--- title: "Diversity techniques and space diversity combining" subject: "Wireless Communication" chapter: 5 key: "semester_7/wireless_communication:diversity-space-combining" source: https://www.bctengineer.com/q/semester_7/wireless_communication/diversity-space-combining site: BCT Engineering --- # Diversity techniques and space diversity combining Wireless Communication · Chapter 5: Equalization and Diversity - Asked in: 2074 Magh, 4+4 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-space-combining - Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-5 ## Question Explain diversity techniques used in wireless communication. Give brief description of combining methods used for space diversity. ## Short answer Space diversity uses separated antennas for independent fading; other techniques are polarization, frequency, and time diversity. For space diversity, combining methods are selection (best SNR branch), feedback/scanning (first above a threshold), maximal ratio combining (co-phase, weight by SNR, sum), and equal gain combining (co-phase, sum with equal weights). ## Answer #### Diversity techniques in wireless communication **Diversity** means sending or receiving the same information over two or more independently fading paths and combining them, so that the probability of a deep fade on all paths together is small. The main types are: 1. **Space (antenna) diversity** – Several receive antennas spaced so their signals fade independently (≈ λ/2 at the mobile, 10λ or more at a base station). 2. **Polarization diversity** – Two antennas with orthogonal polarizations (vertical/horizontal or ±45°) at the same location; scattering makes the two polarizations fade differently. Saves space at base stations. 3. **Frequency diversity** – Same data sent on carriers separated by more than the **coherence bandwidth**, so they fade independently. Used in OFDM with coding and in frequency hopping. 4. **Time diversity** – Same data repeated at intervals greater than the **coherence time**, or spread in time using channel coding with interleaving. RAKE receivers use multipath time diversity. 5. **Angle/pattern diversity** – Directional antennas pointing in different directions receive differently faded components. Diversity can be *microscopic* (against small-scale fading, as above) or *macroscopic* (against shadowing, by receiving from two base stations at different sites). #### Combining methods used for space diversity - **Selection diversity** – The branch with the highest instantaneous SNR is connected to the receiver. Simple; mean SNR gain = Σₖ₌₁ᴹ 1/k. - **Feedback (scanning) diversity** – Branches are checked one by one; the first one above a threshold is used until it falls below it. Uses one receiver; slightly worse than selection. - **Maximal ratio combining (MRC)** – All branches are co-phased, weighted by their signal-to-noise ratio, and summed; output SNR = sum of branch SNRs. Best performance, highest cost. - **Equal gain combining (EGC)** – All branches are co-phased and summed with equal weights. Nearly as good as MRC, easier to build. ```text Ant1 --+ Ant1 --[w1]--+ |--[switch to Ant2 --[w2]--+--(Σ)--> Rx Ant2 --+ max SNR]--> Rx AntM --[wM]--+ Selection / feedback MRC (w ∝ SNR), EGC (w = 1) ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the principle of diversity? A. Send different data on one path B. Increase symbol rate to avoid fading C. Use a single wide-beam antenna D. Receive the same information over independently fading paths and combine them Answer: D. The probability of a deep fade on all paths together is small. (Key point: Diversity principle) 2. How far apart must carriers be for frequency diversity? A. More than the coherence bandwidth B. Less than the coherence bandwidth C. Exactly one chip D. More than the coherence time Answer: A. Carriers separated beyond coherence bandwidth fade independently. (Key point: Frequency diversity) 3. What does macroscopic diversity combat? A. Small-scale fading with close antennas B. Shadowing, by receiving from base stations at different sites C. ISI D. Thermal noise Answer: B. Microscopic diversity handles small-scale fading, macroscopic handles shadowing. (Key point: Macroscopic diversity) 4. Which statement is correct? A. EGC weights by SNR; MRC equal B. Both pick one branch C. MRC weights by SNR; EGC uses equal weights D. Both subtract branches Answer: C. Both co-phase and add all branches but only MRC weights by SNR. (Key point: MRC vs EGC) ## Related in Wireless Communication - [Need for diversity; combining techniques](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adiversity-combining-techniques) — Wireless Communication, chapter 5 - [ISI mitigation and space diversity techniques](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aisi-space-diversity) — 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 - [Need for diversity; maximum ratio combining](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adiversity-need-mrc) — Wireless Communication, chapter 5 - [Feedback vs maximum ratio combining diversity](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Afeedback-vs-mrc-diversity) — Wireless Communication, chapter 5 - [Equalization and diversity; space diversity methods](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aequalization-diversity-space-methods) — Wireless Communication, chapter 5 ## Related in other subjects - [Wired and wireless communication systems](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Awired-wireless-systems) — Basic Electronics Engineering, chapter 5, asked 2× - [Space switch and why TST is used](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aspace-switch-why-tst) — Telecommunications, chapter 4 - [Switching techniques in data communication](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aswitching-techniques-data-comm) — Telecommunications, chapter 8, asked 3×