---
title: "CDMA features and hybrid spread spectrum"
subject: "Wireless Communication"
chapter: 7
key: "semester_7/wireless_communication:cdma-features-hybrid-ss"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/cdma-features-hybrid-ss
site: BCT Engineering
---

# CDMA features and hybrid spread spectrum

Wireless Communication · Chapter 7: Multiple Access in Wireless Communication

- Asked in: 2080 Chaitra, 2+6 marks
- Video: [Spread Spectrum Multiple Access - Mobile Radio Propagation - Mobile Communication System](https://www.youtube.com/watch?v=6r7pOoogNBo) — Ekeeda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/wireless_communication/cdma-features-hybrid-ss
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-7

## Question

List out important features of CDMA in wireless communication. Illustrate hybrid spread spectrum techniques in brief.

## Short answer

CDMA features universal frequency reuse, soft capacity, soft handoff, RAKE multipath diversity, essential power control, privacy, and voice activity gain. Hybrid spread spectrum includes FDMA/CDMA with sub-band CDMA, DS/FHMA that hops a DS-spread signal, TCDMA with one user per time slot per cell, and TDFH that hops each TDMA frame.

## Answer

#### Features of CDMA
CDMA (Code Division Multiple Access) lets all users share the same carrier at the same time; each user is separated by a unique spreading (PN) code. Important features:
- **Universal frequency reuse** – every cell uses the same frequency (reuse factor N = 1), so no frequency planning is needed.
- **Soft capacity** – there is no hard limit on users; adding a user only raises the noise floor slightly (graceful degradation).
- **Soft handoff** – the mobile can talk to two or more base stations at once, giving fewer dropped calls.
- **Multipath resistance** – a RAKE receiver combines delayed copies of the signal (multipath diversity).
- **Power control is essential** – needed to solve the near-far problem.
- **Privacy and anti-jamming** – the signal looks like noise without the code.
- **Voice activity gain** – during silence the user transmits less, raising capacity.

#### Hybrid spread spectrum techniques
Hybrid techniques combine spread spectrum with FDMA, TDMA or frequency hopping to get the benefits of each.

1. **Hybrid FDMA/CDMA (FCDMA)** – The available wide band is divided into a number of narrower sub-bands. Each sub-band is a small DS-CDMA system with its own processing gain. A user is given a sub-band and a code. The sub-bands need not be contiguous, and different users can be given different bandwidths according to their needs.
   - Advantage: flexible use of fragmented spectrum; smaller sub-bands need lower chip rates.
   - Disadvantage: lower processing gain per sub-band than one full-band CDMA system.
2. **Hybrid Direct Sequence / Frequency Hopping (DS/FHMA)** – A DS-modulated signal (narrow PN-spread signal) is made to hop its centre frequency in a pseudo-random way. In each hop only a small part of the band is occupied, so a nearby strong user rarely sits on the same frequency at the same time.
   - Advantage: **avoids the near-far effect**.
   - Disadvantage: complex transmitter/receiver; not suited to soft handoff, because the base station cannot easily stay synchronised to many hopping signals.
3. **Time Division CDMA (TCDMA)** – Each cell uses a different spreading code. Inside a cell, users are given different time slots, so at any instant only **one** CDMA user transmits per cell. During handoff the user's code is changed to the code of the new cell.
   - Advantage: avoids the near-far effect (no two users of the same cell are received together).
   - Disadvantage: needs time synchronisation; capacity per cell is limited by slots.
4. **Time Division Frequency Hopping (TDFH)** – The subscriber hops to a new frequency at the start of each new TDMA frame. This gives frequency diversity against deep fades and spreads co-channel interference. GSM uses this (slow frequency hopping, 217 hops/s).
   - Advantage: avoids long deep fades and bursts of co-channel interference.
   - Disadvantage: needs a central hopping plan and tight frame synchronisation.

```text
 DS/FHMA idea:
  freq
   ^   [DS]            [DS]
   |         [DS]
   |  [DS]                   [DS]
   +----------------------------> time
   each block = DS-spread burst at
   a pseudo-randomly chosen carrier
```

## Check yourself

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

1. What is CDMA's frequency reuse factor?
   A. 1
   B. 3
   C. 7
   D. 12
   Answer: A. Every cell uses the same frequency, so no frequency planning is needed. (Key point: Frequency reuse)

2. What is soft handoff?
   A. Mobile talks to two or more base stations at once
   B. Mobile drops then reconnects
   C. Mobile changes code only
   D. BS changes frequency
   Answer: A. It reduces dropped calls. (Key point: Soft handoff)

3. Why does TCDMA avoid the near-far effect?
   A. Only one CDMA user per cell transmits at a time
   B. All use equal power
   C. It uses no codes
   D. It uses hopping
   Answer: A. Time slots within a cell prevent simultaneous reception of near and far users. (Key point: TCDMA near-far)

4. What is a disadvantage of DS/FHMA?
   A. Complex Tx/Rx and not suited to soft handoff
   B. Suffers near-far
   C. No spreading
   D. Needs contiguous spectrum
   Answer: A. The base station cannot easily stay synchronised to many hopping signals. (Key point: DS/FHMA drawback)

## Related in Wireless Communication

- [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
- [Hybrid spread spectrum and FHMA types](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Ahybrid-ss-fhma-types) — Wireless Communication, chapter 7
- [Spread spectrum disadvantages; CDMA implementation](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspread-spectrum-disadvantages-cdma) — Wireless Communication, chapter 7
- [Self jamming, FHMA and hybrid SS techniques](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aself-jamming-fhma-hybrid) — 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

## Related in other subjects

- [Digital vs analog spread spectrum](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aspread-spectrum-digital-vs-analog) — Data Communication, chapter 6
- [Wired vs wireless media; four-wire and hybrid](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Awired-wireless-four-wire-hybrid) — Telecommunications, chapter 2, asked 2×
- [Spread spectrum: FHSS and DSSS](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aspread-spectrum-fhss-dsss) — Data Communication, chapter 6, asked 4×
- [Short note: Spread spectrum](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aspread-spectrum-short-note) — Data Communication, chapter 6