---
title: "Short note: DSSS versus FHSS"
subject: "Data Communication"
chapter: 6
key: "semester_5/data_communication:dsss-vs-fhss"
source: https://www.bctengineer.com/q/semester_5/data_communication/dsss-vs-fhss
site: BCT Engineering
---

# Short note: DSSS versus FHSS

Data Communication · Chapter 6: Multiplexing and Spreading

- Asked in: 2076 Asoj, 4 marks
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/data_communication/dsss-vs-fhss
- Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-6

## Question

Write a short note on DSSS versus FHSS.

## Short answer

DSSS spreads each bit by multiplying with a high-chip-rate pseudo-noise sequence, occupying the whole band continuously. FHSS spreads by rapidly hopping the carrier among many frequencies. DSSS provides higher data rate but requires precise chip-level sync; FHSS has easier hop-level sync and is used in Bluetooth.

## Answer

Both are spread spectrum techniques that spread a narrow-band signal over a wider bandwidth to resist jamming and interference, but they spread in different ways.

- **DSSS** multiplies each data bit by a high-rate chip sequence, so the signal occupies the whole wide band at once.
- **FHSS** keeps a narrow carrier but moves it among many frequencies in a pseudorandom order.

| Point | DSSS | FHSS |
|---|---|---|
| Spreading | By chip code (XOR with PN sequence) | By hopping carrier frequency |
| Bandwidth use | Whole band continuously | One narrow channel at a time |
| Synchronisation | Needs accurate chip-level sync | Easier, hop-level sync |
| Interference | Averaged out by processing gain | Only the hops that collide are lost |
| Near-far problem | Severe | Less |
| Data rate | Higher | Lower |
| Complexity | Needs high chip rate | Needs fast frequency synthesizer |
| Examples | 802.11b, GPS, CDMA | Bluetooth, early 802.11 |

## Check yourself

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

1. How does DSSS spread the signal?
   A. It hops a narrow carrier among many frequencies
   B. It multiplies each data bit by a high-rate chip sequence
   C. It increases the transmit power of the carrier
   D. It divides the data into time slots
   Answer: B. DSSS XORs each bit with a fast PN chip sequence so the signal fills the whole band at once. (Key point: How DSSS spreads)

2. In FHSS, what happens to the carrier?
   A. It stays fixed while the data rate rises
   B. It occupies the whole band continuously
   C. It stays narrow but moves among frequencies in pseudorandom order
   D. It is replaced by a chip code
   Answer: C. FHSS keeps a narrow carrier and hops it over many frequencies in a pseudorandom pattern. (Key point: How FHSS spreads)

3. Which statement about interference is correct?
   A. DSSS loses only the hops that collide; FHSS averages it out by processing gain
   B. Both lose the whole signal to any interference
   C. DSSS averages interference out by processing gain; FHSS loses only the hops that collide
   D. Neither technique resists interference
   Answer: C. DSSS suppresses narrow-band interference through processing gain, while in FHSS only the colliding hops are lost. (Key point: Interference handling)

4. Which pairing of technique and example is correct?
   A. FHSS: GPS; DSSS: Bluetooth
   B. DSSS: 802.11b, GPS; FHSS: Bluetooth
   C. Both: Bluetooth only
   D. DSSS: Bluetooth; FHSS: CDMA
   Answer: B. DSSS is used in 802.11b, GPS and CDMA; FHSS in Bluetooth and early 802.11. (Key point: Typical use and trade-offs)

## 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×