---
title: "Feedback vs maximum ratio combining diversity"
subject: "Wireless Communication"
chapter: 5
key: "semester_7/wireless_communication:feedback-vs-mrc-diversity"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/feedback-vs-mrc-diversity
site: BCT Engineering
---

# Feedback vs maximum ratio combining diversity

Wireless Communication · Chapter 5: Equalization and Diversity

- Asked in: 2078 Chaitra, 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/feedback-vs-mrc-diversity
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-5

## Question

Compare between feedback and maximum ratio combining technique used in antenna diversity.

## Short answer

Feedback diversity scans branches in order, selects the first above a threshold, and uses one receiver—simple but lower performance. MRC co-phases all branches, weights each by its SNR, and sums them; output SNR equals the sum of branch SNRs, yielding best performance but requiring a receiver per branch.

## Answer

**Feedback (scanning) diversity** scans the antenna branches in a fixed order and stays with the first branch whose signal is above a threshold. When that signal falls below the threshold, it scans again. **Maximal ratio combining (MRC)** uses all branches at once: it co-phases each branch, weights it in proportion to its SNR, and adds them.

```text
 Feedback/scanning            MRC
 Ant1 -+                      Ant1 ->(xG1)-+
 Ant2 -+-[Switch]->Rx         Ant2 ->(xG2)-(Σ)->Rx
 AntM -+   ^                  AntM ->(xGM)-+
     [Compare with threshold]
```

| Point | Feedback (scanning) | Maximal ratio combining |
|---|---|---|
| Branches used | One at a time | All at the same time |
| Selection rule | First branch above threshold | Weighted sum, weight ∝ SNR |
| Receivers needed | Only one | One per branch |
| Co-phasing | Not needed | Needed |
| Output SNR | That of the chosen branch | Sum of all branch SNRs (Σγ_i) |
| Performance | Poorer than other methods | Best of all linear combiners |
| Complexity / cost | Very simple, cheap | Complex, costly |
| Works when all branches are weak | No | Yes, can still give usable SNR |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How does feedback (scanning) diversity choose a branch?
   A. Always picks the highest SNR branch
   B. Sums all branches
   C. Picks a branch at random each symbol
   D. Stays with the first branch above a threshold until it falls below
   Answer: D. It scans in a fixed order and holds the first acceptable branch. (Key point: Feedback rule)

2. How does MRC use the branches?
   A. All at once, co-phased and weighted in proportion to SNR
   B. One at a time
   C. Only the first two
   D. Only the weakest
   Answer: A. MRC adds all weighted, co-phased branches. (Key point: MRC rule)

3. Which statement about output SNR is correct?
   A. Both give the sum of branch SNRs
   B. Feedback gives the chosen branch's SNR; MRC gives the sum of all
   C. Feedback gives the sum; MRC gives one branch
   D. Both give the maximum branch SNR
   Answer: B. MRC output SNR = sum of gamma_i, which is the best of all linear combiners. (Key point: Output SNR)

4. How many receivers do the two methods need?
   A. Feedback: one per branch; MRC: one
   B. Both need one per branch
   C. Feedback: one; MRC: one per branch
   D. Neither needs a receiver
   Answer: C. Feedback switches into a single receiver, MRC needs a receiver and co-phasing per branch. (Key point: Receivers needed)

## Related in Wireless Communication

- [Need for diversity; maximum ratio combining](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adiversity-need-mrc) — 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
- [Need for diversity; combining techniques](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adiversity-combining-techniques) — Wireless Communication, chapter 5
- [Types of antenna diversity compared](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Aantenna-diversity-comparison) — Wireless Communication, chapter 5