--- title: "Spread spectrum: FHSS and DSSS" subject: "Data Communication" chapter: 6 key: "semester_5/data_communication:spread-spectrum-fhss-dsss" source: https://www.bctengineer.com/q/semester_5/data_communication/spread-spectrum-fhss-dsss site: BCT Engineering --- # Spread spectrum: FHSS and DSSS Data Communication · Chapter 6: Multiplexing and Spreading - Asked in: 2079 Baisakh, 4+6 marks; 2076 Chaitra, 4 marks; 2075 Asoj, 6 marks; 2073 Chaitra, 2+6 marks (asked 4 times) - Video: [Spread Spectrum Explained | Basics, Block Diagram, Working, and Significance](https://www.youtube.com/watch?v=p3KbOPEZ39E) — Engineering Funda - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/data_communication/spread-spectrum-fhss-dsss - Whole chapter: https://www.bctengineer.com/md/semester_5/data_communication/chapter-6 ## Question What is spread spectrum? Explain frequency hopping spread spectrum and direct sequence spread spectrum with its block diagram. ## Short answer Spread spectrum deliberately widens a signal's bandwidth using a code independent of the data, providing interference resistance and low probability of intercept. FHSS rapidly hops the carrier among many frequencies following a pseudorandom sequence. DSS multiplies each data bit by a high-rate chip sequence before modulation. Both require the same code at the receiver for despreading. ## Answer #### Spread spectrum **Spread spectrum (SS)** is a technique in which a signal of bandwidth *B* is deliberately spread over a much larger bandwidth *Bss* before transmission, using a code (spreading sequence) that is independent of the data. The receiver uses the same code to despread it. Two requirements: *Bss* must be much larger than *B*, and the spreading code must be independent of the message. Advantages: resistance to jamming and interference, low probability of interception, privacy, and multipath resistance. #### Frequency Hopping Spread Spectrum (FHSS) The available bandwidth is divided into 2^k channels (hop frequencies). A **pseudorandom code generator (PN)** produces a k-bit pattern, and the frequency synthesizer moves the carrier to the corresponding frequency. The carrier stays on each frequency for a short **hop period** and then jumps to the next. Sender and receiver use the same PN sequence, so they hop together. ```text Data --> [Modulator] --> [Freq synthesizer] --> Tx (FSK/PSK) ^ [PN code generator] Rx: Signal --> [Freq synthesizer] --> [Demod] --> Data ^ [PN code generator, same seed] ``` An eavesdropper without the PN sequence hears only short bursts, and narrow-band interference hurts only the hops that coincide with it. Example: Bluetooth hops 1600 times per second over 79 channels of 1 MHz. #### Direct Sequence Spread Spectrum (DSSS) Each data bit is replaced by *n* **chips** using a spreading code (chip sequence), which has a much higher rate. The data signal is XOR-ed (multiplied) with the PN chip sequence, so the bandwidth increases by factor *n* (processing gain). ```text Data --> [XOR] --> [Modulator BPSK] --> Tx ^ [PN chip generator] Rx: Signal --> [Demod BPSK] --> [XOR] --> Data ^ [same PN chips] ``` Example: Wi-Fi 802.11b uses an 11-chip Barker sequence 10110111000. A data bit 1 becomes 10110111000, and 0 becomes its complement. The spread signal looks like noise to receivers without the code. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the key requirement of spread spectrum? A. Bss is much larger than B and the spreading code is independent of the data B. Bss equals B C. The code depends on the message D. The signal bandwidth is reduced Answer: A. The signal is spread over a much wider band using an independent code. (Key point: Spread spectrum) 2. In FHSS, what selects the carrier frequency? A. The data itself B. A PN code generator driving a frequency synthesizer C. The message amplitude D. A fixed table in order Answer: B. A k-bit PN pattern selects one of 2^k frequencies each hop period. (Key point: FHSS mechanism) 3. In DSSS, how is the bandwidth increased? A. Each bit is replaced by n chips at a higher rate, increasing bandwidth by n B. The carrier hops C. The data is repeated slowly D. Filtering Answer: A. Data XOR chips of much higher rate widens the spectrum by factor n. (Key point: DSSS chips) 4. How does the receiver recover data? A. Using the same PN code in sync B. Any random code C. It needs no code D. A higher carrier Answer: A. The same code (and seed) is used for despreading or hopping. (Key point: Receiver) ## Related in Data Communication - [Short note: DSSS versus FHSS](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Adsss-vs-fhss) — 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× - [DSSS bandwidth spreading](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Adsss-bandwidth-spreading) — Data Communication, chapter 6 - [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 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 - [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