--- title: "Spread spectrum and FHSS" subject: "Wireless Communication" chapter: 4 key: "semester_7/wireless_communication:spread-spectrum-fhss" source: https://www.bctengineer.com/q/semester_7/wireless_communication/spread-spectrum-fhss site: BCT Engineering --- # Spread spectrum and FHSS Wireless Communication · Chapter 4: Modulation and Demodulation in Mobile Communication - Asked in: 2079 Chaitra, 2+5 marks - Video: [FHSS - Frequency Hopping Spread Spectrum](https://www.youtube.com/watch?v=CkhA7s5GIGc) — Sunny Classroom - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/wireless_communication/spread-spectrum-fhss - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-4 ## Question Define spread spectrum. Explain frequency hopping spread spectrum with its necessary relevant diagram. ## Short answer Spread spectrum uses a bandwidth much larger than the data rate with a PN code independent of data. In FHSS, a narrowband FSK signal jumps among frequencies selected by a PN code; the synchronized receiver de-hops it back to IF for demodulation. Slow and fast hopping exist, and FHSS is used in Bluetooth and optional GSM. ## Answer #### Spread spectrum **Spread spectrum (SS)** is a modulation technique in which the signal is spread over a bandwidth much larger than the data rate needs, using a **pseudo-noise (PN) code** that is independent of the data. The receiver uses the same synchronized code to despread it. It gives anti-jamming capability, low probability of interception, multipath resistance and multiple access (CDMA). Processing gain PG = B_ss / R_b. #### Frequency hopping spread spectrum (FHSS) In **FHSS**, the carrier frequency of a narrowband signal (usually M-ary FSK) **jumps randomly** among a large set of channels. The hopping pattern is decided by a PN code known to both transmitter and receiver. At any instant the signal occupies only one narrow channel, but over time it covers the whole spread band. **Transmitter** ```text data->[FSK ]--->(x)--->[BPF]---> FH signal [modulator] ^ | [Frequency synthesizer] ^ [PN code generator] ``` **Receiver** ```text FH in-->(x)-->[BPF]-->[FSK ]--> data ^ [demodulator] | [Frequency synthesizer] ^ [PN generator + sync ] ``` - At the transmitter, the PN generator selects a frequency from the synthesizer; mixing shifts the FSK signal to that hop channel. - At the receiver, an identical synchronized synthesizer "de-hops" the signal back to a fixed IF, and a normal FSK demodulator recovers the data. **Time–frequency pattern** ```text f | ■ ■ | ■ ■ | ■ ■ | ■ ■ +------------------------> t one hop per dwell time ``` **Types** - **Slow hopping:** several symbols are sent per hop (hop rate < symbol rate). - **Fast hopping:** one symbol is spread over several hops (hop rate > symbol rate); better against fading and jamming. **Features:** A jammer or deep fade affects only a few hops, which coding and interleaving can correct. FHSS does not need the tight chip-level synchronization of DSSS and suffers much less from the near-far problem. It is used in Bluetooth (1600 hops/s over 79 channels) and optionally in GSM (slow frequency hopping). ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is spread spectrum? A. A technique that uses the minimum bandwidth equal to the data rate B. A technique that varies amplitude with the data C. A technique that spreads the signal over a bandwidth much larger than needed, using a PN code independent of the data D. A technique using orthogonal subcarriers with a cyclic prefix Answer: C. The PN code, known to both ends, spreads the signal and the receiver despreads it, giving anti-jam, LPI, multipath resistance and CDMA. (Key point: SS definition) 2. How does FHSS spread the spectrum? A. Each bit is multiplied by a fast chip sequence B. Data is sent in a single wide channel at all times C. The carrier is held constant while the data is Gaussian filtered D. The carrier of a narrowband (e.g. M-FSK) signal jumps among many channels in a PN-controlled pattern Answer: D. At any instant only one narrow channel is used, but over time the whole band is covered. (Key point: FHSS principle) 3. How does the FHSS receiver recover the data? A. A synchronized synthesizer driven by the same PN code de-hops the signal to a fixed IF, then an FSK demodulator is used B. It multiplies by the PN code and integrates over the chip C. It uses an FFT and one-tap equalizer D. It needs no synchronization Answer: A. The receiver hops in step with the transmitter, so the signal returns to a fixed frequency for normal demodulation. (Key point: De-hopping) 4. What distinguishes fast hopping from slow hopping? A. In fast hopping several symbols are sent per hop B. In fast hopping the hop rate exceeds the symbol rate (one symbol over several hops) C. Fast hopping uses fewer channels D. Fast hopping uses DSSS chips Answer: B. Slow hopping sends several symbols per hop; fast hopping spreads one symbol over several hops, which is better against fading and jamming. (Key point: Slow vs fast hopping) ## Related in Wireless Communication - [CDMA vs TDMA; FHSS multiple access](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Acdma-advantages-fhss-types) — Wireless Communication, chapter 7 - [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 - [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 multiple access variants](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Assma-variants-applications) — Wireless Communication, chapter 7 - [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 - [MSK, GMSK and advantages of spread spectrum](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Amsk-gmsk-spread-spectrum-advantages) — Wireless Communication, chapter 4 ## 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× - [FHSS working principle](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Afhss-working) — 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