--- title: "Diversity and two types of diversity" subject: "Wireless Communication" chapter: 5 key: "semester_7/wireless_communication:diversity-two-types" source: https://www.bctengineer.com/q/semester_7/wireless_communication/diversity-two-types site: BCT Engineering --- # Diversity and two types of diversity Wireless Communication · Chapter 5: Equalization and Diversity - Asked in: 2070 Bhadra, 2+6 marks - Video: [Diversity Techniques in Antennas / Wireless Communication | Antenna and Wave Propagation Module - 6](https://www.youtube.com/watch?v=jJhxMI3jvi0) — THE BACKBENCH ENGINEERING COMMUNITY - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/wireless_communication/diversity-two-types - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-5 ## Question What is diversity? Explain any two types of diversity techniques in detail. ## Short answer Diversity combats fading by combining independently faded signal copies. Space diversity uses multiple antennas spaced λ/2 (mobile) or 10λ (base), with combining techniques like maximal ratio combining. Time diversity transmits the same information at intervals greater than the coherence time, typically using channel coding and interleaving to recover faded bits. ## Answer #### Diversity **Diversity** is a technique in which the receiver gets two or more copies of the same information over independently fading paths (branches) and selects or combines them. Since all branches are unlikely to fade deeply at the same time, the combined signal is much more reliable. It is a powerful, low-cost way to fight small-scale (Rayleigh) fading. #### 1. Space (antenna) diversity - Uses *M* receiving antennas separated so their signals are uncorrelated: about λ/2 at the mobile and 10λ or more at the base station (where scatterers are far away). - No extra bandwidth or transmit power is needed. - Branch signals are combined by: - **Selection** – choose the branch with the highest SNR. - **Feedback/scanning** – stay on a branch until it falls below a threshold, then switch. - **Maximal ratio combining** – co-phase, weight each branch by its SNR and add; output SNR = Σ γᵢ (best). - **Equal gain combining** – co-phase and add with equal weights; close to MRC. ```text Ant1 --[co-phase ×G1]--+ Ant2 --[co-phase ×G2]--+--(Σ / select)--> detector AntM --[co-phase ×GM]--+ ``` - Example: two antennas on a base-station mast several metres apart. - With MRC, *M* branches give *M* times the mean SNR of one branch; with selection the gain is Σₖ₌₁ᴹ 1/k. #### 2. Time diversity - The same information is sent at time intervals **greater than the coherence time** T_c (≈ 0.423/f_d), so the copies fade independently. - Implementations: - **Channel coding with interleaving** – coded bits are spread in time, so a fade causes scattered errors that the decoder corrects (GSM, IS-95, LTE). - **RAKE receiver** – in CDMA, multipath copies separated by more than one chip are despread separately and combined. - Repetition coding and ARQ. - Drawback: adds delay; useless when the mobile is stationary (T_c becomes very long). ```text data->[coder]->[interleaver]->fading channel ->[de-interleaver]->[decoder]->data ``` (Other types: frequency diversity – carriers separated by more than the coherence bandwidth; polarization diversity – orthogonally polarized antennas.) ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does diversity improve reliability against fading? A. It increases transmit power B. It removes all multipath C. It uses a wider bandwidth D. Several independently fading copies are unlikely to all fade deeply together Answer: D. Independent branches rarely fade simultaneously, so the combined signal is more reliable. (Key point: Idea of diversity) 2. For space diversity, what separation is needed at the mobile? A. About lambda/2 B. About 10 lambda C. About 100 lambda D. Zero, antennas are co-located Answer: A. About lambda/2 at the mobile and 10 lambda or more at the base station gives uncorrelated signals. (Key point: Antenna spacing) 3. What does maximal ratio combining do? A. Chooses only the strongest branch B. Adds branches with equal weights without co-phasing C. Co-phases and weights each branch by its SNR, giving output SNR equal to the sum of branch SNRs D. Switches branch only when below a threshold Answer: C. MRC co-phases and weights by SNR, so output SNR = sum of gamma_i, the best of the methods. (Key point: MRC combining) 4. For time diversity, copies must be sent separated by more than what? A. The symbol period B. The coherence time C. The delay spread D. The chip duration Answer: B. Separation greater than coherence time makes the copies fade independently. (Key point: Time diversity condition) 5. What is the role of interleaving with channel coding in time diversity? A. It spreads coded bits so a fade causes scattered errors the decoder can correct B. It increases bit rate C. It removes the need for coding D. It shortens delay Answer: A. Spreading bits in time turns a burst of fading errors into scattered errors that the decoder corrects. (Key point: Interleaving purpose) ## Related in Wireless Communication - [ISI mitigation and space diversity techniques](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aisi-space-diversity) — Wireless Communication, chapter 5 - [Need for diversity; combining techniques](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adiversity-combining-techniques) — Wireless Communication, chapter 5 - [Diversity techniques and space diversity combining](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adiversity-space-combining) — Wireless Communication, chapter 5 - [Space diversity techniques with block diagrams](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aspace-diversity-block-diagrams) — Wireless Communication, chapter 5 - [Importance and types of diversity](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adiversity-importance-types) — Wireless Communication, chapter 5 - [MLSE and time diversity](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Amlse-time-diversity) — Wireless Communication, chapter 5 ## Related in other subjects - [Soil improvement techniques; explain two](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Asoil-improvement-explain-two) — Foundation Engineering, chapter 10, asked 2× - [Linear solution techniques; CG two iterations](https://www.bctengineer.com/question.md?key=semester_8%2Fcomputational_techniques_in_civil_engineering%3Asolution-techniques-cg-numerical-two-iter) — Computational Techniques in Civil Engineering, chapter 2 - [Necessity of appraisal; formulation techniques](https://www.bctengineer.com/question.md?key=semester_7%2Fproject_engineering%3Aappraisal-necessity-formulation-techniques) — Project Engineering, chapter 2, asked 2× - [Appraisal vs formulation; formulation techniques](https://www.bctengineer.com/question.md?key=semester_7%2Fproject_engineering%3Aappraisal-vs-formulation-techniques) — Project Engineering, chapter 2, asked 2×