--- title: "Self jamming, FHMA and hybrid SS techniques" subject: "Wireless Communication" chapter: 7 key: "semester_7/wireless_communication:self-jamming-fhma-hybrid" source: https://www.bctengineer.com/q/semester_7/wireless_communication/self-jamming-fhma-hybrid site: BCT Engineering --- # Self jamming, FHMA and hybrid SS techniques Wireless Communication · Chapter 7: Multiple Access in Wireless Communication - Asked in: 2072 Asoj, 2+4+6 marks - Video: [FHSS : Frequency Hopping Spread Spectrum Explained In Hindi](https://www.youtube.com/watch?v=f4qblYxEEHM) — 5 Minutes Engineering - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/wireless_communication/self-jamming-fhma-hybrid - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-7 ## Question What is self jamming problem in CDMA? Explain the operation of FHMA with the help of block diagram. Explain any two hybrid spread spectrum multiple access techniques along with their advantage and disadvantage. ## Short answer Self-jamming occurs because CDMA codes are not perfectly orthogonal, so other users' signals produce residual interference that grows with user count, limiting capacity. In FHMA, the carrier hops pseudo-randomly via a PN-controlled frequency synthesizer; data is modulated (e.g., FSK), mixed to the hop frequency, and de-hopped at the receiver with a synchronized code. Hybrid DS/FHMA hops a direct-sequence signal, avoiding near-far effect but adding complexity; TCDMA gives each cell a unique code and assigns time slots, eliminating in-cell near-far interference but requiring strict synchronization. ## Answer #### Self-jamming problem in CDMA Self-jamming arises because the spreading codes of different users are **not exactly orthogonal** (especially in the asynchronous uplink, where codes arrive with random delays). When the receiver despreads the wanted user, the other users' signals give non-zero cross-correlation and leave residual interference. This interference comes from users of the same system, so the system "jams itself"; it grows with the number of users and limits CDMA capacity. #### Operation of FHMA In FHMA the carrier frequency of each user hops pseudo-randomly within a wide band according to a PN code. At each instant the signal is narrowband; averaged over time, it covers the whole band. ```text Transmitter data->[FSK modulator]->(x)->[BPF]->[PA]-> antenna ^ [Frequency synthesizer] ^ [PN code generator] Receiver antenna->[BPF]->(x)->[IF filter]->[FSK demod]-> data ^ [Frequency synthesizer] ^ [PN gen] (synchronised to Tx) ``` 1. Data modulates a carrier (BFSK/M-FSK) to give a narrowband signal. 2. The PN generator picks one of many frequencies in the synthesizer; mixing shifts the signal to that hop frequency. 3. The receiver, with the same PN code synchronised, mixes the received signal down to a fixed IF (de-hopping), then demodulates. 4. Other users have different hopping patterns; occasional collisions are corrected by FEC and interleaving. - Slow FH (GSM, 217 hops/s): many symbols per hop. Fast FH: several hops per symbol. #### Two hybrid spread spectrum techniques **1. Hybrid DS/FHMA** - A DS-spread signal whose centre frequency hops pseudo-randomly. - Advantage: avoids the near-far effect, since a strong near user is rarely on the same hop frequency. - Disadvantage: complex; does not support soft handoff easily because the base station must synchronise to many hopping signals. **2. Time Division CDMA (TCDMA)** - Each cell uses a different spreading code; inside a cell, users take turns in time slots, so only one CDMA user per cell transmits at a time. On handoff, the user's code is changed to the new cell's code. - Advantage: no near-far effect within a cell. - Disadvantage: needs strict time synchronisation; capacity per cell limited by the number of slots. (Other hybrids: FCDMA – sub-bands each running DS-CDMA, flexible spectrum use but lower processing gain; TDFH – hop each TDMA frame, gives diversity against fading but needs tight frame sync.) ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What causes self-jamming in CDMA? A. Spreading codes of different users are not exactly orthogonal, leaving residual cross-correlation B. Narrowband jammers C. Different carrier frequencies D. Too few users Answer: A. Non-zero cross-correlation (especially in the asynchronous uplink) leaves interference from users of the same system. (Key point: Self-jamming cause) 2. How does self-jamming vary with the system? A. It decreases as users increase B. It grows with the number of users and limits capacity C. It is independent of users D. It only affects the downlink Answer: B. More users means more residual interference, which limits CDMA capacity. (Key point: Self-jamming effect) 3. In the FHMA transmitter, what picks the hop frequency? A. The BPF B. The FSK modulator C. The PN code generator controlling the frequency synthesizer D. The power amplifier Answer: C. The PN generator selects the synthesizer frequency; mixing shifts the FSK signal to that hop. (Key point: FHMA hop generation) 4. How are occasional collisions in FHMA handled? A. By FEC coding and interleaving B. By power control C. By Walsh codes D. By increasing the hop rate to zero Answer: A. Errors from collisions are corrected by forward error correction and interleaving. (Key point: FHMA collisions) 5. What is a disadvantage of hybrid DS/FHMA? A. Suffers near-far effect B. Needs only one slot C. Complex, and soft handoff is hard because the base must synchronise to many hopping signals D. Has no processing gain Answer: C. Hopping patterns make synchronisation complex and hinder soft handoff. (Key point: DS/FHMA drawback) ## Related in Wireless Communication - [Hybrid spread spectrum and FHMA types](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Ahybrid-ss-fhma-types) — 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 - [CDMA features and hybrid spread spectrum](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Acdma-features-hybrid-ss) — 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 - [Hybrid multiple access techniques](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Ahybrid-multiple-access) — Wireless Communication, chapter 7 ## Related in other subjects - [Frequency counter operation with block diagram](https://www.bctengineer.com/question.md?key=semester_3%2Fdigital_logic%3Afrequency-counter) — Digital Logic, chapter 11, asked 14×