---
title: "Compare FDM and FHSS"
subject: "Data Communication"
chapter: 6
key: "semester_5/data_communication:fdm-vs-fhss"
source: https://www.bctengineer.com/q/semester_5/data_communication/fdm-vs-fhss
site: BCT Engineering
---

# Compare FDM and FHSS

Data Communication · Chapter 6: Multiplexing and Spreading

- Asked in: 2078 Kartik, 3 marks
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/data_communication/fdm-vs-fhss
- Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-6

## Question

Compare FDM and FHSS using a suitable time-frequency graph.

## Short answer

FDM assigns each user a fixed narrow frequency band for continuous use, while FHSS spreads a single user across many frequencies by hopping in a pseudorandom pattern. FDM shares link among multiple users; FHSS resists jamming and interception. Bluetooth uses FHSS.

## Answer

**FDM** gives each user a fixed frequency band all the time; **FHSS** makes one user jump among many frequencies in a pseudorandom pattern.

```text
 FDM                         FHSS
 Freq                        Freq
  f3 |#########              f6 |       #
  f2 |#########              f5 |   #
  f1 |#########              f4 |#
     +---------> t           f3 |          #
  each channel fixed         f2 |      #
  on its own band            f1 |  #
                                +---------> t
                             one user hops over the whole band
```

| Point | FDM | FHSS |
|---|---|---|
| Purpose | Share a link among many signals | Spread one signal to resist jamming / interception |
| Carrier | Fixed per channel | Changes every hop period |
| Bandwidth per user | One narrow band | Whole spread band used over time |
| Code needed | No | PN sequence, shared by Tx and Rx |
| Interference | A narrow-band interferer kills the channel | Affects only a few hops |
| Security | Poor | Good |
| Example | Radio/TV channels | Bluetooth |

## Check yourself

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

1. How do carriers differ?
   A. FDM fixed per channel; FHSS changes every hop
   B. FDM changes; FHSS fixed
   C. Both fixed
   D. Both change
   Answer: A. FHSS hops pseudorandomly. (Key point: Carrier)

2. What is FHSS's purpose?
   A. Spread one signal to resist jamming and interception
   B. Share a link among many
   C. Reduce bandwidth
   D. Amplify
   Answer: A. FDM shares links; FHSS spreads. (Key point: Purpose)

3. A narrow-band interferer affects?
   A. FDM: kills the channel; FHSS: a few hops
   B. Both fully
   C. Neither
   D. FHSS fully
   Answer: A. FHSS hops past it. (Key point: Interference)

## Related in Data Communication

- [Compare FDM, WDM and TDM](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Acompare-fdm-wdm-tdm) — Data Communication, chapter 6
- [FHSS working principle](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Afhss-working) — Data Communication, chapter 6, asked 4×
- [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×
- [Frequency division multiplexing explained](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Afdm-explain) — Data Communication, chapter 6, asked 3×

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