---
title: "Space diversity techniques with block diagrams"
subject: "Wireless Communication"
chapter: 5
key: "semester_7/wireless_communication:space-diversity-block-diagrams"
source: https://www.bctengineer.com/q/semester_7/wireless_communication/space-diversity-block-diagrams
site: BCT Engineering
---

# Space diversity techniques with block diagrams

Wireless Communication · Chapter 5: Equalization and Diversity

- Asked in: 2073 Magh, 4 marks
- Video: [Diversity combining techniques](https://www.youtube.com/watch?v=KP_uO--Tq44) — komathi dayalan
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/wireless_communication/space-diversity-block-diagrams
- Whole chapter: https://www.bctengineer.com/md/semester_7/wireless_communication/chapter-5

## Question

Explain various space diversity techniques along with block diagrams.

## Short answer

In space diversity, multiple antennas provide independent fading copies. Selection diversity chooses the branch with highest SNR. Scanning diversity holds the first branch above a threshold. Maximal ratio combining co-phases and weights each branch by its SNR, summing to yield output SNR equal to the sum of individual SNRs. Equal gain combining simply co-phases with equal gains.

## Answer

**Space diversity** uses *M* receiving antennas separated enough (about λ/2 at the mobile, 10λ or more at the base station) that each receives an independently faded copy of the signal. The copies are then used in one of four ways.

#### 1. Selection diversity

```text
 Ant1-[Rx1]-+
 Ant2-[Rx2]-+--> [select max SNR] --> demodulator
 AntM-[RxM]-+
```
The branch with the highest instantaneous SNR is connected to the output. Simple; output SNR = max(γᵢ).

#### 2. Feedback (scanning) diversity

```text
 Ant1-+
 Ant2-+--[scan switch]--[Rx]--> output
 AntM-+        ^          |
               +--[SNR < threshold?]
```
Branches are scanned in sequence; the first branch above a threshold is held until it falls below it. Needs only one receiver.

#### 3. Maximal ratio combining (MRC)

```text
 Ant1-[co-phase]-[×G1]-+
 Ant2-[co-phase]-[×G2]-+--(Σ)--> detector
 AntM-[co-phase]-[×GM]-+
       Gi ∝ amplitude / noise
```
Each branch is co-phased and weighted by its SNR, then summed. Output SNR = Σ γᵢ (the best result).

#### 4. Equal gain combining (EGC)

Same as MRC but all gains Gᵢ = 1 after co-phasing. Performance is close to MRC and the circuit is simpler.

## Check yourself

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

1. What is the output SNR of selection diversity?
   A. max(gamma_i)
   B. Sum of gamma_i
   C. Mean of gamma_i
   D. min(gamma_i)
   Answer: A. It connects the highest-SNR branch. (Key point: Selection output)

2. How many receivers does feedback diversity need?
   A. M
   B. One
   C. 2M
   D. None
   Answer: B. A scan switch feeds a single receiver. (Key point: Feedback receivers)

3. What are MRC gains proportional to?
   A. Equal for all
   B. Inverse of amplitude
   C. Amplitude divided by noise power
   D. Random
   Answer: C. Gi is proportional to signal amplitude / noise. (Key point: MRC gains)

4. In EGC the gains Gi after co-phasing are...
   A. Proportional to SNR
   B. Zero for weak branches
   C. Inversely proportional to SNR
   D. All equal to 1
   Answer: D. EGC is MRC with unity gains. (Key point: EGC)

## 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
- [Diversity techniques and space diversity combining](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adiversity-space-combining) — 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
- [Need for diversity; combining techniques](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adiversity-combining-techniques) — 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
- [Importance and types of diversity](https://www.bctengineer.com/question.md?key=semester_7%2Fwireless_communication%3Adiversity-importance-types) — Wireless Communication, chapter 5

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