---
title: "Spread spectrum security; pros and cons"
subject: "Data Communication"
chapter: 6
key: "semester_5/data_communication:ss-security-jamming"
source: https://www.bctengineer.com/q/semester_5/data_communication/ss-security-jamming
site: BCT Engineering
---

# Spread spectrum security; pros and cons

Data Communication · Chapter 6: Multiplexing and Spreading

- Asked in: 2069 Chaitra, 3+2 marks
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/data_communication/ss-security-jamming
- Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-6

## Question

How can the use of spread spectrum technique provide security against jamming and interception? List the advantages and disadvantages of spread spectrum.

## Short answer

Spreading over a wide band forces a jammer to either disperse power or hit a small part that despreading corrects. The low power density hides the signal below noise, and without the code it cannot be intercepted. Advantages: jamming resistance, low detection probability, multipath tolerance, CDMA. Disadvantages: large bandwidth, complex synchronisation, near-far problem.

## Answer

#### Security against jamming and interception
- **Jamming:** a jammer puts strong noise in a narrow band to block communication. In spread spectrum the signal is spread over a very wide band, so a jammer must either spread its power over the whole band (so little power in each part, which is harmless) or hit only a small part of the band (which damages only a small fraction of the signal, repaired by despreading and error correction). The jammer would need a huge power. Processing gain also suppresses any narrow-band noise at the receiver after despreading.
- **Interception:** the power of the spread signal is spread so thinly (power density below the noise floor) that it is hard to detect. Without knowing the secret PN code (hop sequence or chip sequence), an eavesdropper cannot despread it and sees only noise-like data. FHSS also hops so an interceptor hears only short random bursts.

#### Advantages
- Resistance to jamming and interference.
- Low probability of detection and interception.
- Resistance to multipath fading.
- Multiple users in one band (CDMA).
- Privacy and precise ranging (GPS).

#### Disadvantages
- Requires much larger bandwidth.
- Complex hardware and exact code synchronisation.
- Self-interference and near-far problem in multi-user systems.
- Lower data rate for the bandwidth used.

## Check yourself

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

1. Why is a spread signal hard to jam?
   A. A jammer can concentrate all power on one frequency to block it entirely
   B. A jammer must spread power over the whole band (too weak per part) or hit only a small part of the signal
   C. The signal uses narrow bandwidth
   D. Jammers cannot transmit
   Answer: B. Spreading means a jammer either becomes harmless per unit band or damages only a small fraction. (Key point: Jamming resistance)

2. Why is interception hard?
   A. The signal is amplified
   B. The signal is encrypted by the modem
   C. Low power density below the noise floor, and the PN code is secret
   D. The signal has no carrier
   Answer: C. Without the secret PN code, the eavesdropper sees only noise-like data. (Key point: Interception)

3. Which is a disadvantage of spread spectrum?
   A. Needs much larger bandwidth and exact code sync
   B. Poor multipath resistance
   C. Cannot give privacy
   D. Low interference immunity
   Answer: A. It requires wider bandwidth, complex hardware and sync, and suffers near-far problems. (Key point: Disadvantage)

4. Which is an advantage listed?
   A. Higher data rate for the bandwidth
   B. Smaller hardware
   C. Multiple users in one band (CDMA)
   D. Self-interference removal
   Answer: C. Different codes allow many users to share the same band. (Key point: Advantage)

## 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
- [Short note: Spread spectrum](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aspread-spectrum-short-note) — Data Communication, chapter 6

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- [Distribution layouts, pros/cons, improvements](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Adistribution-layouts-improvements) — Water Supply Engineering, chapter 7, asked 4×
- [Phase shift method of SSB generation: pros and cons](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Assb-phase-shift-pros-cons) — Communication Systems, chapter 2, asked 2×
- [Direct sequence spread spectrum (DSSS)](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adsss-block-diagram) — Wireless Communication, chapter 4
- [MSK, GMSK and advantages of spread spectrum](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Amsk-gmsk-spread-spectrum-advantages) — Wireless Communication, chapter 4