---
title: "FHSS working principle"
subject: "Data Communication"
chapter: 6
key: "semester_5/data_communication:fhss-working"
source: https://www.bctengineer.com/q/semester_5/data_communication/fhss-working
site: BCT Engineering
---

# FHSS working principle

Data Communication · Chapter 6: Multiplexing and Spreading

- Asked in: 2078 Kartik, 4 marks; 2078 Bhadra, 5 marks; 2072 Kartik, 6 marks; 2069 Asar, 4 marks (asked 4 times)
- Video: [FHSS : Frequency Hopping Spread Spectrum Explained In Hindi](https://www.youtube.com/watch?v=f4qblYxEEHM) — 5 Minutes Engineering
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/data_communication/fhss-working
- Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-6

## Question

Explain the mechanism of frequency hopping spread spectrum (FHSS).

## Short answer

FHSS divides the total bandwidth into many hop channels. A pseudorandom noise generator selects a channel each hop period, and a frequency synthesizer shifts the carrier accordingly. The transmitter hops rapidly among frequencies in a pattern known to the synchronized receiver, which demodulates the data. Narrowband interference affects only occasional hops, and without the code interception is difficult.

## Answer

**Frequency hopping spread spectrum (FHSS)** transmits a signal over a number of different frequencies, switching rapidly from one carrier to another in a pseudorandom pattern known to both transmitter and receiver.

#### Mechanism
1. The total bandwidth *Bss* is divided into 2^k channels (hop frequencies), each wide enough for the data signal.
2. A **pseudorandom number (PN) generator** produces a k-bit pattern for every hop period (*Th*). Each pattern selects one frequency from the table.
3. The **frequency synthesizer** generates a carrier at the selected frequency.
4. The modulator (FSK or BPSK) puts the data on this carrier. The carrier stays at that frequency for *Th* and then hops to the next.
5. The receiver has the same PN generator (same seed, synchronised in time), so it hops to the same frequencies and demodulates the data.

```text
 Freq
  f8 |            #
  f7 |                  #
  f6 |      #
  f5 |
  f4 |   #
  f3 |                        #
  f2 |         #
  f1 |#
     +------------------------> time
      T  T  T  T  T  T  T  T  (hop period Th)
```

Pattern here: f1, f4, f6, f2, f8, f7, f3 ... The order looks random but repeats after the PN cycle.

#### Types
- **Slow hopping**: several bits per hop.
- **Fast hopping**: several hops per bit, better against jamming.

#### Properties
- Bandwidth used is 2^k times the signal bandwidth.
- Narrow-band interference or jamming affects only some hops, and error correction repairs them.
- Hard to intercept without the PN code.
- Several users can use different hop sequences in the same band.

Example: Bluetooth uses 79 channels of 1 MHz in the 2.4 GHz band and hops 1600 times per second.

## Check yourself

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

1. If k bits from the PN generator select a frequency, how many hop frequencies exist?
   A. k
   B. 2k
   C. 2^k
   D. k^2
   Answer: C. A k-bit pattern selects among 2^k channels. (Key point: Channels)

2. How does the receiver follow the hopping?
   A. Same PN generator with the same seed, synchronised in time
   B. It scans all channels
   C. Transmitter sends the frequency list in each hop
   D. Fixed frequency
   Answer: A. Both use the same PN sequence so they hop together. (Key point: Receiver sync)

3. What is fast hopping?
   A. Several bits per hop
   B. Several hops per bit, better against jamming
   C. One hop per second
   D. No hops
   Answer: B. Fast hopping makes several hops per bit. (Key point: Fast hopping)

4. Why does narrow-band interference hurt FHSS little?
   A. It affects only the hops that coincide with it
   B. FHSS filters it out
   C. It is wideband
   D. It cannot occur
   Answer: A. Only a few hops are corrupted and error correction repairs them. (Key point: Interference)

## 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
- [Working mechanism of WDM](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Awdm-working) — Data Communication, chapter 6
- [Compare FDM and FHSS](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Afdm-vs-fhss) — Data Communication, chapter 6

## 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 and FHSS](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspread-spectrum-fhss) — 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
- [Working principle of analog spectrum analyzer](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aanalog-spectrum-analyzer) — Communication Systems, chapter 1