288 M / 9.6 k = 128, which is about 21 dB. (Key point: Processing gain)

3. Why is fast power control needed?
   A. The near-far problem: a nearby mobile can swamp a distant one
   B. To raise chip rate
   C. To increase delay
   D. To generate Walsh codes
   Answer: A. Equal received power at the base station keeps capacity high. (Key point: Power control)

4. Which raises CDMA capacity?
   A. Lower required Eb/N0, voice activity and sectorization
   B. Higher required Eb/N0
   C. Frequency reuse factor 7
   D. Removing power control
   Answer: A. Capacity is about 1 + (W/R)/(Eb/N0), boosted by voice activity and sectorization. (Key point: Capacity)

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