---
title: "Direct sequence and frequency hopped SS"
subject: "Wireless Communication"
chapter: 4
key: "semester_7/wireless_communication:dsss-fhss-techniques"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/dsss-fhss-techniques
site: BCT Engineering
---

# Direct sequence and frequency hopped SS

Wireless Communication · Chapter 4: Modulation and Demodulation in Mobile Communication

- Asked in: 2073 Magh, 4 marks
- Video: [Direct Sequence Spread Spectrum DSSS (Basics, Block Diagram, Working, Waveforms & Applications)](https://www.youtube.com/watch?v=MqnzaHsQ90U) — Engineering Funda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/wireless_communication/dsss-fhss-techniques
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-4

## Question

Describe Direct Sequence and Frequency Hopped Spread Spectrum Techniques.

## Short answer

DSSS multiplies each data bit by a fast PN chip sequence and BPSK modulates, spreading over the full band; FHSS hops the carrier of an FSK signal among many frequencies in a PN pattern. DSSS needs tight chip-level sync and suffers near-far problems; FHSS uses easier hop-level sync and is used in Bluetooth.

## Answer

Both are **spread spectrum** methods that use a PN code to spread a narrowband signal over a wide band.

#### Direct sequence (DSSS)
Each data bit is multiplied by a fast PN chip sequence (T_c ≪ T_b), and the product BPSK-modulates the carrier.
```text
data->(x)->(x)-> s(t)      r(t)->(x)->(x)->[∫]-> data
       ^    ^                     ^    ^
     [PN] [cos]                 [PN] [cos]
```
The receiver multiplies by the same synchronized code to despread the signal. Processing gain PG = T_b/T_c. Used in IS-95 and 802.11b.

#### Frequency hopping (FHSS)
The carrier frequency of an FSK signal hops among many channels in a PN-controlled pattern.
```text
data->[FSK]->(x)-> FH    FH->(x)->[FSK demod]-> data
              ^               ^
       [Synth <- PN]   [Synth <- PN]
```
The receiver hops in step to "de-hop" the signal. Slow or fast hopping. Used in Bluetooth.

| Point | DSSS | FHSS |
|---|---|---|
| Spreading | PN chips multiply data | Carrier hops |
| Instantaneous BW | Full spread band | One narrow channel |
| Sync needed | Chip-level (tight) | Hop-level (easier) |
| Near-far problem | Severe, needs power control | Less severe |

## Check yourself

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

1. How do DSSS and FHSS spread the spectrum differently?
   A. DSSS hops the carrier; FHSS multiplies data by PN chips
   B. DSSS multiplies data by PN chips; FHSS hops the carrier frequency under PN control
   C. Both multiply data by chips
   D. Both use an IFFT
   Answer: B. DSSS spreads instantaneously over the full band; FHSS occupies one narrow channel at a time. (Key point: DSSS vs FHSS spreading)

2. Which synchronization is needed?
   A. FHSS needs tighter sync than DSSS
   B. Neither needs synchronization
   C. DSSS needs tight chip-level sync; FHSS needs easier hop-level sync
   D. Both need only carrier sync
   Answer: C. Chip periods are much shorter than hop dwell times, so DSSS timing is more demanding. (Key point: Synchronization)

3. Which spread spectrum technique suffers a severe near-far problem needing power control?
   A. FHSS
   B. Neither
   C. OFDM only
   D. DSSS
   Answer: D. In DSSS CDMA a strong nearby user swamps weak ones; FHSS is less affected. (Key point: Near-far)

## Related in Wireless Communication

- [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 DSSS transmitter/receiver](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspread-spectrum-dsss-tx-rx) — Wireless Communication, chapter 4
- [OFDM principle and spread spectrum types](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aofdm-spread-spectrum-types) — Wireless Communication, chapter 4
- [Frequency hopped multiple access (FHMA)](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Afhma-operation) — Wireless Communication, chapter 7, asked 5×
- [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
- [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

## 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: 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