---
title: "Short note: Spread spectrum"
subject: "Data Communication"
chapter: 6
key: "semester_5/data_communication:spread-spectrum-short-note"
source: https://www.bctengineer.com/q/semester_5/data_communication/spread-spectrum-short-note
site: BCT Engineering
---

# Short note: Spread spectrum

Data Communication · Chapter 6: Multiplexing and Spreading

- Asked in: 2070 Chaitra, 5 marks
- Video: [Spread Spectrum (Basics, Block Diagram, Working, Need & Types) Explained in Digital Communication](https://www.youtube.com/watch?v=xltpukBncs8) — Engineering Funda
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/data_communication/spread-spectrum-short-note
- Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-6

## Question

Write a short note on spread spectrum.

## Short answer

Spread spectrum transmits a signal over a bandwidth much larger than the data rate using a spreading code independent of the data. The receiver uses the same code to despread. Techniques: frequency hopping (FHSS) and direct sequence (DSSS). It gives processing gain, resists jamming and interference, has low probability of detection, and enables CDMA.

## Answer

**Spread spectrum** is a technique in which a signal is transmitted over a bandwidth much greater than the minimum needed for the information, using a spreading code that is independent of the data. The receiver uses the same code to despread the signal and recover the data.

**Principle:** the spreading code makes the signal bandwidth *Bss* ≫ the data bandwidth *B*. The ratio *Bss*/*B* is the **processing gain**. Spread signals have low power density, so they look like noise to others.

```text
 Data -> [Modulator] -> [Spreader] -> channel
                           ^ code
 channel -> [Despreader] -> [Demod] -> Data
                ^ same code
```

#### Main techniques
- **FHSS**: carrier hops among frequencies in a pseudorandom sequence (Bluetooth).
- **DSSS**: each bit is multiplied by a fast chip sequence (802.11b, GPS, CDMA).

#### Advantages
- Resists jamming and narrow-band interference.
- Low probability of detection and interception, so secure.
- Resists multipath fading.
- Allows many users in the same band using different codes (CDMA).

#### Disadvantages
- Needs a much wider bandwidth.
- Complex transmitter and receiver, with precise code synchronisation.

#### Applications
Military communication, cellular CDMA, wireless LAN, Bluetooth, GPS.

## Check yourself

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

1. What defines spread spectrum?
   A. Transmitting in the minimum bandwidth needed
   B. Transmitting over a much wider bandwidth using a code independent of the data
   C. Using a code that depends on the data
   D. Using only one frequency
   Answer: B. The signal bandwidth is far larger than the data needs, spread by an independent code. (Key point: Definition)

2. What is processing gain?
   A. Bss/B, the ratio of spread to data bandwidth
   B. Transmit power / noise power
   C. Data rate / chip rate
   D. Number of users
   Answer: A. Processing gain is the ratio of spread bandwidth to data bandwidth. (Key point: Processing gain)

3. Which is an advantage?
   A. Needs less bandwidth
   B. Simple receivers
   C. Low probability of interception and resistance to jamming
   D. No synchronisation needed
   Answer: C. Low power density and secret codes give secure, jam-resistant transmission. (Key point: Advantages)

4. Which is a disadvantage?
   A. Much wider bandwidth and precise code synchronisation
   B. Weak against multipath
   C. Cannot support CDMA
   D. Easily detected
   Answer: A. Spread spectrum needs wide bandwidth and complex sync. (Key point: Disadvantages)

## Related in Data Communication

- [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×
- [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
- [Spread spectrum security; pros and cons](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Ass-security-jamming) — Data Communication, chapter 6

## Related in other subjects

- [Spread spectrum frequency hopping in Bluetooth](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Afreq-hopping-bt) — Instrumentation II, chapter 5, asked 2×
- [Spread spectrum and DSSS transmitter/receiver](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspread-spectrum-dsss-tx-rx) — Wireless Communication, chapter 4
- [Direct sequence spread spectrum (DSSS)](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adsss-block-diagram) — Wireless Communication, chapter 4
- [Spread spectrum and FHSS](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspread-spectrum-fhss) — Wireless Communication, chapter 4