--- title: "Hybrid spread spectrum and FHMA types" subject: "Wireless Communication" chapter: 7 key: "semester_7/wireless_communication:hybrid-ss-fhma-types" source: https://www.bctengineer.com/q/semester_7/wireless_communication/hybrid-ss-fhma-types site: BCT Engineering --- # Hybrid spread spectrum and FHMA types Wireless Communication · Chapter 7: Multiple Access in Wireless Communication - Asked in: 2081 Bhadra, 6+4+2 marks - Video: [Spread Spectrum Multiple Access - Mobile Radio Propagation - Mobile Communication System](https://www.youtube.com/watch?v=6r7pOoogNBo) — Ekeeda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/wireless_communication/hybrid-ss-fhma-types - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-7 ## Question Explain any two types of hybrid spread spectrum multiple access techniques along with their advantage and disadvantage. Describe the operation of FHMA. What are the types of FHMA? ## Short answer Hybrid FDMA/CDMA divides spectrum into sub-bands each running CDMA, giving flexible allocation but lower processing gain. Hybrid DS/FHMA first spreads with DS code then frequency-hops, avoiding near-far problems but adds complexity. FHMA operates by assigning each user a PN-controlled hopping pattern; the transmitter modulates (FSK) and hops, while a synchronized receiver de-hops to a fixed IF for demodulation. Types: slow FHMA (several symbols per hop) and fast FHMA (several hops per symbol). ## Answer Hybrid spread spectrum techniques combine two basic multiple access methods to get the advantages of both. #### Hybrid spread spectrum multiple access techniques **1. Hybrid FDMA/CDMA (FCDMA)** - The available wideband spectrum is divided into several narrower sub-bands (FDMA); inside each sub-band, a separate CDMA system with its own PN codes serves a group of users. - *Advantages:* the sub-bands need not be contiguous, so spectrum can be allocated flexibly; each sub-band has lower bandwidth, so cheaper hardware and lower chip rate; sub-bands can be switched on/off to avoid interference. - *Disadvantage:* processing gain in each sub-band is lower than full-band CDMA, and frequency-selective fading is less averaged out. **2. Hybrid Direct Sequence / Frequency Hopped Multiple Access (DS/FHMA)** - The signal is first spread by a DS code, then the **centre frequency of the DS signal hops** pseudo-randomly. Each user has a unique DS code and hopping pattern. - *Advantages:* avoids the **near–far problem** of pure DS-CDMA, since a near user rarely shares the same hop frequency; combines anti-jamming of FH with multipath resistance of DS. - *Disadvantage:* complex transmitter and receiver; needs fast, accurate synthesizers and code synchronisation. **3. Time Division CDMA (TCDMA)** - Different spreading codes are assigned to different **cells**, and inside a cell only one user transmits in a given **time slot** (TDMA). So a user meets interference only from other cells. - *Advantages:* no near–far problem within a cell; easier power control. *Disadvantage:* needs time synchronisation and has TDMA overhead. **4. Time Division Frequency Hopping (TDFH)** - A user hops to a new frequency at the start of each new TDMA frame (as in GSM slow frequency hopping). - *Advantages:* avoids severe fades or co-channel interference on one channel for long; gives frequency diversity. *Disadvantage:* needs synchronised hopping across frames and more complex synthesizers. #### Operation of FHMA **Frequency Hopped Multiple Access (FHMA)** lets many users share a wide band by making each user's narrowband carrier hop pseudo-randomly among many channels. At any instant a user uses one channel, and different users follow different hop patterns. ```text TRANSMITTER data->[FSK modulator]->(×)->[BPF]-> antenna ^ [PN code gen.]->[frequency synthesizer] RECEIVER antenna->(×)->[BPF at IF]->[FSK demodulator]-> data ^ [PN code gen.]->[frequency synthesizer] (same code, synchronised to Tx) ``` 1. Data modulates the carrier (usually FSK). 2. A PN code generator drives a frequency synthesizer, which sets a new carrier frequency every hop period. 3. The transmitted signal thus occupies a different channel in each hop. 4. The receiver, using the same PN code in synchronism, de-hops the signal to a fixed IF and demodulates it. 5. When two users hop to the same channel at the same time (collision), errors occur in that hop only; channel coding and interleaving correct them. **Example:** with 8 channels, user A may follow f₂, f₆, f₁, f₇ … and user B f₅, f₀, f₃, f₄ …; they share the band without colliding in these hops. #### Types of FHMA - **Slow FHMA:** several symbols per hop (hop rate < symbol rate), e.g. GSM SFH at 217 hops/s. - **Fast FHMA:** several hops per symbol (hop rate > symbol rate); better diversity and anti-jamming, more complex synthesizer. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the main disadvantage of hybrid FDMA/CDMA? A. Sub-bands must be contiguous B. Lower processing gain per sub-band than full-band CDMA C. No code is needed D. Higher chip rate Answer: B. Narrower sub-bands give less processing gain and less averaging of frequency-selective fading. (Key point: FCDMA) 2. Why does DS/FHMA avoid the near-far problem? A. A near user rarely shares the same hop frequency as a far user B. It uses higher power C. It has no spreading D. All users share one hop Answer: A. Hopping puts users on different frequencies most of the time. (Key point: DS/FHMA advantage) 3. In Time Division CDMA, how are users separated? A. Different codes per cell, and one user per time slot within a cell B. One code for all cells C. Different frequencies per user D. Random access Answer: A. Only one user transmits per slot in a cell, so interference comes from other cells only. (Key point: TCDMA) 4. Slow FHMA means which? A. Several symbols per hop (hop rate < symbol rate) B. Several hops per symbol C. One bit per chip D. No hopping Answer: A. Slow hopping sends several symbols per hop, e.g. GSM SFH at 217 hops/s. (Key point: Types of FHMA) ## Related in Wireless Communication - [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 - [Spread spectrum multiple access; FDMA vs CDMA](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspread-spectrum-ma-fdma-cdma) — Wireless Communication, chapter 7 - [Hybrid multiple access techniques](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Ahybrid-multiple-access) — Wireless Communication, chapter 7 - [Spread spectrum multiple access variants](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Assma-variants-applications) — Wireless Communication, chapter 7 - [FHMA principle and hybrid techniques for near-far](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Afhma-hybrid-near-far) — Wireless Communication, chapter 7 - [Frequency hopped multiple access (FHMA)](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Afhma-operation) — Wireless Communication, chapter 7, asked 5× ## Related in other subjects - [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 - [Multiple access protocols](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Amultiple-access-protocols) — Computer Networks and Security, chapter 3, asked 2× - [Multiple access in IEEE 802.5](https://www.bctengineer.com/question.md?key=semester_7%2Fcomputer_networks_and_security%3Aieee-802-5-access) — Computer Networks and Security, chapter 3 - [Multiplexing vs multiple access](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Amux-vs-multiple-access) — Data Communication, chapter 6