---
title: "DSSS bandwidth spreading"
subject: "Data Communication"
chapter: 6
key: "semester_5/data_communication:dsss-bandwidth-spreading"
source: https://www.bctengineer.com/q/semester_5/data_communication/dsss-bandwidth-spreading
site: BCT Engineering
---

# DSSS bandwidth spreading

Data Communication · Chapter 6: Multiplexing and Spreading

- Asked in: 2070 Asar, 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 Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/data_communication/dsss-bandwidth-spreading
- Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-6

## Question

Explain how DSSS (Direct Sequence Spread Spectrum) achieves bandwidth spreading.

## Short answer

DSSS multiplies each data bit by a higher-rate pseudo-noise code, converting bits to chips. The chip rate Rc = n × Rb, so the signal bandwidth expands by factor n. At the receiver, correlating with the same code despreads the desired signal and spreads narrowband interference, providing processing gain equal to n (e.g.,11 for Barker-11).

## Answer

In DSSS every data bit is replaced by *n* smaller pulses called **chips**, taken from a pseudo-noise (PN) spreading code. The chip rate is *n* times the bit rate, so the bandwidth grows by the same factor *n*.

#### Steps
1. Data signal has bit rate *Rb* and bandwidth about *Rb*.
2. A PN code generator produces chips at rate *Rc* = *n* × *Rb*.
3. Data and chips are combined by XOR (or multiplication of ±1 values), producing a signal at chip rate *Rc*.
4. The result is modulated (usually BPSK), so the required bandwidth is proportional to *Rc*, i.e. *n* times wider.

```text
 Data bit    : 1       0
 PN chips    : 1011 0111 000 (n = 11 Barker)
 Sent chips  : chips   inverted chips
 Rate  : Rc = n * Rb   =>  Bss = n * B
```

Example with Barker n = 11: a 1 Mbps stream becomes 11 Mchip/s, and the bandwidth rises about 11 times. **Processing gain** *Gp* = *Bss*/*B* = *n* = 11, i.e. 10 log10(11) ≈ 10.4 dB. At the receiver the same chips are used to despread the signal, which compresses the wanted signal back to narrow band while narrow-band interference is spread out and reduced.

## Check yourself

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

1. What replaces each data bit in DSSS?
   A. One pulse of half the width
   B. n smaller pulses (chips) from a PN code
   C. A burst of random noise
   D. A different carrier frequency
   Answer: B. Each bit becomes n chips, so the chip rate is n x the bit rate. (Key point: Chips)

2. If the chip rate is n times the bit rate, the spread bandwidth is
   A. n times wider
   B. unchanged
   C. n times narrower
   D. n^2 times wider
   Answer: A. Bandwidth is proportional to chip rate, so Bss = n x B. (Key point: Bandwidth relation)

3. For an 11-chip Barker code, the processing gain is about
   A. 1.1 dB
   B. 10.4 dB
   C. 11 dB exactly
   D. 20.8 dB
   Answer: B. Gp = 11, and 10 log10(11) = 10.4 dB. (Key point: Processing gain)

4. What does the receiver do with the same chip code?
   A. Spreads the wanted signal further
   B. Compresses the wanted signal and spreads/reduces narrow-band interference
   C. Removes the carrier
   D. Increases the bit rate of the interference
   Answer: B. Despreading restores the narrow-band data while interference is spread out and reduced. (Key point: Despreading)

## 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
- [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

- [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
- [Direct sequence and frequency hopped SS](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adsss-fhss-techniques) — Wireless Communication, chapter 4